MASARYKOVA UNIVERZITA PŘÍRODOVĚDECKÁ FAKULTA ÚSTAV BOTANIKY A ZOOLOGIE
PREDAČNÍ STRATEGIE STENOFÁGNÍCH PAVOUKŮ Disertační práce
Eva Líznarová
Vedoucí práce: prof. Mgr. Stanislav Pekár, Ph.D.
Brno 2016
Bibliografický záznam Autor:
Mgr. Eva Líznarová Přírodovědecká fakulta, Masarykova univerzita Ústav botaniky a zoologie
Název práce:
Predační strategie stenofágních pavouků
Studijní program:
Biologie
Studijní obor:
Ekologie
Vedoucí práce:
prof. Mgr. Stanislav Pekár, Ph.D.
Akademický rok:
2016/2017
Počet stran:
59+77
Klíčová slova:
Adaptace; Evoluce; Pavouci; Predace; Potravní specializace; Trofická nika;
Bibliographic Entry Author
Mgr. Eva Líznarová Faculty of Science, Masaryk University Department of Botany and Zoology
Title of Thesis:
Predatory strategies of stenophagous spiders
Degree programme:
Biology
Field of Study:
Ecology
Supervisor:
prof. Mgr. Stanislav Pekár, Ph.D.
Academic Year:
2016/2017
Number of Pages:
59+77
Keywords:
Adaptations; Evolution; Predation; Prey specialisation; Spiders; Trophic niche;
Abstrakt Pavouci jsou predátoři, kteří loví především hmyz a jiné členovce. Potravní nároky a predační strategie jednotlivých druhů pavouků jsou velice různorodé, což z nich dělá vhodnou modelovou skupinu pro studium potravní ekologie predátorů. Z dosavadních znalostí potravní ekologie pavouků vyplývá, že stenofágní druhy, které loví jen určitý typ kořisti, jsou vzácnější než druhy euryfágní. Nicméně u většíny druhů pavouků chybí ucelené informace o jejich potravní ekologii. V této disertační práci jsem se zabývala dosud málo prozkoumanými predačními strategiemi stenofágních druhů pavouků. Zaměřila jsem se na studium pavouků lovících mravence (myrmekofágie), termity (termitofágie) a stejnonožce (oniskofágie). U vybraných druhů jsem zkoumala jejich trofickou niku a stupeň potravní specializace pomocí analýzy kořisti v přírodě a prostřednictvím laboratorních experimentů. Dále jsem u stenofágních druhů pavouků testovala přítomnost behaviorálních, metabolických a jedových adaptací, které jim pomáhají lovit jejich preferovanou kořist efektivněji než alternativní typy kořisti. Zjistila jsem, že stupeň specializace na určitý typ kořisti je u stenofágních pavouků velice rozmanitý a může se u jednotlivých druhů pavouků velice lišit. Některé druhy se živí jedním typem kořisti pouze na určité lokalitě, ale stále si zachovávají schopnost lovit i jiné typy kořisti. Jiné druhy jsou naopak specializované natolik, že jsou schopny lovit kořist pouze jednoho druhu. Specializovaní pavouci si k ulovení a zpracování preferované kořisti vyvinuli různé adaptace, které jim umožňují zužitkovat danou kořist efektivněji než nespecializovaným pavoukům, ale na druhou stranu je tyto adaptace mohou omezovat při využívání alternativních typů kořisti.
Abstract Spiders are predators hunting mostly insect and other arthropods. Trophic niches and predatory strategies differ greatly among spider species, which makes them ideal model group to study trophic ecology of predators. A far as we know, stenophagous spider species, which prefer a specific prey type, are rarer than euryphagous species. However, the data about trophic ecology of most spider species are still very scarce. In this dissertation thesis, I focused on little known predatory strategies of stenophagous spider species. I studied trophic niche and a degree of prey specialisation in spider species that feed on ants (myrmecophagy), termites (termitophagy), and isopods (oniscophagy). In my research, I combined natural prey analysis with laboratory experiments. Specifically, I tested behavioural, metabolic and venom adaptations in hunting preferred prey in selected stenophagous spider species. I found that the degree of prey specialisation differed greatly among tested stenophagous spider species. Some species were locally specialised and have generalised adaptation for the capture of several prey types, whereas another spider species possessed specialised adaptations in the capture of preferred prey. The latter predators were not able to feed efficiently on alternative prey types. Furthermore, some spider species were so strictly specialised that they can hunt only prey of one species. Specialised spiders evolved various adaptations, which enable them to catch and utilise preferred prey more efficiently than generalist spiders. However, these adaptations may constrain spider’s ability to utilise alternative prey types.
© Eva Líznarová, Masarykova univerzita, 2016
Poděkování Tato práce je završením mého desetiletého vysokoškolského studia, které by zdaleka nebylo tak skvělé bez těch, kterým bych chtěla touto cestou poděkovat. Největší dík patří Stanovi Pekárovi, který se mě hned zezačátku studia ujal a ukázal mi, že i pavouci jsou skvělá a zajímavá zvířata, která stojí za to studovat. Celé ty roky mě trpělivě vedl a snad mi i dokázal předat něco málo ze svých bohatých znalostí. I díky finanční podpoře, kterou mi po celou dobu poskytoval, jsem se mohla podívat za pavouky do různých koutů světa a účastnit se konferencí, na kterých jsem se seznámila se spoustou úžasných a inspirativních osobností arachnologického a behaviorálního světa. Velmi bych chtěla poděkovat také svým rodičům, kteří mi dali naprostou volnost ve výběru toho, čemu se chci v životě věnovat, a nenutili mě do studia nějakého „lukrativnějšího“ oboru. Rovněž mi poskytnuli zázemí, díky kterému jsem této volby nikdy nelitovala. Příteli Míšovi děkuji za domácí pohodu, vaření večeří, venčení psa, povídání si o kukačkách a pavoucích a přečtení první verze této práce a cenné připomínky k ní. Vandabandu, Bioikonám a všem Biolidem, se kterými jsem měla tu čest strávit množství šílených bioexkurzí, beček a hydrohospod, děkuji za neutuchající důkazy toho, že věda opravdu nemusí a nesmí být nudná. A nakonec děkuji všem kamarádům, kteří za ty roky prošli naší arachnoskupinou, a kterým vděčím za zábavné terény, pomoc při sběru dat a debaty o vědě i nevědě u kafe, vína, piva…ostatně za to poslední děkuji i všem svým nearachnologickým kamarádům.
Prohlášení Prohlašuji, že jsem svoji disertační práci vypracovala samostatně s využitím informačních zdrojů, které jsou v práci citovány.
Brno 26. září 2016
……………………………… Eva Líznarová
OBSAH 1 PŘEDMLUVA.............................................................................................................. 10 2 ÚVOD ........................................................................................................................... 11 2.1 Potravní ekologie ................................................................................................... 11 2.1.1 Potravní ekologie pavouků .......................................................................................... 12 2.1.2 Metody studia potravní ekologie pavouků .................................................................. 15
2.2 Stenofágní a specializovaní pavouci ...................................................................... 17 2.3 Adaptace pavouků k lovu kořisti............................................................................ 20 2.3.1 Behaviorální adaptace ................................................................................................. 20 2.3.2 Morfologické adaptace ................................................................................................ 21 2.3.3 Jedové adaptace ........................................................................................................... 21 2.3.4 Metabolické adaptace .................................................................................................. 22
2.4 Lov nebezpečné kořisti .......................................................................................... 23 2.5 Kvantifikace efektivity lovu pomocí funkční odpovědi......................................... 25 3 CÍLE PRÁCE ................................................................................................................ 28 4 SHRNUTÍ JEDNOTLIVÝCH RUKOPISŮ ................................................................. 29 4.1 Rukopis A .............................................................................................................. 30 4.2 Rukopis B ............................................................................................................... 32 4.3 Rukopis C ............................................................................................................... 34 4.4 Rukopis D .............................................................................................................. 36 4.5 Rukopis E ............................................................................................................... 38 4.6 Rukopis F ............................................................................................................... 40 4.7 Rukopis G .............................................................................................................. 42 5 ZÁVĚR ......................................................................................................................... 44 6 SEZNAM POUŽITÉ LITERATURY .......................................................................... 48 7 PŘÍLOHY ..................................................................................................................... 60
1 PŘEDMLUVA Předložená disertační práce se skládá z několika nezávislých studií, jejichž výsledky byly
sepsány
do
sedmi
samostatných
vědeckých
rukopisů
a
vydány
v odborných časopisech s IF. Všechny práce se zabývají pavouky a jejich predačními strategiemi. Rukopisy A˗D jsem vypracovala jako první autor, na rukopisech E˗G jsem se podílela jako spoluautor. Data použitá v rukopise A jsem získala už během magisterského studia, ale samotný rukopis jsem vypracovala až během doktorského studia. Data použitá v rukopise B vychází z mé diplomové práce a byla tedy také získána během magisterského studia, do podoby rukopisu byla ale opět zpracována až v průběhu doktorského studia. Sběr dat použitých v ostatních rukopisech a jejich sepsání bylo provedeno v průběhu doktorského studia. V úvodní části disertační práce se zabývám predační ekologií, především predační ekologií pavouků a možnými metodami jejího studia. Podrobněji se pak věnuji stenofágním a specializovaným pavoukům a zabývám se pozorovanými adaptacemi při lovu preferované kořisti. Dále se věnuji lovu nebezpečné kořisti v souvislosti s evolučními závody ve zbrojení mezi predátorem a kořistí. Nakonec v úvodní části popisuji, jak je možné kvantifikovat efektivitu lovu kořisti pomocí analýzy funkční odpovědi. Jednotlivé vytyčené cíle disertační práce se vztahují ke konkrétním rukopisům, jejichž hlavní zjištění následně stručně popisuji. V závěru práce shrnuji a diskutuji zjištěné poznatky a uvádím možnosti budoucího výzkumu dané problematiky. Přílohy obsahují vydané rukopisy vztahující se k tématu disertační práce, popis mého podílu na jednotlivých rukopisech, seznam všech mých dosud publikovaných prací a seznam mých příspěvků na mezinárodních a zahraničních konferencích.
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2 ÚVOD 2.1 Potravní ekologie Pojem ekologická nika zahrnuje soubor všech životních podmínek, které umožňují existenci populace určitého biologického druhu. Tyto podmínky mohou být abiotické (např. teplota, vlhkost) nebo biotické (např. potrava, predátoři). Teoretické nároky druhu na prostředí jsou označovány jako fundamentální nika (Roughgarden, 1974), kdežto skutečný prostor, který daný organismus v konkrétním prostředí zaujímá, je označován jako nika realizovaná (Polechová a Storch, 2008). Realizovaná nika bývá často jen určitým výsekem fundamentální niky a může se výrazně lišit mezi jednotlivými populacemi druhu nebo dokonce mezi samotnými jedinci, jelikož závisí i na přítomnosti jiných organismů, např. konkurentů nebo predátorů (Begon a kol., 2006). Potravní zdroje jsou nezbytné pro všechny aspekty života zvířat a množství a typ konzumované potravy přímo ovlivňuje jejich přežívání, růst a reprodukci (Wilder, 2011). Možná právě proto je trofická nika nejčastěji zkoumanou částí ekologické niky. Pojem trofická nika tedy vyjadřuje potravní nároky daného druhu a její šířka je hodnocená především různorodostí potravy (Bearhop a kol., 2004). Jednotlivé druhy se z hlediska trofické niky pohybují na kontinuu, které plynule přechází od druhů, které konzumují široké spektrum potravy, a mají tedy širokou trofickou niku (euryfágní druhy), až po druhy s velice úzkou trofickou nikou (stenofágní druhy) konzumující jen určitý typ potravy (Pekár a Toft, 2015). Stenofágní druhy najdeme častěji mezi herbivory, parazity a parazitoidy, kdežto většina predátorů, kteří musí během svého života ulovit a zkonzumovat více jedinců kořisti (Begon a kol., 2006), je považována za druhy euryfágní. Doposud se však mnohem méně studií zabývalo potravní ekologií obligátních predátorů ve srovnání s výzkumem potravní ekologie herbivorů, parazitů a omnivorů. Je tedy možné, že s podrobnějším výzkumem potravní ekologie predátorů budou i stenofágní predátoři častější, než se dosud předpokládalo. Potravní ekologii druhu lze kromě šířky trofické niky hodnotit také pomocí dalších parametrů. Jedním z nich je stupeň specializace na určitý typ potravy (Morse, 1980; Fox a Morrow, 1981). Bez ohledu na šířku trofické niky se u některých druhů vyvinuly specifické adaptace, které jim umožňují využívat preferovaný typ potravy efektivněji než alternativní typy potravy. Stupeň specializace se pak u různých druhů liší, pro zjednodušení můžeme jako generalisty označit ty druhy, které mají generalizované adaptace vhodné k využívání různých typů potravy, zatímco specialisté jsou velmi dobře 11
přizpůsobeni k využívání určitého typu potravy (více viz Pekár a Toft, 2015). Specializované adaptace se častěji vyskytují u stenofágních druhů, kterým umožňují využívat preferovanou potravu efektivněji než alternativní potravu. Ovšem existují i takzvaní polyspecialisté, kteří dokáží velice efektivně využívat více různých typů potravy, a ke zpracování každého typu potravy mají vyvinuté jiné, ale vždy vysoce specifické adaptace (Nylin a Janz, 2009; Friberg a Wiklund, 2010; Jackson a Cross, 2015). Dále je možné zjišťovat potravní preference jednotlivých zvířat, které vyjadřují, co by daný jedinec konzumoval za ideálních podmínek. Potravní preference je možné odhadovat například z přirozeného složení potravy a z chování při výběru potravy (Huseynov a kol., 2008). Nicméně musíme mít na paměti, že preference zkoumaného jedince se mohou od reálného složení potravy velmi lišit, jelikož ne vždy je preferovaná potrava dostupná (Horn a kol., 1982; Peterson a Renaud, 1989; Jackson a Cross, 2015). Pro úplné pochopení potravní ekologie druhu je tak nutné kombinovat různé přístupy a odpovídat na různé otázky vhodně zvolenými metodami (viz dále kapitolu 2.1.2). 2.1.1 Potravní ekologie pavouků Pavouci s více než 46 000 druhy (World Spider Catalog: http://www.wsc.nmbe.ch) tvoří velice diverzifikovanou skupinu predátorů, rozšířenou a početnou ve většině terestrických biotopů (Nyffeler a Benz, 1987; Coddington a Levi, 1991). Pavouci mají díky přímému i nepřímému vlivu na další organismy velký význam ve fungování celých společenstev (např. Moran a Hurd, 1997; Fagan a kol., 2002; Křivan a Schmitz, 2004). Kromě přímého vlivu na populace své kořisti samotnou konzumací může jiné organismy ovlivňovat i například pouhá přítomnost chemických signálů pavouků. Ta může mít za následek sníženou konzumaci rostlin herbivory nebo změnu jejich potravních preferencí, což má následně efekt i na společenstva rostlin a celkový koloběh živin (Schmitz, 2003). Navíc jsou potravní nároky a predační strategie různých druhů pavouků velice různorodé (Cardoso a kol., 2011), což z pavouků dělá vhodnou modelovou skupinou pro studium potravní ekologie predátorů. Pavouci se živí velice pestrou potravou a většina druhů je považována za euryfágní a generalizované predátory. To ale neznamená, že by pavouci lovili bez rozdílu každou kořist, na kterou narazí. Naopak, pavouci jsou schopni aktivního výběru kořisti, například na základě vhodné velikosti kořisti, její nebezpečnosti, nutričního složení nebo množství dostupného jedu (Uetz a Hartsock, 1987; Pasquet a Krafft, 1992; Toft, 1999; 12
Wullschleger a Nentwig, 2002). Pavouci mohou rovněž při lovu používat různé lovecké strategie a podle konkrétního loveného typu kořisti zvolit strategii, která vede k jejímu nejefektivnějšímu ulovení (Jackson a Hallas, 1986; Pasquet a Krafft, 1992; Edwards a Jackson 1993; Nelson a Jackson, 2011). Nicméně naprostá většina pavouků loví převážně hmyz, případně jiné bezobratlé živočichy (Nentwig, 1987). Ale existují pavouci, kteří dokáží ulovit i obratlovce, kterými se živí buď pouze příležitostně, jako někteří síťoví pavouci, kteří do svých sítí občas uloví i ptáky (Brooks, 2012) nebo plazy (Hódar a Sánchez‐Piñero, 2002). Poměrně běžně se obratlovci živí například někteří lovčíci, kteří dokáží pod vodou ulovit malé ryby nebo pulce žab (Bleckmann a Lotz, 1987). O jednom druhu skákavky je dokonce známo, že preferuje čerstvě nasáté komáří samice, a tak se nepřímo živí krví obratlovců (Jackson a kol., 2005). Někteří pavouci se s různou frekvencí mohou přiživovat i na rostlinném materiálu, např. na pylu (Smith a Mommsen, 1984), nektaru (Taylor a Pfannenstiel, 2008), nebo třeba na specializovaných, na bílkoviny bohatých tělíscích vyrůstajících na konci listů akácií (Meehan a kol., 2009). Mezi predátory mají pavouci celou řadu unikátních vlastností, které ovlivňují jejich potravní ekologii. Jejich trávení je mimotělní a přijímají tedy potravu výhradně v tekuté podobě. Do ulovené kořisti vypouští trávící šťávy, které tkáně kořisti rozpustí a pavouci tuto tekutinu následně nasají pomocí savého žaludku do trávicí trubice. Výhodou tohoto způsobu trávení je možnost konzumovat i relativně větší kořist s těžce stravitelnou kutikulou a rovněž zvýšená efektivita extrakce a koncentrace živin z kořisti (Cohen, 1995). Nicméně mimotělní trávení vyžaduje dodatečnou investici času oproti predátorům, kteří svoji kořist hned po ulovení celou pozřou. U pavouků tak existuje riziko, že budou během poměrně dlouhého procesu trávení vyrušeni (konkurenty, predátory nebo jinými náhodnými vlivy), a přijdou o svoji kořist, a s ní i o již investovaný čas a eventuálně další zdroje použité při lovu (jed, vlákno, trávící enzymy) (Nelson a Jackson, 2011). Pavouci natrávenou kořist nasávají do rozvětveného střeva, jehož některé slepé větve zasahují v hlavohrudi až do kyčlí nohou a v zadečku dokonce vyplňují jeho podstatnou část (Cohen, 1995). Kutikula v oblasti zadečku je navíc velice elastická a pavouci jsou schopni i během konzumace jedné větší kořisti viditelně zvětšit jeho objem. Díky těmto nutričním zásobám v jednotlivých větvích střeva jsou pak pavouci schopni přežít i mnoho dní bez příjmu potravy (Tanaka a Itô, 1982; Forster a Kavale, 1989).
13
Velká diverzita pavouků je také zřejmě spojena s klíčovou evoluční novinkou typickou pro většinu pavouků, kterou je použití vlákna při lovu kořisti (Bond a Opell, 1998). Vlákno pavouků je tvořeno ve snovacích žlázách, kterých existuje několik různých typů a které produkují různé typy vlákna lišící se svými fyzikálními a chemickými vlastnostmi (Vollrath, 1999) a následně také způsobem použití. Pavouci totiž vlákno nepoužívají pouze k lovu kořisti, ale například také ke stavbě svých úkrytů nebo ochranných obalů pro vajíčka (Nentwig a Heimer, 1987; Vollrath, 1999). Vlastní vlákno se dostává na povrch těla snovacími bradavkami umístěnými typicky na konci zadečku. Jedna čeleď pavouků (Scytodidae) navíc tvoří lepkavá vlákna i zcela netypicky ve žlázách umístěných v hlavohrudi a na svoji kořist je plive z klepítek (Gilbert a Rayor, 1985). Použití vlákna při samotném lovu kořisti je velice variabilní a liší se druh od druhu. Pavouci tak mohou vlákno používat pouze pro přenos signálu, který pavouka informuje, že je kořist nablízku, nebo si mohou z vláken budovat pasivní lapací sítě či vlákno používat aktivně přímo při lovu kořisti (Vollrath a Selden, 2007; Blackledge a kol., 2009). Podle způsobu použití vlákna při lovu kořisti můžeme pavouky zjednodušeně rozdělit do dvou základních skupin, a to na pavouky, kteří loví kořist bez pomoci lapacích sítí, a na pavouky budující si sítě určené k lovu kořisti. Zároveň ale existují i druhy pavouků, které mohou lovit oběma zmíněnými způsoby a využívají je podle své aktuální situace a typu lovené kořisti (Jackson a Blest, 1982). Pavouci lovící bez sítí na svoji kořist buď pasivně čekají na vhodném místě, například na rostlině nebo v úkrytu, a přepadají ji ze zálohy, nebo pobíhají po okolí a kořist aktivně vyhledávají. Nicméně i nesíťoví pavouci mohou při lovu používat vlákno, a to například v podobě signálních vláken vybíhajících ven z úkrytu, která je informují o přítomnosti kořisti v jejich blízkosti, nebo jako záchytná lana při skoku na kořist, anebo mohou při lovu kořist do vlákna omotat, a tak jí znemožnit únik a obranu (Gilbert a Rayor, 1985; Nentwig 1987; Nentwig a Heimer, 1987). Většina druhů pavouků si ovšem k lovu kořisti nějaký typ sítě staví (Blackledge a kol., 2009). Samotná diverzita typů pavoučích sítí je ohromná, pavouci mohou stavět jen malé síťky tvořené změtí několika málo chaoticky spletených vláken až po velice komplikované sítě ohromných rozměrů. Sítě mohou být dvourozměrné nebo trojrozměrné a pavouka buď jen informují o přítomnosti kořisti a zpomalují její pohyb, nebo kořist přímo zachycují. Kořisti může být v síti zachycena buď pomocí kapek lepu umístěných na jednotlivých vláknech anebo pomocí velice jemného (kribelátního) 14
vlášení (Vollrath a Selden, 2007), do kterého se otrněné nohy kořisti lehce zamotají (Vollrath, 2006). Do sítí se teoreticky může zachytit jakákoliv kořist, nicméně pavouci často již umístěním a tvarem sítě předurčují typ kořisti, který se do nich bude chytat nejčastěji (Rypstra, 1982). Do sítí umístěných na povrchu půdy se tak chytají nejvíce zemní členovci, kdežto sítě umístěné nad vodou slouží k lovu hmyzu vyvíjejícího se a vyletujícího z vody. Sítě s velkými mezerami mezi jednotlivými vlákny nezachytí malou kořist, kdežto do drobné síťky se zase nelapí velká kořist. Některé sítě jsou univerzálnější a vhodné k lovu širokého spektra kořisti, zatímco jiné, vysoce specializované sítě, napomáhají při lovu jinak obtížně ulovitelné kořisti, například motýlů (Stowe, 1978; Stowe, 1986; Stowe a kol., 1987). Navíc i síťoví pavouci mohou kořist zachycenou v síti, pokud je například nebezpečná, ještě dodatečně omotat do vlákna (Nentwig, 1987). Vlákno pavoukům při lovu slouží především k zachycení kořisti. K samotnému usmrcení kořisti používá převážná většina pavouků jed. Ten je tvořen v jedových žlázách, které ústí na povrch těla drápkem klepítka. Párová jedová žláza je uložena buď v bazálním drápku klepítek (u mygalomorfních pavouků) nebo
v hlavohrudi
(u araneomorfních pavouků) (Foelix, 1982). Samotný jed je tvořen rozmanitou směsí látek, převážně peptidů a proteinů s různými účinky cílenými na různé typy kořisti (Kuhn-Nentwig a kol., 2011). Jen asi u tří čeledí pavouků se vyvinuly jiné strategie usmrcení kořisti a jedová žláza druhotně zanikla. Například pavouci z čeledi pakřižákovití (Uloboridae) kořist nejprve zachytí do sítě a následně ji velice důkladně zabalí do vlákna. Do takto vzniklého balíčku pak vpustí tělní tekutinu, která obsahuje velice agresivní trávící enzymy, případně toxiny, které naruší membrány a vnitřní tkáně kořisti a kořist rychle usmrtí (Weng a kol., 2006). 2.1.2 Metody studia potravní ekologie pavouků Potravní ekologii pavouků je možné zkoumat různými metodami. Nejvhodnějším způsobem, jak zjistit, co pavouci v přirozeném prostředí loví, je samozřejmě přímé pozorování v přírodě. Nicméně snadno pozorovat při lovu můžeme často jen velké a neskrytě žijící druhy. Tato metoda je navíc poměrně časově náročná. U pavouků lovících kořist do sítí lze v ideálním případě posbírat sítě s ponechanými zbytky vysáté kořisti a tyto zbytky následně v laboratoři identifikovat (Nyffeler, 1999; Líznarová a kol., 2013). Nevýhodou této metody je, že ne všichni síťoví pavouci si po konzumaci ponechávají zbytky kořisti v síti, ne všechna kořist zachycená v síti je pavoukem skutečně 15
zkonzumována, a navíc ne všechny zbytky kořisti je možné správně identifikovat (Nentwig, 1983). U pavouků lovících bez použití sítí
je situace ještě složitější, jelikož
pravděpodobnost, že pozorovaný pavouk bude zrovna konzumovat kořist nebo bude dokonce přistižen přímo při jejím lovu, je velmi nízká (Nyffeler, 1999; Huseynov, 2005; 2006; 2007). Aby bylo možné získat dostatečně velký vzorek ulovené kořisti k vyvozování závěrů o složení potravy daného druhu, je potřeba značné trpělivosti a štěstí. Zároveň s přímým pozorováním ulovené kořisti v přírodě je také vhodné zjistit potenciální zastoupení kořisti, která se na zkoumané lokalitě vyskytuje. Pokud tyto dvě zjištění následně srovnáme pomocí vhodných indexů diverzity, můžeme zjistit, nakolik byl výběr ulovené kořisti selektivní a nakolik jen odráží její dostupnost na lokalitě (Líznarová a kol., 2013; Líznarová a Pekár, 2015). Oproti tomu analýza obsahu střeva pavouků nám pomůže zjistit, čím se pavouci v přírodě živí, bez nutnosti jejich přímého pozorování. Jelikož pavouci přijímají potravu výhradně v tekutém stavu, nelze analyzovat obsah střeva pomocí pitvy a následného přímého určování zbytků těla kořisti, jako je možné například u obratlovců (Pierce a Boyle, 1991) nebo některých skupin hmyzu (Ingerson-Mahar, 2002). K detekci zkonzumované kořisti pavouky je nutné použít jiné přístupy, např. analýzu radionukleotidů (Breene a kol., 1988), chromatografickou analýzu (Putman, 1967), elektroforézu (Solomon a kol., 1996), sérologii (Greenstone, 1996) nebo analýzu monoklonálních protilátek (Greenstone, 1996).
Nicméně v současné době je
nejefektivnější a nejpoužívanější metodou přímé identifikace zkonzumované kořisti detekce její DNA pomocí polymerázové řetězové reakce (PCR) (Symondson, 2002), využívající druhově specifické DNA primery (King a kol., 2008). Výhodou této metody je v dnešní době už relativní cenová dostupnost technologie potřebné pro vývoj specifických DNA primerů a zároveň velké množství již zmapovaných úseků DNA u hmyzu, které je možné jako primery použít. Nevýhodami této metody jsou časová náročnost, nutnost usmrcení zkoumaných jedinců a rychlá degradace DNA ve střevech. Další metodou, jak získat informace o potravní ekologii pavouků, jsou laboratorní experimenty. S jejich pomocí můžeme u jednotlivých druhů pavouků zjistit akceptaci různých typů kořisti (Nentwig, 1983; 1986; Líznarová a kol., 2013), jejich potravní preference (Jackson a kol., 1998; Jackson a Cross, 2015), predační potenciál (Mansour a Heimbach, 1993; Líznarová a Pekár, 2013), efektivitu lovu kořisti (Pruitt a Riechert; 2010; Líznarová a kol., 2013; Líznarová a Pekár, 2015) anebo testovat přítomnost 16
různých adaptací k lovu kořisti (Rovner, 1980; Jackson a kol., 1998; Líznarová a Pekár, 2016). Nicméně oproti přímému pozorování v přírodě mají laboratorní experimenty i řadu nevýhod, např. možné zkreslení výsledků kvůli nepřirozeným podmínkám, nabízení kořisti, se kterou se pavouci v přírodě nemusí setkat, odmítání jakékoliv kořisti pavouky v zajetí a podobně. Pokud jsou ovšem laboratorní pokusy dobře naplánované, je možné s jejich pomocí zodpovědět celou řadu otázek a přispět tak k celkovému poznání predační ekologie zkoumaných druhů pavouků. Poslední možností studia potravní ekologie pavouků jsou experimenty přímo v přírodě. Při této metodě se všechny interakce odehrávají v přirozeném nebo polopřirozeném prostředí. Se zkoumanými pavouky a jejich kořistí se nějakým způsobem manipuluje a výsledný efekt se srovnává s kontrolní plochou, se kterou manipulováno nebylo. Touto metodou je například možné kvantifikovat predační tlak pavouků na různé typy kořisti, například zemědělských škůdců. Výsledky pak mohou být následně využity při výzkumu využití pavouků v biologickém boji proti daným škůdcům (např. Mansour a kol., 1980; Oraze a Grigarick, 1989; Riechert a Lawrence, 1997). Tato metoda ale není používaná příliš často, jelikož na zkoumané organismy kromě studovaných faktorů často působí i faktory jiné, které nemůžeme ovlivnit nebo odfiltrovat, a které mohou zjištěné výsledky zkreslovat. Pomocí všech těchto metod a nejlépe jejich kombinací je možné získat důležité informace o potravní ekologii pavouků. Z dosavadních znalostí vyplývá, že stenofágní druhy pavouků, které loví jen určitý typ kořisti, jsou vzácnější než druhy euryfágní, lovící široké spektrum kořisti (Pekár a kol., 2012). Nicméně u většiny druhů pavouků zatím neexistují žádné nebo existují pouze neúplné informace o jejich potravní ekologii. V současné době tak s tím, jak roste počet studií věnujících se potravní ekologii pavouků, stoupá i počet zjištěných stenofágních druhů pavouků (např. Pekár a Toft, 2009; Jackson a Cross, 2015; Petráková a kol., 2015).
2.2 Stenofágní a specializovaní pavouci Pavouci se nejčastěji specializují na kořist, která je v daném ekosystému hojná a/nebo se vyskytuje agregovaně. Zároveň je časté, že tato kořist není pro generalistu vhodná, protože jí nedokáže efektivně ulovit nebo strávit (Pekár a Toft, 2015). Bylo navrženo několik různých hypotéz, proč a jak se predátoři vůbec začali na určitou kořist specializovat (Price, 1982; Tauber a kol., 1993; Thompson, 1994), nicméně důkazů podporujících dané hypotézy u pavouků existuje prozatím velmi málo. Jedna z možných 17
evolučních cest vedoucích ke specializaci predátorů na jeden typ kořisti začíná u předka‒generalisty, který tuto kořist loví. Následná evoluce vedoucí ke specializaci predátora na tuto kořist zahrnuje tři kroky. Prvním krokem je vznik vztahu mezi predátorem a danou kořistí, často daný společným prostorovým a časovým výskytem. Druhým krokem je následné vylepšení tohoto vztahu vznikem specifických adaptací, které predátorovi pomáhají lovit danou kořist efektivněji než jiné typy kořisti. A nakonec třetím krokem je zachování vztahu mezi predátorem a jeho kořistí pomocí mechanismů, které omezují lov alternativní kořisti a zajišťují reprodukční izolaci mezi specializovanými jedinci a jejich nespecializovanými příbuznými (Tauber a kol., 1993). Nejčastější typy kořisti, na které se pavouci specializují, jsou jiní pavouci (araneofágie), motýli (lepidopterofágie), dvoukřídlí (dipterofágie), stejnonožci (oniskofágie), termiti (termitofágie) a mravenci (myrmekofágie) (Pekár a kol., 2012). Ačkoliv většina druhů pavouků občas uloví pavouka jiného nebo stejného druhu, araneofágní pavouci se na lov jiných pavouků specializují a tato kořist tvoří více než 90 % jejich potravy (Jackson a Blest, 1982). Pavouci se sice většinou nevyskytují agregovaně, nicméně patří mezi nejhojnější terestrické bezobratlé predátory, a tak pravděpodobnost, že pavouk potká pavouka jiného nebo stejného druhu, je poměrně vysoká. Lov jiného pavouka ale může být poměrně riskantní záležitost, jelikož je tato kořist vybavena podobným loveckým arsenálem jako lovec sám (tzn. klepítky s vyústěním jedových žláz, pevným a pružným vláknem a efektivní strategií lovu), a lovec se tak může snadno stát kořistí (Jackson a kol., 1998). Proto se u araneofágních pavouků vyvinuly převážně takové adaptace, které jim pomáhají předcházet zranění způsobené lovenou kořistí (Clark a Jackson, 2000; Clark a kol., 2000; Pekár a kol., 2011; Wood a kol., 2012; viz dále kapitolu 2.3). Lepidopterofágní pavouci se specializují na lov motýlích dospělců. Motýli většinou nejsou pro predátory nijak nebezpečnou ani nechutnou kořistí. Jelikož se ale nevyskytují agregovaně, tak je většina pavouků loví pouze příležitostně. Navíc mají motýli na křídlech šupinky, které se při nárazu motýla do pavoučí sítě snadno odlupují, a samotný motýl se tak v síti často nezachytí. U pavouků specializovaných na lov motýlů se vyvinuly především adaptace, které umožňují kořist nalákat do jejich blízkosti (agresivní chemické mimikry) a následně ji efektivně ulovit (Eberhard, 1977; Stowe a kol., 1987; Haynes a kol., 2002). Dipterofágní pavouci se specializují na lov dvoukřídlého hmyzu. Lov této kořisti není, stejně jako lov motýlů, pro pavouky nebezpečný. Tento typ kořistí je navíc velice 18
hojný v mnoha různých biotopech a tvoří základ potravy mnoha euryfágních druhů pavouků, a to jak pavouků, kteří loví kořist pomocí sítí, tak i těch, kteří si sítě k lovu nestaví a používají jiné lovecké strategie. U pavouků specializovaných na lov dvoukřídlých se podobně jako v předchozím případě vyvinuly hlavně adaptace, které jim slouží k přilákání a následnému ulovení této kořisti (Tietjen a kol., 1987; Yeargan a Quate, 1996; 1997; Jackson a kol., 2005). Oniskofágní pavouci se specializují na lov stejnonožců (Isopoda), což jsou převážně suchozemští korýši. Stejnonožce můžeme najít poměrně běžně a často se vyskytují agregovaně v mnoha terestrických biotopech. I když je tato kořist hojná, pavouci ji loví pouze výjimečně, protože její ulovení není vůbec jednoduché. Stejnonožci využívají ke své obraně repelentní žlázy, silně sklerotizované tělo a navíc různé obranné chování, například stočení se do kuličky nebo pevné přitisknutí těla k substrátu, které jejich ulovení značně znesnadňuje (Schmalfuss, 1984; Villani a kol., 1999). Z tohoto důvodu se u pavouků specializovaných na lov stejnonožců vyvinuly rozmanité morfologické, behaviorální a metabolické adaptace, které jim napomáhají překonat tyto obranné strategie (Řezáč a kol., 2008, Pekár a kol., 2016). Termitofágní pavouci se specializují na lov termitů, což je sociálně žijící hmyz, který se vyskytuje agregovaně a ve svém areálu je dosti hojný. Nicméně termiti žijí často skrytě, ať už v hnízdech ukrytých v zemi, v nadzemních termitištích či v kusech mrtvého dřeva. Jejich aktivita na povrchu je navíc prostorově a časově nepředvídatelná. Kasta termitích dělníků je sice se svým měkkým tělem pro pavouky poměrně vhodnou a málo nebezpečnou kořistí, avšak kasta vojáků si vyvinula efektivní morfologické a behaviorální adaptace sloužící k obraně kolonie před predátory (Noirot a Darlington, 2000). U pavouků specializovaných na lov termitů se tak vyvinuly adaptace, které jim pomáhají termity vyhledat a překonat jejich obranu (Eberhard, 1991; DippenaarSchoeman a kol., 1996; Henschel, 1997). Myrmekofágní
pavouci
se specializují
na lov
mravenců. Mravenci
jsou
nejpočetnějším hmyzem ve většině terestrických biotopů. Žijí sociálně a vyskytují se agregovaně ve velkých počtech na zemi a na vegetaci. Nicméně, na rozdíl od termitů, jsou i samotné dělnice mravenců pro predátory nebezpečné. Disponují žahadly, kyselinou mravenčí a silnými kusadly a navíc dokáží při útoku nebo obraně kooperovat (Hölldobler a Wilson, 1990). Proto je loví jen málo euryfágních predátorů a většina se jim vyhýbá (Huseynov a kol., 2008). Pavouci, kteří se na lov mravenců specializují, si vyvinuli různé adaptace, které jim pomáhají obejít jejich obranné mechanismy a přežít 19
v jejich bezprostřední blízkosti (Carico, 1978; Porter a Eastmond, 1982; Castanho a Oliveira, 1997; Pekár, 2004b; 2005).
2.3 Adaptace pavouků k lovu kořisti Jelikož se výše vyjmenované typy kořisti velmi liší, například svým výskytem, chováním, antipredačními strategiemi a nutričním složením, tak se u stenofágních a specializovaných pavouků vyvinuly velice rozmanité adaptace, umožňující efektivní lov a následné zpracování jednotlivých typů kořisti. Tyto adaptace se projevují jak při vyhledávání a rozpoznání kořisti, tak při samotném lovu a následně také během jejího zpracování. Přítomnost různých adaptací tedy významně ovlivňuje šířku trofické niky a strategii lovu u konkrétního druhu pavouka. Adaptace jsou považované za specializované, pokud jsou vysoce efektivní při lovu určitého typu kořisti a méně efektivní při lovu alternativní kořisti (Pekár a Toft, 2015). Jednotlivé adaptace můžeme pro zjednodušení rozdělit do kategorií podle typu znaku, který ovlivňují. U specializovaných
pavouků
jsou
nejčastěji
zkoumány
adaptace
behaviorální,
morfologické, jedové a metabolické. 2.3.1 Behaviorální adaptace Behaviorální adaptace pavouků zahrnují různé taktiky používané při vyhledávání a lovu kořisti. Specializovaní pavouci, kteří kořist vyhledávají aktivně (např. Ammoxenidae, Salticidae, Zodariidae), musí být schopni nejprve preferovanou kořist lokalizovat. Oproti tomu pavouci, kteří na kořist pasivně čekají, musí být schopni rozpoznat místa s vysokou pravděpodobností jejího výskytu (např. Araneidae, Oecobiidae, Theridiidae). Kořist nebo vhodné místo k lovu může být rozpoznáno pomocí vizuálních, chemických nebo mechanických signálů, které spouští následné chování predátora jako je zahájení lovu nebo postavení sítě (např. Riechert a Gillespie, 1986; Morse, 1993; Persons a Uetz, 1996; Johnson a kol., 2011). Druhy pavouků, které se specializují na více typů kořisti (tzv. euryfágní specialisté neboli polyspecialisté), dokonce volí mezi různými taktikami lovu v závislosti na tom, jakou kořist loví (Jackson a Blest, 1982; Jarman a Jackson, 1986; Harland a Jackson, 2000). Oproti tomu někteří stenofágní specialisté používají při lovu různých typů kořisti stále stejnou taktiku lovu, která je však efektivní pouze při lovu preferované kořisti (Pekár, 2004a). U stenofágních specialistů, kteří loví preferovanou kořist již od prvního instaru, se rovněž předpokládá existence vrozeného obrazu preferované kořisti (search 20
image) nebo jeho vtištění během raného stádia ontogenetického vývoje (Jackson a Li, 2004; Pekár a Cárdenas, 2015). U euryfágních specialistů hraje pravděpodobně větší roli učení během života jedince (Jackson a Li, 2004; Cross a Jackson, 2010). 2.3.2 Morfologické adaptace Některé morfologické znaky, jako například tvar klepítek nebo tvar a otrnění nohou, přímo souvisí s typem lovené kořisti a predační strategií daného druhu pavouka. Tyto morfologické znaky pak ovlivňují efektivitu lovu a následné zpracování kořisti. Oniskofágní druhy pavouků (Dysderidae) se vyznačují značnou variabilitou v morfologii ústního ústrojí, která úzce souvisí s konkrétní obrannou strategií preferované kořisti (Řezáč a kol., 2008). Myrmekofágní pavouci (Zodariidae) mají splynuté klepítka (Pekár a kol., 2013), což jim pravděpodobně umožňuje přesně cílené kousnutí do nohy mravence během velmi rychlého útoku. Juvenilní jedinci a samci pavouků rodu Mastophora (Araneidae) se specializují na lov dvoukřídlého hmyzu, a k jejich ulovení používají první dva páry nohou, které jsou silně otrnělé (Yeargan a Quate, 1996, 1997). Toto otrnění zaniká u dospělých samic, které se specializují na lov motýlů a které k jejich ulovení používají zcela jinou strategii (Eberhard, 1977). Araneofágní pavouci z čeledi Archaeidae mají extrémně prodloužená klepítka, což jim pravděpodobně slouží k tomu, aby si loveného pavouka drželi bezpečně daleko od svého těla (Wood a kol., 2012). Jiná skupina araneofágních pavouků (Palpimanidae) má zase masivní a prodloužený první pár nohou, který jim pomáhá loveného pavouka pevně uchopit. Zároveň jsou tito predátoři vyzbrojeni velice silnou kutikulou, která jim slouží jako obrana před kousnutím loveným pavoukem (Pekár a kol., 2011). Araneofágní skákavky (Salticidae), které se specializují na lov jiných skákavek s výborným zrakem, svým vzezřením napodobují kus detritu, což jim umožňuje přiblížit se ke své kořisti nepozorovaně (Harland a Jackson, 2001). 2.3.3 Jedové adaptace Většina pavouků svoji kořist znehybňuje a zabíjí pomocí jedu. Stále se ovšem velice málo ví o jeho chemickém složení. Všeobecně se předpokládá, že jed stenofágních pavouků by měl být více specifický, to znamená méně diverzifikovaný ve složení peptidů a proteinů, než u euryfágních pavouků (Kuhn-Nentwig a kol., 2011). Selekce by měla preferovat rychlou účinnost jedu zvláště u pavouků, kteří loví nebezpečnou kořist, jako například jiné pavouky, termití vojáky nebo mravence. Tyto nebezpečné typy 21
kořisti mohou totiž predátora během jejich lovu zranit či dokonce usmrtit a je proto žádoucí, aby usmrcení kořisti nastalo v co nejkratším čase (Jackson a kol., 1998; Líznarová a Pekár, 2013). Srovnáním složení jedu u dvou druhů pavouků ze stejné čeledi (Zodariidae) s různým stupněm potravní specializace se ukázalo, že euryfágní pavouci mají skutečně vyšší diverzitu peptidů než stenofágní specialisté (Pekár a kol., 2013). U myrmekofágních pavouků (Zodariidae) bylo navíc zjištěno, že jejich jed není například vůbec účinný při lovu termitů (Pekár, 2004a) a v rámci mravenců je dokonce efektivnější při lovu pouze určitých druhů mravenců (Pekár, 2005; Pekár a kol., 2008). Obecně se zdá, že složení jedu stenofágních specialistů je natolik specifické, že neumožňuje efektivní lov alternativní kořisti (Pekár a Toft, 2015). 2.3.4 Metabolické adaptace Jedno z možných vysvětlení vzniku stenofágních specialistů u predátorů, a tedy i pavouků, navrhuje hypotéza o fyziologickém omezení (physiological trade-off; Singer, 2001). Podle této hypotézy by specializovaní predátoři měli mít určité metabolické adaptace, které jim napomáhají při trávení preferované kořisti, a zároveň by je tyto adaptace měly omezovat ve schopnosti trávit alternativní typy kořisti. Stenofágní specialisté jsou poměrně běžní u herbivorních členovců, kde je velká část druhů vázaná pouze na určitou hostitelskou rostlinu (Jaenike, 1990). Vznik potravní specializace u herbivorů je často podporován sníženým přežíváním při konzumaci alternativních rostlin (Futuyma a Moreno, 1988; Jaenike, 1990; Thompson, 1994; Singer, 2001), což může být důsledkem právě odlišných metabolických adaptací, například schopnosti detoxikace různých obranných látek rostlin. Existují studie na herbivorech a jejich hostitelských rostlinách, pro které tato hypotéza platí, zatímco u jiných specializovaných herbivorů potvrzena nebyla (Gould, 1979; Karban, 1989; Fry, 1990; Karowe, 1990; Via, 1991; Mackenzie, 1996; Agosta a Klemens, 2009; GarcíaRobledo a Horvitz, 2011). Predátoři jsou obecně méně specializovaní ve své potravě než herbivoři, a často loví a konzumují poměrně široké spektrum kořisti (Thompson, 1994). Euryfágní predátoři tak mohou dosáhnout optimálního příjmu energie a živin konzumací různých typů kořistí (Mayntz a kol., 2005). Zároveň ale mohou dočasně preferovat určitý typ kořisti podle svých aktuálních nutričních požadavků (Greenstone, 1979; Jensen a kol., 2012). Konzumace různých typů kořisti během života euryfágního predátora má prokazatelně 22
pozitivní efekt na jeho přežívání a vývoj (Waldbauer a Friedman, 1991; Uetz a kol., 1992; Wallin a Chiverton, 1992; Bilde a Toft, 2000). Nicméně i euryfágní predátoři se při lovu setkávají s kořistí, která pro ně není nutričně vhodná nebo je schopná obrany, a tak predátorům neposkytuje dostatečný energetický a/nebo nutriční zisk (Toft a Wise, 1999a;1999b; Bilde a Toft, 2000). Takovouto kořist pak euryfágní predátoři loví, jen pokud se s ní ještě předtím nesetkali nebo v důsledku dlouhého hladovění a nedostupnosti vhodnější kořisti (Skelhorn a Rowe, 2006; Halpin a kol., 2012). Nicméně stenofágní predátoři často loví právě kořist, které je schopná obrany (Pekár & Toft, 2015). Například mravenci, termiti a pavouci představují pro bezobratlé predátory nebezpečnou kořist, a přesto existují myrmekofágní, termitofágní a araneofágní predátoři, kteří tuto kořist loví a dokonce se na její lov specializovali (např. Jackson a Whitehouse, 1986; Hölldobler a Wilson, 1990; Pekár a Toft, 2015). Tito predátoři musejí být schopni získat všechny potřebné živiny a energii pouze z jednoho typu kořisti (Pekár a kol., 2008, 2010; Toft, 2013), nicméně přítomnost metabolických adaptací u nich byla zkoumána pouze okrajově. U stenofágních pavouků, u kterých byly metabolické adaptace studovány, bylo potvrzeno, že pokud byli krmeni preferovanou kořistí, tak přežívali lépe a vyvíjeli se rychleji (Li a Jackson, 1997; Řezáč a Pekár, 2007; Pekár a kol., 2008; Pekár a Toft, 2009). Metabolické adaptace při konzumaci jednoho typu kořisti ale mohou mít za následek snížení fitness při konzumaci alternativní kořisti, což bylo prokázáno jak u araneofágních (Li a Jackson, 1997), tak u oniskofágních (Řezáč a Pekár, 2007) a myrmekofágních pavouků (Pekár a kol., 2008; Pekár a Toft, 2009).
2.4 Lov nebezpečné kořisti Lov nebezpečné kořisti si vyžaduje takové adaptace, které predátorovi pomáhají ulovit tuto kořist efektivně a zároveň snižují riziko jeho zranění během lovu. Tyto adaptace predátora k lovu a následné obranné adaptace jeho kořisti mohou být jedním z příkladů koevoluce a evolučních závodů ve zbrojení (arms races). Koevoluce je evoluční změna znaků dvou (nebo více) druhů způsobená jejich vzájemnou interakcí. Závody ve zbrojení jsou pak příkladem antagonistické koevoluce, kdy spolu dva těsně evolučně provázané druhy soupeří o přežití (Dawkins a Krebs, 1979). Při závodech ve zbrojení vyvolává adaptace jedné strany navazující adaptaci na straně druhé, a následně zase i tato adaptace druhé strany vyvolává selekci a evoluční odpověď na straně první, a tak dále. Ne všechny koevoluční procesy, jako např. mutualistické vztahy mezi druhy nebo 23
frekvenčně závislé cyklické změny znaků mezi dvěma konkurenty, proto kritéria závodů ve zbrojení splňují (Brodie, 1999). Ačkoliv je koevoluční vztah mezi predátorem a jeho kořistí často popisován právě jako závody ve zbrojení, většinou se jedná pouze o asymetrickou selekci, která působí silněji na kořist než na predátora. Důsledkem jsou pak pouze jednostranné obranné adaptace kořisti (Brodie, 1999). Výjimkou je ale vztah mezi predátorem a kořistí, která je pro predátora nebezpečná, a ve kterém může bát síla selekce na obou stranách vyrovnaná (symetrická selekce). Nebezpečná kořist může disponovat různými morfologickými (eg. Davenport a kol., 1999), behaviorálními (eg. Jackson a Wilcox, 1990) nebo chemickými adaptacemi (eg. Brodie a Brodie, 1990), které nejenže znesnadňují její ulovení, ale predátora přímo ohrožují na životě. Predátoři, kteří nebezpečnou kořist loví, jsou tak při neúspěchu vystaveni vážným (ale zároveň předvídatelným) následkům, a tedy i silné selekci. V důsledku této selekce se může predátor začít nebezpečné kořisti úplně vyhýbat a/nebo si může vylepšit svou predační dovednost při lovu dané kořisti, což následně může vést ke zmíněným závodům ve zbrojení (Brodie, 1999). I u pavouků lovících nebezpečnou kořist může být vznik různých adaptací k lovu kořisti důsledkem závodů ve zbrojení. Příkladem může být koevoluce araneofágní skákavky druhu Portia fimbriata a její kořisti, skákavky rodu Euryattus. Skákavky rodu Euryattus jsou specifické tím, že si samice staví úkryt ze zavěšeného a stočeného listu. Z tohoto úkrytu se je predátor, skákavka P. fimbriata, snaží vylákat pomocí vibrací, kterými napodobuje vibrace vydávané samci této kořisti (agresivní mimikry). Navíc P. fimbriata při lovu ze zálohy napodobuje vhledem i pohybem detrit a znesnadňuje tak pavoukům (včetně skákavek s výborným zrakem) její včasné zpozorování. Nicméně právě skákavky rodu Euryattus jsou schopny blížící se skákavky P. fimbriata rozpoznat a buď se jim vyhnout, nebo se aktivně bránit jejich útoku. Skákavky rodu Euryattus jsou pro skákavky rodu Portia zřejmě tak důležitou kořistí, že si oba dva druhy vyvinuly velice specifické adaptace a protiadaptace, které zdokonalují na straně predátora jeho lov a na straně kořisti její obranu (Jackson a Wilcox, 1990). Mravenci jsou pro pavouky také velmi nebezpečnou kořistí a pavouci, kteří je pravidelně loví, vykazují velice specifické adaptace k jejich lovu (Castanho a Oliveira, 1997; Huseynov a kol., 2008; Pekár a kol., 2014). Nicméně dosud se ví velmi málo o protiadaptacích mravenců sloužících k jejich obraně před myrmekofágními pavouky. Jelikož jsou mravenci sociální hmyz a dělnice, které se nejčastěji stávají kořistí pavouků, 24
představují nereprodukující se kastu (Hölldobler a Wilson, 1990), je selekční tlak na jedince této kořisti pravděpodobně o dost menší než v případě jiných typů kořisti. Ulovení několika dělnic pavouky pravděpodobně nesnižuje reprodukční zdatnost konkrétního mravenčího genotypu natolik, aby byl selekční tlak dostatečně silný a dal vzniknout specializovanějším obranným adaptacím. Mravenci tak spíše investují do obecné nebezpečnosti proti více různým predátorům, aby snížili celkovou mortalitu dělnic. Jedná se tedy opět o asymetrickou selekci, v tomto případě ale působí silněji na konkrétního predátora, než na kořist. A podobně tomu bude pravděpodobně i u termitofágních pavouků lovících termity, kteří mají podobný způsob života a rozmnožování jako mravenci.
2.5 Kvantifikace efektivity lovu pomocí funkční odpovědi Jedním z možných způsobů, jak objektivně vyjádřit nebo kvantifikovat efektivitu predátorů při lovu kořisti, je použití funkční odpovědi. Funkční odpověď popisuje množství ulovené kořisti jedním predátorem za jednotku času při měnících se hustotách kořisti (Solomon, 1949). Znalost funkční odpovědi může být využita v různých oblastech studia zkoumaného druhu a lze ji zjišťovat různými způsoby. V populační ekologii je znalost funkční odpovědi zajímavá, jelikož spojuje různé trofické úrovně, a je v ní vhodné použít dlouhodobé průměrné funkční odpovědi mnoha jedinců. Z evolučního hlediska je znalost funkční odpovědi rovněž žádoucí, jelikož zahrnuje důležité faktory fitness jako je příjem energie a riziko mortality, a nejvíce přínosné jsou v tomto ohledu dlouhodobé funkční odpovědi jednotlivých jedinců. I etologické studie mohou pracovat s funkčními odpověďmi, jelikož chování zvířat je často schopné adaptace, a ty jsou opět ovlivněny příjmem energie a rizikem mortality. V etologických studiích jsou využívané zejména krátkodobé funkční odpovědi jednotlivých jedinců. A nakonec je možné využít znalost funkční odpovědi i při studiu fyziologie zvířat, a v této oblasti jsou využívané krátkodobé i dlouhodobé odpovědi jednotlivých jedinců (Jeschke a kol., 2004). Při studiu funkční odpovědi můžeme zjišťovat odpovědi na různé otázky. Základní otázkou je, jaký je typ funkční odpovědi predátora, což nás zajímá, pokud se snažíme zjistit, je-li hustotně závislá predace stabilizujícím faktorem v populační dynamice predátora a jeho kořisti. Rovněž bychom na tuto otázku měli znát odpověď před tím, než pozorovanými daty proložíme nějaký konkrétní matematický model. Další možná otázka, kterou si můžeme při studiu funkčních odpovědí položit, je, jaké jsou nejlepší 25
odhady parametrů modelu funkční odpovědi. To je důležité, pokud se snažíme použít funkční odpověď v modelu popisujícím predaci nebo kompetici o zdroje. Na to navazuje další otázka, a to zda se parametry popisující dvě nebo více funkčních odpovědí signifikantně liší. Odpověď na tuto otázku nás zajímá, pokud se pokoušíme porovnat rozdílné druhy predátorů, abychom určili, který z nich je efektivnější v lovu určitého typu kořisti. Také můžeme porovnávat různé druhy nebo populace kořisti a testovat tak rozdílnou koevoluci mezi predátorem a jeho kořistí. Anebo můžeme srovnávat efektivitu predátora lovícího v různém prostředí (Juliano, 1993). Prozatím byly popsány čtyři základní typy funkčních odpovědí (Holling, 1961). V prvním typu funkční odpovědi stoupá počet ulovené kořisti lineárně s rostoucí hustotou kořisti až po dosažení určité maximální úrovně, kdy zůstává počet ulovené kořisti konstantní bez ohledu na hustotu kořisti. Tento typ je charakteristický například pro filtrátory (Jeschke a kol., 2004). Ve druhém typu funkční odpovědi počet ulovené kořisti rovněž stoupá s rostoucí hustotou kořisti, nikoliv však lineárně, ale asymptoticky se přibližuje maximální úrovni. Tento typ funkční odpovědi je nejčastěji pozorovaným typem v experimentech, a to i v případech, kdy jsou jako predátoři použiti pavouci (např. Heong a kol., 1991; Hardman a Turbull, 1974; Smith a Wellington, 1986). Tvar křivky funkční odpovědi typu tři je esovitý, jelikož po překročení určité hustoty kořisti dochází k rychlejšímu nárůstu v počtu ulovené kořisti, než by odpovídalo přímé úměře mezi lovem a hustotou. Dříve se předpokládalo, že je tento typ specifický pouze pro obratlovce (Holling 1965), ale dnes už byl prokázán i u bezobratlých predátorů a parazitoidů (Hassell, 2000). Všechny tyto typy jsou rostoucími funkcemi hustoty kořisti. Průběh funkční odpovědi typu čtyři je zpočátku také rostoucí a podobá se funkční odpovědi typu dvě nebo tři, avšak po překročení určité hustoty kořisti dochází k poklesu v počtu ulovené kořisti pod dosažené maximum. Tento typ byl zpočátku pouze teoreticky odvozen z matematického modelu a jeho existence se předpokládala pouze u obratlovců (Holling, 1965). Nicméně později byl tento typ funkční odpovědi nalezen i u bezobratlých predátorů (Mori a Chant, 1966; Tostowaryk, 1972), avšak stále zůstává nejméně prozkoumaným typem funkční odpovědi. V experimentech, ve kterých byla pozorována funkční odpověď typu čtyři, docházelo k poklesu v počtu ulovené kořisti po dosažení určité hustoty kořisti v důsledku různých faktorů, které ovlivňovaly buď samotného predátora, nebo jeho kořist. Jednou z možných příčin vzniku funkční odpovědi typu čtyři je zmatení predátora při vysokých hustotách kořisti, které často nastává, pokud se kořist pohybuje v hejnu 26
(Welty, 1934; Jeschke a Tollrian, 2007). Při zmatení predátora tak dochází ke snížení množství úspěšných útoků v důsledku neschopnosti predátora odlišit z hejna jednoho jedince kořisti a zaměřit na něj svůj útok (Krause a Ruxton, 2002). Ke snížení množství ulovené kořisti při jejích vysokých hustotách může docházet i v případě, kdy pohybující se jedinci kořisti predátora při lovu přímo vyrušují (Mori a Chant, 1966; Sadana, 1991). S rostoucí hustotou kořisti roste i pravděpodobnost jejího střetu s predátorem a množství ulovené kořisti klesá pod dosažené maximum. Některé typy kořisti jsou často schopny nejen predátora vyrušovat svojí přítomností, ale navíc se i aktivně bránit. A pokud jsou jedinci kořisti navíc schopni při obraně spolupracovat, je efektivita jejich obrany hustotně závislá, a s rostoucí hustotou kořisti tak postupně klesá i množství kořisti, které predátor uloví (Tostowaryk, 1972). Právě funkční odpověď typu 4 můžeme očekávat i u pavouků, kteří loví nebezpečnou kořist vyskytující se přirozeně při vysokých hustotách, jako jsou například mravenci nebo termiti.
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3 CÍLE PRÁCE Cílem této disertační práce bylo prostudovat predační strategie stenofágních druhů pavouků. V jednotlivých rukopisech se věnuji těmto dílčím cílům: 1) Zjistit
stupeň
potravní
specializace
pavouka
druhu
Oecobius
navus
pomocí analýzy složení kořisti v přírodě a laboratorních experimentů u dvou vzdálených populací (Rukopis A). 2) Kvantifikovat pomocí funkční odpovědi stupeň potravní specializace na lov nebezpečné kořisti u pavouků (Rukopis B). 3) Zjistit trofickou niku a srovnat efektivitu lovu dvou typů kořisti u pavouka druhu Oecobius maculatus (Rukopis C). 4) Porovnat efektivitu v trávení dvou typů kořisti u myrmekofágních pavouků druhu Euryopis episinoides a otestovat tak přítomnost metabolických adaptací (Rukopis D). 5) Zjistit přítomnost adaptací, které pomáhají myrmekofágním pavoukům druhu Zodarion cyraneicum lovit několikanásobně větší kořist (Rukopis E). 6) Prozkoumat trofickou niku a adaptace k lovu kořisti u termitofágního pavouka druhu Ammoxenus amphalodes (Rukopis F). 7) Otestovat vhodnost stejnonožců jako kořisti pro tři druhy pavouků s různým stupněm potravní specializace (Rukopis G).
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4 SHRNUTÍ JEDNOTLIVÝCH RUKOPISŮ
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4.1 Rukopis A Líznarová, E., Sentenská, L., García, L. F., Pekár, S. a Viera, C. (2013). Local trophic specialisation in a cosmopolitan spider (Araneae). Zoology, 116, 20-26. Potravní specializace může nabývat dvou různých forem: specializace fundamentální, kdy jsou všichni jedinci jednoho druhu specializovaní na jeden typ kořisti, a pak specializace lokální, kdy jsou na určitou kořist specializovaní pouze jedinci z konkrétní populace. Aby bylo možné tyto dvě formy rozlišit, je nutné zkoumat několik různých populací nebo generací jednoho druhu. U predátorů je ale často zkoumána pouze jediná populace, a to ještě během krátkého časového období. A tak druhy, které na základě studia jediné populace považujeme za fundamentální specialisty, mohou být specializované pouze lokálně. Lokální specialisté by měli mít velice plastické adaptace, jelikož získávání různých typů potravy vyžaduje odlišné kognitivní, morfologické, behaviorální a metabolické adaptace. Oproti tomu fundamentální specialisté mají často velice stereotypní a specializované adaptace, které jim pomáhají efektivně zpracovávat pouze jeden typ potravy, a tyto adaptace jsou navíc často geneticky fixované. My jsme studovali stupeň potravní specializace u kosmopolitně rozšířeného pavouka druhu Oecobius navus. U tohoto pavouka se na základě kusých informací o jeho potravní ekologii předpokládalo, že se specializuje na lov mravenců. Nicméně dosud chyběly podrobnější informace o složení jeho kořisti v přírodě a existenci různých adaptací k lovu kořisti. Tito pavouci si staví pod kameny nebo na zdech domů síťku, která jim slouží jako úkryt. Tato síťka je složená ze dvou na sobě položených pavučinových plachetek, mezi kterými se ukrývá samotný pavouk. Ze síťky vybíhají do okolí signální vlákna, která schovaného pavouka informují o přítomnosti kořisti v jeho blízkosti. Jakmile kořist pohybující se v blízkosti této síťky zavadí o signální vlákno, pavouk vyběhne ze svého úkrytu a začne kořist velice rychle obíhat a pomocí zadního páru končetin na ni ze snovacích bradavek nahazovat vlákno. Teprve když je kořist pečlivě omotaná do vlákna, tak se k ní pavouk přiblíží blíž a kousnutím do ní vpustí jed. Poté si kořist přichytí ke snovacím bradavkám a odnese si ji do blízkosti své síťky, kde ji začne konzumovat. Jelikož tito pavouci svoji kořist vysávají vcelku, je následně možné zkonzumovanou kořist identifikovat podle zbytků těl ponechaných v okolí síťky.
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V tomto rukopise jsme srovnávali trofickou niku dvou vzdálených populací pavouka O. navus, a to na lokalitách v Portugalsku a v Uruguayi. Zjistili jsme, že tito pavouci v přírodě lovili nejčastěji kořist, která byla na dané lokalitě nejhojnější. V Uruguayi pavouci lovili převážně mravence, kteří tam byli zároveň nejhojnější dostupnou kořistí, a na lokalitě v Portugalsku, kde se mravenci téměř nevyskytovali, pavouci lovili převážně velmi hojné dospělce dvoukřídlého hmyzu. V laboratorních pokusech jsme navíc zjistili, že tito pavouci jsou schopni lovit poměrně široké spektrum různých typů kořisti. Rovněž jsme změřili a srovnali efektivitu lovu dvou typů kořisti, mravenců a dvoukřídlých, u pavouků z obou populací. Zjistili jsme, že jedinci z každé populace lovili efektivněji ten typ kořisti, který byl na jejich lokalitě nejhojnější. Jelikož jsme ale v pokusu použili dospělé jedince pavouků odchycené přímo na zkoumaných lokalitách, může být vyšší efektivita lovu kořisti způsobena předchozí zkušeností jedinců s danou kořistí a schopností učit se lovit známou kořist efektivněji, spíše než genetickými rozdíly mezi populacemi. Naše výsledky tak naznačují, že pavouci druhu O.navus jsou stenofágní generalisté, kteří jsou schopni efektivně lovit více typů kořisti. Nicméně v závislosti na lokalitě se dokáží živit pouze jedním typem lokálně hojné kořisti. Je zajímavé, že i další druhy pavouků rodu Oecobius jsou na základě předběžných pozorování považovány za myrmekofágní specialisty (O. cellariorum, O. civitas, O. interpellator, O. maculatus, O. templi). To je pravděpodobně způsobeno tím, že mravenci se často vyskytují na podobných stanovištích, na kterých si tito pavouci často staví své síťky (kamenné zídky, pod kameny), a zároveň jsou tito pavouci schopni mravence poměrně efektivně lovit. Pokud tedy tito pavouci loví nejhojnější kořist na lokalitě, na mnoha lokalitách jsou touto kořistí právě mravenci.
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4.2 Rukopis B Líznarová, E. & Pekár, S. (2013). Dangerous prey is associated with a type 4 functional response in spiders. Animal Behaviour, 85, 1183-1190. Některé typy kořisti mohou predátorům znesnadňovat jejich lov, například tím, že snižují efektivitu lovu nebo prodlužují dobu nutnou k úspěšnému ulovení. Určité typy kořisti mohou být i pro samotného predátora vyloženě nebezpečné a mohou způsobit jeho zranění nebo dokonce smrt. Takováto kořist používá ke své obraně různé morfologické nebo behaviorální adaptace nebo disponuje chemickými látkami, které mohou predátora zranit či zabít. Jedinci koloniálně nebo sociálně žijící kořisti mohou navíc při obraně před predátorem spolupracovat a efektivita obrany se s počtem jedinců kořisti ještě zvyšuje, a je tedy hustotně závislá. Ke kvantifikaci hustotně závislého efektu obrany kořisti na predátorovu efektivitu lovu je možné použít funkční odpověď, která udává množství zabité kořisti v závislosti na její hustotě. Doposud byly popsány čtyři základní typy funkčních odpovědí, z nichž první tři v sobě nezahrnují efekt obrany kořisti, a proto jsou vhodné k popisu lovu kořisti, která není pro predátora nijak nebezpečná. Zato čtvrtý typ funkční odpovědi má ve svém modelu obsaženu proměnnou, kterou je možné vztáhnout právě k obraně kořisti. Průběh funkční odpovědi typu čtyři je specifický v tom, že po překročení určité hustoty kořisti dochází k poklesu ve frekvenci lovu predátora pod dosažené maximum. Příčinou poklesu ve frekvenci lovu při vysoké hustotě kořisti může být například zmatení predátora kořistí pohybující se v hejnu, nebo vyrušování predátora při lovu dalšími jedinci kořisti. V tomto rukopise jsme zkoumali funkční odpověď u pavouků lovících mravence. Mravenci jsou pro většinu pavouků nebezpečná kořist, a proto je pavouci loví jen zřídka anebo vůbec. Přesto se někteří pavouci na lov mravenců specializovali. My jsme použili funkční odpověď ke srovnání efektivity lovu mravenců u tří druhů pavouků s různým stupněm specializace na mravence. Zároveň jsme srovnávali i efektivitu při obraně u tří rodů mravenců s různým stupněm agrese. Ve všech případech jsme pozorovali funkční odpověď typu čtyři, ve které došlo k poklesu ve frekvenci lovu při vysokých hustotách mravenců pravděpodobně kvůli schopnosti mravenců kooperovat při obraně před predátorem.
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Pro pozorovanou funkční odpověď jsme navrhli nový matematický model, který ji popisoval lépe, než dosud navržené modely pro funkční odpověď typu čtyři. Parametry odhadnuté tímto modelem (vyhledávací účinnost, doba zpracování kořisti, inhibice kořistí) jsme pak srovnali mezi jednotlivými druhy pavouků. Zároveň jsme srovnali mortalitu pavouků v průběhu experimentu. Zjistili jsme, že myrmekofágní pavouci rodu Zodarion lovili mravence nejefektivněji a zároveň měli nejnižší mortalitu, a to i v přítomnosti velkého počtu mravenců. To naznačuje, že jsou tito pavouci velmi dobře adaptovaní na život v těsné blízkosti mravenišť, kde loví svoji kořist. Euryfágní pavouci rodu Xysticus lovili mravence méně efektivně a jejich mortalita byla nejvyšší, což naznačuje, že tito pavouci v přírodě loví mravence pouze příležitostně a spíše osamocené dělnice vyskytující se dál od hnízda. Euryfágní pavouci rodu Pardosa mravence nelovili vůbec a jejich mortalita byla poměrně nízká, a pravděpodobně jsou tak schopni se mravencům v přírodě efektivně vyhýbat. Mravenci všech tří použitých rodů byli vůči pavoukům agresivní. S jejich rostoucí agresivitou se snižovala i frekvence jejich lovu pavouky, jelikož pavouci museli trávit více času vlastní obranou a méně času jim zbylo na samotný lov. Námi navržený matematický model funkční odpovědi typu 4 je možné aplikovat i na jiné systémy predátora a jeho kořisti, ve kterých je kořist pro predátora nebezpečná a pokles v počtu ulovené kořisti při vysokých hustotách kořisti je způsoben společnou obranou kořisti.
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4.3 Rukopis C Líznarová, E. & Pekár, S. (2015). Trophic niche of Oecobius maculatus (Araneae: Oecobiidae): evidence based on natural diet, prey capture success, and prey handling. The Journal of Arachnology, 43, 188-193. I samotní predátoři jsou během lovu kořisti ohrožení tím, že budou uloveni jiným predátorem, a proto se snaží dobu věnovanou lovu minimalizovat. Na druhou stranu lov nebezpečné kořisti vyžaduje opatrnost ze strany predátora, a tak je nebezpečná kořist zpravidla zpracovávána déle než kořist neškodná. Navíc predátor často do lovu nebezpečné kořisti musí investovat i více jiných zdrojů, než jen svůj čas. Konkrétně pavouci musí často použít při lovu nebezpečné kořisti více vlákna a/nebo jedu než při lovu snadno ulovitelné kořisti. Lov nebezpečné kořisti je tak energeticky náročnější, a tak se predátoři takovéto kořisti buď úplně vyhýbají, nebo si vyvinou adaptace k jejímu lovu, které jim pomáhají lovit ji efektivněji. Pro většinu pavouků představují nebezpečnou kořist mravenci, a tak se jim většina euryfágních pavouků bez specializovaných adaptací k jejich lovu vyhýbá. Nicméně je známo několik druhů pavouků, kteří se na lov mravenců specializují, například pavouci rodu Zodarion si k jejich lovu vyvinuli velice specifickou taktiku lovu, kdy minimalizují riziko svého zranění a zároveň i vystavení se predátorům. Rovněž mají velice efektivní jed, kdy jediné krátké kousnutí mravence vede k jeho úplné a rychlé paralýze. I u pavouků rodu Oecobius se předpokládalo, že se na lov mravenců specializují, ale nedávná detailní studie ukázala (viz Rukopis A), že loví převážně nejhojnější kořist, která se v jejich okolí vyskytuje. A jelikož jsou právě mravenci poměrně hojnou kořistí na podobných stanovištích, kde se žijí i pavouci rodu Oecobius, loví je tito pavouci poměrně často. Nicméně zatím nebylo podrobněji zkoumáno, jak efektivně dokáží mravence ulovit ve srovnání s jinými typy kořisti. V tomto rukopise jsme se věnovali stupni potravní specializaci a efektivitě lovu dvou typů kořisti u pavouka druhu Oecobius maculatus. Z pozorování v přírodě a pomocí laboratorních experimentů jsme prozkoumali potravní ekologii populace žijící na ostrově Brač v Chorvatsku. Analýza kořisti z přírody odhalila, že pavouci sice lovili kořist z pěti různých hmyzích řádů, ale nejčastěji lovenou kořistí byli mravenci, kteří byli zároveň nejhojnější potenciální kořistí na zkoumané lokalitě. Laboratorní experimenty dále ukázaly, že pavouci byli schopni lovit poměrně široké spektrum kořisti. Termity, 34
octomilky a chvostoskoky dokonce lovili s větší pravděpodobností než mravence. Efektivitu lovu jsme měřili a srovnávali u dvou typů kořisti, kde mravenci představovali nebezpečnou kořist a octomilky snadno ulovitelnou kořist. Srovnávali jsme celkovou dobu lovu kořisti a zároveň i investici vlákna a jedu použitou k ulovení jednoho jedince kořisti. Lov mravenců trval výrazně déle, než lov octomilek a pavouci do ulovení mravence investovali více vlákna a jedu než do ulovení octomilky. Lov octomilek byl tedy celkově efektivnější, nicméně zůstává otázkou, zda je získaná energie a živiny z jedné octomilky srovnatelná s výtěžkem získaným konzumací mravence stejné velikosti. Námi získaná data potvrzují hypotézu, že pavouci druhu O. maculatus jsou stenofágní generalisté. Nepozorovali jsme žádné specifické adaptace, které by jim pomáhaly lovit mravence efektivněji než jiné typy kořisti. Jelikož tito pavouci v laboratoři ochotně akceptovali různé typy kořisti, je u nich složení kořisti v přírodě dáno spíše preferencí určitého loveckého místa, než preferencí konkrétního typu kořisti. A protože byli námi studovaní pavouci nalezeni v blízkosti hojného výskytu mravenců, tak právě mravenci tvořili převážnou část jejich kořisti.
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4.4 Rukopis D Líznarová, E. & Pekár, S. (2016). Metabolic specialisation on preferred prey and constraints in the utilisation of alternative prey in an ant-eating spider. Zoology (online). U potravních specialistů se předpokládá přítomnost různých adaptací, které jim pomáhají při zpracování preferované potravy. Tyto adaptace ale mohou zároveň omezovat schopnost využívat alternativní typy potravy. Jedním možným přizpůsobením na preferovanou potravu jsou metabolické adaptace. Přítomnost metabolických adaptací je poměrně dobře prozkoumaná u herbivorů, jelikož rostliny se před herbivory často brání přítomností různých toxických chemikálií ve svých tkáních a herbivoři si proto museli vyvinout schopnost tyto toxiny tolerovat. Nicméně přizpůsobení svého trávení na jeden druh rostliny často znamená, že herbivor pak není schopen využívat stejně efektivně i jiné druhy rostlin s rozdílným obsahem toxinů. Euryfágní predátoři během svého života konzumují více různých typů kořisti a optimálního příjmu potřebných živin mohou dosáhnout jejich kombinací. Oproti tomu stenofágní predátoři konzumují během svého života pouze jeden typ kořisti a musí z něj být schopni získat všechny potřebné živiny. Stenofágní predátoři navíc často loví kořist, která je pro generalisty poměrně málo nutričně výživná. Proto se u stenofágních predátorů předpokládá existence metabolických adaptací, které jim napomáhají při trávení preferované kořisti. Nicméně tyto metabolické adaptace mohou mít za následek neschopnost získávat potřebné živiny z alternativní kořisti. Stenofágní pavouci se poměrně často specializují na lov mravenců. Ačkoliv jsou mravenci velice hojní ve většině terestrických biotopů, nejsou pro řadu druhů pavouků vhodnou kořistí. Samotný jejich lov je náročný a nebezpečný, jelikož jsou vybaveni silnými kusadly, žahadlem nebo kyselinou mravenčí. Navíc pro většinu predátorů nejsou ani chutnou kořistí, jelikož mají poměrně subtilní a sklerotizované tělo obsahující různé chemické látky sloužící k jejich obraně. Pavouci druhu Euryopis episinoides jsou ale schopni lovit mravence poměrně efektivně, jelikož na ně z dálky vrhají pomocí zadních končetin vlákno, a tím je znehybní ještě před tím, než se přiblíží a kousnutím vpraví do jejich těla jed, který mravence usmrtí. Doposud se ale nevědělo, jak jsou tito pavouci efektivní v získávání potřebných živin a energie z mravenců ve srovnání s jinými typy kořisti. 36
V tomto rukopise jsme testovali hypotézu, že jsou myrmekofágní pavouci druhu E. episinoides metabolicky adaptovaní k trávení mravenců, a jako důsledek této adaptace mají sníženou efektivitu v trávení alternativní kořisti. Čerstvě vylíhnutá mláďata těchto pavouků jsme rozdělili do dvou skupin, kde první skupina byla po celou dobu vývoje krmena pouze mravenci a druhá skupina pouze octomilkami. Octomilky použité ke krmení jsme chovali na speciálně obohaceném médiu, takže byly svým nutričním složením vhodné pro euryfágní pavouky. U těchto dvou skupin pavouků jsme srovnávali jejich přežívání, nárůst hmotnosti, rychlost ontogenetického vývoje a reprodukční zdatnost. Mláďata byla do skupin rozdělena sice náhodně, ale zároveň tak, aby byli v obou skupinách rovnoměrně zastoupeni potomci od všech samic. To nám dále umožnilo otestovat vliv genetické variability a efekt matky na prospívání mláďat. Zjistili jsme, že přežívání a nárůst hmotnosti byl významně lepší u mláďat krmených mravenci, ale celková doba jejich ontogenetického vývoje ani počet nakladených kokonů se mezi těmito dvěma skupinami nelišil. Nicméně u pavouků krmených mravenci byl výrazně větší počet vajíček v jednom kokonu a rovněž úspěšnost líhnutí mláďat z vajíček byla vyšší než u pavouků krmených octomilkami. Efekt matky byl významný hned u několika pozorovaných proměnných, především u přežívání a vývoje mláďat krmených octomilkami. Většina testovaných pavouků sice dokázala octomilku ulovit, ale jen několik málo jedinců se bylo schopno vyvíjet a rozmnožovat, pokud byli krmeni pouze octomilkami. To naznačuje existenci omezení při trávení alternativní kořisti v důsledku metabolické specializace na mravence.
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4.5 Rukopis E Pekár, S., Šedo, O., Líznarová, E., Korenko, S. & Zdráhal, Z. (2014). David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey. Naturwissenschaften, 101, 533-540. Většina druhů pavouků je považována za euryfágní predátory, kteří loví různé typy kořisti. Velikost těla vhodných typů kořisti pro euryfágní predátory je zpravidla menší, než velikost těla predátorů. Oproti tomu specializovaní stenofágní predátoři jsou schopni ulovit stejně velkou nebo dokonce větší kořist, než jsou oni sami. Ulovení kořisti větší než je predátor vyžaduje speciální adaptace, u pavouků k tomuto účelu slouží například specifické taktiky lovu, lapací sítě nebo velice účinný jed. Během ontogenetického vývoje se tělo pavouků zvětšuje a euryfágní pavouci tak v dospělosti loví větší typy kořisti, než lovili jako juvenilové. K tomu jim může pomáhat i větší množství dostupného jedu v dospělosti. Specializovaní stenofágní pavouci ale během celého svého vývoje často loví pouze jeden typ kořisti, a tak jsou juvenilové vystaveni nutnosti lovit stejnou kořist jako dospělci, která oproti nim může být i mnohonásobně větší. V tomto rukopise jsme zjišťovali složení kořisti u myrmekofágního pavouka druhu Zodarion cyrenaicum a jejich adaptace k lovu kořisti. Tito pavouci byli pozorováni při lovu mravenců druhu Messor arenarius, jehož dělnice jsou sice velikostně polymorfní, ale i ta nejmenší kasta je stále větší, než studovaní pavouci. Z dat získaných v přírodě jsme srovnali velikost kořisti lovenou juvenilními jedinci a velikost kořisti lovenou dospělci. V laboratoři jsme testovali přítomnost behaviorálních a jedových adaptací, které by pavoukům mohly umožnit lov větší kořisti. Předpokládali jsme, že jelikož mají juvenilní jedinci velice malé jedové žlázy ve srovnání s dospělci a produkují tedy poměrně malé množství jedu, musí být jejich jed velice efektivní při paralýze mravenců druhu M. arenarius. Provedli jsme srovnání jedu u dospělců a juvenilů, abychom zjistili, zda se jeho složení mění během ontogenetického vývoje. Na studované lokalitě se vyskytovalo pět druhů mravenců, nicméně pavouci Z. cyrenaicum lovili téměř výhradně mravence druhu M. arenarius. Dospělé samice lovily velké dělnice mravence M. arenarius, zatímco drobní juvenilové lovili menší dělnice. Nicméně v obou případech byla kořist mnohonásobně větší než pavouci. Dospělci i juvenilové pavouků používali k lovu mravenců efektivní predační strategii, která je chránila před obrannými útoky mravenců. Dospělci a část juvenilů útočili na 38
mravence ze strany a kousnutí mířili nejčastěji na některou ze zadních končetin nebo zadeček mravence. Po kousnutí se pak okamžitě stáhli a čekali, dokud jejich jed mravence neparalyzuje. Teprve potom se k němu vrátili a začali jej konzumovat. Část juvenilů ale použila k lovu několikanásobně většího mravence jinou strategii. Tito drobní pavouci se vyšplhali na dorzální stranu mravence a zakousnuli se do jeho zadečku nebo do tělní stopky a tam zůstali pevně přichyceni a čekali, dokud jed mravence neparalyzuje. Většině pavouků stačilo ke znehybnění mravence jedno rychlé kousnutí. Ačkoliv byla jedová žláza dospělců mnohem větší než žláza juvenilů, doba paralýzy juvenilů byla jen nepatrně delší. Složení jedu se u juvenilů a dospělců nelišilo v poměru peptidů a proteinů. Získané výsledky ukazují, že behaviorální a jedové adaptace napomáhají potravně specializovaným pavoukům lovit několikanásobně větší kořist, a to už během juvenilních stadií.
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4.6 Rukopis F Petráková, L., Líznarová, E., Pekár, S., Haddad, C. R., Sentenská, L. & Symondson, W. O. (2015). Discovery of a monophagous true predator, a specialist termite-eating spider (Araneae: Ammoxenidae). Scientific reports, 5: 14013. Predátoři musí za svůj život ulovit a zkonzumovat více jedinců kořisti a jsou tak většinou euryfágní, tedy lovící různé typy kořisti. Ale existují i příklady stenofágních predátorů, kteří se specializují na lov pouze jednoho typu kořisti. Kořist stenofágních predátorů je často tvořena jen několika zástupci jedné čeledi nebo několika druhy jednoho rodu. Monofágní predátor, který by po celý život lovil výhradně jen jeden druh kořisti, dosud objeven nebyl. Stenofágní pavouci se často specializují na lov sice hojné, ale obtížně ulovitelné kořisti. Termiti jsou v místech svého výskytu hojnou kořistí, ale jen poměrně málo pavouků se specializuje na jejich lov. To je pravděpodobně způsobeno skrytým způsobem života termitů, kteří tráví většinu života v hnízdech v zemi, v mrtvém dřevě nebo v nadzemních termitištích, a na povrch vylézají jen v krátkých a nepravidelných intervalech. Nicméně všichni pavouci rodu Ammoxenus (Ammoxenidae) se zdají být k lovu termitů přizpůsobeni. Některé druhy pavouků z této čeledi byly pozorovány, že se živí termitem druhu Hodotermes mosambicus, což je jediný druh tohoto rodu vyskytující se v jihovýchodní Africe. Tito termiti obývají podzemní hnízda a ven vychází dělnice jen na velice rychlé a nepravidelné výpravy, při kterých sbírají stébla rostlin, na kterých následně ve svých hnízdech pěstují houby. Tyto termity loví v Jižní Africe endemičtí pavouci druhu Ammoxenus amphalodes, kteří si budují pavučinové komůrky ukryté v písku, často v blízkosti vstupů do termitišť, a na povrch vylézají lovit jen v omezené době, právě když termiti opouští svá hnízda. Vhodný okamžik pravděpodobně poznají pomocí chemických nebo vibračních signálů vydávaných termity. V tomto rukopise jsme testovali hypotézu, že pavouci druhu A. amphalodes jsou monofágní specialisté, kteří loví výhradně termity druhu H. mosambicus. Abychom důkladně prozkoumali potravní ekologii tohoto druhu pavouka, zkombinovali jsme pozorování pavouků v přírodě s laboratorními experimenty. Na lokalitě v Jižní Africe jsme nasbírali pavouky a jejich potenciální kořist, kterou jsme následně určili v laboratoři. Přirozenou kořist, kterou tito pavouci lovili v přírodě, jsme zkoumali 40
pomocí molekulární analýzy obsahu střeva pavouků. V laboratoři jsme dále provedli akceptační experimenty, kdy jsme pavoukům nabízeli různé typy kořisti a pozorovali jejich lov, a následně jsme měřili dobu paralýzy kořisti jedem pavouka. Naprostá většina sekvencí, nalezených ve střevě pavouků, byla tvořena jedním druhem termita, a to termitem druhu H. mosambicus. V akceptačních experimentech pak tito pavouci nelovili žádné jiné typy kořisti než termity druhu H. mosambicus, dokonce ani jiné druhy termitů. Lov termitů H. mosambicus byl velice krátký, poté co se pavouk vyhrabal z písku a lokalizoval termita, jej rychle kousnul za hlavu na boční straně hrudi mezi sklerity. Termit sice bezprostředně po kousnutí bojoval, nicméně pavouk jej po celou dobu pevně držel za hlavou, dokud nebyl úplně paralyzován. Poté se pavouk i s termitem zahrabal zpátky do písku, kde jej mohl v bezpečí zkonzumovat. Doba paralýzy termita jedem byla průměrně 82 sekund a nelišila se mezi samci a samicemi a ani poměr velikosti mezi pavoukem a termitem neměl na dobu paralýzy vliv. Výsledky z přírody i z laboratoře naznačují, že pavouk A. amphalodes by mohl být monofágní specialista, adaptovaný pouze k lovu termitů druhu H. mosambicus.
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4.7 Rukopis G Pekár, S., Líznarová, E. & Řezáč, M. (2016). Suitability of woodlice prey for generalist and specialist spider predators: a comparative study. Ecological Entomology, 41, 123–130. Predátoři si vybírají svoji kořist na základě jejího nutričního složení a energie z ní získané. Během lovu se euryfágní predátoři často setkávají s kořistí, která je nebezpečná nebo těžce ulovitelná a investice do jejího lovu je pak často větší než následný zisk z této kořisti. Takovéto kořisti se pak euryfágní predátoři vyhýbají a neloví ji. Naopak stenofágní predátoři se často specializují právě na těžce ulovitelnou kořist. Pavouci se například specializují na lov nebezpečných mravenců, jiných pavouků nebo termitů. Z daného typu kořisti pak musí být stenofágní pavouci schopni získat jak energii, tak potřebné živiny. Nicméně není známo, jak je tato kořist výživná pro euryfágní predátory, a jestli se jí vyhýbají jenom kvůli obtížnému lovu nebo i z důvodu nízké nutriční kvality. Hojným typem kořisti teoreticky dostupným pro pavouky jsou suchozemští stejnonožci. Stejnonožci jsou v epigeonu běžní a nejsou nijak rychlí ani nebezpeční. Nicméně jsou proti predátorům poměrně dobře chránění silně inkrustovanou kutikulou, chemickými látkami a obranným chováním. Behaviorální obrana u nich spočívá v převážně noční aktivitě a ve schopnosti stočit se v nebezpečí do kuličky nebo se silně přitisknout k substrátu, takže se predátor nedostane k jejich málo sklerotizované ventrální straně těla. Tyto obranné mechanismy mají patrně za následek, že je mnoho predátorů neloví. Ze všech možných predátorů se na jejich lov specializují pouze některé druhy mravenců a pavouků. Z našich druhů pavouků se na lov stejnonožců adaptovaly některé druhy z čeledi Dysderidae. Pavouci specializovaní k lovu stejnonožců si k jejich lovu vyvinuli různé morfologické, behaviorální a metabolické adaptace. Cílem tohoto rukopisu bylo pomocí laboratorního experimentu srovnat nutriční a energetickou hodnotu stejnonožců jako kořisti u tří druhů pavouků s různým stupněm specializace na stejnonožce. Použili jsme pavouky druhu Dysdera crocata, kteří jsou specializovaní na lov stejnonožců, pavouky druhu Pholcus phalangioides, což jsou oligofágní generalisté a stejnonožce také loví, a nakonec euryfágní generalisty druhu Tegenaria domestica, kteří stejnonožce běžně neloví. Jedinci všech tří druhů pavouků byli chováni na třech různých dietách; část byla krmena pouze svinkami rodu Armadillidium, část pouze stínkami rodu Porcelio a poslední kontrolní skupina byla 42
krmena směsí různých členovců. Pozorovali jsme frekvenci lovu kořisti a různé parametry fitness u pavouků, konkrétně jejich přežívání, rychlost růstu a ontogenetický vývoj. Zjistili jsme, že specialisté druhu D. crocata nejlépe prosperovali na stínkách rodu Porcelio, zatímco oba generalisté, P. phalangioides a T. domestica prosperovali nejlépe na dietě složené ze směsi členovců. Získané výsledky ukázaly, že stejnonožci jsou nevhodnou kořistí pro oligofágní a euryfágní generalisty.
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5 ZÁVĚR V této disertační práci jsem se zabývala různými predačními strategiemi stenofágních druhů pavouků. Především jsem u zkoumaných druhů pavouků zjišťovala stupeň potravní specializace a přítomnost různých adaptací k lovu preferovaného typu kořisti. Potravní ekologii jednotlivých druhů pavouků jsem zkoumala z různých hledisek a pomocí různých metod, kdy jsem často kombinovala analýzu složení kořisti v přírodě s laboratorními experimenty. U druhů, u kterých nebylo o jejich potravní ekologii známo téměř nic, jsem poodhalila jejich potravní nároky, a u druhů, u kterých již byl předpoklad jejich specializace na konkrétní druh kořisti, jsem tento předpoklad pomohla potvrdit nebo naopak vyvrátit. U pavouků rodu Oecobius, u kterých se předpokládalo, že jsou specializovaní na lov mravenců, jsme zjistili, že se na daných lokalitách sice živí téměř výhradně mravenci, ale pravděpodobně pouze z ekologických důvodů. V laboratorních experimentech totiž nevykazovali žádné behaviorální, morfologické ani jedové adaptace, které by jim pomáhaly lovit mravence efektivněji, než jiné typy kořisti. Nicméně jsme u těchto pavouků kvůli nedostatku vhodných jedinců nestudovali přítomnost metabolických adaptací. Takový experiment by případně mohl ukázat, že ačkoliv tito pavouci mravence neloví efektivněji než jiné typy kořisti, mohou být schopni z nich efektivněji získávat energii a živiny, a proto se jim náročnější lov této kořisti vyplatí. Právě přítomnost metabolických adaptací jsme zkoumali u jiného myrmekofágního pavouka rodu Euryopis. Tito pavouci byli sice schopni také ulovit i jinou kořist než mravence, ale pokud byli dlouhodobě krmeni alternativní kořistí, výrazně se zhoršilo jeho přežívání a vývoj. To naznačuje, že je u těchto pavouků schopnost lovit alternativní kořist výhodná pouze dočasně, například pokud není preferovaná kořist krátkodobě dostupná anebo u dospělých samic, které potřebují v krátké době před kladením kokonů získat co nejrychleji co nejvíce živin. O pavoucích rodu Zodarion již bylo známo, že jsou specializovaní k lovu mravenců, a že jinou kořist neloví. Nyní jsme však pomocí experimentu ke zjištění funkční odpovědi zjistili, že lov nebezpečné kořisti generuje vzácně pozorovanou funkční odpověď typu čtyři. Navrhnuli jsme nový matematický model, který pozorovanou funkční odpověď popisoval lépe než doposud navržené modely, a s jeho pomocí jsme byli schopni kvantifikovat efektivitu lovu specializovaných pavouků ve srovnání s pavouky nespecializovanými. U pavouků druhu Zodarion cyrenaicum jsme dále 44
srovnávali jejich schopnost lovit mravence během různých vývojových stadií a zjistili jsme, že i ta nejmenší mláďata jsou schopná lovit několikanásobně větší mravence, a to díky velice specifické taktice lovu a vysoce účinnému jedu. O pavoucích rodu Ammoxenus se sice předpokládalo, že loví převážně termity, ale ucelenější studie o jejich potravní ekologii provedena nebyla. My jsme kombinací výzkumu složení potravy v přírodě a pomocí laboratorních experimentů prokázali, že námi studovaná populace těchto pavouků loví výhradně termity druhu Hodotermes mosambicus. Jelikož ani v laboratorních podmínkách žádnou jinou kořist nelovili, předpokládáme, že jsou to monofágní specialisté lovící výhradně kořist jednoho druhu. Nicméně je možné, že by byli teoreticky schopni lovit i jiný druh termita rodu Hodotermes, ale areál výskytu příbuzného druhu se v přírodě nepřekrývá s areálem výskytu námi zkoumaného druhu pavouka. Oniskofágní pavouci rodu Dysdera, specializovaní na lov suchozemských korýšů, používají k jejich lovu morfologické a behaviorální adaptace. Nás ovšem zajímalo, jak efektivně tito pavouci stejnonožce tráví, a jak kvalitní potravu pro ně tedy představují. Zjistili jsme, že stejnonožci jsou pro tyto pavouky nutričně vhodná kořist na rozdíl od alternativní kořisti. Zároveň jsme pozorovali, že pro nespecializované pavouky jsou stejnonožci nutričně nedostačující kořist, která jim neumožňuje další vývoj. To naznačuje přítomnost metabolických adaptací u oniskofágních pavouků, které jim umožňují získávat potřebnou energii a živiny z kořisti, která je jinak pro euryfágní predátory nevhodná. Nicméně v mnoha případech nebyli nespecializovaní pavouci schopni vůbec stejnonožce ulovit, a tak v podstatě hladověli. V takovém případě nelze mluvit o případné nízké nutriční hodnotě kořisti a metabolických adaptacích predátora, jelikož k trávení kořisti v podstatě vůbec nedošlo. Jednotlivé typy adaptací k lovu kořisti jsou spolu často tak výrazně provázané, že je nelze při výzkumu jednoho typu adaptace snadno oddělit a studovat zvlášť. Z našich výsledků vyplývá, že každý námi zkoumaný druh pavouka je unikátní stupněm potravní specializace, přítomností jednotlivých adaptací a případnými omezeními při lovu nepreferované kořisti. Jelikož se ale větší část studovaných druhů pavouků nevyskytuje na území České republiky, byli jsme v našem výzkumu omezeni množstvím jedinců, které se nám podařilo nasbírat během zahraničních pracovních cest. Bohužel jsme tak zatím nebyli schopni provést všechny plánované experimenty a odpovědět na všechny dosud nezodpovězené otázky týkající se potravní ekologie zkoumaných druhů. 45
V budoucnu by tak například bylo vhodné testovat z možných behaviorálních adaptací, kromě samotné strategie lovu a schopnosti lovit alternativní kořist, rovněž preferenci určité kořisti. Tu by bylo možné zjistit například experimentem, ve kterém jsou predátorovi nabídnuty současně dva typy kořisti, a ten si může vybrat, kterou kořist uloví. Rovněž by bylo zajímavé zjistit, zda a jak se během života jedince jeho potravní preference mění. Dále by bylo také velice přínosné otestovat, na základě jakých signálů specializovaní pavouci poznají preferovanou kořist, a zda je preference dané kořisti vrozená, nebo získaná během života jedince. V případě jedových adaptací jsme dosud testovali jen dobu paralýzy kořisti za podmínek, kdy pavouk sám určoval množství jedu, které do kořisti vpustí. Nicméně je možné, že jsou pavouci schopni regulovat množství jedu, které při lovu použijí, podle typu kořisti. Určitě by bylo užitečné vyvinout metodu odebrání jedu pavoukům a kontrolovanou injikaci daného množství jedu do kořisti. To by mohlo ukázat reálnou efektivitu jedu na jednotku objemu. Při studiu metabolických adaptací by určitě bylo vhodné testovat více typů kořisti, než jsme dosud použili, a jejich kombinace, nicméně tyto experimenty vyžadují sledování pavouků po celou dobu jejich vývoje, což je časově náročné a vyžaduje velké množství jedinců. Zároveň by bylo zajímavé manipulovat nutriční složení kořisti a zjistit, zda a jak tyto změny ovlivní přežívání a vývoj pavouků na dané kořisti. Díky molekulární detekci DNA kořisti ve střevech pavouků se nám otevírají možnosti zkoumání složení kořisti v přírodě různých vývojových stadií pavouků, srovnání různých populací s odlišnou potenciální kořistí, srovnání potravy příbuzných druhů a podobně. Nicméně k průkazným výsledkům získaných touto metodou je potřeba poměrně velkého počtu testovaných jedinců, jelikož ne u všech jsme schopni DNA zkonzumované kořisti detekovat, pravděpodobně kvůli rychlé degradaci DNA kořisti ve střevech pavouků. Zatím totiž nevíme, jak dlouho trvá, než DNA kořisti ve střevech pavouků degraduje, a jak se tato doba liší v závislosti na stupni specializace nebo typu kořisti.
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S tím, jak postupně získáváme další informace o potravně specializovaných pavoucích, tak zároveň vyvstává i množství nových otázek a možností studia jejich potravní ekologie. Nicméně věřím, že nám další výzkum této problematiky pomůže na tyto otázky postupně odpovědět a budeme si moci udělat ucelenější pohled na potravní ekologii již zkoumaných nebo i dalších druhů pavouků. Jakmile totiž budeme mít více informací o potravní ekologii většího počtu druhů, budeme schopni dělat obecnější závěry a scénáře týkající se potravní specializace pavouků a případně i dalších predátorů.
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7 PŘÍLOHY 7.1 Rukopisy studenta vztahující se k tématu disertační práce 7.2 Podíl studenta na jednotlivých rukopisech 7.3 Seznam publikací 7.4 Příspěvek na mezinárodních konferencích 7.5 Příspěvek na domácích konferencích
7.1 Rukopisy studenta vztahující se k tématu disertační práce Rukopis A Líznarová, E., Sentenská, L., García, L. F., Pekár, S. & Viera, C. (2013). Local trophic specialisation in a cosmopolitan spider (Araneae). Zoology, 116, 20–26. Rukopis B Líznarová, E. & Pekár, S. (2013). Dangerous prey is associated with a type 4 functional response in spiders. Animal Behaviour, 85, 1183–1190. Rukopis C Líznarová, E. & Pekár, S. (2015). Trophic niche of Oecobius maculatus (Araneae: Oecobiidae): evidence based on natural diet, prey capture success , and prey handling. The Journal of Arachnology, 43, 188–193. Rukopis D Líznarová, E. & Pekár, S. (2016). Metabolic specialisation on preferred prey and constraints in the utilisation of alternative prey in an ant-eating spider. Zoology (online). Rukopis E Pekár, S., Šedo, O., Líznarová, E., Korenko, S. & Zdráhal, Z. (2014). David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey. Naturwissenschaften, 101, 533–540. Rukopis F Petráková, L., Líznarová, E., Pekár, S., Haddad, C. R., Sentenská, L. & Symondson, W. O. (2015). Discovery of a monophagous true predator, a specialist termite-eating spider (Araneae: Ammoxenidae). Scientific reports, 5: 14013. Rukopis G Pekár, S., Líznarová, E. & Řezáč, M. (2016). Suitability of woodlice prey for generalist and specialist spider predators: a comparative study. Ecological Entomology, 41, 123– 130.
Rukopis A
LOCAL TROPHIC SPECIALISATION IN COSMOPOLITAN SPIDER (ARANEAE) Líznarová E., Sentenská L., García L. F., Pekár S., Viera C. (2013) Zoology, 116, 20–26
Oecobius navus
© Macek
Zoology 116 (2013) 20–26
Contents lists available at SciVerse ScienceDirect
Zoology journal homepage: www.elsevier.com/locate/zool
Local trophic specialisation in a cosmopolitan spider (Araneae) Eva Líznarová a , Lenka Sentenská a , Luis Fernando García b , Stano Pekár a,∗ , Carmen Viera b,c a
Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotláˇrská 2, 611 37 Brno, Czech Republic Laboratorio de Ecología del Comportamiento, Instituto de Investigaciones Biológicas Clemente Estable, Avenida Italia 3318, Montevideo, Uruguay c Sección Entomología, Facultad de Ciencias, Universidad de la República, Iguá 4225, Montevideo, Uruguay b
a r t i c l e
i n f o
Article history: Received 2 March 2012 Received in revised form 19 June 2012 Accepted 24 June 2012 Available online 30 November 2012 Keywords: Oecobius navus Stenophagy Myrmecophagy Feeding strategy
a b s t r a c t Trophic specialisation can be observed in species with long-term constant exploitation of a certain prey in all populations or in a population of a species with short-term exploitation of a certain prey. While in the former case the species would evolve stereotyped or specialised trophic adaptations, the trophic traits of the latter should be versatile or generalised. Here, we studied the predatory behavioural adaptations of a presumed myrmecophagous spider, Oecobius navus. We chose two distinct populations, one in Portugal and the other in Uruguay. We analysed the actual prey of both populations and found that the Portuguese population feeds mainly on dipterans, while the Uruguayan population feeds mainly on ants. Indeed, dipterans and springtails in Portugal, and ants in Uruguay were the most abundant potential prey. In laboratory trials O. navus spiders recognised and captured a wide variety of prey. The capture efficiency of the Portuguese population measured as components of the handling time was higher for flies than for ants, while that of the Uruguayan population was higher for ants. We found phenotypic plasticity in behavioural traits that lead to increased capture efficiency with respect to the locally abundant prey, but it remains to be determined whether the traits of the two populations are genetically fixed. We conclude that O. navus is a euryphagous generalist predator which shows local specialisation on the locally abundant prey. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction Many organisms exhibit some degree of dietary specialisation. A narrow dietary range or stenophagy is a common trophic strategy of herbivores (e.g., Bach, 1980; Novotny´ and Basset, 2005; Shipley et al., 2009), parasites (e.g., Little et al., 2006) and parasitoids (e.g., Strand and Obrycki, 1996), but is rather rare in carnivorous predators (Pekár et al., 2012). Within the latter it has been found only in some taxa, such as coccinellid beetles (e.g., Hodek and Honˇek, 1996), syrphid flies (e.g., Sadeghi and Gilbert, 2000), horned lizards (e.g., Montanucci, 1989) and zodariid spiders (e.g., Pekár, 2004). In these prey specialists the narrow diet is rather constant across populations, genetically fixed, and thus expressed over their life history. Studies on trophic specialisation in herbivores have shown that stenophagy can be a flexible attribute of a population. The high degree of chemical distinctiveness of many plant species demands the ability to tolerate, detoxify or metabolise a broad spectrum of qualitatively different chemicals. Many herbivore generalists thus act as dietary specialists in local communities exploiting a narrow range of plants that are locally abundant (Fox and Morrow, 1981).
∗ Corresponding author. E-mail address:
[email protected] (S. Pekár). 0944-2006/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.zool.2012.06.002
Local specialisation is regarded as an optimal foraging strategy. If the abundant food declines, a predator can shift to other abundant prey or expand its diet by including less favoured items (Robinson and Wilson, 1998). Separate populations of a species may then differ due to various reasons, such as spatial or temporal variation in the availability of prey (Fox and Morrow, 1981). Trophic specialisation may thus have two distinct forms, fundamental (strict) and realised/local (sensu Bolnick et al., 2003). To distinguish between them, data on several populations and generations of the same species are needed. Yet, the diet breadth in carnivorous invertebrates is typically investigated in a single population over a short time period (e.g., Lawton, 1970; Nyffeler and Benz, 1987; Al-Zyoud and Sengonca, 2004). Thus, many species with a stenophagous habit regarded as fundamental stenophages may only be local specialists. Evidence for local specialisation in carnivorous predators is very scarce. For example, a garter snake, Thamnophis sp., has a widely generalised diet over its whole range, but some local populations have much narrower prey preferences. Specialised populations of this species are able to digest their preferred prey more efficiently than generalised ones due to physiological adaptations (Britt et al., 2006). Local trophic specialisation implies plasticity in a series of trophic adaptations: cognitive adaptations secure the recognition of different prey, morphological and behavioural adaptations provide predators with the means to efficiently capture various
E. Líznarová et al. / Zoology 116 (2013) 20–26
prey, and physiological adaptations enable nutrient utilisation from a different prey (Rowe, 1994; Mayntz et al., 2005). On the other hand, in fundamental stenophages, adaptations should be stereotyped or specialised – efficient only against the focal prey (e.g., Pekár, 2004; Yeargan, 1988) and genetically fixed. Predatory versatility is documented in some vertebrates (Curio, 1976), but it has rarely been studied in invertebrates, including spiders. Some evidence is available for jumping (e.g., Forster, 1977) and orb-web spiders (Robinson, 1969; Harwood, 1973; Viera, 1995; Japyassú and Viera, 2002). In our study we chose a species that has a large range of distribution (worldwide) and for which anecdotal evidence suggests stenophagy. The spider Oecobius navus Blackwall is a cosmopolitan (circumglobal subtropical and tropical) species frequently associated with urban areas, such as walls of buildings (Voss et al., 2007). It has spread its range from North Africa to other parts of the world with a subtropical and tropical climate (Shear and Benoit, 1974), including South and North America (Santos and Gonzaga, 2003), Australia, Europe, and Asia (Platnick, 2010). Available anecdotal data on its natural diet from Southern Europe and Australia suggest that the most common items are ants (Glatz, 1967; Voss et al., 2007). Ant-eating or myrmecophagy was further supported in laboratory observations, where spiders did not consume any other arthropods but ants (Glatz, 1967). It has most likely specialised locally on different prey, though probably mostly on ants, which are present in all kinds of terrestrial habitats and are often superabundant (Hölldobler and Wilson, 1990). Although available evidence suggests that O. navus is a fundamental trophic specialist (Glatz, 1967), we expected that this species is a generalist predator like the majority of spider species (Pekár et al., 2012). Thus, we predicted that all populations of this species capture a variety of prey and possess versatile or generalised adaptations in predatory features. To investigate this hypothesis, we analysed the potential and actual prey of two distinct populations of O. navus, one from southern Europe (Portugal) and the other from South America (Uruguay), and performed experiments on their capture efficiency regarding a variety of prey.
2. Materials and methods 2.1. Study animals O. navus lives in a tent-like web consisting of two parallel sheets from which signal threads run out into its vicinity (Hingston, 1925). The spider spends most of the time hiding in the web with legs touching the signal threads. It runs out of the retreat and attacks prey that touches a thread (see online supplementary video). Once the prey is subdued, the spider consumes the prey in its web and deposits the prey remains by the web. As Oecobius spiders do not chew their prey during feeding, it is possible to determine the diet of each individual spider by analysing the prey remains attached to its web. Individuals of O. navus were collected from buildings at the Universidade de Évora in Valverde da Mitra (Portugal) and from the walls of the Instituto de Investigaciones Biológicas Clemente Estable, Montevideo (Uruguay). Each specimen was placed singly in a plastic tube along with its web containing prey remains. Altogether, 41 individuals at different developmental stages with their own webs were collected in Portugal and 141 individuals in Uruguay. Potential prey were collected from the walls around the spiders’ webs by means of a pooter. The potential prey of the Portuguese population was collected in spring (April) on two days, one time in the morning and the other in the evening, to obtain both diurnal and nocturnal potential prey. On each date, four people collected potential prey for half an hour, amounting to 120
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‘man-minutes’. The potential prey of the Uruguayan population was also collected in spring (September) using a similar method. Two people collected potential prey over four sessions both during the day and in the evening, with a combined sampling effort of 360 ‘man-minutes’. The size of the spiders (prosoma width) and the total body length of all potential and actual prey items were measured to the nearest 0.1 mm using an ocular micrometre in a stereomicroscope. Usually the prey remnants were well preserved and if only part of the prey was found the whole body size was estimated from conspecific individuals. Oecobius spiders were identified using the keys provided by Nentwig et al. (2010) and Santos and Gonzaga (2003). All prey, potential and actual (from webs), were identified to the lowest taxonomic level allowed by the physical condition of the specimens. In most cases the prey specimens could be assigned to a particular order. Diptera were split into Nematocera and Brachycera, and Hymenoptera into Formicidae and others.
2.2. Behavioural experiments After transfer to the laboratory, living spiders from the Portuguese population (31 individuals) and from the Uruguayan population (20 individuals) were placed singly in Petri dishes (diameter 30 mm) and left for three days to build a web. Before and between the trials, the spiders were kept under the following conditions: room temperature – approximately 22 ◦ C; room humidity – approximately 43%; natural L:D – 14:10. Moisture in the dishes was maintained by adding a drop of water to the bottom of the dish. Seven days before the beginning of the experiments, the spiders were fed with fruit flies to satiation to standardise their hunger level. The experiments were also performed at room temperature. In the behavioural trials, the prey from several different invertebrate orders was used. To the spiders from Portugal, the following types of prey were offered: wingless fruit flies (Drosophila melanogaster, Diptera, average body length 2 mm), termites (Reticulitermes sp., Isoptera, 3.5 mm), springtails (Tomocerus sp., Collembola, 4 mm), crickets (Acheta domestica, Orthoptera, 5 mm), spiders (Zodarion sp., Araneae, 3 mm), ants (Lasius sp., Hymenoptera, 3 mm), beetles (Curculionidae, Coleoptera, 3 mm), moths (Lithocolletinae, Lepidoptera, 4 mm), aphids (Aphidinae, Hemiptera, 2.5 mm) and true bugs (Miridae, Heteroptera, 5 mm). Fruit flies, termites, springtails and crickets were taken from laboratory-reared cultures; spiders, ants, beetles, moths, aphids and true bugs were taken from the field. To the spiders from Uruguay, the following types of prey were offered: ants (Myrmicinae, 3 mm), termites (Nasutermitidae, 3.5 mm), cockroaches (Blatella germanica, first nymphal stage, 3.5 mm), aphids (Aphidiidae, 3 mm) and flies (D. melanogaster, 2.5 mm). Although the flies used with spiders from Uruguay were able to fly, unlike those used with spiders from Portugal, the size of the experimental dishes constrained their flight ability to short jumps. The prey types were offered to the spiders randomly. The prey was removed 10 min after the first contact between spider and prey if the spider did not start hunting. If the spider attacked and caught the offered prey, the prey was allowed to be consumed. Another prey type was offered to the same spider after 4 days. Such a period was found to result in moderate hunger levels in trials performed prior to the present study. If the spiders did not attack one prey type, another type of prey was offered the next day. The prey given to the spider was within the size range of 0.5–2 times the total body length of the spider. Each prey and spider were chosen randomly, but for each prey type at least eight independent replications were done. Each individual spider was thus used repeatedly with various prey types but only once with each prey type.
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Trials began when a prey was introduced to the dish occupied by a spider. All trials were recorded using a video camera attached to a stereoscopic microscope (Olympus SZX 9). Video footage with hunting sequences was analysed. In each trial we recorded whether the spider attacked and consumed the prey. In addition, we measured the handling time (time elapsed from first contact with the prey to the start of consumption) and the number of bites delivered. Handling time was composed of two components (wrapping and waiting), which were quantified separately: (i) time for prey wrapping (total wrapping time) and (ii) waiting time (i.e., waiting at a safe distance). The following events of prey capture were distinguished: approach – the spider approached the prey; wrap – the spider circled around the prey and wrapped it in silk; bite – the spider bit the prey; wait – the spider retreated from the prey and waited for a while at a distance; carry – the spider dragged the immobilised prey to the web; feed – the spider started to consume the prey. The sequences and frequencies of hunting behaviour following approach and ending with successful subduing (feeding) were recorded to construct flow diagrams with transition frequencies for selected prey types. The probability of the approach to wrap transition was estimated from a single observation of each spider individual with each prey. But the probabilities of all other transitions were estimated from repeated observations of the same individual as the same event could occur repeatedly during one trial. 2.3. Statistical analyses All analyses were performed within the R environment (R Development Core Team, 2010). For both actual and potential prey, we estimated the niche breadth as the inverse of the Simpson index (Levins, 1968). The proportions of prey types in the diet were compared by Morisita’s index (Horn, 1966). The relationship between the size of the spiders and their prey was studied using the Pearson product correlation. The relative frequencies of prey found in the webs and the prey available on the walls were compared using the chi-square test. The results of the laboratory feeding experiments were analysed using linear methods that could handle correlations resulting from repeated use of the same individuals (repeated measures design). Generalised estimating equations (GEE) and generalised least squares (GLS) were used for non-normally and log-normally distributed response variables, respectively (Pinheiro and Bates, 2000). To compare the handling time, GEE with gamma errors (GEE-g) was used. Since handling time and its components (wrapping, biting and waiting) may be influenced by the size of a prey, the effect of prey size was added to the linear predictor. To compare proportions, GEE with binomial errors (GEE-b) was used. GEE require a correlation structure to be specified. We used ‘independence’ or ‘exchangeable’ due to the small block size (Hardin and Hilbe, 2003). Post hoc comparisons among estimated parameters were made using treatment contrasts based on the Wald test. The transition frequencies were compared using the chi-square test.
Table 1 Proportion of the prey taxa of Oecobius navus spiders found in webs (actual) and on the walls of buildings (potential) in Valverde da Mitra (Portugal) and in Montevideo (Uruguay). Order
Actual
Uruguay Potential
Actual
Potential
0.87 0.01
0.47 0.04
0.07 0.02
0.01 0.01
0.03 0.02 0.00 0.02 0.01 0.03 0.00 0.00 0.00 306 1.30
0.00 0.00 0.00 0.46 0.01 0.01 0.01 0.00 0.00 140 2.25
0.54 0.02 0.20 0.00 0.03 0.10 0.00 0.00 0.03 105 1.54
0.63 0.01 0.18 0.05 0.06 0.01 0.00 0.02 0.00 146 1.85
on the walls was 71% (Morisita’s index). In contrast, the most abundant potential and actual prey of the Uruguayan population were ants followed by hemipterans (Table 1). The similarity between prey taxa present in the webs and those collected on the walls in Uruguay was 93% (Morisita’s index). The potential prey of the two O. navus populations differed significantly at taxonomic level (chi-square test: 211 = 9656, P < 0.0001). The diversity of potential prey of the Portuguese population was two times larger than the diversity of actual prey, as indicated by the inverse of the Simpson index (Table 1), while that of the Uruguayan population was only slightly larger. The inverse of the Simpson index for actual prey of the Portuguese individuals varied between 1 and 3. Most of the O. navus specimens clearly captured prey from a single taxon only, but a few captured prey from three taxa. The size (total length) of actual prey varied between 0.9 mm and 2.5 mm. Prey size was not related to the size of the spider (Pearson correlation, r = 0.04, P = 0.82, Fig. 1). The ratio between the size of prey caught in webs and spider size (prosoma width) varied between 0.8 and 4.6, so spiders were able to catch prey ranging 2.5 2.3 2.1 1.9 1.7 1.5 1.3 1.1
3. Results
0.9
3.1. Actual and potential prey
0.7
Representatives from nine invertebrate orders were found in the webs (actual prey) of O. navus. The most abundant prey in the webs of the Portuguese population were dipterans (Table 1), while the most frequent potential prey occurring on the walls around the spider webs were dipterans (47%) and springtails (46%). The similarity between prey taxa present in the webs and those collected
Portugal
Diptera Nematocera Brachycera Hymenoptera Formicidae Other Hemiptera Collembola Araneae Coleoptera Ephemeroptera Acari Isopoda No. of prey items 1/Simpson index
Prey size [mm]
22
0.5
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Spider size [mm] Fig. 1. Relationship between the average size of actual prey captured and the size of Oecobius navus spiders (prosoma width) from the Portuguese population. A linear model is included to emphasise the lack of correlation.
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0.6 0.4 0.0
0.2
Capture success
0.8
1.0
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2
3
4
5
6
7
8
Size ratio (prey/spider) Fig. 2. Relationship between the relative size ratio (total length of prey to spider prosoma width) and the capture efficiency of Oecobius navus in laboratory trials with 10 different prey species. The logit model is displayed.
from smaller than their prosoma to almost five times larger than their prosoma. In the laboratory, capture efficiency decreased with increasing prey size (GEE-b, 21 = 6.4, P = 0.01): 50% capture efficiency was achieved when total prey size was 300% larger than the prosoma of the spider (Fig. 2). 3.2. Predatory behaviour Both Portuguese and Uruguayan spiders attacked all prey types offered. Hunting always started once the prey had touched the signal thread. Then, the spider ran out of the tent-like web. The predatory behaviour used to catch different prey types was preyspecific and differed slightly, but not significantly, for the same prey type between the two examined populations (X2 < 21.1, P = 0.072, Fig. 3). All spiders of both populations began hunting ants by wrapping them with silk threads. Then, the majority of spiders bit the ant (92%, N = 26). Most spiders waited after biting until the ant was motionless (65%, N = 26); the rest of the spiders carried the ant to the web or began feeding immediately after biting. On the whole, the hunting of ants (Fig. 3A and B) was more similar between the two populations than the hunting of flies (Fig. 3C and D), which was far more variable. Most spiders started to wrap the flies after first contact (90%, N = 20). Only half of the spiders from Portugal (50%, N = 10) and even fewer spiders from Uruguay (40%, N = 10) bit the flies, using venom to subdue them. 3.3. Capture efficiency The interaction between population and prey (ants, flies) was significant (GEE-g: 23 = 15.1, P < 0.0001): handling time for flies was longer than that for ants in Uruguayan spiders, but significantly shorter in Portuguese spiders. Handling time for flies was significantly longer in Uruguayan spiders than in Portuguese spiders (contrasts, P < 0.001), while handling time for ants was not shorter in Uruguayan spiders than in Portuguese spiders (contrasts, P = 0.236, Fig. 4). The effect of prey size on handling time was not significant (GEE-g: 21 = 1.6, P = 0.2). Handling time of the Uruguayan population differed significantly among prey types (GEE-g: 24 =
Fig. 3. Flow diagrams of the hunting sequences of Oecobius navus with the relative frequencies of transitions for two main prey types in both populations: (A) ants in Uruguay (N = 15), (B) ants in Portugal (N = 11), (C) flies in Uruguay (N = 10), (D) flies in Portugal (N = 10). The transition frequencies between events are also indicated by the widths of the lines.
33.0, P < 0.0001): the shortest handling time was for ants. It was not significantly different from the handling times for cockroaches and flies (contrasts, P > 0.06). Termites and aphids were handled for a significantly longer time (contrasts, P < 0.0001). The longest handling time was found for aphids, more than 15 min on average (Fig. 5). Handling time of the Portuguese population also differed significantly among prey types (GEEg: 29 = 128.9, P < 0.0001): the shortest handling time was for flies. It was not significantly different from the handling times for moths, springtails and termites (contrasts, P > 0.123). Other prey types took a significantly longer time to handle (contrasts, P < 0.015). The longest handling time was found for true bugs (Fig. 5).
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Fig. 4. Comparison of the prey handling times of Uruguayan (N = 19) and Portuguese (N = 15) populations of Oecobius navus for two different prey types. Points are means, whiskers are 95% confidence intervals.
4. Discussion Analysis of the natural prey of the two populations showed that O. navus catches different prey at different locations. The Uruguayan population supports the view that O. navus is a fundamental myrmecophagous specialist (Voss et al., 2007). However, the Portuguese population contradicts this view, as this population captured mainly dipterans. In both locations O. navus spiders captured more than one prey order, though all the other orders were represented at a markedly lower frequency. This shows that
Fig. 5. Comparison of the prey handling times of Uruguayan (N = 20) and Portuguese (N = 31) populations of Oecobius navus for different prey types. Bars are means, whiskers are 95% confidence intervals.
O. navus has the capacity to catch and feed on a wide range of arthropods, as was also confirmed in laboratory experiments. Thus, the fundamental trophic niche of this species is wide, suggesting that O. navus is a euryphagous predator. Individual spiders, however, displayed various levels of specialisation: analysis of prey items in webs revealed that some individuals captured only prey of one order while others captured several orders. Instead of consuming smaller quantities of a variety of prey, most O. navus individuals captured a large amount of one prey type. While in the Uruguayan population the actual prey spectrum corresponded closely to the potential one, the Portuguese population of O. navus selected only one of two major types of prey available (dipterans and springtails). We assume that O. navus spiders developed a preference for dipterans due to the smaller size of springtails in comparison with dipterans. That is, springtails may constitute a less profitable prey for O. navus. Thus, both O. navus populations display a certain level of dietary specialisation (Holbrook and Schmitt, 1992). Such a narrow breadth of diet may be the result of an optimal foraging strategy, i.e., O. navus concentrates on the most profitable prey (Robinson and Wilson, 1998). For the Portuguese population the most profitable prey were dipterans, whereas for the Uruguayan population, they were ants. In our study we used only adult specimens which had had previous experience with prey from the field. This experience could result in the formation of a search image, as has been documented for other spider species. It is known that in some spiders even a single encounter with prey results in search image formation that further increases the efficiency of prey recognition (Jackson and Cross, 2011). Thus, from the obtained results it is not possible to distinguish whether our findings show genetically fixed adaptations in the two study populations or phenotypic plasticity after learning. Additional studies using naive spiderlings would have to be carried out to determine this. Fox and Morrow (1981) suggested that local specialisation results from different defensive abilities of host/prey organisms that require different adaptations. Euryphagy and local stenophagy in O. navus is obviously enabled by plasticity in a series of trophic traits, which reduces costs in handling different prey types. Locally specialised herbivores tend to avoid novel plants due to new toxins (McEachern et al., 2006). Similarly, specialised carnivorous predators should reject, or should not recognise or attack, novel prey. We used several prey types that were not among the actual or potential prey (true bugs, aphids, crickets) and thus represented novel prey. Yet O. navus did recognise, capture and consume them. This reveals that they have generalised cognitive and versatile behavioural predatory adaptations. Therefore, O. navus is not a fundamental specialist, in contrast to Zodarion spiders, which capture only ants. The predatory behaviour of Zodarion spiders is stereotyped – constrained to such an extent that they are not able to capture alternative prey, except for termites that behave similarly to ants (Pekár, 2004). Similarly, other fundamentally stenophagous spiders, such as Mastophora, use a very specific capture tactic employing bolas that is efficient for moths, but not for other prey (Yeargan, 1988). Versatility in predatory behaviour was particularly apparent in the use of wrapping. Small prey, such as aphids, springtails, flies and termites, were wrapped only for a few seconds. True bugs and crickets required a longer wrapping time due to their size and strong legs, which had to be restrained. Prey that may inflict serious harm on a spider by a kick or a bite, such as crickets, ants or spiders, were wrapped for a considerably longer time before the spider risked biting them. Indeed, wrapping is an efficient tactic for the capture of dangerous prey such as spiders and ants. This tactic is also used by gnaphosid and theridiid spiders when capturing ants (Soyer, 1943; Carico, 1978; Porter and Eastmond, 1981). Dangerous prey items were bitten in nearly all cases only after being restrained by wrapping.
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The number of bites and the waiting time are indicators of venom efficacy (Pekár, 2009): a low number of bites and a short waiting time reflect rapid paralysis (and thus, high venom efficacy). For subduing small prey, such as flies or springtails, a single bite by O. navus was sufficient. Multiple bites were used for large prey, such as spiders or ants, in which case the waiting time was also longer. As all types of prey were immobilised by O. navus, the venom seems to be generalised rather than specific. Most spiders catch prey smaller than themselves (Nentwig, 1987). Fundamentally stenophagous species that hunt prey without the use of a web capture prey which is several times larger (Pekár et al., 2005). Thus, a high ratio of prey size to spider size seems to be indicative of stenophagy. O. navus spiders captured prey that was about two to three times the size of their prosoma. This would suggest that these spiders are fundamental stenophages if they did not use wrapping to catch prey. Euryphagous spider species are, by means of wrapping, able to catch prey items about 400% of their prosoma size (corresponding to about 200% of their own size) (Nentwig, 1987). There is some evidence that locally specialised herbivores have also evolved physiological adaptations that increase food utilisation (McEachern et al., 2006) similar to those of fundamental ˇ stenophages (Rezᡠc and Pekár, 2007; Pekár and Toft, 2009). We have not yet studied the physiological adaptations of O. navus; this will be the goal of our future work. We expect that O. navus possesses generalised physiological adaptations, i.e. that they attain the highest level of fitness on a mixture of prey, as do other euryphagous spider species (Toft and Wise, 1999). Predatory behaviour, including web architecture, was rather similar in the two populations. The Portuguese population of O. navus was quite specialised on the most abundant prey and was most efficient on flies. We did not observe such a distinct difference as can be found in the orb-weaving spider Parawixia bistriata Rennger, which spins two kinds of webs differing in size and architecture. The specificity of these webs is supported by the synchronisation of the time of web building with the flight activity of the most abundant prey (Sandoval, 1994). In the non-web building araneophagous spider Portia labiata Thorell, effective predatory tactics were observed for the capture of sympatric prey populations. Nevertheless, these were not used for the capture of allopatric prey populations (Jackson et al., 2002). Interestingly, anecdotal prey records for other Oecobius species across the world also suggest ant-eating. These species include the cosmopolitan O. cellariorum Dugès (Glatz, 1967), O. civitas Shear (Shear, 1970) from Mexico, O. interpellator Shear (Shear, 1970) from North America, O. maculatus Simon (E. Líznarová, unpublished data) from the Mediterranean, O. templi O. P.-Cambridge (Debski, 1923) from North Africa, Oecobius sp. from Iran (Hingston, 1925) and Oecobius sp. from Colombia (L.F. García, unpublished data). The distribution range of these species is smaller than that of O. navus. Therefore, if there is a correlation between diet breadth and geographical range, as there is in some herbivores (Janz and Nylin, 2008), then these species could be fundamental stenophages. All the obtained evidence strongly suggests that O. navus is not a fundamental stenophage but a euryphagous generalist that is opportunistically specialised on the most abundant prey. Such a type of specialisation is enabled by versatile behavioural adaptations.
Acknowledgements We would like to thank S. Korenko and M. Lacava for their help with sampling in the field, and two reviewers for very useful comments. The study was supported by grant no. 526/09/H025 provided by the Czech Science Foundation.
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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.zool.2012.06.002.
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Rukopis B
DANGEROUS PREY IS ASSOCIATED WITH A TYPE 4 FUNCTIONAL RESPONSE IN SPIDERS Líznarová, E., Pekár, S. (2013) Animal Behaviour, 85, 1183–1190
Zodarion rubidum
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Animal Behaviour 85 (2013) 1183e1190
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Dangerous prey is associated with a type 4 functional response in spiders Eva Líznarová*, Stano Pekár 1 Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotlár ská, Czech Republic
a r t i c l e i n f o Article history: Received 13 August 2012 Initial acceptance 23 October 2012 Final acceptance 18 February 2013 Available online 13 April 2013 MS. number: 12-00624R Keywords: ant anteater antipredator defence model Pardosa Xysticus Zodarion
Prey can defend themselves against predators in many different ways. Social insects, such as ants, possess particularly effective defensive systems. Some predators are better adapted to prey defence than others. We compared the capture and defence efficiency in three spider species that differ in their level of myrmecophagy. We used three ant species differing in body size and aggression in a functional response experimental set-up that measured capture frequency at different prey densities. We found a type 4, dome-shaped functional response, and we propose a new mechanistic model to describe this type. Estimated parameters (searching efficiency, handling time, inhibition by prey) were then compared among spider and ant species to quantify density-dependent defensive effects on the predator’s capture efficiency. We also compared survival of spiders during experiments. We found that myrmecophagous Zodarion spiders hunted ants with the highest capture efficiency and had the highest survival, suggesting that these spiders are adapted to living with high densities of ants. Polyphagous Xysticus spiders captured ants with lower efficiency and had the lowest survival, indicating that these spiders are adapted to the capture of solitary ant workers. Polyphagous nonanteating Pardosa spiders did not capture ants but had high survival, and are apparently adapted to living with high densities of ants. The new proposed model of the type 4 functional response can be applied to other predatoreprey systems in which the prey is dangerous and a decrease in predator hunting has a similar dependence on prey density. Ó 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Individual prey are capable of inflicting costs on foraging predators in many ways. Prey can simply make handling by the predator more difficult, specifically, inducible prey defence can lead to a lower predation rate by increasing the predator’s handling time, decreasing the attack efficiency or both (Havel & Dodson 1984; Jeschke & Tollrian 2000). Some prey can even be dangerous to predators. Such prey possess morphological structures or behavioural adaptations, or contain chemical substances, that may cause injury or lead to the death of the predator (e.g. Edmunds 1974; Caro 2005). Numerous examples are available: the locking spines of ictalurid catfish can choke gape-limited predators, such as herons and grebes, during swallowing (Forbes 1989); zebras can defend themselves by kicking and biting when attacked by predators (Goodall & van Lawick 1970); and consumption of the toxic collembolan Folsomia candida by spiders Pardosa prativaga reduces survival, development and growth rate (Fisker & Toft 2004). Colonial or social prey can use group defence (Tener 1965; Holmes & Bethel 1972; May & Robinson 1985) and the efficiency of such defence increases with the number of prey individuals, giving rise to density dependence (Jeschke 2006). Predators then try to * Correspondence: E. Líznarová, Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotlárská 2, 611 37, Czech Republic. E-mail address:
[email protected] (E. Líznarová). 1 E-mail address:
[email protected] (S.Pekár).
avoid patches of high prey densities (Schaller 1972). Since prey density in natural environments will rarely be constant (Baldwin 1996), information about density dependence is essential for understanding evolutionary arms races and for predicting the predatore prey dynamics. To quantify density-dependent defensive effects on a predator’s capture efficiency the functional response can be used. It is an important component of predation (Solomon 1949) and describes the relationships between an individual predator and its prey in terms of capture frequency. Holling (1961) distinguished four types of functional response (see Table A1 and Fig. A1 in Appendix 1 for the summary of functional response types, their descriptions and examples). Of these, types 1 to 3 do not take into account prey defence and are thus useful for modelling the functional response of a predator capturing innocuous prey. Type 4 includes an additional component that can be related to prey defence. The existence of type 4 was first derived only theoretically and was expected to occur only in vertebrates (Holling 1961). In types 1 to 3 the capture frequency reaches a maximum at a certain prey density and then remains constant. Type 4 is the only type for which the capture frequency decreases at high density below the maximum achieved. Holling (1965) ascribed the decline to situations in which the predator develops a ‘nonsearching’ image of the prey and gives up hunting it. Studies have investigated a range of potential causes for
0003-3472/$38.00 Ó 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anbehav.2013.03.004
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the decrease in hunting frequency at higher prey densities: for instance, the predator may become confused when hunting prey in swarms or aggregations (Welty 1934; Jeschke & Tollrian 2007); prey may disturb predators (Mori & Chant 1966); predators avoid prey treated with a neurotoxic insecticide (Toft & Jensen 1998; Claver et al. 2003); heterogeneous surroundings provide refuges for prey (Vucic-Pestic et al. 2010); the nutrient content of prey is imbalanced (Bressendorff & Toft 2011); and predators’ mobility is limited in tall and dense grass swards (Heuermann et al. 2011). Our aim in this study was to investigate the functional response of true predators (capturing many prey items and killing them immediately after attack) hunting dangerous social prey. Spiders are ideal for such a study because they feed on a variety of prey and have a variety of capture adaptations (Pekár et al. 2012). Most spider species hunt innocuous prey (Pekár et al. 2012), but some catch dangerous prey, such as other spiders (e.g. Whitehouse 1987), ants (e.g. Pekár 2004) or termites (e.g. Eberhard 1991). The latter two prey types are social, possessing a unique set of defences. In our study we focused on ant-eating predators. Ants are very dangerous prey because they are also predators, possess strong mandibles, can sting, and have effective group defence (Hölldobler & Wilson 1990). Most polyphagous spiders are not able to hunt ants without risk of harming themselves, so they avoid them (e.g. Huseynov et al. 2008). With respect to predatory strategy, predators, such as spiders, can be divided into five categories: (1) species that do not hunt ants at all, (2) species that are able to catch ants but prefer other prey types, (3) species that hunt ants as frequently as other prey types, (4) species that prefer ants over other prey but still accept other prey and (5) species that exclusively hunt ants (Huseynov et al. 2008). We hypothesized that the capture of ants would give rise to the type 4 functional response, as supported by a previous study (Pekár 2005), owing to the ants’ defensive ability increasing with group size. Fitting an appropriate model allowed us to estimate the capture efficiency of different spider species, which is influenced by capture tactics that increase capture rate and reduce counterattack. We used three spider species differing in the level of myrmecophagy (category 1, 3 and 5) and three ant species (see below). Parameters estimated from the functional response model (handling time, searching efficiency, prey inhibition) were then used as a quantitative measure of level of defensive abilities in the ant species and the spiders’ adaptations for the capture of ants. Observed survival was used to assess defensive adaptations of spiders to hunting dangerous prey. Although spiders from category 1 do not consume ants, they frequently encounter ants of different densities in their microhabitat and thus must have evolved a strategy to survive encounters. Measurement of capture rate is irrelevant in such a case and only defence efficiency of these spiders was studied. METHODS The Model Three models have been proposed for the type 4 functional response (Tostowaryk 1972; Hassell 1977; Fujii et al. 1986). We fitted all three models to our data and found their fits unsatisfactory (see Appendix 2); therefore, we proposed a new mechanistic model. We followed Holling (1965) who based his equation on particular components of predation. The total time (T) of one capture cycle (1) is the sum of the time spent in a digestive pause (TD), searching for prey (TS), handling prey (TP) and consuming prey (TE): Tð1Þ ¼ TDð1Þ þ TSð1Þ þ TPð1Þ þ TEð1Þ (Holling 1965). We ignored the time spent in digestive pause (TD) as we examined the functional response over a short time period. We also ignored the time spent on consumption (TE) because the predators mostly consumed the killed prey for only a short time owing to interruptions by other prey individuals. A new
component (TO) was added, which includes the time that the predator had to spend defending itself against dangerous prey. One capture cycle (the time it takes to catch one prey item) then included the following components: Tð1Þ ¼ TSð1Þ þ TPð1Þ þ TOð1Þ . Total searching time (TS) increases with the number of prey caught (A) and decreases with prey density (N) and the predator’s searching efficiency (a). The total time spent hunting prey (TP) increases with the number of prey caught (A) and prey handling time (th) (Holling 1965). Furthermore, we hypothesized that the total time of predator defence (TO) increases with the number of prey caught (A) and with prey density (N) owing to cooperative defence at some rate (c). We called this rate ‘inhibition by prey’ after Tostowaryk (1972). It was assumed to be positively related to ant ‘aggression’, in terms of attacks on spiders. Unlike Tostowaryk (1972) who assumed a quadratic relationship between prey inhibition and prey density, we observed a linear relationship between spider mortality and ant density. Therefore, the assumption of linearity was satisfied for our data (see Appendix 2). Total hunting time (T) with a known number of cycles (a known number of prey caught) is then given by:
T ¼
A þ Ath þ cAN: aN
Thus, the number of prey caught (A) during a given time period T is:
A ¼
aTN : 1 þ ath N þ acN2
Experiments We used three spider species from different families and with different hunting strategies. Wolf spiders (Lycosidae) of the genus Pardosa (Pardosa agrestis) are actively hunting spiders that do not hunt ants (category 1) but live around their nests and must deal with the danger of being attacked by ants (Nentwig 1986). Crab spiders (Thomisidae) of the genus Xysticus (Xysticus cristatus) use a sit-andwait strategy to capture ants (category 3) as well as other prey (Nyffeler & Breene 1990). Ant-eating spiders (Zodariidae) of the genus Zodarion (Zodarion rubidum) hunt only ants (category 5; Pekár 2004). Pardosa spiders were reared in the laboratory from eggsacs, which had been collected together with females from grassland in Brno (4915013.480 N, 16 34016.790 E, Czech Republic). After hatching and leaving the females, the spiderlings were fed with a mixture of small insects (springtails, flies, termites). The spiderlings were used in experiments when they had achieved a body size of about 3e 3.5 mm, which corresponded to the second instar. In total, 58 Pardosa spider individuals were used. Xysticus spiders were also collected from grassland in Brno (49150 5.520 N, 16 34015.940 E). Juvenile and subadult individuals with an average body size of 3.5 mm were used. In total, 106 Xysticus spider individuals were used. Specimens of Zodarion spiders were collected in the vicinity of the railway station in Brno (491501.480 N, 16 350 24.770 E). Individuals of Zodarion used in experiments were juveniles or subadults of both sexes and adult females. The average body size of spiders was 3 mm. In total, 226 Zodarion spider individuals were used. All spider individuals were kept separately in test tubes with plaster of Paris at the bottom. Humidity was maintained by adding a few drops of water to the plaster each week. They were kept at room temperature (ca. 23 C) and a natural light:dark cycle (approximately 14:10 h). Prey were offered to spiders ad libitum 5 days before running the experiments to standardize their level of hunger. Zodarion spiders were fed with Tetramorium ants; Xysticus and Pardosa spiders were fed with vestigial Drosophila flies. One day before the experiments, the spiders were moved separately to 7 cm high plastic containers with circular bases 4.5 cm in
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Data Analysis To model the type 4 functional response and to compare the responses of two different spider species and three different ant species, we used generalized linear models (GLM). The total number of ants killed by each spider throughout the experiment was weighted by the duration of the experiment, because some spiders were killed by ants during the experiment and, therefore, their hunting time was shorter than 6 h. As a result the response variable was the number of prey killed/h. Our model was linear in parameters, and therefore suitable for fitting with a GLM. The linear 1 predictor was mi ¼ a þ b þ gNi . The number of attacks was Ni T assumed to come from a gamma distribution: wGamma ðmi ;4Þ. Ai Thus the gamma error structure (GLM-g) and inverse link were used to fit the model (Pekár & Brabec 2009). Estimated values of a, b, and g were used to estimate the parameters of the functional b is the handling time for a single prey (th), the inverse of response: a b b is the inhibition by prey (c). b is the searching efficiency (a), and g The fitted model was an analogue of a two-way ANCOVA with interaction. The response variable was number of ants killed/h; ant density was a covariate and the spider (Pardosa, Xysticus, Zodarion) and ant (Tetramorium, Lasius, Formica) species were factors. Nonsignificant interactions were removed using backward elimination and parameters were estimated from the most adequate model. Treatment contrasts were used to compare parameter estimates. The probability of survival for different spiders and ants at different ant densities was modelled using GLM with binomial error structure and the logit link (GLM-b). The model was the analogue of a two-way ANCOVA with interaction, with the same
covariate and factors as above. The estimated probabilities for different spiders and ants were compared using the odds ratio. All analyses were performed in the R environment (R Development Core Team 2010).
RESULTS Functional Response Spiders of the genus Pardosa did not kill any ant at any density; thus it was omitted from the analysis. The observed functional response of the other two spider species was type 4 (Figs 1, 2). There was a significant effect of spider species (GLM-g: F1, 209 ¼ 254.1, P < 0.001), but the interaction with the ant density was not significant (GLM-g: F1, 202 ¼ 70.7, P > 0.6). Zodarion spiders killed significantly more Tetramorium and Lasius ants at all densities than Xysticus spiders. Estimates of a differed significantly between the spider species (contrast: t ¼ 10.8, P < 0.001). The handling time (th) of both ant species was much longer for Xysticus spiders than for Zodarion. The searching efficiency (a) and inhibition by ants (c) were similar for both spider species (Table 1). Zodarion spiders killed significantly more Tetramorium ants at all densities than ants of Lasius and Formica. At the same time, they killed more Lasius ants at all densities than Formica ants (Fig. 1). Interaction between ant species and the g parameter of the model was significant (GLM-g: F2, 205 ¼ 4.5, P ¼ 0.01). Estimates of a did not differ between ant species (contrast: t ¼ 0.31, P > 0.76). Estimates of b differed significantly between Formica and Tetramorium ants (contrast: t ¼ 2.6, P ¼ 0.01), but did not differ significantly between Lasius and Tetramorium ants (contrast: t ¼ 1.4, P ¼ 0.17) or between Lasius and Formica ants (contrast: t ¼ 1.9, P ¼ 0.06). Estimates of g differed significantly between Formica and Tetramorium b for Lasius was closer ants (contrast: t ¼ 2.2, P ¼ 0.02). The value of g to the value for Formica than for Tetramorium. Xysticus spiders killed more Tetramorium ants than Lasius ants at all densities (Fig. 2). However, the interaction between ant species
7
Tetramorium Lasius Formica
6 Number of killed ants/h
diameter (bottom area 15.9 cm2). Filter paper was glued on the bottom of the containers and a layer of fluon was applied to the sides to prevent ants escaping, restricting them to the bottom. All the experiments were performed at room temperature (ca. 23 C). We used three ant species as prey in the experiments: Tetramorium caespitum, Lasius niger and Formica rufibarbis. These species are natural prey of the myrmecophagous spider species we used. The three ant species differ in body size: Tetramorium workers were 2.3e3.2 mm long, Lasius ant workers 2.5e4 mm long and Formica workers 6e7.5 mm long. All ants were collected directly from, or near, their nest. Collected individuals from each nest were kept separately in plastic containers (150 ml) with added moistened filter paper. The containers with ants were kept at a temperature of 15 C prior to the experiment. At the beginning of the experiment the ants of one species from the same colony were placed into a container occupied by a spider at a given density. The containers were checked every 15 min and dead ants were replaced by living ants to maintain a constant density of live prey. The entire experiment lasted 6 h. At the end of the experiment the number of ants killed was recorded for each spider individual. For each density of each ant and spider species, at least five replicates were done. Experiments with Zodarion spiders were done at densities of 1, 3, 5, 7,10, 15, 20, 30 and 40 individuals of Tetramorium ants and at densities of 1, 3, 5, 7, 10 and 15 individuals of Lasius and Formica ants. Experiments with Xysticus spiders were done at densities of 1, 3, 5, 7, 10 and 15 individuals of Tetramorium and Lasius ants. Formica ants were not used with Xysticus spiders because of the high mortality of spiders in the experiments with smaller ant species. Experiments with Pardosa spiders were done at densities of 1, 3, 5, 7, 10, 15, 20, 30 and 40 individuals using Tetramorium ants only. The maximum density was selected according to the decrease in functional response.
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5 4 3 2 1 0
0
10
20 Density of ants
30
40
Figure 1. Functional response of Zodarion spiders to density of Tetramorium, Lasius and Formica ants. Points are means, vertical bars are quartiles. The fitted models are: Tetx x , Lasius y ¼ and Formica ramorium y ¼ 0:1x þ 0:29 þ 0:01x2 0:18x þ 0:88 þ 0:06x2 x y ¼ . 0:3x þ 3:12 þ 0:15x2
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1
Tetramorium Lasius
1.2
0.8 Survival of spiders
Number of killed ants/h
1
0.8
0.6
0.4
0.6
0.4
0.2
0.2
0
Tetramorium Lasius Formica
0
5
0
15
10
0
10
Density of ants
and parameters of the model was not significant (GLM-g: F1, 201 ¼ 2.4, P > 0.12). Survival of Spiders The three-way interaction of ant species, spider species and ant density was not significant (GLM-b: c230 ¼ 0:3, P ¼ 0.57); nor was, the interaction between density of ants and ant species (GLM-b: c234 ¼ 5:2, P ¼ 0.07). The survival probability for Zodarion spiders decreased significantly with the density of ants (GLM-b: c219 ¼ 6:5, P ¼ 0.01) and differed significantly between ant species (GLM-b: c217 ¼ 65:9, P < 0.001). The interaction of ant density with ant species was not significant (GLM-b: c215 ¼ 3:0, P ¼ 0.22). Tetramorium ants did not kill any spider up to a density of 10 ants, but at higher densities the survival of spiders declined to 43% (N ¼ 7). In contrast, null mortality in the presence of Lasius and Formica ants was only at a density of one ant (Fig. 3). Compared with the mortality caused by Formica ants, the mortality caused by Lasius ants was slightly lower, except at the highest density. The survival curves did not differ significantly between Lasius and Formica ants (contrast: z ¼ 0.6, P ¼ 0.55). With Tetramorium ants, the survival chance of Zodarion was 55 times higher than with Lasius ants and 67 times higher than with Formica ants. The survival probability for Xysticus spiders decreased significantly with the density of ants (GLM-b: c210 ¼ 15:2, P < 0.001) and was significantly different between ant species (GLM-b: c29 ¼ 7:5, P ¼ 0.006). The interaction between the density of ants and ant species was not significant (GLM-b: c28 ¼ 0:09, P ¼ 0.76). No spider was killed in the presence of the Lasius ants at a density of 1 to 5, but at the Table 1 List of estimated parameters: handling time (th), searching efficiency (a) and prey inhibition (c) for two spider species and three ant species Spider species
Ant species
a
th (h)
c
Zodarion Zodarion Zodarion Xysticus Xysticus
Tetramorium Lasius Formica Tetramorium Lasius
3.40 1.14 0.32 3.40 3.40
0.10 0.18 0.30 1.47 1.55
0.01 0.06 0.15 0.01 0.01
30
40
Figure 3. Change in the probability of survival of Zodarion spiders with the density of Tetramorium, Lasius and Formica ants. Points are means, vertical bars are SDs (for the binomial distribution). Estimated logit models are shown.
highest density 29% (N ¼ 7) of spiders were killed. Survival of Xysticus spiders in the presence of Tetramorium ants was 100% only for densities 1 and 3; at higher densities the survival decreased more steeply than with Lasius ants. At the highest density 44% (N ¼ 9) of spiders were killed (Fig. 4). The chance of survival was five times higher in the presence of Lasius than in the presence of Tetramorium ants. The interaction between the density of Tetramorium ants and spider species (Zodarion, Xysticus, Pardosa) was not significant (GLMb: c218 ¼ 1:4, P ¼ 0.50), but the additive effect of spider species was highly significant (GLM-b: c220 ¼ 25:4, P < 0.001). This shows that all spider species had parallel survival trends when exposed to Tetramorium ants but with a different intercept. Zodarion spiders exhibited the highest survival rate (Fig. 3) and Pardosa spiders exhibited a slightly lower survival rate, in which the survival decreased steeply above a density of 15 ants and 82% (N ¼ 11) of spiders were killed at the highest density (Fig. 4). The survival of Zodarion spiders did not
1
Xysticus-Tetramorium Xysticus-Lasius Pardosa-Tetramorium
0.8 Survival of spiders
Figure 2. Functional response of Xysticus spiders to density of Tetramorium and Lasius ants. Points are means, vertical bars are quartiles. The fitted models are: Tetramorium x x y ¼ and Lasius y ¼ . 1:47x þ 0:29 þ 0:009x2 1:55x þ 0:29 þ 0:009x2
20 Density of ants
0.6
0.4
0.2
0
0
10
20 Density of ants
30
40
Figure 4. Change in the probability of survival of Xysticus and Pardosa spiders with the density of Tetramorium and Lasius ants. Points are means; vertical bars are SDs (for the binomial distribution). Estimated logit models are shown.
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differ from the survival of Pardosa spiders (contrast: z ¼ 1.7, P ¼ 0.09). The survival of Xysticus spiders was significantly lower (contrast: z ¼ 3.5, P < 0.001) than in the other two genera (Fig. 3). The probability of survival was highest for Zodarion spiders, 2.4 times higher than for Pardosa spiders and 18.6 times higher than for Xysticus spiders. The chance of survival for Pardosa spiders was 7.8 times higher than for Xysticus spiders. With Lasius ants, Zodarion spiders had significantly lower survival than Xysticus spiders (GLM-b: c29 ¼ 22:5, P ¼ 0.005). In Zodarion, survival dropped from 100% at the lowest ant density to 24% (N ¼ 7) at the density of 15 ants, while the survival of Xysticus spiders dropped from almost 100% to 70% (N ¼ 7) at the density of 15 ants. DISCUSSION We observed that the functional response was of type 4. Both Zodarion and Xysticus spiders exhibited high capture efficiency at low ant densities but at high densities their efficiency decreased. The decrease was caused by the ants’ active defence. We did not study explicitly whether the defence by ants was a form of cooperative group defence or a sum of individual defences as this would be difficult to distinguish. According to the dangerous prey hypothesis, predators may need more time to handle defended prey individuals than undefended ones (Forbes 1989). This leads to a lower maximum per capita predation rate (Altwegg et al. 2006) and partial consumption at high prey densities. All this should result in a reduction of handling times (Sih 1980), but as handling is a function of the risk of injury to the predator, dangerous prey should be handled more carefully and for a longer time. All three ant species are aggressive towards intruders. Their attack rate is a function of the encounter rate, which is a function of ant speed, which is a function of body size (and leg length). So we assumed that Tetramorium ants, the smallest, had the lowest attack rate towards spiders, followed by Lasius, with Formica being the largest and therefore the most ‘aggressive’. Given this assumption, increased ‘aggression’ led to a decrease in searching efficiency (a) in Zodarion spiders, that is, attacking the ant became more complicated, but the handling time (th) increased with ant ‘aggression’ because it was necessary to bite a quicker ant repeatedly and it took longer to paralyse the ant (Pekár 2005). With increasing ‘aggression’ of ants, inhibition by ants (c) increased, too. Spiders had to spend more time defending themselves and less time hunting. ‘Aggression’ of ants had a similar effect on the functional response in Xysticus spiders as in Zodarion spiders. The parameter estimates of searching efficiency and inhibition by prey were identical for Tetramorium and Lasius ants. Both parameters depended on the number of contacts with prey. As Xysticus spiders do not seek their prey actively, the parameters depended only on ant density. The parameter of handling time increased significantly with attack rate of ants because the paralysis time as well as consumption took longer for more ‘aggressive’ Lasius ants. How to handle prey is a major decision that must be made by animals that hunt dangerous prey (Lima & Dill 1990). Predators either take dangerous prey less often (Forbes 1989) or they become specialized in hunting. Specialized methods by which predators handle different prey items may be innate or they may be learned through experience during encounters with different prey (O’Connell & Formanowicz 1998). Some animals may change the order of foraging actions or add another action to their foraging repertoire to circumvent prey defences (Helfman 1990). Examples of capture tactics used for dangerous prey include wrapping chemically defended insects with silk by orb-weaving spiders (Eisner & Dean 1976), retaliation to ant attack in medium-sized blind snakes (Webb & Shine 1993), careful subduing of ants by
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antlions and preventing them from spraying formic acid (Eisner et al. 1993), and learning and modification of foraging tactics in sticklebacks (Croy & Hughes 1991). One characteristic shared by these predators is that the foraging methods used when handling dangerous prey species are different from those employed when handling innocuous prey (O’Connell & Formanowicz 1998). The efficiency of the antipredator behaviour is measured through predation rate so that the more efficient the behaviour, the lower the predation rate (Ives & Dobson 1987). Zodarion spiders killed more individuals of all ant species than Xysticus spiders and at all densities. The spider species thus differed only in the parameter estimates of handling time not of searching efficiency. Xysticus spiders held the attacked ant in their chelicerae during the entire handling period and were not able to catch another ant at the same time. By contrast, attack and consumption in Zodarion spiders was separated by a waiting period, when these spiders were able to attack another ant specimen; they thus killed a high number of ants in a short time. The defence efficiency of ant species was expected to increase with their density, possibly because of both cooperative and individual defensive behaviour in ant workers. Indeed, the frequency of capture and the survival of spiders declined steeply with increasing density of ants. The efficiency of defence by ants can be read from the value of the prey inhibition parameter, c (the larger the value the higher their efficiency). However, the value of this parameter is also influenced by the efficiency of the predator’s counterattack strategies and the dangerousness of ants. Since we did not estimate the dangerousness it was not possible to estimate the contribution of these effects. When hunting dangerous prey, predators may counter prey defences with a variety of handling behaviours, which serve to kill prey and avoid counterattack. The Pardosa spiders do not usually hunt dangerous prey and employ fast speed of movement as defence against predators, including ants. The Xysticus spiders hold prey tightly by the chelicerae (forelegs outstretched) during the entire handling period (Nentwig 1986) so that the mandibles of a dangerous ant are directed away from the spider’s body. A similar defence is used by other thomisid spiders (Castanho & Oliveira 1997). This defence can be used only when the prey is not much larger than the predator. Zodarion spiders have developed a specialized predatory behaviour based on a quick approach, attack of the ant’s appendages, followed by retreat to a safe distance (Pekár 2004). Such a tactic is effective even for prey larger than the spider. When hunting dangerous social prey, predators need to adopt strategies effective also against other individuals of prey in the colony. Zodarion and Pardosa spiders used rather similar defence tactics based on avoidance of direct contact with ants; thus, their survival rates during the experiments were similar. In addition, Zodarion spiders could use the body of a dead ant as a shield against other approaching ants (Pekár & Král 2002). Avoidance of contact, however, became harder with increasing ant density so the survival of spiders declined steeply. Xysticus spiders defend themselves by staying motionless with legs crouched. In our experiments, ants often ignored them. If attacked by ants, spiders could cast off a limb. Leg autotomy is a well-known defence strategy of spiders (Foelix 1996): it has been observed in Zodarion (Pekár & Král 2002) and also in lycosid spiders (Punzo 1997; Amaya et al. 2001; Brueseke et al. 2001). The survival of Xysticus spiders was rather similar to the survival of Zodarion and Pardosa spiders. Xysticus spiders are able to catch ants but are probably not adapted to hunting ants at high abundance as they also feed on other prey. In nature they usually hunt solitary workers seeking food further away from the nest (Nyffeler & Breene 1990). Zodarion spiders, instead, occur in the vicinity of ant nests where the density of ants is high (e.g. Pekár 2004). The prey type often affects the shape of the functional response (e.g. Holling 1965; Tostowaryk 1972; Jeschke & Tollrian 2000; Bressendorff & Toft 2011). Here, we found the type 4 functional
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response to be the result of hunting dangerous prey. We used the functional response to compare foraging and defence efficiency among predators that possess different behavioural capture and defence adaptations. The new mechanistic model estimated three biologically relevant parameters; two of them concern the level of adaptation of the predator and one describes the strength of the prey’s defence. This model can also be applied to other predatoreprey systems in which the prey is dangerous and the decrease in predator hunting has a similar dependence on prey density as described here. Acknowledgments We thank three referees for useful comments to the manuscript. This study was supported by grant no. MUNI/A/0937/2012 provided by MU. References Altwegg, R., Eng, M., Caspersen, S. & Anholt, B. R. 2006. Functional response and prey defence level in an experimental predatoreprey system. Evolutionary Ecology Research, 8, 115e128. Amaya, C. C., Klawinski, P. D. & Formanowicz, D. R. 2001. The effects of leg autotomy on running speed and foraging ability in two species of wolf spider (Lycosidae). American Midland Naturalist, 145, 201e205. Baldwin, I. T. 1996. Inducible defenses and population biology. Trends in Ecology & Evolution, 11, 104e105. Bressendorff, B. B. & Toft, S. 2011. Dome-shaped functional response induced by nutrient imbalance of the prey. Biology Letters, 7, 517e520. Brueseke, M. A., Rypstra, A. L., Walker, S. E. & Persons, M. H. 2001. Leg autotomy in the wolf spider Pardosa milvina: common phenomenon with few apparent costs. American Midland Naturalist, 146, 153e160. Burnham, K. P. & Anderson, D. R. 2002. Model Selection and Multimodel Inference: a Practical Information-theoretic Approach. New York: Springer. Caro, T. 2005. Antipredator Defenses in Birds and Mammals. Chicago: University of Chicago Press. Castanho, L. M. & Oliveira, P. S. 1997. Biology and behaviour of the neotropical antmimicking spider Aphantochilus rogersi (Araneae: Aphantochilidae): nesting, maternal care and ontogeny of ant-hunting techniques. Journal of Zoology, 242, 643e650. Claver, M. A., Ravichandran, B., Khan, M. M. & Ambrose, D. P. 2003. Impact of cypermethrin on the functional response, predatory and mating behaviour of a non-target potential biological control agent Acanthaspis pedestris (Stal) (Het.: Reduviidae). Journal of Applied Entomology, 127, 18e22. Croy, M. I. & Hughes, R. N. 1991. The role of learning and memory in the feeding behaviour of the fifteen-spined stickleback, Spinachia spinachia L. Animal Behaviour, 41, 149e159. Eberhard, W. G. 1991. Chrosiotes tonala (Araneidae:Theridiidae): a web-building spider specializing on termites. Psyche, 98, 7e20. Edmunds, M. 1974. Defence in Animals: a Survey of Anti-Predator Defences. Harlow: Longman. Eisner, T. & Dean, J. 1976. Ploy and counterploy in predatoreprey interactions: orbweaving spiders versus bombardier beetles. Proceedings of the National Academy of Sciences, U.S.A., 73, 1365e1367. Eisner, T., Baldwin, I. T. & Conner, J. 1993. Circumvention of prey defense by a predator: ant lion vs. ant. Proceedings of the National Academy of Sciences, U.S.A., 90, 6716e6720. Everitt, B. S. 2002. Cambridge Dictionary of Statistics. 2nd edn. Cambridge: Cambridge University Press. Fisker, E. N. & Toft, S. 2004. Effects of chronic exposure to a toxic prey in a generalist predator. Physiological Entomology, 29, 129e138. Foelix, R. F. 1996. Biology of Spiders. Oxford: Oxford University Press. Forbes, L. S. 1989. Prey defences and predator handling behaviour: the dangerous prey hypothesis. Oikos, 55, 155e158. Fujii, K., Holling, C. S. & Mace, P. M. 1986. A simple generalized model of attack by predators and parasites. Ecological Research, 1, 141e156. Goodall, J. & van Lawick, H. 1970. Innocent Killers. London: Collins. Hassell, M. P. 1977. The Spatial and Temporal Dynamics of HosteParasitoid Interactions. New York: Oxford University Press. Havel, J. E. & Dodson, S. I. 1984. Chaoborus predation on typical and spined morphs of Daphnia pulex: behavioral observations. Limnology and Oceanography, 29, 487e494. Helfman, G. S. 1990. Mode selection and mode switching in foraging animals. Advances in the Study of Behavior, 19, 249e298. Heong, K. L., Bleih, S. & Rubia, E. G. 1991. Prey preference of the wolf spider, Pardosa pseudoannulata (Boesenberg et Strand). Researches on Population Ecology, 32, 179e186. Heuermann, N., van Langevelde, F., van Wieren, S. E. & Prins, H. H. T. 2011. Increased searching and handling effort in tall swards lead to a Type IV functional response in small grazing herbivores. Oecologia, 166, 659e669.
Holling, C. S. 1959. Some characteristics of simple types of predation and parasitism. Canadian Entomologist, 91, 385e398. Holling, C. S. 1961. Principles of insect predation. Annual Review of Entomology, 6, 163e182. Holling, C. S. 1965. Functional response of predators to prey density and its role in mimicry and population regulation. Memoirs of the Entomological Society of Canada, 45, 1e60. Hölldobler, B. & Wilson, E. O. 1990. The Ants. Cambridge, Massachusetts: Harvard University Press. Holmes, J. C. & Bethel, W. M. 1972. Modification of intermediate host behaviour by parasites. Zoological Journal of the Linnean Society, Supplement, 51, 123e149. Huseynov, E. F., Jackson, R. R. & Cross, F. R. 2008. The meaning of predatory specialization as illustrated by Aelurillus m-nigrum, an ant-eating jumping spider (Araneae: Salticidae) from Azerbaijan. Behavioural Processes, 77, 389e399. Ives, A. R. & Dobson, A. P. 1987. Antipredator behavior and the population dynamics of simple predatoreprey systems. American Naturalist, 130, 431e447. Jeschke, J. M. 2006. Density-dependent effects of prey defenses and predator offenses. Journal of Theoretical Biology, 242, 900e907. Jeschke, J. M. & Tollrian, R. 2000. Density-dependent effects of prey defences. Oecologia, 123, 391e396. Jeschke, J. M. & Tollrian, R. 2007. Prey swarming: which predators become confused and why? Animal Behaviour, 74, 387e393. Jeschke, J. M., Kopp, M. & Tollrian, R. 2004. Consumer-food systems: why type I functional responses are exclusive to filter feeders. Biological Reviews, 79, 337e 349. Lima, S. L. & Dill, L. M. 1990. Behavioral decisions made under the risk of predation: a review and prospectus. Canadian Journal of Zoology, 68, 619e640. Luck, R. F., van Lenteren, J. C., Twine, P. H., Juenen, L. & Unruh, T. 1979. Prey or host searching behavior that leads to a sigmoid functional response in invertebrate predators and parasitoids. Researches on Population Ecology, 20, 257e264. Mansour, F., Rosen, D. & Shulov, A. 1980. Functional response of the spider Cheiracanthium mildei (Arachnida: Clubionidae) to prey density. Entomophaga, 25, 313e316. May, R. M. & Robinson, S. K. 1985. Population dynamics of avian brood parasitism. American Naturalist, 126, 475e494. Mori, H. & Chant, D. A. 1966. Influence of humidity on activity of Phytoseiulus persimilis Athias-Henriot and its prey Tetranychus urticae (C. L. Koch) (Acarina: Phytoseiidae, Tetranychidae). Canadian Journal of Zoology, 44, 863e871. Nentwig, W. 1986. Non-webbuilding spiders: prey specialists or generalists? Oecologia, 69, 571e576. Nyffeler, M. & Breene, R. G. 1990. Spiders associated with selected European hay meadows, and the effects of habitat disturbance, with the predation ecology of the crab spiders, Xysticus spp. (Araneae, Thomisidae). Journal of Applied Entomology, 110, 149e159. O’Connell, D. J. & Formanowicz, D. R., Jr. 1998. Differential handling of dangerous and non-dangerous prey by naive and experienced Texas spotted whiptail lizards, Cnemidophorus gularis. Journal of Herpetology, 32, 75e79. Pekár, S. 2004. Predatory behavior of two European ant-eating spiders (Araneae, Zodariidae). Journal of Arachnology, 32, 31e41. Pekár, S. 2005. Predatory characteristics of ant-eating Zodarion spiders (Araneae: Zodariidae): potential biological control agents. Biological Control, 34, 196e203. Pekár, S. & Brabec, M. 2009. Modern Analysis of Biological Data. 1. Generalised Linear Models in R. Prague: Scientia [in Czech]. Pekár, S. & Král, J. 2002. Mimicry complex in two central European zodariid spiders (Araneae: Zodariidae): how Zodarion deceives ants. Biological Journal of the Linnean Society, 75, 517e532. Pekár, S., Coddington, J. A. & Blackledge, T. 2012. Evolution of stenophagy in spiders (Araneae): evidence based on the comparative analysis of spider diets. Evolution, 66, 776e806. Punzo, F. 1997. Leg autotomy and avoidance behavior in response to a predator in the wolf spider Schizocosa avida (Araneae, Lycosidae). Journal of Arachnology, 25, 202e205. R Development Core Team 2010. R: A Language and Environment for Statistical Computing. Vienna: R Foundation for Statistical Computing. http://www. R-project.org. Schaller, G. B. 1972. The Serengeti Lion: A Study of PredatorePrey Relations. Chicago: University of Chicago Press. Sih, A. 1980. Optimal foraging: partial consumption of prey. American Naturalist, 116, 281e290. Smith, R. B. & Wellington, W. G. 1986. The functional response of a juvenile orbweaving spider. In: Proceedings of the Ninth International Congress of Arachnology (Ed. by W. G. Eberhard, Y. D. Lubin & B. C. Robinson), pp. 275e279. Panama: Smithsonian Institution Press. Solomon, M. E. 1949. The natural control of animal populations. Journal of Animal Ecology, 18, 1e35. Tener, J. S. 1965. Muskoxen. Ottawa: Queens Printer. Toft, S. & Jensen, A. P. 1998. No negative sublethal effects of two insecticides on prey capture and development of a spider. Pesticide Science, 52, 223e228. Tostowaryk, W. 1972. Effect of prey defense on functional response of Podisus modestus (Hemiptera: Pentatomidae) to densities of sawflies Neodiprion swainei and N. pratti banksianae (Hymenoptera: Neodiprionidae). Canadian Entomologist, 104, 61e69. Vucic-Pestic, O., Birkhofer, K., Rall, B. C., Scheu, S. & Brose, U. 2010. Habitat structure and prey aggregation determine the functional response in a soil predatoreprey interaction. Pedobiologia, 53, 307e312.
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Appendix 1 Table A1 Overview of the four types of functional response Functional Description (see Fig. A1 for response shapes) Type 1
Type 2
Type 3
Type 4
Organisms
Source
Intake rate increases with prey density up to a maximum; handling time is negligible
Filter feeders, Smith & web-building Wellington 1986; spiders Jeschke et al. 2004 Intake rate decelerates with Hunting Mansour et al. increasing prey density; spiders 1980; Heong handling time for one prey item et al. 1991 is constant but with increasing number of captured prey total handling time increases Intake rate accelerates at low Shrews, deer Holling 1959; prey densities owing to e.g. mice, Luck et al. 1979 formation of a search image; at parasitoid higher densities the rate is wasps decelerating as in type 2 Intake rate decelerates with Predatory Mori & Chant increasing prey density up to a mites, 1966; maximum limited by total pentatomid Tostowaryk handling time, thereafter bugs 1972 handling time for one prey item increases with prey density owing to e.g. prey defence
Type 1 Type 2
Capture frequency
Type 3 Type 4
Prey density Figure A1. Shapes of four types of functional response.
Appendix 2 To model the type 4 functional responses, three mathematical models have been proposed so far. All three models are derived from the Holling disc equation for type 2 functional responses (Holling 1959). Hassell (1977) found that predator searching efficiency (a) may not be constant and can be a function of prey density
1189
(N). The searching efficiency then indicates a predator’s learning ability. The number of attacked prey (A) is then related to prey density as follows:
A ¼
uTN2 1 þ uth N2 þ vN
(A1)
Here u and v are unknown parameters. The model is used to describe the type 3 functional response and if the value of v is negative, the resulting curve shape corresponds to type 4. This model should be particularly useful when confusion of the predator is the crucial factor. Tostowaryk (1972) altered the handling time of the prey instead. He observed that the handling time of one prey item increased with increasing prey density owing to prey defence, which is characterized by the inhibition parameter (c). Thus, the number of killed prey is:
A ¼
aTN 1 þ ath N þ acN3
(A2)
The term cN2 (in linear form) suggests that the effectiveness of prey defence is proportional to the square of the prey density. This model should be used to describe situations with defence by prey or disturbance of the predator. A generalized model, used for the description of all four types of functional response, was proposed by Fujii et al. (1986):
A ¼
TaNecN 1 þ th aNecN
(A3)
Here, c is an auxiliary parameter. If c < 0, type 4 occurs. This model could be used to fit any kind of mechanism mentioned above causing a decline in capture frequency. We fitted the four different models, which differ in the number of parameters, to the data on Zodarion spiders and Tetramorium ants (see Methods). Two of the models (A2 and the new one) were linear in parameters, therefore fitted using generalized linear models (GLM). Gamma error structure (GLM-g) and inverse link were used to fit both models. Two other models (A1 and A3) were not linear in parameters, therefore fitted using nonlinear regression. Values of a, th and c were directly estimated from the fit, except for a in A1 where the fit provides estimates of v and u. The density of ants at which the hunting frequency of spiders reached a maximum was estimated by finding the root of the first derivative of the final models. The models were compared on the basis of several criteria. Overall fit to the data was assessed by the generalized coefficient of determination (R2) and standard diagnostics plots of residuals. R2 for models A2 and the new model was estimated as a proportion of explained to total deviance (Pekár & Brabec 2009), while for models A1 and A3 it was estimated using the squared correlation between observed and predicted values (Everitt 2002). We expected that the estimated parameters should be meaningful and biologically interpretable and that the model should be parsimonious, which was assessed by means of the Akaike information criterion (AIC), which measures goodness of fit. Akaike weights (Burnham & Anderson 2002) were also used to select the best among competing models. Model A1 differed considerably from all the others. It gave the worst fit, dramatically overestimated or underestimated the number of killed prey, and predicted the maximum hunting frequency at a density of 2.6 ants. A negative estimate of v resulted in a negative value of a for most densities (Table A2). All other models provided a much better fit and differed in the predicted maximum number of ants killed and the rate of decrease at higher densities.
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Model A2 predicted the highest hunting frequency at a density of 8.1 ants and model A3 at a density of 8.8 ants. The new model predicted the maximum number of killed ants at a density of 5.4 ants and had a steeper decrease in the hunting frequency than the previous two models, which in fact corresponded better to the observed data. For the two models, A2 and the new one, the diagnostic plots were similar. The new model had the best fit in comparison with other models in terms of R2 and AIC (Table A2, Fig. A2). According to Akaike weights the new model has the highest relative likelihood to be the best.
Table A2 Comparison of four models used to describe the type 4 functional response for Zodarion spiders and Tetramorium ants Model
AIC
w
R2
a
th (min)
c
(1) Hassell (1977)
298
0.00003
0.11
0.30
e
(2) Tostowaryk (1972) (3) Fujii et al. (1986) (4) New model
280 292 278
0.27 0.0007 0.73
0.19 0.19 0.30
1:52N 1 0:77N 4.65 4.46 3.40
0.17 0.14 0.10
0.0002 0.1140 0.0100
Akaike information criterion (AIC, preferred model is the one with the lowest value), Akaike weight (w, preferred model is the one with the highest value), coefficient of determination (R2) and parameter estimates (a, th, c) are shown.
7
1 2 3 4
6 Number of killed ants/h
1190
5 4 3 2 1 0
0
10
20 Density of ants
30
40
Figure A2. Fit of four models (see Table A2 for explanation) to observed functional response of Zodarion spiders to density of Tetramorium ants. Points are means of observed data, vertical bars are quartiles.
Rukopis C
TROPHIC NICHE OF OECOBIUS MACULATUS (ARANEAE: OECOBIIDAE): EVIDENCE BASED ON NATURAL DIET, PREY CAPTURE SUCCESS , AND PREY HANDLING Líznarová, E., Pekár, S. (2015) The Journal of Arachnology, 43, 188–193
Oecobius maculatus
© Macek
Trophic niche of Oecobius maculatus (Araneae: Oecobiidae): evidence based on natural diet, prey capture success, and prey handling Author(s): Eva Líznarová and Stano Pekár Source: Journal of Arachnology, 43(2):188-193. Published By: American Arachnological Society DOI: http://dx.doi.org/10.1636/J14-56 URL: http://www.bioone.org/doi/full/10.1636/J14-56
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2015. Journal of Arachnology 43:188–193
Trophic niche of Oecobius maculatus (Araneae: Oecobiidae): evidence based on natural diet, prey capture success, and prey handling Eva Lı´znarova´ and Stano Peka´r: Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotla´rˇska´ 2, 611 37 Brno, Czech Republic. E-mail:
[email protected] Abstract. Field and laboratory observations of the feeding ecology (natural diet, prey capture success, and prey handling) of Oecobius maculatus Simon 1870 were combined in this study to reveal the level of trophic specialization by this species. Natural prey were investigated on the Croatian island of Brac. Field observations revealed that although spiders captured prey belonging to five invertebrate orders, the most frequently captured prey were ants, which were also the most abundant available prey in the locality. In laboratory experiments, O. maculatus spiders accepted three other prey types with a higher probability than ants and were significantly more efficient at capturing and handling flies than ants. These results suggest that this species is a stenophagous generalist with a narrow prey range due to ecological circumstances. Keywords:
Ants, diet breadth, myrmecophagous, spider, trophic specialization
Most generalist spiders with no specialized adaptations for ant capture are not able to hunt ants without risk of harm, and thus they avoid ants as prey. However, a few spider species with specialized adaptations for ant capture are stenophagous specialists and feed primarily on ants. For example, members of the genus Zodarion Walckenaer 1826 (Zodariidae) use specialized adaptations to overcome ant defenses: they use a unique hunting strategy which involves quickly approaching and attacking an ant’s appendage, after which they retreat to a safe distance (Peka´r 2004, 2005). Their venom is potent on ants – a single bite suffices to paralyse an individual (Peka´r et al. 2014). However, venom specialization carries trade-offs: in particular, it is not so effective on other prey types (Peka´r 2004). Another spider genus that has been considered to be a stenophagous ant-eating specialist is Oecobius Lucas 1846 (Oecobiidae). This assumption was based on anecdotal observation of the natural diet of Oecobius navus Blackwall 1859 (Glatz 1967; Voss et al. 2007). Exclusive ant-eating in this species was further supported by laboratory observations in which the spiders accepted no arthropods other than ants as prey (Glatz 1967). However, more recent evidence shows that O. navus is not strictly stenophagous. The natural diet of two populations of O. navus was studied (Lı´znarova´ et al. 2013). One population fed mostly on ants, whereas the other population fed primarily on flies and springtails. The most commonly captured prey in each locality corresponded closely to the most abundant potential prey (Lı´znarova´ et al. 2013), suggesting that O. navus is more likely a stenophagous generalist with its diet in different localities dependent on available prey. Similarly, Garcia et al. (2014) revealed that, in nature, Oecobius concinnus Simon 1893 captured mostly ants and dipterans. We studied the natural diet, prey capture success, and prey handling of Oecobius maculatus Simon 1870. The predatory biology of this species has not been systematically investigated. In nature, this species was observed preying primarily on ants (Lı´znarova´, personal observation). We tested whether this species possesses specialized adaptations for ant capture and, moreover, if this specialization carries some trade-offs, namely reduced proficiency at capturing an alternative prey. We
The degree of trophic specialization can be viewed in two contexts. The ecological context describes the diet breath of species in nature: euryphagous species have a wide diet range and stenophagous species have a narrow diet range (Peka´r & Toft 2014). The second context focuses on evolutionary adaptations for feeding on a range of food: generalists are able to feed and perform on a wide diet range, while specialists exhibit enhanced handling and use of one or several food types, often associated with poorer performance on alternative food types (Peka´r & Toft 2014). Specialists do well when a habitat is stable and the preferred food type is abundant; however, they are vulnerable to changes in their environment (Goldstein 2009). Predators seem to be less frequently adapted to a narrow diet range than herbivores, parasites or parasitoids (Thompson 1994). Spiders are predators that capture several prey taxa throughout their lives, including a wide variety of invertebrate and vertebrate prey. The number of prey taxa accepted by spiders varies considerably among species. The majority of spider species seem to be euryphagous or oligophagous with a slightly restricted diet (Nentwig 1987) and only a few species are stenophagous, feeding on restricted prey types. The most frequent type of stenophagy in spiders is myrmecophagy or anteating (Peka´r et al. 2011). This is probably because ants are highly abundant in many different terrestrial habitats (Ho¨lldobler & Wilson 1990). However, ants often have slender bodies with limited usable body mass and can be dangerous because they are often predatory, have effective defense mechanisms, and are similar to spiders in size (Ho¨lldobler & Wilson 1990). Small predators, such as spiders, are often exposed to predation themselves when handling prey, so they tend to minimize handling time. Nevertheless, dangerous prey such as ants should be handled more carefully than innocuous prey due to the risk of harm by the prey (Lima & Dill 1990). Furthermore, besides time, spiders usually need to invest more silk and venom when dealing with dangerous prey (Nentwig & Heimer 1987; Malli et al. 1999). Thus, the handling of dangerous prey is energetically costly. Predators can take dangerous prey less often (Forbes 1989) or become specialized and decrease the handling time and energetic costs by increasing the effectiveness of their capture. 188
´ & PEKA ´ R—TROPHIC NICHE OF OECOBIUS MACULATUS LI´ZNAROVA
combined field observations of its natural diet with laboratory experiments, investigating the spider’s capture success for different prey types and its handling tactics with respect to ants and flies. We selected these two prey types because a previous study indicated that Oecobius spiders could feed predominantly either on ants or dipterans (Lı´znarova´ et al. 2013). METHODS Spiders of the genus Oecobius build tent-like webs (double sheet webs) that consist of two parallel sheets and several signal threads that run out into the vicinity (Hingston 1925). Spiders use the webs as retreats and most of the time sit hidden between the two sheets with their legs touching the signal threads. When the prey touches the thread, the spider quickly runs out of the web and starts to subdue the prey by circling around it and throwing silk over it. The subdued prey is then taken to the web retreat and consumed; prey remnants remain attached to the web. We collected Oecobius maculatus spiders in the town of Sumartin (GPS: 43u 179 50 N, 16u 529 190 E) on the island of Brac (Croatia) from the stone walls around houses where they were especially abundant. The spider individuals collected (n 5 29) were at different developmental stages. We placed each collected spider into a plastic tube together with silk from a web that contained prey remnants. Oecobius spiders do not chew their prey during feeding; thus, prey remnants are usually preserved in a good condition, enabling their identification. To obtain information about potential prey in the spiders’ vicinity, we collected invertebrate individuals within 50 cm of spiders’ webs using a pooter (aspirator) and placed them in vials with ethanol at the time of collection. The collection was performed on two days in September 2010 for two hours each day (one hour in the morning, one hour in the afternoon) by one person, amounting to 240 ‘person-minutes.’ We identified spiders using Nentwig et al. (2010) and Santos & Gonzaga (2003). We identified all prey, potential and actual (i.e. prey remnants found in webs), to the lowest taxonomic level allowed by the physical condition of the specimens. In most cases, we identified the prey specimens to order level. We split Diptera into Nematocera and Brachycera, and Hymenoptera into Formicidae and others. We identified ants collected as potential prey and ant remnants from the webs to species level using Collingwood & Prince (1998). After transfer to the laboratory, we placed living spiders (n 5 24, 10 adult females, 14 juveniles) in individual Petri dishes (30 mm diameter 3 10 mm height) and left them for three days, during which they built normal webs. Before and during the trials, the spiders were kept at room temperature (approximately 22 uC) and under a natural 14L:10D photoperiod. We maintained moisture levels by adding a drop of water to the bottom of the dish. Seven days before the beginning of experiments, we fed the spiders with fruit flies until satiated to standardize their hunger level. After each trial we fed the spiders with fruit flies ad libitum and then left them for seven days without prey prior to the next trial. In the first experiment, we observed the spiders’ capture success for different prey types. We used prey from nine invertebrate orders, most of which occur on Brac. From laboratory-reared cultures, we took fruit flies (Drosophila sp.,
189
Diptera, mean body length 2.0 mm), termites (workers of Reticulitermes sp., Isoptera, 3.5 mm), springtails (Sinella curviseta, Collembola, 4.0 mm), crickets (Acheta domestica, Orthoptera, 5.0 mm), and beetles (Oryzaephilus surinamensis, Coleoptera, 3.0 mm). From the field, we collected spiders (Zodarion sp., Araneae, 3.0 mm), millipedes (Julidae, Diplopoda, 4.0 mm), ants (workers of Lasius sp., Hymenoptera, 3.0 mm), beetles (Curculionidae, Staphylinidae, Coleoptera, 3.0 mm), and aphids (Aphidinae, Hemiptera, 2.5 mm). We placed one prey individual in each dish occupied by a spider (n 5 24) on the opposite side of the dish to where the spider web was built. The webs were usually built on the base of the dish or in the angle between the base and wall; thus, opening the dish did not damage the web. In each trial, we recorded whether the spider captured and then consumed the prey. If the spider did not attack the prey within one hour, then we removed the prey from the dish. We used a randomised incomplete block design so that each prey type was used with at least ten spider individuals in a random order. In the second experiment, we compared the handling of ants (Lasius sp.) and fruit flies (Drosophila melanogaster). We offered ants and flies to individual spiders successively in a random order. As before, we placed the prey in the dish occupied by the spider on the opposite side of the dish to where the spider web was built and observed the prey capture behaviour in detail. We recorded the duration of the handling time (the time that elapsed from the first contact with the prey to the start of consumption) using a stopwatch. We measured the following two components of the handling time separately: (1) the time required to wrap the prey and (2) the waiting time (when the spider waited at a safe distance from the prey after envenomation). We also recorded the number of bites delivered to the prey individual. There were 21 replications with ants and 24 with fruit flies. We performed all analyses with R (R Development Core Team 2010). For both actual and potential prey, we estimated the niche breadth as the inverse of the Simpson index, because it is not affected by rare species that could appear accidentally in a sample (Levins 1968). We compared the proportions of prey types in the diet using Morisita’s index, which is suitable when comparing samples of different size and diversity (Horn 1966). Using the Chi-square test, we compared the relative frequencies of prey found in the webs and the prey available on the walls. For each ant species, we measured the selectivity of attack according to the Savage selectivity index (W) (Manly et al. 2002). The results of the prey-capture success experiments and the results of the prey handling experiments were analysed using Generalised Estimating Equations (GEE), which is a linear method that can handle correlations resulting from repeated use of the same spider individuals. We used GEE for non-normally and log-normally distributed response variables (Peka´r & Brabec 2012). To compare the acceptance of prey, GEE with binomial errors (GEE-b) were used because the response variable was composed of binary scores. To compare the handling time, wrapping time, and waiting time, GEE with Gamma errors (GEE-g) were used because the response variables were expected to have a Gamma distribution. We used GEE with Poisson errors (GEE-p) to compare the numbers of bites, because the response variable was expected to have a Poisson distribution.
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Table 1.—Relative frequencies of prey taxa found in the webs (5 actual prey) and around the webs (5 potential prey) of Oecobius maculatus in the town of Sumartin on the island of Brac (Croatia). Order
Actual
Diptera Nematocera
0.03
0.00
Hymenoptera Formicidae Other
0.87 0.01
0.91 0.01
0.01 0.03 0.03 0.00 0.00 0.00 70
0.01 0.03 0.00 0.01 0.01 0.01 157
Hemiptera Araneae Acari Isopoda Psocoptera Gastropoda Total (N)
Potential
RESULTS Natural prey analysis.—Representatives from five invertebrate orders were found in the webs (actual prey) of O. maculatus and representatives from six invertebrate orders were found in the vicinity of spiders’ webs (potential prey) (Table 1). The most abundant prey in the webs were ants (89%) and ants were also the most frequent prey occurring around the spiders webs (91%). There were three ant species from two subfamilies found around spiders’ webs: Crematogaster scutellaris (Olivier) (Myrmicinae), Pheidole pallidula (Nylander) (Myrmicinae), and Lepisiota frauenfeldi (Mayr) (Formicinae). The similarity between prey taxa composition found in the webs and that of the potential prey was 99.8% (Morisita’s index) and did not differ significantly (Chi-square test: x29 5 0.01, P 5 0.99). The diversity of potential prey expressed by Simpson’s reciprocal index was 1.27, indicating that one prey type was dominant in the sample. Selectivity was computed for each ant species (i) according to the Savage selectivity index (Wi); there was almost no selection in C. scutellaris (W 5 0.89), positive selection in P. pallidula (W 5
Figure 2.—Capture probability for nine different prey types compared to the average (thick horizontal line) prey capture probability (5 0.495) of the spider Oecobius maculatus.
5.99), and negative selection in L. frauenfeldi (W 5 0.60) (Fig. 1). Prey-capture success.—All of the offered prey types were captured by some (at least two) spider individuals. The capture success differed significantly among prey types (GEE-b, x21 5 21974, P 5 0.001). The only prey type that was captured with significantly less than the average probability of 49.5% was aphids (8%, n 5 24, contrasts, P 5 0.007). Millipedes (25%, n 5 12), beetles (38%, n 5 21), spiders (42%, n 5 19), and crickets (42%, n 5 19) were captured with less than average probability, but not significantly less (P . 0.05). In contrast, termites (100%, n 5 19, contrasts, P 5 0.001) and fruit flies (78%, n 5 23, contrasts, P 5 0.039) were captured with significantly higher than average probability. Springtails (62%, n 5 21) and ants (57%, n 5 23) were captured with only slightly higher than average probability (Fig. 2). All prey individuals that spiders captured were consumed, except beetles, where 62% (n 5 21) of individuals were not killed following the spider’s attack or were rejected by the spider after being subdued. Prey handling.—The handling times for flies and for ants differed significantly (GEE-g: x21 5 3.33, P 5 0.002). Ants were handled for a significantly longer time than flies (Fig. 3). When the components of the handling process were compared separately, the wrapping time did not differ significantly between flies and ants (GEE-g: x21 5 20.9, P 5 0.38), but the number of bites used during the attack was significantly higher for ants (GEE-p: x21 5 2.3, P 5 0.02). Also, the waiting time for prey paralysis was significantly longer for ants than for flies (GEE-g: x21 5 2.4, P 5 0.02). DISCUSSION
Figure 1.—Relative frequencies of ant species in the composition of potential and actual prey of the spider Oecobius maculatus.
Natural prey analysis showed that O. maculatus spiders captured mainly ants at the study site. From such data alone, it would be tempting to claim that O. maculatus are stenophagous predators specialised on ants. A similarly
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Figure 3.—Comparison of the total handling time (A), number of bites (B), wrapping time (C), and waiting time (D) of Oecobius maculatus for two different prey types, fruit flies (n 5 17) and ants (n 5 13). Bars are means; whiskers are 95% confidence intervals.
erroneous conclusion was made in the past with respect to O. navus. This species was formerly considered as a stenophagous specialist (Glatz 1967); however, recent laboratory experiments revealed that this species is able to successfully subdue a wide variety of prey, indicating that O. navus is only a stenophagous generalist (i.e., local trophic specialist) (Lı´znarova´ et al. 2013). This may also apply to many other Oecobius species. It is assumed that foraging individuals should discriminate among prey types on the basis of their relative profitability (Reichert & Luczak 1982), usually expressed as a cost:benefit ratio (Krebs 1978). Commonly unprofitable prey types with higher costs than benefits for spiders are animals that are beyond the spiders’ size range, are distasteful prey, and are predatory species that can attack the spider (Reichert & Luczak 1982). Our laboratory experiments showed that termites, fruit flies, and springtails were captured by O. maculatus spiders at a higher frequency than ants, suggesting that these prey types are actually more profitable for the studied spiders than ants. These prey types are most likely to be of optimal size, palatable, and non-dangerous. In contrast, many aphid species are distasteful or even noxious for spiders
(e.g. Malcolm 1989; Toft 1995), which could be the reason why aphids were captured by O. maculatus spiders with the lowest frequency. In addition, the foraging tactics of spiders may also differ when hunting different prey types—in this case, either innocuous or dangerous prey (O’Connell & Formanowicz 1998). Oecobius maculatus was able to discriminate between innocuous fruit flies and dangerous ants and to employ different hunting tactics. To subdue ants, it was necessary to use more bites, possibly to inject more venom. The waiting time was also longer for ants than for flies; possibly, the venom took longer to paralyze ants or the spiders were more vigilant when hunting dangerous prey and waited until the ants were completely paralyzed before feeding. By contrast, spiders were able to consume flies immediately after an attack without risk of harm. This is in line with the dangerous prey hypothesis, which states that predators may need more time to handle individuals exhibiting strong defensive behaviour compared to defenseless prey (Forbes 1989). In general, Oecobius spiders use the tactic that is efficient for hunting dangerous prey for all prey types. Since these spiders throw their silk from some distance and face away from their prey in
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a similar fashion as ant-eating Euryopis spiders (Carico 1978; Porter & Eastmond 1982), they are able to entangle the prey in silk before it can retaliate. Specialists have often evolved morphological, metabolic, venomic, and behavioral traits used in prey capture and processing that are absent in generalists. These adaptations increase the efficiency of capture of the principal prey; however, they may constrain the spiders’ ability to catch alternative prey. Despite that, there are also examples of polyspecialists, which are simultaneously specialized on more than one type of prey (e.g., Jackson & Blest 1982). Ant-eating Zodarion spiders use specialized predatory behavior (Peka´r 2004) and selectively potent venom to subdue ants effectively (Peka´r et al. 2014). We did not observe any of these adaptations in O. maculatus. Furthermore, it is not known whether Oecobius spiders are able to develop and grow on ants exclusively, as are Zodarion spiders (Peka´r et al. 2010), and thus possess metabolic adaptations which enable them to balance the nutrient intake and extract all essential nutrients from just one prey type, or whether they require additional nutrients from other prey species. In nature, Oecobius maculatus captured ants of three species. Yet, we observed some degree of selectivity in O. maculatus among these ant species. The spiders hunted P. pallidula more often than expected and L. fraunfeldi less than expected. This suggests some ability by O. maculatus to discriminate even between ant species and actively choose more profitable ones. Pheidole ants are primarily scavengers that occasionally hunt small insects (Detrain & Deneubourg 1997); therefore, they may be less aggressive, and thus less dangerous, than strictly predatory Lepisiota ants (Sekamatte et al. 2003). Alternatively, the selection could be passive, due to the spider’s lower success rate at subduing Lepisiota ants. The obtained data support the view that O. maculatus spiders are stenophagous generalists rather than stenophagous specialists, because we did not observe any specialized adaptations in capturing ants. Their hunting tactics are well adapted to a wide variety of prey types including ants and the successful capturing of ants does not result in trade-offs in capturing alternative prey types. Because Oecobius spiders captured a variety of insects in the laboratory, their sedentary foraging mode suggests selection for a preferred habitat rather than prey (Uetz et al. 1992), which is rather frequent in spiders (Reichert 1981). Because the Oecobius species studied were typically found near high ant aggregations (Glatz 1967; Voss et al. 2007; Lı´znarova´ et al. 2013; Garcia et al. 2014), their prey choice in nature is probably made indirectly by choosing spots with high ant densities. Here, we show that in order to understand the trophic ecology of a predatory species completely, it is necessary to combine both field and laboratory approaches. Field observations of natural prey reveal what the predator actually eats, whereas laboratory experiments help to determine the level of trophic specialization of the studied species, e.g., by exploring the predator’s likelihood of catching different prey types and the respective efficiencies with which they are caught. ACKNOWLEDGMENTS We would like to thank Robert R. Jackson and one anonymous referee for their very useful comments which
considerably improved this manuscript. This study was supported by grant number MUNI/A/0888/2013. LITERATURE CITED Carico, J. 1978. Predatory behavior in Euryopis funebris (Hentz) (Araneae: Theridiidae) and the evolutionary significance of web reduction. Symposia of the Zoological Society of London 42: 51–58. Collingwood, C.A. & A. Prince. 1998. A guide to ants of continental Portugal (Hymenoptera: Formicidae). Sociedade Portuguesa de Entomologia, Suppl. 5:1–49. Detrain, C. & J.L. Deneubourg. 1997. Scavenging by Pheidole pallidula: a key for understanding decision-making systems in ants. Animal Behaviour 53:537–547. Forbes, L.S. 1989. Prey defences and predator handling behaviour: the dangerous prey hypothesis. Oikos 55:155–158. Garcı´a, L.F., M. Lacava & C. Viera. 2014. Diet composition and prey selectivity by the spider Oecobius concinnus (Araneae: Oecobiidae) from Colombia. Journal of Arachnology 42:199–201. Glatz, L. 1967. Zur Biologie und Morphologie von Oecobius annulipes (Araneae: Oecobiidae). Zoomorphology 64:185–214. Goldstein, N. 2009. Animal Hunting and Feeding. Infobase Publishing, New York. Hingston, R.W.G. 1925. Nature at the Desert’s Edge, Studies and Observations in the Bagdad Oasis. Witherby, London. Ho¨lldobler, B. & E.O. Wilson. 1990. The Ants. Harvard Belknap, Cambridge, Massachusetts. Horn, H.S. 1966. Measurement of ‘‘overlap’’ in comparative ecological studies. American Naturalist 100:419–424. Jackson, R.R. & A.D. Blest. 1982. The biology of Portia fimbriata, a web-building jumping spider (Araneae, Salticidae) from Queensland: utilization of webs and predatory versatility. Journal of Zoology 196:255–293. Krebs, J.R. 1978. Optimal foraging: decision rules for predators. Pp. 23–63. In Behavioural Ecology: An Evolutionary Approach. (J.R. Krebs, N.B. Davies, eds.). Blackwell Scientific Publications, Oxford. Levins, R. 1968. Evolution in Changing Environments: Some Theoretical Explanations. Princeton University Press, Princeton. Lima, S.L. & L.M. Dill. 1990. Behavioral decisions made under the risk of predation: a review and prospectus. Canadian Journal of Zoology 68:619–640. Lı´znarova´, E., L. Sentenska´, L.F. Garcı´a, S. Peka´r & C. Viera. 2013. Local trophic specialisation in a cosmopolitan spider (Araneae). Zoology 116:20–26. Malcolm, S.B. 1989. Disruption of web structure and predatory behavior of a spider by plant-derived chemical defenses of an aposematic aphid. Journal of Chemical Ecology 15:1699–1716. Malli, H., L. Kuhn-Nentwig, H. Imboden & W. Nentwig. 1999. Effects of size, motility and paralysation time of prey on the quantity of venom injected by the hunting spider Cupiennius salei. Journal of Experimental Biology 202:2083–2089. Manly, B.F.J., L.L. McDonald, D.L. Thomas, T.L. McDonald & W.P. Erickson. 2002. Resource Selection by Animals: Statistical Analysis and Design for Field Studies. Nordrecht, Kluwer. Nentwig, W. 1987. The prey of spiders. Pp. 249–263. In Ecophysiology of Spiders. (W. Nentwig, ed.). Springer-Verlag, Berlin. Nentwig, W. & S. Heimer. 1987. Ecological aspects of spider webs. Pp. 211–225. In Ecophysiology of Spiders. (W. Nentwig, ed.). Springer-Verlag, Berlin. Nentwig, W., A. Ha¨nggi, C. Kropf & T. Blick. 2010. Central European Spiders – Determination Key. Online at http://www. araneae.unibe.ch/ O’Connell, D.J. & D.R. Formanowicz, Jr. 1998. Differential handling of dangerous and non-dangerous prey by naive and experienced
´ & PEKA ´ R—TROPHIC NICHE OF OECOBIUS MACULATUS LI´ZNAROVA Texas spotted whiptail lizards, Cnemidophorus gularis. Journal of Herpetology 32:75–79. Peka´r, S. 2004. Predatory behavior of two European ant-eating spiders (Araneae, Zodariidae). Journal of Arachnology 32:31–41. Peka´r, S. 2005. Predatory characteristics of ant-eating Zodarion spiders (Araneae: Zodariidae): potential biological control agents. Biological Control 34:196–203. Peka´r, S. & M. Brabec. 2012. Modern Analysis of Biological Data. 2. Linear Models with Correlation R. Masaryk University Press, Brno [in Czech]. Peka´r, S. & S. Toft. 2014. Trophic specialisation in a predatory group: the case of prey specialised spiders (Araneae). Biological Reviews (in press). Peka´r, S., J. Sˇobotnı´k & Y. Lubin. 2011. Armoured spiderman: morphological and behavioural adaptations of a specialised araneophagous predator (Araneae: Palpimanidae). Naturwissenschaften 98:593–603. Peka´r, S., D. Mayntz, T. Ribeiro & M.E. Herberstein. 2010. Specialist ant-eating spiders selectively feed on different body parts to balance nutrient intake. Animal Behaviour 79:1301–1306. Peka´r, S., O. Sˇedo, E. Lı´znarova´, S. Korenko & Z. Zdra´hal. 2014. David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey. Naturwissenschaften 101:1–8. Porter, S.D. & D.A. Eastmond. 1982. Euryopis coki (Theridiidae), a spider that preys on Pogonomyrmex ants. Journal of Arachnology 10:275–277.
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R Development Core Team. 2010. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. Reichert, S.E. 1981. The consequences of being territorial: spiders, a case study. American Naturalist 117:871–892. Reichert, S.E. & J. Luczak. 1982. Spider foraging: behavioral responses to prey. Pp. 353–385. In Spider Communication: Mechanisms and Ecological Significance. (P.N. Witt & J.S. Rovner, eds.). Princeton University Press, New Jersey. Santos, A.J. & M.O. Gonzaga. 2003. On the spider genus Oecobius Lucas, 1846 in South America (Araneae, Oecobiidae). Journal of Natural History 37:239–252. Sekamatte, B.M., M. Ogenga-Latigo & A. Russell-Smith. 2003. Effects of maize–legume intercrops on termite damage to maize, activity of predatory ants and maize yields in Uganda. Crop Protection 22:87–93. Thompson, J.N. 1994. The Coevolutionary Process. University of Chicago Press, Chicago. Toft, S. 1995. Value of the aphid Rhopalosiphum padi as food for cereal spiders. Journal of Applied Ecology 32:552–560. Uetz, G.W., J. Bischoff & J. Raver. 1992. Survivorship of wolf spiders (Lycosidae) reared on different diets. Journal of Arachnology 20:207–211. Voss, S.C., B.Y. Main & I.R. Dadour. 2007. Habitat preferences of the urban wall spider Oecobius navus (Araneae, Oecobiidae). Australian Journal of Entomology 46:261–268. Manuscript received 10 October 2014, revised 18 February 2015.
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METABOLIC SPECIALISATION ON PREFERRED PREY AND CONSTRAINTS IN THE UTILISATION OF ALTERNATIVE PREY IN AN ANT-EATING SPIDER Líznarová, E., Pekár, S. (2016) Zoology (online)
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Metabolic specialisation on preferred prey and constraints in the utilisation of alternative prey in an ant-eating spider Eva Líznarová ∗ , Stano Pekár ∗ Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotláˇrská 2, 611 37 Brno, Czech Republic
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Article history: Received 8 January 2016 Received in revised form 11 March 2016 Accepted 25 April 2016 Available online xxx Keywords: Euryopis episinoides Ant-eating spider Stenophagy Trophic specialisation
a b s t r a c t Trophic specialists are expected to possess adaptations that increase the efficiency of handling preferred prey. Such adaptations may constrain the ability to utilise alternative prey. Here we tested whether the ant-eating spider Euryopis episinoides possesses metabolic specialisations with increased efficiency in utilising preferred prey and decreased efficiency in utilising alternative prey. In addition, we investigated the contribution of genetic variation via maternal effects. We reared E. episinoides spiders from the first instar on two different diets, either ants (preferred prey) or fruit flies (alternative prey). Spider survival rate and increases in body mass were significantly higher on the ant diet. The total development time did not differ between diet groups, nor did the number of egg sacs per female or the incubation period. However, the number of eggs per egg sac and hatching success were higher on the ant diet. There was a genetic variation in several offspring traits. Our data support the hypothesis that stenophagous anteating E. episinoides have a metabolic specialisation on ant utilisation indicated by higher efficiency in utilising ants than fruit flies. While most individuals of E. episinoides were able to capture fruit flies, only very few spiders were able to develop and reproduce on a pure fruit fly diet, suggesting the existence of within-species genetic variation regarding the tolerance to alternative prey. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Feeding on a narrow diet range (i.e. stenophagy) may be a response to specific ecological conditions, e.g. a dominance of a certain food type (Líznarová et al., 2013). Evolution of stenophagous specialists (sensu Pekár and Toft, 2015) can be explained by a number of hypotheses such as the enemy-free space hypothesis (Brower, 1958), neural constraint hypothesis (Jermy et al., 1990), optimal foraging hypothesis (Singer, 2008), or physiological tradeoff hypothesis (Singer, 2001). The latter hypothesis predicts that predators possess some metabolic specialisations which enhance the utilisation of the preferred food type but may constrain the utilisation of alternative food types. Stenophagous specialists are quite common in herbivorous arthropods, where most of the species are highly host-specific, i.e. feeding only on a small fraction of the plant species that they encounter (Jaenike, 1990). According to the physiological tradeoff hypothesis, metabolic specialisation in herbivores is favoured by means of trade-offs in performance on alternative host plants
∗ Corresponding authors. E-mail addresses:
[email protected] (E. Líznarová),
[email protected] (S. Pekár). http://dx.doi.org/10.1016/j.zool.2016.04.004 0944-2006/© 2016 Elsevier GmbH. All rights reserved.
(Futuyma and Moreno, 1988; Jaenike, 1990; Thompson, 1994; Singer, 2001) and can be the result of differences in metabolic adaptation to plant defences, such as chemical detoxification ability. Such a trade-off exists if a genotype well suited to one food plant has relatively poor performance on others due to antagonistic pleiotropy or genetic linkage equilibrium (Agosta and Klemens, 2009). There is evidence both for and against such trade-offs from several plant–herbivore systems (Gould, 1979; Karban, 1989; Fry, 1990; Karowe, 1990; Via, 1991; MacKenzie, 1996; Agosta and Klemens, 2009; García-Robledo and Horvitz, 2011). In general, true predators seem to be less frequently specialised than herbivores, accepting and feeding on a wider diet range (Thompson, 1994). Although not all potential prey is suitable and beneficial for generalist predators (Bilde and Toft, 1997; Toft and Wise, 1999a,b; Sigsgaard et al., 2001), prey possessing these qualities is still diverse. Predators can then balance their specific nutritional demands via the consumption of a variety of prey types (Mayntz et al., 2005). The positive effect of dietary mixing on the performance of euryphagous predators (i.e., those capturing a wide variety of prey) was confirmed in several studies (Waldbauer and Friedman, 1991; Uetz et al., 1992; Wallin and Chiverton, 1992; Bilde and Toft, 1994). However, feeding on any kind of available prey might not always be beneficial for a predator (Bilde and Toft, 2000). Negative effects of prey mixing may emerge if one of the available
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prey species is toxic and can cause poor performance or even death of a predator. In that case, feeding on one high-quality prey species alone might be safer and sufficient to sustain high reproduction (Marcussen et al., 1999). The diet range of stenophagous true predators is composed predominantly or exclusively of a prey from a single (higher) taxon (Toft, 2013). For example, stenophagous spiders feed on termites ˇ (Dippenaar-Schoeman et al., 1996), woodlice (Rezᡠc et al., 2008), ants (e.g., Pekár, 2004), or even other spiders (e.g., Li and Jackson, 1997; Pekár et al., 2011). All these prey types are hard to handle. Spiders exploiting such prey are expected to evolve a variety of specific adaptations which enable them to overcome prey defences, such as highly potent venom (Pekár et al., 2014) or an extremely thick cuticle (Pekár et al., 2011), or to develop specific hunting tactics (Jackson and Hallas, 1986; Dippenaar-Schoeman et al., 1996; Jackson et al., 1998; Pekár, 2004). However, these specific adaptations may constrain the exploitation of an alternative prey (Pekár, 2005; Cárdenas et al., 2012). Evolutionary constraints on adaptations to one prey type can be so extreme that predators do not accept alternative prey even in a state of severe hunger and die (Pekár and Toft, 2009; Petráková et al., 2015). Metabolic adaptations and physiological trade-off in specialised predators have only been studied in a small number of species. For example, the aphid specialist Coccinella septempunctata Linnaeus survived similarly on a single-species and a mixed-species aphid diet (Hauge et al., 1998). Specialised spiders had better survival and growth rates when fed with a diet consisting of a preferred prey than when fed with alternative prey (Li and Jackson, 1997; Pekár and Toft, 2009). This suggests the existence of metabolic adaptations enhancing the utilisation of preferred prey connected with physiological trade-offs when feeding on alternative prey. Here, we tested the hypothesis that Euryopis episinoides (Walckenaer, 1847) (Theridiidae), which appears to prey only on ants in nature, is a stenophagous specialist with metabolic adaptations for ant utilisation and that in this species there exist trade-offs in the utilisation of alternative prey. We compared the performance (survivorship, development, body mass increase and reproduction) of E. episinoides on two monotypic diets, ants and fruit flies. We used fruit flies reared on an enhanced medium, which are believed to be optimal prey for euryphagous spiders (Mayntz and Toft, 2001). We predicted that the studied spiders would exhibit increased metabolic (assimilation) efficiency in the utilisation of preferred prey (ants) and there would be constraints on the utilisation of alternative prey (fruit flies). As we used a split-brood design (Bernardo, 1996) with seven mothers, we also investigated the contribution of genetic variation to performance on these two diets.
2. Materials and methods 2.1. Study species For the present study we used spiders of the species Euryopis episinoides (family Theridiidae). There are more than 70 species of the genus Euryopis worldwide. Published data on their prey indicate that in nature Euryopis spiders were observed to prey mostly on ants, suggesting they are stenophagous: e.g., Levi (1954) listed several instances of Euryopis spiders preying on ants; Berland (1933) reported that E. episinoides captured Crematogaster ants; Carico (1978) described Euryopis funebris (Hentz, 1850) feeding on Camponotus ants and observed that although the spiders captured only ants in the field they also accepted fruit flies in the laboratory; Gertsch (1979) observed females of Euryopis texana Banks, 1908 preying upon a moving line of small ants. Porter and Eastmond (1982) frequently observed spiders of the species Euryopis coki Levi
(1954) closely associated with Pogonomyrmex ants. We observed E. episinoides spiders feeding only on ants in nature, but in the laboratory they occasionally also accepted other prey types such as fruit flies, termites, springtails, crickets and bugs (Líznarová, unpublished data).
2.2. Rearing experiment Individuals of E. episinoides of different ages were collected in an abandoned grass field in Lagoa do Santo Andre, Portugal and reared in the laboratory. The spiders were fed with a mixture of ant species during their whole development and during adulthood. Once the females and males became adult, they were mated. The produced egg sacs were then incubated under controlled conditions (26 ◦ C, L:D = 16:8). Hatched spiderlings from seven mothers were assigned at random using a split-brood design to one of two diet treatments with a similar number of individuals coming from each egg sac and labelled with a specific ID referring to the mother. We used mother affiliation to measure the presence of genetic variability. Spiders from the first instar were housed singly in plastic tubes (diameter 5 mm, length 50 mm) with a layer of plaster of Paris (hemihydrated calcium sulfate) at the bottom. The tubes were plugged with foam rubber and maintained under controlled conditions (26 ◦ C, L:D = 16:8). The plaster of Paris was moistened with a few drops of water at 4-day intervals. Two different diets were used in the experiment: (1) an ant diet (a mixture of the following ant species, which occur in the natural habitats of E. episinoides: Tetramorium sp., Myrmica sp., Lasius sp., Formica sp., Messor sp.); (2) a fruit fly diet (Drosophila melanogaster Meigen, 1830 with vestigial wings). Ant species were regularly collected outside (from a single nest) and kept in plastic bottles (250 ml) with moistened paper gauze under controlled conditions (10 ◦ C to reduce metabolic rate, L:D = 16:8 h). Fruit flies were reared in the laboratory on a mixture of Drosophila medium and crushed dog food (Chappi brand) to enrich their nutritional value. Such enriched fruit flies had been shown to be optimal for euryphagous predators including spiders (Mayntz and Toft, 2001). We used 40 spider individuals on the ant diet and 39 individuals on the fruit fly diet. Spiderlings were fed with prey at 3-day intervals. Such feeding frequency is a pattern characteristic of spiders foraging without a web in the natural environment (Zimmermann and Spence, 1989; Wise, 1993) and was sufficient for the development of the studied species according to previous experiments (Líznarová, pers. observ.). Spiders subjected to the ant diet treatment were offered one ant individual (the ant species were offered in random order) and spiders subjected to the fruit fly diet treatment received 2–3 fruit flies depending on the ant size to compensate any prey weight difference. To prevent the ant from killing the spider, the mandibles of each ant were crushed with forceps before it was introduced into a tube occupied by a spider. After such a procedure the ant was still alive but was not able to counter-attack the spider. All prey, consumed or not, were removed the next day. Rearing lasted for 130 days, until all spiders died or reached adulthood. Mortality and moulting were recorded daily. Spiders were weighed using a Kern balance with a precision of 0.001 mg, approximately at 10-day intervals until reaching adulthood. Mass was compared only until day 70, because then most of the spiders reached the highest mass and after that their mass started to decrease due to egg laying in adult females and due to limited prey consumption in adult males. Adult females were mated at least once with a male from the same diet treatment but we avoided mating between sisters and brothers. In females which laid egg sac(s) we measured the latency to egg sac production, the number of egg sacs, and the numbers of eggs per egg sac, longevity, incuba-
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tion period, hatching success, and size of offspring prosoma (both width and length). We also recorded the clutch size and the proportion of hatched spiderlings from eggs laid by other females we collected at the adult stage in the field in Portugal for a comparison. The field-collected females usually laid egg sacs soon after they had been brought to the laboratory; they were fed twice a week with ants during the egg-laying period. They were kept in the laboratory until they died. Additionally, as we expected low spider survival on the fruit fly diet, we reared more freshly hatched individuals of E. episinoides (N = 368) from several mothers on the same fruit fly diet as described above. In these individuals we observed only the number of individuals which reached adulthood and their sex, and in mated females we observed clutch size and the proportion of individuals hatched from eggs.
2.3. Data analyses Data analyses were performed using various methods within the R environment (R Core Team, 2014). The survival of spiders was compared between treatments using the frailty mixed-effect Cox model in order to estimate the maternal effect by setting the ID of the mother as a random effect. The proportion of individuals which developed to the next instar and the proportion of spiderlings which hatched from eggs were compared using a twosample test for the equality of proportions. The maternal effect on selected study characteristics (development time, instar duration) was estimated as a variance component using generalised linear mixed models (GLMM). The sex ratio was tested using generalised linear models (GLM) with Poisson error structure. Instar duration and total development time were tested using linear mixed-effects models (LME) with log-transformed data and both the varPower variance structure due to heteroscedasticity and an exchangeable correlation structure due to the presence of autocorrelation. The mass change was tested using LME with the varPower variance structure and an exchangeable correlation structure. Final female body mass, latency of laying egg sacs, incubation period, female longevity, and size of spiderlings were compared using LME with log-transformed data to improve homoscedasticity and normality of residuals. Numbers of egg sacs per female and clutch size were compared using LME. The maternal effect in all LME models was estimated by comparing models with and without mother ID as a random effect. The clutch sizes of females collected in nature and those reared in the laboratory on ants were compared using GLM with Poisson error structure. Sex ratio and the proportion of individuals hatched from eggs were compared by the two-sample test for the equality of proportions.
3. Results 3.1. Survival There was a significant effect of diet on spider survival (mixed effects Cox model, X2 1 = 27.8, P < 0.0001, Fig. 1): the hazard on the fruit fly diet was 1.7 times higher than on the ant diet. 53% of spiders on an ant diet and 7.3% of spiders on a fruit fly diet reached adulthood. For the ant diet group the maternal effect was not significant (mixed effects Cox model, 2 = 0.15, X2 1 = 0.7, P = 0.26). However, within the fruit fly group the maternal effect was highly significant (mixed effects Cox model, 2 = 2.46, X2 1 = 13.8, P = 0.0002) and two of the three individuals which reached adulthood on fruit flies were offspring of the same mother. 6.3% of spiders (N = 368) on the fruit fly diet in the additional experiment reached adulthood. The survival rate did not differ from
Fig. 1. Comparison of the survival of Euryopis episinoides spiders reared on two diets (ants and fruit flies) during ontogenesis (max 130 days). +marks represent censored cases (before day 20 mortality resulting from manipulation, and after 20 days maturation of spider individuals). Table 1 Number of individuals of Euryopis episinoides which moulted to particular instars on two diets (ants and fruit flies). Diet
1st instar
2nd instar
3rd instar
4th instar
5th instar
Ant Fruit fly
39 40
33 9
30 6
25 5
21 3
Table 2 Variance component estimates obtained from GLMM models for moulting of Euryopis episinoides spiders at particular instars.
Var (mother) Var (individual) Var (residual)
1st instar
2nd instar
3rd instar
4th instar
0.37 0.62 0.02
<0.01 0.99 <0.01
<0.01 <0.01 0.99
<0.01 0.99 <0.01
that for spiders reared on the fruit fly diet in the first experiment (two-sample test for equality of proportions, X2 1 = 0, P = 1). 3.2. Development All spiders that survived until adulthood passed through five instars. Significantly more spiders on the ant diet (85%, total N = 40, Table 1) than on the fruit fly diet (23%, total N = 39) moulted from the first to the second instar (two-sample test for equality of proportions, X2 1 = 2.42, P < 0.001, Fig. 2). However, there were no significant differences between treatments in the proportion of moulted individuals in the next instars: 91% of spiders (N = 33) on the ant diet and 67% on the fruit fly diet (N = 9) moulted to the third instar (two-sample test for equality of proportions, X2 1 = 1.7, P = 0.19), 83% of spiders (N = 30) on the ant diet and 83% (N = 6) on the fruit fly diet moulted to the fourth instar (two-sample test for equality of proportions, X2 1 = 0, P = 1.00); and finally 84% of spiders (N = 25) on the ant diet and 60% of spiders on the fruit fly diet (N = 5) moulted to the final fifth instar (two-sample test for equality of proportions, X2 1 = 0.375, P = 0.54). The maternal effect was significant for moulting from the first to the second instar only (Table 2). The sex ratio at final instar was different (male:female = 1:2 on the fruit fly diet and 1:1.1 on the ant diet) but not significantly so due to low sample size (two-sample test for equality of proportions, X2 1 = 0,
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2.5
1.0
4
m 1.5
0.6 0.0
0.5
0.2
1.0
0.4
Proportion
2.0
0.8
Ant
2
1
4
3
10
20
30
40
[ Fig. 2. Comparison of the proportion of moulted Euryopis episinoides spiders from the previous to the next instar during four moulting events on two different diets (ants and fruit flies).
80
P = 1). The sex ratio (male:female) from the additional experiment with spiders reared on fruit flies was 1:1.1 and did not differ significantly from that in the first experiment (two-sample test for equality of proportions, X2 1 = 0, P = 1). Concerning the duration of the instars, there was no significant difference between ant and fruit fly diets for the first instar (LME, F1,34 = 0.1, P = 0.38, Fig. 3), the second instar (LME, F1,28 = 2.2, P = 0.074, Fig. 3), and the fourth instar (LME, F1,16 = 1.0, P = 0.166, Fig. 3). Nevertheless, there was a significant difference in the duration of the third instar (LME, F1,22 = 16.8, P < 0.001, Fig. 3). Only in the third instar was there a significant maternal effect
50
60
70
]
Fig. 4. Comparison of mass change (mean ± SE) of Euryopis episinoides spiders on two diets (ants and fruit flies) during the 70 days of the experiment.
(LME, logLik = –13.63, F1,22 = 16.85, P < 0.001). The total development time was not significantly different between diet treatments (LME, F1,22 = 0.2, P = 0.69, Fig. 3) and there was no maternal effect on development time (LME, logLik = –6.33, F1,22 = 0.058, P = 0.13). 3.3. Body mass Spiders on the ant diet increased their body mass at a significantly higher rate than those on the fruit fly diet (LME, F2,180 = 72.3, P < 0.001, Fig. 4). There was a significant maternal effect on the body mass increase (LME, logLik = –141.97, F1,1 = 4.40, P = 0.018, Fig. 5). However, the final body mass of females which reached adulthood did not differ significantly between diet treatments (LME, F1,8 = 2.59, P = 0.146), and there was no maternal effect to female final body mass (LME, logLik = −3.94, F1,1 = 0.083, P = 0.77).
Ant
40 0
20
Duration [days]
60
3.4. Reproduction
1
2
3
4
total
Instar Fig. 3. Comparison of the duration of instars and the total development time of Euryopis episinoides spiders on two diets (ants and fruit flies). Bars are means; whiskers represent standard error of the mean.
From all adult females reared on the ant diet, about 73% (total N = 11) laid at least one egg sac; the rest died before egg laying. Only two females survived until adulthood on the fruit fly diet and both of them laid at least one egg sac. Females on the ant diet oviposited significantly earlier after becoming adult than those on the fruit fly diet (LME, F1,8 = 13.0, P = 0.007). There was no maternal effect on the oviposition latency (LME, logLik = –2.53, F1,1 = 0.28, P = 0.60). The mean number of egg sacs laid by one female did not differ among diets (LME, F1,8 = 0.2, P = 0.936); there was no maternal effect on the number of egg sacs (LME, logLik = –11.26, F1,1 = 0.48, P = 0.48). Nevertheless, females on the ant diet had a significantly larger clutch size than those on the fruit fly diet (LME, F1,8 = 8.8, P = 0.003, Table 3); there was no maternal effect on clutch size (LME, logLik = –30.88, F1,1 = 3.10, P = 0.31). The incubation period did not differ between diet treatments (LME, F1,10 = 0.036, P = 0.85) and lasted about 15 days on average; there was no maternal effect on incubation period (LME, logLik = 32.40, F1,1 = 3.08, P = 0.88). However, the proportion of spiderlings hatched from eggs was significantly higher in females on the ant diet (82%, total N = 455) compared to females on the fruit fly diet (51%, total N = 39) (two-sample test for equality of
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Mean number of eggs in clutch (±SE) Proportion of hatched individuals (%)
Ant diet
Fruit fly diet
10.34 ± 0.54 82.22 (N = 455)
6.50 ± 0.43 51.28 (N = 39)
proportions, X2 1 = 19.39, P < 0.001, Table 3). There was no difference in the length of the spiderling prosoma (LME, F1,30 = 2.67, P = 0.11), but the width of the prosoma was significantly larger in spiderlings from females reared on the ant diet (LME, F1,30 = 6.8, P = 0.014). There was no grand-mother’s effect on spiderling size (LME, logLik = 25.68, F1,1 = 2.95, P = 0.37). We found no significant difference in female longevity, with females on both diets living a mean of 81 days after reaching adulthood (LME: F1,8 = 0.04, P = 0.86); there was no maternal effect on longevity (LME, logLik = −6.42, F1,1 = 6.79, P = 0.5). Females collected in nature had a significantly larger clutch size (15.78 eggs ± 2.4 SEM) when compared with females reared on the ant diet in the laboratory (10.34 ± 0.54 SEM) (GLM-p, X2 60 = 30.02, P < 0.001). Also, the proportion of spiderlings hatched from eggs was higher in females collected in nature (89%) when compared with females reared on the ant diet in the laboratory (82%) (two-sample test for equality of proportions, X2 1 = 5.27, P = 0.022). The clutch size (GLM-p, X2 12 = 0, P = 1) and the proportion of individuals hatched from eggs in the additional experiment with spiders reared on the fruit fly diet (two-sample test for equality of proportions, X2 1 = 0.314, P = 0.58) did not differ significantly from spiders reared on the fruit fly diet in the first rearing experiment. 4. Discussion The most prominent benefits of the pure ant diet in E. episinoides when compared with the pure enriched fruit fly diet were higher survival rates, greater weight increase, higher fecundity, and higher fertility. This suggests the existence of a metabolic specialisation which is expressed by higher efficiency in the utilisation of the preferred prey (e.g. access to certain essential elements in the ant diet) and a decreased efficiency in the utilisation of alternative prey. Ants, although the most abundant arthropods in many terres¨ trial habitats (Holldobler and Wilson, 1990), are no suitable prey for most spider species because they are dangerous (equipped with a sting and strong mandibles) and can also be unpalatable because of the chemical substances they use to defend themselves. Thus, the majority of euryphagous spiders seem to avoid them as prey
Mass [mg]
fixed
4 3 2
M1/1
5
(e.g., Edwards and Jackson, 1994; Nelson et al., 2004; Líznarová and Pekár, 2013). Yet, Euryopis spiders are able to capture ants quite efficiently, as their typical hunting technique consists of casting silk over their prey from a distance, thus preventing counter-attack (Carico, 1978). However, specific adaptations for eating ants could carry constraints on capturing and utilising alternative prey. Ant-eating Zodarion spiders are an example of stenophagous specialists which became specialised in their behaviour and sensory system to a degree that, as a trade-off, makes them largely ineffective at preying on conventional prey (e.g., fruit flies) (Pekár and Toft, 2009). Some Zodarion individuals did not capture fruit flies even in a state of severe hunger and starved to death. It is still unknown whether starving Zodarion spiders are unable to bite fruit flies, which usually escape from attack by jumping or flying away, or whether their venom is not efficient enough to paralyse them. Although some Zodarion individuals captured and fed on fruit flies, their growth was very slow compared with those on an ant diet and all these spiders died before the end of the experiment. Thus, the fruit fly diet was also unsuitable for them nutritionally (Pekár and Toft, 2009), suggesting the existence of a metabolic specialisation on ants which constrains the utilisation of alternative prey. In the present study, most of the Euryopis spiders accepted fruit flies as prey and readily fed on them. Nevertheless, our results suggest that a pure fruit fly diet, even though nutritionally enriched (and optimal for euryphagous spiders, as reported by Mayntz and Toft (2001), is not suitable for Euryopis spiders, supporting the existence a physiological trade-off that results from metabolic specialisation to a certain prey type. Feeding on an alternative prey had a similar effect in the araneophagous spider Portia fimbriata. Their survival, developmental rate and body size at maturation were highest when they were fed with spiders. When they were fed with a mixture of spiders and insects or with insects only, most individuals died before reaching maturity (Li and Jackson, 1997). The reason why Euryopis spiders are able to capture and eat alternative prey could be similar to that found in aphidophagous ladybirds (Evans et al., 1999) where adult females make use of all potential prey during a short period before egg-laying when they need to gain mass quickly. In the present study, this possibility is supported by the fact that the clutch size and the hatching success of females collected in the field (whose diet was unknown) were greater than those of females reared in the laboratory on a pure ant diet. In nature, they probably captured and fed on more or on different ant species. Trophic specialisation is affected and constrained by both ecological and genetic factors. The main ecological factor is food availability, which can vary in space and time; thus a specialisation
mother
M2/2
individual
M2/3 4 3 2
1
1
0
0 10 20 30 40 50 60 70
10 20 30 40 50 60 70
Time [days] Fig. 5. Weight increase of spiders on a fruit fly diet which reached adulthood with fixed, individual, and maternal effects. Points indicate measured values. The first ID indicates affiliation to a mother, followed by the ID of a spider after the slash. Individuals 2 and 3 have the same mother (M2).
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on a certain food type is beneficial in a predictable environment (Laukkanen et al., 2013) with a constant supply of such food. Nevertheless, even in a predictable environment, a lack of genetic variation in performance on different food types may inhibit adaptation to, and specialisation on, a current and novel food (Futuyma et al., 1995). In our experiment a few spiders survived, developed to adulthood, and even produced offspring on the alternative fruit fly diet. Moreover, the maternal effect, which can probably explain parts of offspring genetic variance (Bernardo, 1996), seems to be an important factor in our study, as two of the three individuals which reached adulthood on fruit flies were offspring of the same mother. Nevertheless, the maternal effect could also be due to a non-genetic effect as was the case in the spider Dicymbium brevisetosum Locket, 1962, where survival of offspring reared on poor quality prey was affected entirely by the diet their mother had been feeding on. When the mother had been reared on an enriched diet, the survival of offspring was higher (Bilde and Toft, 2000). However, the effect of the mother’s diet on the survival on the alternative diet was negligible in our experiment as all spider mothers were raised on a similar diet. Maternal effect was more likely due to within-species genetic variation as some siblings of spiders which developed successfully on the fruit fly diet did not survive to adulthood on an ant diet. Such within-species genetic variation could determine the ability of individuals to capture, feed and utilise different food types (Bolnick et al., 2003). In our case, the offspring of one mother could be more metabolically specialised on ant utilisation than the offspring of another mother, which could be, on the other hand, more tolerant to alternative prey. Similarly, some level of genetic variation was found in the non-specialised spider species Leptyphantes tenuis (Blackwall, 1852) regarding its tolerance to feed on aphids (Beck and Toft, 2000). Many populations of euryphagous species are actually composed of stenophagous individuals whose trophic niche is narrow (Araújo et al., 2011). Similarly, some individuals of specialised species are more tolerant to other than focal prey than other individuals. Such variation in the degree of specialisation could be, for example, important for population survival when the preferred prey becomes extinct or very rare in a habitat and individuals are forced to feed on non-preferred prey. The three E. episinoides specimens that managed to reach adulthood on the fruit fly diet did so in the same time period as spiders reared on the ant diet, and adult females weighed the same as females reared on the ant diet. Thus, they were able to compensate a slower weight increase during juvenile development and, in general, utilise a sufficient amount of energy and nutrients from fruit flies. The main constraint on females on the fruit fly diet became evident when we compared their reproductive success. Females on the fruit fly diet needed more time before laying the first egg sac, probably due to the necessity to obtain more essential nutrients from prey. Furthermore, even though the mean number of egg sacs was similar for females on both diets, the actual clutch size and the proportion of living offspring which hatched from those eggs were distinctly reduced in females reared on fruit flies. It was proposed that low hatching success indicates a limitation in nutrients (Toft, 1995). Moreover, spiderling body size was also decreased in offspring produced by females reared on fruit flies. Similarly, Bilde and Toft (2001) found reduced offspring size in Erigone atra Blackwall, 1833 spiders when reared on a pure aphid diet instead of a mixed aphid/fruit fly diet. There is evidence in spiders that offspring performance increases with offspring size (Walker et al., 2003). Furthermore, there is a hypothesis that says that parental fitness is maximised by producing as many offspring as possible given constraints on a minimum viable offspring size (Walker et al., 2003). Thus, fewer and smaller offspring of spiders reared on a fruit fly diet imply lower fitness.
Our results indicate that E. episinoides spiders have a high metabolic efficiency with respect to consuming ants, which enables them to develop on a monotypic diet consisting only of ant species. Moreover, our findings showing that these spiders have a low efficiency in fruit fly utilisation, with very high mortality during first instar development, suggest that the metabolic specialisation of E. episinoides on one prey type comes with trade-offs in the utilisation of alternative prey. Still, there seems to be some selection pressure for maintaining the ability of E. episinoides to capture, feed on and at least partially utilise alternative prey, as they accepted several alternative prey types and a few individuals were even able to develop fully on a pure fruit fly diet, suggesting the existence of within-species genetic variation at the level of specialisation in the studied species. However, individuals that grew to adulthood purely on fruit flies exhibited reduced fecundity, indicating that their ability to feed on alternative prey is probably advantageous only for short time periods, such as when they are exposed to a temporary lack of focal prey or during the egg-laying period. Regarding further lines of investigation, it would be appropriate to test the performance of E. episinoides on a mixed diet containing other nonant prey types or to test if this species is more metabolically adapted to a particular ant subfamily as was found in Zodarion spiders (Pekár et al., 2008). Acknowledgments This study was supported by grant no. MUNI/A/1484/2014 and the Czech Science Foundation (GA15-14762S). We thank two anonymous referees for their useful comments that improved this manuscript. References Agosta, S.J., Klemens, J.A., 2009. Resource specialization in a phytophagous insect: no evidence for genetically based performance trade-offs across hosts in the field or laboratory. J. Evol. Biol. 22, 907–912. Araújo, M.S., Bolnick, D.I., Layman, C.A., 2011. The ecological causes of individual specialisation. Ecol. Lett. 14, 948–958. Beck, J.B., Toft, S., 2000. Artificial selection for aphid tolerance in the polyphagous predator Lepthyphantes tenuis. J. Appl. Ecol. 37, 547–556. Berland, L., 1933. Contribution a l´ıetude de la biologie des Arachnides. Arch. Zool. Expér. 76, 1–23. Bernardo, J., 1996. Maternal effects in animal ecology. Am. Zool. 36, 83–105. Bilde, T., Toft, S., 1994. Prey preference and egg production of the carabid beetle Agonum dorsale. Entomol. Exp. Appl. 73, 151–156. Bilde, T., Toft, S., 1997. Consumption by carabid beetles of three cereal aphid species relative to other prey types. Entomophaga 42, 21–32. Bilde, T., Toft, S., 2000. Evaluation of prey for the spider Dicymbium brevisetosum Locket (Araneae: Linyphiidae) in single-species and mixed-species diets. Ekologia (Bratislava) 19, 9–18. Bilde, T., Toft, S., 2001. The value of three cereal aphid species as food for a generalist predator. Physiol. Entomol. 26, 58–68. Bolnick, D.I., Svanbäck, R., Fordyce, J.A., Yang, L.H., Davis, J.M., Hulsey, C.D., Forister, M.L., 2003. The ecology of individuals: incidence and implications of individual specialization. Am. Nat. 161, 1–28. Brower, L.P., 1958. Bird predation and foodplant specificity in closely related procryptic insects. Am. Nat. 92, 183–187. Cárdenas, M., Jiroˇs, P., Pekár, S., 2012. Selective olfactory attention of a specialised predator to intraspecific chemical signals of its prey. Naturwissenschaften 99, 597–605. Carico, J., 1978. Predatory behavior in Euryopis funebris (Hentz) (Araneae: Theridiidae) and the evolutionary significance of web reduction. Symp. Zool. Soc. Lond. 42, 51–58. Dippenaar-Schoeman, A., De Jager, M., Van den Berg, A., 1996. Ammoxenus species (Araneae: Ammoxenidae) − specialist predators of harvester termites in South Africa. Afr. Plant. Prot. 2, 103–109. Edwards, G.B., Jackson, R.R., 1994. The role of experience in the development of predatory behaviour in Phidippus regius, a jumping spider (Araneae, Salticidae) from Florida. New Zeal. J. Zool. 21, 269–277. Evans, E.W., Stevenson, A.T., Richards, D.R., 1999. Essential versus alternative foods of insect predators: benefits of a mixed diet. Oecologia 121, 107–112. Fry, J.D., 1990. Trade-offs in fitness on different hosts: evidence from a selection experiment with a phytophagous mite. Am. Nat. 136, 569–580. Futuyma, D.J., Moreno, G., 1988. The evolution of ecological specialization. Annu. Rev. Ecol. Syst. 19, 207–234.
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Pekár, S., Toft, S., 2015. Trophic specialisation in a predatory group: the case of prey-specialised spiders (Araneae). Biol. Rev. 90, 744–761. Pekár, S., Toft, S., Hruˇsková, M., Mayntz, D., 2008. Dietary and prey-capture adaptations by which Zodarion germanicum, an ant-eating spider (Araneae: Zodariidae), specialises on the Formicinae. Naturwissenschaften 95, 233–239. ˇ Pekár, S., Sobotník, J., Lubin, Y., 2011. Armoured spiderman: morphological and behavioural adaptations of a specialised araneophagous predator (Araneae: Palpimanidae). Naturwissenschaften 98, 593–603. ˇ Pekár, S., Sedo, O., Líznarová, E., Korenko, S., Zdráhal, Z., 2014. David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey. Naturwissenschaften 101, 533–540. Petráková, L., Líznarová, E., Sentenská, L., Haddad, C.R., Pekár, S., Symondson, W.O.C., 2015. Discovery of a monophagous true predator, a specialist termite-eating spider (Araneae: Ammoxenidae). Sci. Rep. 5, 14013. Porter, S.D., Eastmond, D.A., 1982. Euryopis coki (Theridiidae), a spider that preys on Pogonomyrmex ants. J. Arachnol. 10, 275–277. R Core Team, 2014. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria http://www.r-project/ org. ˇ Rezáˇc, M., Pekár, S., Lubin, Y., 2008. How oniscophagous spiders overcome woodlouse armour. J. Zool. 275, 64–71. Sigsgaard, L., Toft, S., Villareal, S., 2001. Diet-dependent fecundity of the spiders Atypena formosana and Pardosa pseudoannulata: predators in irrigated rice. Agr. Forest Entomol. 3, 285–295. Singer, M.S., 2001. Determinants of polyphagy by a woolly bear caterpillar: a test of the physiological efficiency hypothesis. Oikos 93, 194–204. Singer, M.S., 2008. Evolutionary ecology of polyphagy. Specialization, speciation, and radiation. In: Tilmon, K.J. (Ed.), The Evolutionary Biology of Herbivorous Insects. University of California Press, Berkeley, pp. 29–42. Thompson, J.N., 1994. The Coevolutionary Process. University of Chicago Press, Chicago. Toft, S., 1995. Value of the aphid Rhopalosiphum padi as food for cereal spiders. J. Appl. Ecol. 32, 552–560. Toft, S., 2013. Nutritional aspects of spider feeding. In: Nentwig, W. (Ed.), Spider Ecophysiology. Springer, Berlin, pp. 373–384. Toft, S., Wise, D.H., 1999a. Growth, development, and survival of a generalist predator fed single- and mixed-species diets of different quality. Oecologia 119, 191–197. Toft, S., Wise, D.H., 1999b. Behavioral and ecophysiological responses of a generalist predator to single- and mixed-species diets of different quality. Oecologia 119, 198–207. Uetz, G.W., Bischoff, J., Raver, J., 1992. Survivorship of wolf spiders (Lycosidae) reared on different diets. J. Arachnol. 20, 207–211. Via, S., 1991. The genetic structure of host plant adaptation in a spatial patchwork: demographic variability among reciprocally transplanted pea aphid clones. Evolution 45, 827–852. Waldbauer, G., Friedman, S., 1991. Self-selection of optimal diets by insects. Annu. Rev. Entomol. 36, 43–63. Walker, S.E., Rypstra, A.L., Marshall, S.D., 2003. The relationship between offspring size and performance in the wolf spider Hogna helluo (Araneae: Lycosidae). Evol. Ecol. Res. 5, 19–28. Wallin, H., Chiverton, P., 1992. Diet: fecundity and egg size in some polyphagous predatory carabid beetles. Entomol. Exp. Appl. 65, 129–140. Wise, D.H., 1993. Spiders in Ecological Webs. Cambridge University Press, New York. Zimmermann, M., Spence, J.R., 1989. Prey use of the fishing spider Dolomedes triton (Pisauridae, Araneae): an important predator of the neuston community. Oecologia 80, 187–194.
Rukopis E
DAVID AND GOLIATH: POTENT VENOM OF AN ANT-EATING SPIDER (ARANEAE) ENABLES CAPTURE OF A GIANT PREY Pekár, S., Šedo, O., Líznarová, E., Korenko, S., Zdráhal, Z. (2014) Naturwissenschaften, 101, 533–540
Zodarion cyrenaicum
© Michálek
Naturwissenschaften DOI 10.1007/s00114-014-1189-8
ORIGINAL PAPER
David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey Stano Pekár & Onřej Šedo & Eva Líznarová & Stanislav Korenko & Zdeněk Zdráhal
Received: 17 March 2014 / Revised: 13 May 2014 / Accepted: 20 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract It is rare to find a true predator that repeatedly and routinely kills prey larger than itself. A solitary specialised ant-eating spider of the genus Zodarion can capture a relatively giant prey. We studied the trophic niche of this spider species and investigated its adaptations (behavioural and venomic) that are used to capture ants. We found that the spider captures mainly polymorphic Messor arenarius ants. Adult female spiders captured large morphs while tiny juveniles captured smaller morphs, yet in both cases ants were giant in comparison with spider size. All specimens used an effective prey capture strategy that protected them from ant retaliation. Juvenile and adult spiders were able to paralyse their prey using a single bite. The venom glands of adults were more than 50 times larger than those of juvenile spiders, but the paralysis latency of juveniles was 1.5 times longer. This suggests that this spider species possesses very potent venom already at the juvenile stage. Comparison of the venom composition between juvenile and adult spiders did not reveal significant differences. We discovered here that specialised Communicated by: Sven Thatje S. Pekár (*) : E. Líznarová Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic e-mail:
[email protected] O. Šedo : Z. Zdráhal Research Group Proteomics, Mendel Centre for Plant Genomics and Proteomics, Central European Institute of Technology, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic O. Šedo : Z. Zdráhal National Centre for Biomolecular Research, Faculty of Science, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic S. Korenko Department of Agroecology and Biometeorology, Faculty of Agrobiology, Food and Natural Resources, Czech University of Life Sciences, Kamýcká 129, 165 21 Prague 6-Suchdol, Czech Republic
capture combined with very effective venom enables the capture of giant prey. Keywords Adaptations . Ant-eater . Araneae . Venom . Specialist
Introduction True predators catch a number of prey items during their lifetime in contrast to parasites and parasitoids. They usually take prey that is smaller than themselves (Griffiths 1980) because capture success is a function of relative prey/predator size: it declines as the ratio increases (Balgooyen 1976). Predators feeding on tiny plankton have very high capture success (100 %). However, predators catching large prey have a lower capture success, which can be, for example, only about 30 % (Schaller 1972; Kruuk 1972). The capture of large prey is a challenge that requires either a specialised capture adaptation to evolve in solitary foragers, such as a spider web, specialised hairs in ants, or poison in spiders (Enders 1975; Dejean et al. 2010; Gregorič et al. 2011), specialised capture behaviour in assassin bugs (e.g. Bulbert et al. 2014), or cooperation to evolve among colonial or social arachnid predators (e.g. Zeh and Zeh 1990). There is evidence that group-hunting predators (lions or social spiders) capture larger prey than solitary ones (e.g. cheetahs or solitary spiders) (e.g. Kruuk 1972; Avilés and Tufiňo 1998) using a similar capture strategy. Spiders are the most diversified and the most abundant terrestrial true predators (Coddington and Levi 1991). Among spiders, most species are euryphagous and capture prey smaller than their bodies (Nentwig 1987). In such species, the size of prey correlates well with the size of the spider (e.g. Turner 1979). A few spider species are, however, stenophagous and specialised on prey that is large. For example, individuals of
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moth-eating Mastophora and ant-eating Zodarion spiders have been reported to catch prey that was larger than themselves (Eberhard 1980; Pekár et al. 2011). Some of these prey, such as ants, are also dangerous. The capture of large and dangerous prey entails significant difficulty in subduing the prey and includes increased costs (Griffiths 1980). The predator must possess effective morphological and/or behavioural adaptations that prevent counter attack by such dangerous prey. Indeed, many ant-eating predators use a specialised attack strategy to avoid retaliation (e.g. Jackson and Li 2001; Pekár 2004; Bulbert el al. 2014). In venomous predators, capture adaptations may include potent venom. It is well known that the venom functions in the immobilisation of prey and aids in prey digestion and defence against predators (McCue 2005). Highly potent venom may also enable predators to catch large prey. There is anecdotal evidence that insectivorous mammals, such as shrews or hedgehogs, may subdue prey much larger than themselves (other vertebrates) by means of neurotoxic venom (Dufton 1992). Specialised capture adaptations are under strong selection to evolve, as they are used frequently during the lifetime of the true predator. As the body of a predator increases during ontogenetic development, there is prey shift to larger and often different prey items (Nakazawa et al. 2013). The ontogenetic shift to larger prey may be associated with the use of larger venom quantities if the predator switches between developmental stages of the same prey species (e.g. Sasa 1999). For predators switching to distinct prey types, the venom composition may change during ontogenesis (Herzig et al. 2004). Here, we investigated the natural prey and predatory behaviour of the ant-eating species Zodarion cyrenaicum Denis, which has specialised to feed on a big ant species, Messor arenarius (Fabricius). Although the ant is polymorphic in size, the small workers are still giant when compared to spiderlings (6 vs 2 mm). We hypothesised that juveniles should use especially effective capture strategy to overcome such giant prey. Thus, we performed field observations and laboratory experiments to reveal behavioural capture adaptations. Furthermore, we tested our prediction that juveniles posses very effective venom against M. arenarius ants as they have much smaller venom glands than adults. So we compared the venom composition of both juvenile and adult spiders and in order to disclose whether the venom composition undergoes an ontogenetic change.
Methods Observations were performed in the Mashabbim sand dunes, the Negev desert, Israel, in October 2009 and September 2011, when both adult and early juvenile spiders of Z. cyrenaicum occur. On each census day, the survey was
conducted early in the morning (between 5:00 and 9:00) when the spiders were foraging as this spider species is mainly nocturnal (Pekár et al. 2005a). Sunrise was at 6:30. To investigate potential prey, the ant nests were counted along three 100 m transects. A worker ant from each nest was collected, put in alcohol, and later identified in the lab. As the most abundant species, a hundred M. arenarius workers were collected from a single nest in order to obtain a sample of potential prey size. The total body size of each worker was measured with callipers and body mass was measured using a Kern 770 balance with a precision of 0.00001 g. To investigate actual prey, Zodarion spiders were searched for around each ant nest and around foraging trails by inspecting immobilised ants, as the juveniles were too small (body size 1–3 mm) to be easily spotted. Spiders and their ant prey were collected and brought to the laboratory, where each specimen was identified to species level using Levy (1992). In each spider specimen, the prosoma length was measured using a calliper and body mass was measured using a Kern 770 balance. The composition of potential ant prey (i.e. frequency of each prey species) was compared with the composition (frequency) of the actual prey using a goodness of fit test. The test compared observed (actual) frequencies with the expected (potential) frequencies. Ant body size was compared between spider sexes and among spider stages using ANOVA. Linear regression (LM) was used to study the relationship between the sizes and masses of ants and spiders. We collected 18 adult female and 24 juvenile specimens of Z. cyrenaicum to be used in the capture efficacy experiments. Live spiders were kept for 7 days without prey before the following experiment. A juvenile or an adult female spider was placed singly in a Petri dish (6 cm diameter) with a thin layer of sand on the bottom. A worker ant of M. arenarius (size 10–17 mm, weight 0.01–0.08 g) was released into the dish occupied by the spider. We recorded the latency to the first attack, the number of attacks, the body part that was attacked, the behaviour of the bitten ant, and the latency to paralysis (i.e. the time between the bite and the collapse of the ant on one side). The symptoms of the attacked ant were observed. The attack counts were compared with Generalised Linear Models with a Poisson error structure (GLM-p); the latencies were compared between females and juveniles using GLM w ith a G amma error structure due to the heteroscedasticity in residuals (GLM-g) (Pekár and Brabec 2009). The capture behaviour of spiders was recorded and compared between females and juveniles. The analysis of venom gland contents was performed for ten juvenile and six adult individuals of Z. cyrenaicum. Due to the existence of very tiny chelicera, pure venom could not be extracted. Pairs of venom glands were dissected and placed into 10 μl of a physiological solution of 0.9 % NaCl. The dimensions of the glands—the widths (2r) and the lengths (d)—were measured using an ocular micrometer attached to an
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Olympus SX stereomicroscope. The volume of the gland (V) was estimated by assuming a cylindrical shape (V=dπr2). The gland was then squeezed and the sample stored at −20 °C prior to analysis. One sample consisted of venom glands from five juvenile or three adult individuals. The venom samples were subjected to MALDI-TOF mass spectrometric analysis using an Ultraflex III instrument (Bruker Daltonik, Bremen, Germany) operated in linear positive arrangement under FlexControl 3.3 software. External calibration of the mass spectra was performed using Escherichia coli DH5 alpha standard peaks. Samples were deposited on three positions (wells) of the sample plate to a volume of 0.3 μl and, after drying at room temperature, overlain with 0.3 μl of saturated alpha-cyano-4hydroxycinnamic acid in an acetonitrile/water/TFA (50:47.5:2.5, v/v) mixture. Five independent spectra, each comprising of 1,000 laser shots, were obtained from each of the wells. The mass spectra were processed using Flex Analysis software (version 3.4; Bruker Daltonik). Alpha-cyano-4hydroxycinnamic acid was obtained from Bruker Daltonik (Leipzig, Germany). Trifluoroacetic acid and acetonitrile were obtained from Merck (Darmstadt, Germany). Water was prepared on a Milli-Q plus 185 apparatus (Millipore, Bilerica, MA, USA). All chemicals were of analytical grade. Signals reproduced in at least 11 out of the total of 15 spectra obtained from each sample were taken into account. The mass spectra were recorded within the mass range 2–20 kDa. Peptides/ proteins from different samples were considered homologous if the masses differed less than 0.3 % of molecular weight. In addition to MALDI-MS analysis, the venom samples were subjected to SDS PAGE. The samples were mixed with 3 μl of sample buffer (1.2 g sodium dodecyl sulphate; 6 mg bromphenol blue; 4.7 ml glycerol; 1.2 ml Tris–HCl 0.5 M, pH 6.8; 0.93 g dithiothreitol in 10 ml of the stock solution). The mixture was incubated for 5 min at 95 °C and subsequently loaded on 12 % 1D-SDS-PAGE gels. Separation was carried out in a Mini-PROTEAN device (Bio-Rad) for 1 h under a separation voltage of 200 V. Bands from each sample were analysed by comparing migration distances to the distances of standards, and the relative amount of each band was analysed by comparing the intensities of bands using a GelAnalyzer (Lazar 2010). The compositions of peptides and proteins in the venom of females and juveniles were compared using a multivariate method, namely Redundancy Analysis (RDA), because the gradient was expected to be linear (Lepš and Šmilauer 2003). The intensities were logarithmically transformed. These analyses were performed using the vegan package (Oksanen et al. 2012). All statistical analyses were performed within the R environment (R Core Team 2012). Standard model diagnostic was performed following each regression analysis (Pekár and Brabec 2009).
Results Natural prey The ant fauna of the Mashabbim dunes was represented by five species, with M. arenarius predominant (Table 1). Z. cyrenaicum spiders captured 98.2 % (N = 110) of M. arenarius and 1.8 % of Cataglyphis niger. However, the first instar juveniles captured only M. arenarius (N=92). Thus, the actual (captured) prey was significantly different from the potential (available) one (X25 =594, P<0.0001). Female spiders chose significantly smaller ant morphs than those available (ANOVA, F3,228 =68.9, P<0.0001, Fig. 1). Juvenile spiders (of different instars) captured smaller morphs than females (contrast, P=0.004). Thus, there was a positive relationship between the size of the spider and the ant prey (LM, F1,108 =29.5, P<0.0001, R2 =0.21, Fig. 2a), so that the average body size of the first instar juvenile’s prey was 3.7 times larger than that of the spider. The mass of the captured ant increased significantly with the mass of the first instar juvenile spiders (LM, F1,76 = 24.8, P < 0.0001, R2 = 0.24, Fig. 2b). The first instar juveniles captured prey with a mass that was, on average, 6.5 times heavier, while females captured ants that were, on average, 0.65 times their mass. The trend of relationship of spider and ant mass was similar for juveniles and females (LM, F1,72 =1.9, =0.19).
Capture efficiency Z. cyrenaicum spiders (juveniles and adult females) attacked ants (Fig. 3a) on different body parts but most frequently on the legs (75 %, N=95) followed by the abdomen (19 %). In 60 % (N=75) of attacks on legs, the hind leg was bitten. Following a bite, the ant stopped moving and stood still with opened mandibles. Meanwhile, the bitten limb contracted, and the gaster bent under the thorax (Fig. 3b). Such a C-shaped position lasted for several minutes; then the ant collapsed, falling on one side. At this moment, the spiders approached and began to feed (Fig. 3c). Both females and juveniles tended to bite once or twice, but juveniles did not necessarily bite more frequently than females: 42 % (N=42) of juvenile spiders (of different instars) made only a single bite. There was no significant difference in the number of attacks between females and juveniles (GLM-p, X21 =0.26, P=0.61). Paralysis latency increased significantly with ant mass in both females and juveniles (GLM-g, F1,28 = 6.3, P=0.01, Fig. 4). However, the paralysis latency for ants attacked by juveniles was 1.5 times longer than the paralysis latency for ants attacked by females (GLM-g, F1,29 =14.5, P= 0.0007). While, on average, females paralysed ants in 10.6 min (SE=0.97), juveniles paralysed ants in 17.2 min (SE=1.78).
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Species
% colonies
Messor arenarius (Fabricius 1787) Cataglyphis niger (André 1881) Camponotus fellah Dalla Torre 1893 Crematogaster nigriceps Forel 1911
56.0 21.0 6.0 3.0
Tetramorium sp. Total no. of colonies
14.0 101
A
16 14 12
Ant size [mm]
Table 1 Relative frequency of ant colonies found at the Mashabbim sands of the Negev desert estimated from the linear transect
10 8 6 4
Venom composition The venom glands were cylindrical. The volume of the venom glands in females (on average 1.67 mm long, 0.36 mm in diameter, 0.17 μl) was 54 times larger than those in juveniles (0.39×0.09 mm, 0.003 μl). MALDI-TOF MS of venom gland extracts revealed 75 peptides/proteins with masses ranging between 3 and 8 kDa (Fig. 5). There were major peaks (above 30 % of intensity) at m/z 4,025, 4,604, 4,626, 4,642, 4,869, 4,886, 4,908, 4,924, 4,945, 5,199, 5,220, and 5,838. Differences in peptide/protein
0 0
1
2
3
4
5
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7
Spider size [mm]
B
1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 -0.5
0
0.5
1
1.5
2
log(Spider mass [mg])
Fig. 2 a Relationship between spider body size (both juveniles and females) and captured ant body size. b Relationship between juvenile spider mass and captured ant mass (both logarithmically transformed). Linear regression models are shown
14
12
composition between females and juveniles were not significant (RDA, F=2.0, P=0.35). In the venom gland extracts of Z. cyrenaicum, 26 bands (proteins) in total were found by 1-SDS-PAGE analysis, with the highest intensities at <16 kDa and at about 75 kDa (Fig. 6). Differences in protein composition between females and juveniles were not significant (RDA, F=0.6, P=0.66).
10
Ant size [mm]
2
log(Ant mass [mg])
Females and juveniles differed in their capture behaviour. While all females (N=18) and most juveniles attacked the ant laterally on an appendage or the abdomen and then quickly retreated, some juveniles (29 %, N=24) used an alternative strategy: they climbed the dorsal side of the ant and bit it on the abdomen or petiole and stayed attached to the ant. Within juveniles, the paralysis latency was not significantly different between attacks on legs or the dorsal side of the abdomen or petiole (GLM-g, F1,10 =0.06, P=0.81).
8
6
4
2
Discussion
0
Available
Juvenile
Male
Female
Fig. 1 Comparison of the mean body size of available M. arenarius ants and those captured by female, juvenile, and male spiders of Z. cyrenaicum. Bars are means, whiskers are SE
Z. cyrenaicum was found to exclusively capture ants in nature similarly to other Zodarion species investigated so far (Pekár 2004, 2005; Pekár et al. 2005, 2011). Although at the study
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Fig. 3 Capture sequence of Z. cyrenaicum juvenile attacking M. arenarius worker. a Juvenile attacking ant on its third leg. b Attacked ant standing in a C-shaped position with venom taking effect. The spider biting its hind leg. c Juvenile feeding on the gaster of the paralysed ant
site in the Negev desert, several ant species occurred syntopically, all developmental stages of Z. cyrenaicum were predominantly engaged in catching a single ant species, M. arenarius. This ant species was the most abundant species among the five ant species found foraging upon the ground. Yet, our data indicate that M. arenarius was not captured randomly but actively selected by Z. cyrenaicum against all other available prey. Thus, the realised trophic niche of this spider is very narrow. The active selection likely results from their ability to recognise a specific component of a pheromone produced by M. arenarius as has been found for another Zodarion spider species (Cárdenas et al. 2012). M. arenarius exhibits considerable size polymorphism. Thus, it was not surprising to find that the size of captured prey increased with the body size of Z. cyrenaicum individuals. Yet, even the smallest ant morphs are large in comparison with the body of spider juveniles, showing that juveniles catch larger prey than expected from the adult spider/ant relationship. Although some small juveniles were found to feed on very large ant morphs, there was trophic niche partitioning between adult females and juvenile spiders similar to the one 30
Paralysis latency [min]
25 20 15 10 5
Females Juveniles
0 0
0.02
0.04
0.06
0.08
0.1
Ant mass [g]
Fig. 4 Relationship between paralysis latency and captured ant body mass for female and juvenile spiders of Z. cyrenaicum. Estimated models are shown
observed for another Zodarion species (Pekár et al. 2011). Zodarion male spiders cease ant hunting and kleptoparasitize on the female’s or juvenile’s prey (Martišová et al. 2009). The capture of giant prey (more than 30 times heavier than the predator) has been reported for social foragers such as ants (Dejean et al. 2010), spiders (Yip et al. 2008), or pseudoscorpions (Zeh and Zeh 1990), in which the prey is overcome by several individuals, which also share it during feeding. Here, we observed that giant ants were captured solely by a single individual. This was possible because Zodarion spiders have evolved specific behavioural and venomic adaptations. They use a specific capture strategy that includes a fast attack from behind followed by a retreat to avoid counter attack by the ants (Pekár 2004). Such strategy is commonly used by other anteating spiders (e.g. Li et al. 1999). Here, we observed that tiny juveniles also used a different strategy. Thanks to their tiny size, they were able to climb on the dorsal side of an ant and deliver a bite to the abdomen or petiole, thus avoiding retaliation by the immediately helpless prey. A similar strategy has been observed in spiders that hunt another dangerous prey, other spiders (Harland and Jackson 2006). The venom of predators includes compounds that first constrain prey mobility so that the prey is less likely to retaliate against the spider (if dangerous), crawl away, or become lost (e.g. when the ant is in its nest). Finally, the compounds lead to prey paralysis and aid digestion. The venom of spiders is a complex mixture of biologically active substances (Rash and Hodgson 2002; Kuhn-Nentwig et al. 2011), which are used to perform these tasks. The venom composition of Zodarion spiders is not known, as no compound has been identified so far in any species of this genus. This is because it is difficult to obtain venom samples from these tiny spiders. We tried to identify proteins from the venom samples from adult females. As the amount of venom was limited, the identification was problematic. For analysis of the venom composition, we used methods that are widely used in venomic research. 1D-SDS-PAGE represents a basic method employed routinely in proteomics (Laemmli 1970). The main advantages of 1D-SDS-PAGE consist in its simplicity, versatility, and, after employing ingel digestion, compatibility with mass spectrometry (Shevchenko et al. 2006). Performance of the method is
Naturwissenschaften Fig. 5 Comparison of MALDITOF MS profiles of the venom of females (a) and juveniles (b) of Z. cyrenaicum spiders. m/z, molecular weight
insufficient for effective separation of highly complex and heterogeneous protein mixtures (Rabilloud et al. 2009). On the contrary to 1D-SDS-PAGE, direct protein profiling by MALDI-TOF mass spectrometry was established in routine laboratory practice including venomics quite recently (Escoubas et al. 1997). Due to suppression effects connected with the desorption/ionization process, only relatively small, basic, and abundant proteins are detected from complex protein mixtures by MALDI-TOF MS profiling. To elucidate the venom action in Z. cyrenaicum, we used data on the venom composition of Lachesana tarabaevi Zonstein et Ovtchinnikov, a single zodariid spider whose venom has been investigated and from which a few
compounds have been identified. Lachesana spiders are, however, polyphagous, though they also catch ants (Pekár and Lubin 2009). The venom of L. tarabaevi is particularly rich in linear cytolytic peptides, these constituting more than 50 % of the active compounds (Kuzmenkov et al. 2013). Specifically, from the venom of L. tarabaevi, latarcins (molecular weight 2.4–4.4 kDa) (Kozlov et al. 2006), latartoxins (6.5—7.7 kDa), and cyto-insectotoxins (7.8–8.6 kDa) (Vassilevski et al. 2008) have been identified. All these peptides possess insecticidal cytolytic activity (Kuzmenkov et al. 2013), as they bind to and subsequently damage the lipid bilayers of muscle membranes. In the venom of Z. cyrenaicum, the major constituents were peptides with molecular weights between 4 and 6 kDa. These
Naturwissenschaften
Fig. 6 SDS-PAGE gel of the venom of Z. cyrenaicum female and juvenile spiders. MW, molecular weight marker
most likely correspond to linear cytolytic peptides. However, using 1D electrophoresis, we also found proteins with high molecular weights, so far unreported for zodariid spiders. We selected four bands with high molecular weights (25, 60, 75, and 90 kDa). These were subjected to in-gel digestion using trypsin and subsequently analysed using LC-MS/MS (detailed data not shown). By means of comparison with the NCBI database, we identified only two proteins that have previously been isolated from other spiders (disulfide isomerase and troponin 1). These proteins might be part of the glandassociated tissue as the former is an enzyme in the endoplasmic reticulum and the latter is often found in muscles. Z. cyrenaicum change their body size considerably during ontogenetic development but captured the same prey species. We compared the venom composition of adult females and juveniles of Z. cyrenaicum and failed to find significant differences either in low or high molecular compounds. Thus, their shift from smaller to larger prey during ontogenesis was not associated with a change in venom composition. We observed higher capture efficiency in adult spiders than in juveniles. This was likely achieved by the envenomation of a large quantity of venom, as has been found in snakes and other spiders. It is not known whether the venom of spiders can be preyspecific. Recent research on diverse predatory taxa has brought increasing evidence of an adaptive response of venom
composition to prey. For example, high venom specificity has been reported for Conus snails and Crotalus snakes, which are specialised on certain prey (Duda and Lee 2009; Mackessy 2010). The venom of Zodarion spiders may be adapted to their exclusive prey, as it is ineffective against other prey (Pekár 2004). The specificity of Z. cyrenaicum venom is indirectly supported by the fact that the paralysis latency with juvenile spiders was about twice longer than that with female spiders which posses much larger venom glands, yet the venom efficacy of females was only little improved. Using data on the paralysis procedure, we assume that in the venom of Z. cyrenaicum, cytolytic and neurotoxic compounds have a synergistic effect. The ant showed typical neurotoxic symptoms after an attack, which are, in insects, characterised by limb hyperextensions, body jerking and quivering, and uncoordinated movement (e.g. Franklin 1988). Zodarion spiders deliver an attack to the tibia of the hind legs and the venom is injected far away from neural ganglia situated in the ant's thorax (Hölldobler and Wilson 1990). The injected venom seems to act locally—to cramp the leg—and then be transferred to the other body parts. Cytolytic peptides may clear the way at the envenomation point by means of their cytolytic activity (Vassilevski et al. 2008) and open a path for the neurotoxic proteins to reach ant neurons. Then, the neurotoxins may target Na2+ channels that control the coordination of various processes, such as locomotion (Rash and Hodgson 2002), and cause the formation of cation-permeable pores in neuronal tissues (Rohou et al. 2007) that lead to complete paralysis. Here, we described adaptations in a specialised ant-eating predator that enable it to catch giant prey. We discovered yet another important function of venom in venomous animals: the capture of giant prey. Acknowledgments We would like to thank Y. Lubin for providing us with a collection permit at the Mashabbim sand dunes. We are grateful to G. Corcobado, M. Guererro, J. Král, L. Mestre, and J. Niedobová for help with spider collection in the field, and to L. Sentenská and D. Fridrichová for help in the laboratory. We would also like to thank C. Komposch for his suggestion for the title of this paper. This work was supported by CEITEC project (CZ.1.05/1.1.00/02.0068), CEITEC open access project (LM2011020), and the grant provided by the Czech Science Foundation. SK was supported by the project of European Science Foundation and Ministry for Education and Youth of the Czech Republic CZ.1.07/2.3.00/ 30.0040 and “Účelová podpora na specifický vysokoškolský výzkum” provided by Ministry for Education and Youth of the Czech Republic.
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Levy G (1992) The spider genera Palaestina, Trygetus, Zodarion and Ranops (Araneae, Zodariidae) in Israel with annotations on species of the Middle East. Israel J Zool 38:67–110 Li D, Jackson RR, Harland DP (1999) Prey-capture techniques and prey preferences of Aelurillus aeruginosus, A. cognatus and A. kochi, anteating jumping spiders (Araneae: Salticidae) from Israel. Isr J Zool 45:341–359 Mackessy SP (2010) Crotalus viridis viridis (Prairie Rattlesnake). Noxious weeds as a hazard to snakes. Herpetol Rev 41(3):363 Martišová M, Bilde T, Pekár S (2009) Sex-specific kleptoparasitic foraging in ant-eating spiders. Anim Behav 78:1115–1118 McCue MD (2005) Enzyme activities and biological functions of snake venoms. Appl Herpetol 2:109–123 Nakazawa T, Ohba S, Ushio M (2013) Predator–prey body size relationships when predators can consume prey larger than themselves. Biol Lett 9:20121193 Nentwig W (1987) The prey of spiders. In: Nentwig W (ed) Ecophysiology of spiders. Springer, Berlin, pp 249–263 Oksanen J, Blanchet FG, Kindt R, Legendre P, Minchin PR, O'Hara RB, Simpson GL, Solymos P, Stevens MHH, Wagner H (2012) Package 'vegan' version 2.0-2 Pekár S (2004) Predatory behavior of two European ant-eating spiders (Araneae, Zodariidae). J Arachnol 32:31–41 Pekár S (2005) Predatory characteristics of ant-eating Zodarion spiders (Araneae: Zodariidae): potential biological control agents. Biol Control 34(2):196–203 Pekár S, Král J, Lubin Y (2005) Natural history and karyotype of some ant-eating zodariid spiders (Araneae: Zodariidae) from Israel. J Arachnol 33(1):50–62 Pekár S, Brabec M (2009) Modern analysis of biological data. 1. Generalised Linear Models in R. Scientia, Praha Pekár S, Lubin Y (2009) Prey and predatory behaviour of two zodariid spiders (Araneae, Zodariidae). J Arachnol 37(1):118–121 Pekár S, Bilde T, Martišová M (2011) Intersexual trophic niche partitioning in an ant-eating spider (Araneae: Zodariidae). PLoS One 6(1):e14603 R Core Team (2012) R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL http://www.R-project.org/ Rabilloud T, Vaezzadeh AR, Potier N, Lelong C, Leize-Wagner E, Chevallet M (2009) Power and limitations of electrophoretic separations in proteomics strategies. Mass Spectrom Rev 28:816–843 Rash LD, Hodgson WC (2002) Pharmacology and biochemistry of spider venoms. Toxicon 40:225–254 Rohou A, Nield J, Ushkaryov YA (2007) Insecticidal toxins from black widow spider venom. Toxicon 49(4):531–49. Sasa M (1999) Diet and snake venom evolution: can local selection alone explain intraspecifc venom variation? Toxicon 37:249–252 Schaller GB (1972) The Serengeti lion. University of Chicago Press, Chicago Shevchenko A, Tomas H, Havlis J, Olsen JV, Mann M (2006) In-gel digestion for mass spectrometric characterization of proteins and proteomes. Nat Protoc 1:2856–2860 Turner M (1979) Diet and feeding phenology of the green lynx spider, Peucetia viridans (Araneae: Oxyopidae). J Arachnol 7(2):149–154 Vassilevski AA, Kozlov SA, Samsonova OV, Egorova NS, Karputin DV, Pluzhnikov KA, Feofanov AV, Grishin EV (2008) Cytoinsectotoxins, a novel class of cytolytic and insecticidal peptides from spider venom. Biochem J 411:687–696 Yip EC, Powers KS, Avilés L (2008) Cooperative capture of large prey solves scaling challenges faced by spider societies. PNAS 105(33): 11818–11822 Zeh JA, Zeh DW (1990) Cooperative foraging for large prey by Paratemnus elongatus (Pseudoscorpionida, Atemnidae). J Arachnol 18:307–311
Rukopis F
DISCOVERY OF A MONOPHAGOUS TRUE PREDATOR, A SPECIALIST TERMITE-EATING SPIDER
(ARANEAE: AMMOXENIDAE) Petráková, L., Líznarová, E., Pekár, S., Haddad, C. R., Sentenská, L., Symondson, W. O. (2015) Scientific reports, 5: 14013
Ammoxenus amphalodes
© Michálek
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received: 10 April 2015 accepted: 12 August 2015 Published: 11 September 2015
Discovery of a monophagous true predator, a specialist termite-eating spider (Araneae: Ammoxenidae) Lenka Petráková1, Eva Líznarová1, Stano Pekár1, Charles R. Haddad2, Lenka Sentenská1 & William O. C. Symondson3 True predators are characterised by capturing a number of prey items during their lifetime and by being generalists. Some true predators are facultative specialists, but very few species are stenophagous specialists that catch only a few closely related prey types. A monophagous true predator that would exploit a single prey species has not been discovered yet. Representatives of the spider family Ammoxenidae have been reported to have evolved to only catch termites. Here we tested the hypothesis that Ammoxenus amphalodes is a monophagous termite-eater capturing only Hodotermes mossambicus. We studied the trophic niche of A. amphalodes by means of molecular analysis of the gut contents using Next Generation Sequencing. We investigated their willingness to accept alternative prey and observed their specific predatory behaviour and prey capture efficiency. We found all of the 1.4 million sequences were H. mossambicus. In the laboratory A. amphalodes did not accept any other prey, including other termite species. The spiders attacked the lateral side of the thorax of termites and immobilised them within 1 min. The paralysis efficiency was independent of predator:prey size ratio. The results strongly indicate that A. amphalodes is a monophagous prey specialist, specifically adapted to feed on H. mossambicus.
Predators can be classified into herbivores, parasites, parasitoids and true predators1. These categories differ not only in the type of food consumed but also in the width of their trophic niche. While some herbivores, many parasites and parasitoids frequently feed only on a few host species and are often specialists, true predators typically capture a wide variety of prey. As true predators capture a number of prey items during their lifetime they are usually generalists2. Some species of true predators, such as snakes3 or spiders4, only capture a few prey types. These are, however, examples of facultative stenophagy or local specialisation, with exploitation of a locally abundant prey5. Such predators lack specialised capture adaptations6. Stenophagous specialised true predators (sensu6) that capture only certain prey and possess specialised adaptations are rather rare. The trophic width of these predators is restricted yet it includes a few representatives of a family or a few species of one genus. For example, acarophagous mites catch several other mite species7, araneophagous spiders catch spiders from several families8, aphidophagous coccinellids catch aphids of a single genus9, lepidopterophagous bolas spiders capture male moths of a few species belonging to distinct families10, and myrmecophagous spiders feed on several species of a single subfamily11. A theoretical monophagous true predator, one in which all individuals are adapted to 1
Department of Botany and Zoology, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic. 2Department of Zoology & Entomology, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa. 3Cardiff School of Biosciences, Cardiff University, Sir Martin Evans Building, Museum Avenue, Cardiff CF10 3AX, United Kingdom. Correspondence and requests for materials should be addressed to S.P. (email:
[email protected]) Scientific Reports | 5:14013 | DOI: 10.1038/srep14013
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www.nature.com/scientificreports/ exclusively exploit a single prey species throughout their life and ignore even prey closely related to the focal species, has not been discovered yet. Spiders are the largest (most diversified) order of terrestrial true predators12. Most species are generalist predators, some are oligophagous, while a few are stenophagous13. Stenophagous specialised spiders have been found to catch other spiders, woodlice, dipterans, lepidopterans, ants, and termites6. Although termites are locally abundant and highly palatable, they are only exploited by a few termite-eating spiders: Salticidae14, Theridiidae15, Zodariidae16 and Ammoxenidae17. This is probably because the majority of termites live underground or in wood and have highly unpredictable and brief activity on the surface, restricting their accessibility to spiders18. All representatives of the family Ammoxenidae appear to be adapted to catch termites19. Some Ammoxenidae have been reported to feed only on the harvester termite, Hodotermes mossambicus (Hagen) (Isoptera: Hodotermitidae), which is the only species of the genus occurring in southern and eastern Africa20. Hodotermes mossambicus inhabits subterranean nests and forage on grass upon the surface during short activity periods21,22. Anecdotal observations suggest that some Ammoxenus species may be specialists of this termite species17,21–25. Rigorous analysis of the trophic niche of a predator can be based on direct observations of prey consumption or on the morphological analysis of prey remnants. The latter can be successfully used only in sedentary species, such as web-building spiders, that store remnants in their web26. In cursorial species it is difficult to investigate the trophic niche using any of these methods, as they feed cryptically (e.g. during the night or under vegetation) and do not store prey remnants. Molecular approaches provide useful tools for determining the dietary breadth of such predators. Prey DNA sequences in the gut content of predators have been successfully analysed in many studies27–30. Next generation sequencing (NGS), in particular, allows reliable, rapid and simultaneous amplification, and subsequent identification, of thousands of prey DNA sequences from the guts of many individual predators separately31,32. It is necessary to target short prey amplicons (to ensure they survived digestion in the predator27,33), amplify part of the barcoding region of the cytochrome c oxidase I gene (to maximise the chances of identification from databases such as GenBank and BOLD, the Barcoding of Life Database), and, where possible, design primers that will amplify the DNA of a range of prey but not that of the predator28. Such primers must amplify sequences that are variable enough to distinguish taxa to the highest level possible34,35. Here we focused on Ammoxenus amphalodes Dippenaar & Meyer, in which preliminary observations suggest that it may only capture H. mossambicus harvester termites21. We tested the hypothesis that A. amphalodes are monophagous, feeding on a single termite species. Ammoxenus amphalodes is a common, widespread species that is endemic to South Africa. It mainly inhabits open plains in grassland and savannah habitats, and usually lives in sandy soils near to H. mossambicus nests. We investigated the natural prey of these spiders by means of NGS. Then we investigated their willingness to accept alternative sympatrically-occurring prey types under standardised laboratory conditions. Finally, we observed their predatory behaviour and prey capture efficiency to reveal their behavioural and venomic adaptations.
Results
Trophic niche. The primers (AMF1 and AMR1) successfully amplified DNA from all potential prey
taxa. The new primers were specific enough to amplify DNA from all the potential prey, but not that of A. amphalodes. Altogether, prey DNA fragments were successfully amplified from 46 females, 34 males and seven juveniles. In four females and three males (7.5%) no fragments were amplified. Ion Torrent sequencing gave us, after filtering, 1,755,965 sequences (70% of all reads). 1,371,138 sequences were used in further analyses after removing rare haplotypes and sequences with indels changing the reading frame. The number of sequences per spider varied from 5 to 101,242. Generally, fewer sequences were obtained from samples that were extracted from agarose gels (due to presence of very strong dimers during PCRs) than from samples that were purified without cutting from the gels. MOTU analysis showed 16 variants, ranging from 1 to 4 “MOTUs” (groups of haplotypes differing by 4 bp) per spider. Such variation was probably caused by sequencing errors; all but one “MOTUs” contained only 2–4 sequences. The great majority of sequences were assigned to H. mossambicus (99.8% of valid sequences) when compared to GenBank and BOLD databases. Almost all those sequences were similar to H. mossambicus reference sequences above 99%. Odontotermes species differed from H. mossambicus only by 4 bases in the amplified fragment. When searching for the specific mutations, no base combinations corresponding to the Odontotermes sp. were found. Only 32 valid sequences were less than 99% similar to H. mossambicus (according to BOLD database criterion) but were not assigned to any other species. Two identical sequences were not assigned to any arthropod taxon with a match higher than 95%. There was a significant difference in the number of sequences (from DNA not cut from gels, N = 46) obtained from females, males and juveniles (GLM-p: F2,43 = 32204, P < 0.0001) (Fig. 1). The highest number of termite sequences was found in juveniles. Seven arthropod orders were found as potential prey of A. amphalodes spiders (Table 1) in the two field plots. Hodotermes mossambicus was not a frequent prey item on the soil surface, where A. amphalodes foraged. The majority of potential prey were ants (91%, N = 701). Comparison of the potential and actual prey showed a significant difference (χ 29 = 459.6, P < 0.0001). Smith’s index of trophic niche is 0.07 indicating very high level of stenophagy.
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Figure 1. Comparison of the mean numbers of Hodotermes mossambicus termite sequences found in the guts of females (N = 17), males (N = 23) and juveniles (N = 7) of Ammoxenus amphalodes spiders.
Prey
Potential
Actual
H. mossambicus (Isoptera)
0.079
1.000
Odontotermes sp. (Isoptera)
0.001
0.000
Anoplolepis sp. (Formicidae)
0.633
0.000
Monomorium sp. (Formicidae)
0.152
0.000
Pheidole sp. (Formicidae)
0.026
0.000
Crematogaster sp. (Formicidae)
0.021
0.000
Orthoptera
0.062
0.000
Heteroptera
0.007
0.000
Coleoptera
0.011
0.000
Mantodea
0.003
0.000
Araneae
0.005
0.000
758
87
Total
Table 1. Comparison of relative frequency of potential and actual prey of Ammoxenus amphalodes spiders. Potential prey is the proportion of prey individuals found at the study site. Actual prey is the proportion of Ammoxenus individuals with corresponding prey sequences in their gut.
Prey acceptance. All 32 female spiders accepted H. mossambicus as prey but none even attempted to catch other termite species or any other prey. They did not even emerge from the soil to make contact with the alternative prey offered to them. Predatory behaviour. The hunting sequence was composed of the following events: emerge (the
spider emerged from the sand), chase (chasing the moving termite), attack (the spider aimed to attack the termite), bite (the spider bit the termite and held it), and dig in (the spider dragged the immobilized termite and dug with it into the soil) (see Supporting information). The flow diagram (Fig. 2) shows that 72% (N = 32) of spiders needed more than one attempt to catch the termite, repeatedly digging themselves in and out of the soil and making sequential attacks. The remaining individuals (28%) successfully captured the termite at the first attempt. A successful bite was inflicted behind the termite’s head on the lateral side of the thorax. On average (1.96 ± 0.22 (SE)), spiders captured the termite on the second attempt. The termite struggled hard after being bitten, but the spider held it firmly while lying on its back. Then the spider dug itself into the soil together with the immobilized termite and fed on the termite while hidden in the soil. It took a mean of 82 s (SE = 10.5) to immobilize a termite completely. The paralysis latency did not differ between male and female spiders (N = 40; GLM-g: χ 21 = 0.05, P = 0.83), and there was no relationship between termite/spider size ratio and the paralysis latency (N = 40; GLM-g: χ 21 = 0.05, P = 0.84, Fig. 3).
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Figure 2. Flow diagram of hunting sequence of Ammoxenus amphalodes spiders (N = 32) when hunting Hodotermes mossambicus termites. Percentages signify the proportion of individuals transferring from previous step to the next one. Pictures show particular behaviours (attack, holding and carrying, burying beneath prey). Photographs made by Stano Pekár.
700
Female
Paralysis latency [s]
600
Male
500 400 300 200 100 0 0
2
4
6
8
10
12
14
Termite/spider size ratio
Figure 3. Relationship between paralysis latency after final bite and termite/spider body size ratio in female and male Ammoxenus spiders (N = 40). The GLM revealed no relationship (GLM-g: χ 21 = 0.05, P = 0.84).
Discussion
This paper appears to be the first where invertebrate predators have been individually screened using NGS. It is also one of very few papers to use NGS to analyze the diet of an invertebrate species36. The combination of prey DNA analysis with behavioural experiments allowed us to obtain more reliable data Scientific Reports | 5:14013 | DOI: 10.1038/srep14013
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www.nature.com/scientificreports/ on a species’ diet than any single approach. The results of the DNA analyses were in close concordance with data from the behavioural experiments. Unlike all other papers29,31,32,35–38 using NGS to analyse diet to date only one prey species was detected and identified (99.8% of valid sequences). Although different “MOTUs” were isolated, all had their closest match to one termite species, H. mossambicus. Although some MOTU could have belonged to different termite haplotypes, or possibly nuclear copies of the mtDNA sequences within the nuclear genome were present, we think it more likely that most of the differences were caused by sequencing errors. Evidence for this comes from the very low sequence numbers (< 4) of different MOTUs found within individual spider predators. Several factors have quantitative affects on DNA amplification during gut content analysis, particularly primer efficiency, DNA copies number per cell in different prey species, the effect of temperature and predator activity on DNA degradation and differences in the digestibility of different prey29,39 (e.g. soft-bodies vs. chitinous invertebrate prey). Given that (1) we used the same primers throughout, (2) there was only one prey species, and (3) all spiders and termites were living under the same microclimatic conditions (temperature), we can make an approximate comparison of the relative quantities of ingested prey. The highest number of sequences per individual was found in juvenile spiders. This is likely because juveniles may catch prey more often than adults due to their investment in ontogenetic development40,41. Our results confirm that A. amphalodes is a monophagous termite-eater. However, we expected that it would catch and exploit several termite species. Although sequences for H. mossambicus and Odontotermes sp. collected at the same site were so similar, when compared to the databases no sequence was assigned to any Odontotermes sp. (among 34 sequences of the barcoding region available in the BOLD database), not even with a lower percentage similarity. Therefore, the likelihood that the sequences were incorrectly assigned to H. mossambicus is very low. The two termite species differ considerably in their ecology: H. mossambicus forages upon the soil surface and is thus available to A. amphalodes, whereas Odontotermes sp. inhabits wood and is thus inaccessible. Furthermore, in the laboratory A. amphalodes spiders did not accept other termite species. There are two other harvester termite species occurring in South Africa, Trinervitermes trinervoides and Microhodotermes viator (Latreille 1804), which have been reported to be prey of other Ammoxenus species19,21,24. However, we have not found any sequence that would correspond to either of those two species. Microhodotermes viator does not occur in the Free State Province where this study was conducted20. Thus, all our evidence shows that the population of A. amphalodes at our study site is a monophagous termite-eating true predator. If A. amphalodes was a facultative stenophage then it would accept alternative prey when available, under laboratory conditions. Yet, our results show that not only were different prey types refused, but other sympatrically-occurring termite species were rejected. The experiments were conducted with experienced adult spiders, which might have developed a preference for H. mossambicus during their ontogenetic development. A previous study showed that even freshly hatched spiderlings capture H. mossambicus25, supporting suggestions for an obligate preference. Previous continuous pitfall-trap sampling over two years revealed that both A. amphalodes and H. mossambicus are active throughout the year. However, A. amphalodes has two seasonal peaks—it is a bivoltine species with one reproductive period in September and the other in March. This coincides with seasonal activity of H. mossambicus termites but not with other sympatrically occurring termite species, namely T. trinervoides (Haddad, unpublished). In some places A. amphalodes was even observed to hide in the mounds of the latter termite species but results of our study, both field and laboratory, show that A. amphalodes does not feed on it. Additionally, this study found that there is a synchronised spatial co-occurrence of A. amphalodes and H. mossambicus. In a monophagous predator all populations and all developmental stages would have to catch a single prey species. Our data were obtained from a single A. amphalodes population occurring in the central South Africa. Previous work on the trophic ecology of A. amphalodes conducted in the north-eastern part of South Africa provided evidence that this species fed exclusively on H. mossambicus there as well17,21,25. This supports our view that not only is the population we studied monophagous, but that the species as a whole is a monophagous termite-eater. Furthermore, the distributional overlap of A. amphalodes and H. mossambicus supports this view20,42. Specialist predators must be adapted to deal with extreme predator-prey size differences, with tiny juveniles capable of capturing prey as large as those attacked by adults. Prey specialists seem to be adapted to catch extremely large prey. For example, ant-eating Zodarion cyrenaicum Denis that occurs in the Negev Desert captured mainly large Messor arenarius Fabricius ants43. Surprisingly, even the first instar juveniles captured this ant species, even though the ants are gigantic when compared to the body size of the spiders44. All ontogenetic stages of A. amphalodes were found to catch H. mossambicus21,25. The body size of workers of this termite species is sexually dimorphic45, so it might be possible that tiny spiderlings select the smaller female morphs while adult spiders select the larger male morphs. A similar trend has been observed in ant-eating Zodarion spiders44. Although several ant-eating spiders of the genus Zodarion appear to be monophagous, as they only captured one prey species in the wild44, in the laboratory they accepted other ant species, though the capture was less efficient. In A. amphalodes, other termites did not even elicit an attack. The difference stems from the fact that prey selection in specialists can take place at various levels during the predation sequence: encounter—detection—recognition—immobilisation—capture—processing46,47. At each step Scientific Reports | 5:14013 | DOI: 10.1038/srep14013
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www.nature.com/scientificreports/ of the predation sequence, specific adaptations may function as filters that exclude some of the available prey types, so that fewer prey types are left at the next step. A high level of specificity will correspond to the filtering at earlier stages in the predatory sequence. Specifically, in A. amphalodes alternative prey did not even elicit the hunting behaviour, suggesting that the filter is at the level prior to direct encounter, possibly through the detection of vibrations of foraging termites in the substrate24. This again points to monophagy of A. amphalodes. It is not known which cues produced by H. mossambicus are used to elicit the predatory behaviour in A. amphalodes. The cues must be very specific and different from those produced by sympatrically occurring alternative prey. Alarm, trail or sex pheromones produced by prey have been reported for prey-specialists48,49, but these are used at a greater distance. During inactivity, Ammoxenus spiders hide singly in a silk cell buried in soil mounds around the entrance to the termite nest17. As Hodotermes termites begin their foraging activity their movement upon the soil surface may produce vibratory cues that subsequently elicit the predatory behaviour of A. amphalodes24. Ammoxenus amphalodes seem to possess very effective venom to paralyse large individuals of H. mossambicus. The prey was paralysed within approx. 1 min and the paralysis efficiency was independent of predator-prey body size ratio, even for extreme ratios, when the termite was ten times larger than the spider prosoma. A paralysis latency of 30–35 s was reported for adult A. pentheri feeding on H. mossambicus25, while 2nd and 3rd instars took approximately 1 min to paralyze prey. In a study on paralysis efficiency of myrmecophagous spider specialists we found that the efficiency decreases with an increasing body size ratio44. This is likely because larger prey requires a larger volume of venom to take its effect. Absence of a relationship here suggests that either A. amphalodes spiders adjusted the amount of injected venom perfectly according to the prey size, or they possess very effective venom compounds. A similarly high venom efficacy has been found in ant-eating Zodarion spiders44. Ammoxenus amphalodes seems to consume one termite per week, which amounts to dozens of individuals during their life cycle. The termites have, however, unpredictable foraging activity, often staying inactive for several days at a time. How does the spider deal with their absence? The silken retreats of A. amphalodes containing up to four immobile H. mossambicus termites were observed22. Even more surprisingly, they were not dead, but only paralyzed. The authors suggested that they could serve as food storage and be used during periods when termites are inactive. We conclude that both the DNA analyses and laboratory experiments support the hypothesis that A. amphalodes is a specialized predator of a single termite species, H. mossambicus. The evidence suggest that this is the first case of a monophagous true predator, although we cannot entirely exclude the possibility that in other sites in Africa, where different sympatric termite species are found, they may take other species too. As in specialised herbivores, parasites and parasitoids, monophagy in true predators seems to evolve when the prey is considerably larger than the predator. Other ammoxenid species may show similar trophic ecology. Indeed, there is anecdotal evidence that A. pentheri Simon is a specialised predator of H. mossambicus23, whereas Rastellus sabulosus Platnick & Griffin feeds only on Psammotermes allocerus Silvestri (Isoptera: Rhinotermitidae)50.
Methods
Potential prey. Fieldwork was conducted in a grassland at the Amanzi Private Game Reserve near
Brandfort (S 28° 35’ 53″ E 26° 25’ 04″), South Africa in March 2013. Ammoxenus spiders are rare free-living soil-dwellers hiding in the termite mounds25, thus are difficult to catch. Observations from previous years revealed that in March there is a seasonal peak of adult occurrence (and reproduction) of A. amphalodes (Haddad, unpublished). At this time females were expected to be maximising prey capture, therefore the chances to observe prey capture should be high. Ammoxenus amphalodes were found in a strip of short grass and bare soil, about 20 m wide and 200 m long, between two fields of cultivated Pangola grass (Digitaria eriantha). Previous observations revealed that A. amphalodes are active during the day (Haddad, pers. observ.). To investigate the potential prey of A. amphalodes, two square plots, each 10 × 10 m, were marked in the morning when the spiders were active. These plots were at least 20 m from each other. Arthropods occurring in the plots with body size between 3–15 mm were counted and recorded by visual census of the entire area which lasted for a period of 2 hours. The survey provided not complete but basic idea of the potential prey. Most arthropods were determined to order level, but ants were determined to genus level. Three representatives of each arthropod taxon found were stored in 100% ethanol. These included Hippodamia sp. (Coleoptera: Coccinellidae), Zonocerus sp. (Orthoptera: Acrididae), Hodotermes mossambicus, Odontotermes sp. (Isoptera: Termitidae: Termitinae), Trinervitermes trinervoides (Sjöstedt) (Isoptera: Termitidae: Nasutitermitinae), Ligariella sp. (Mantodea: Mantidae), Pheidole sp. (Hymenoptera: Formicidae: Myrmicinae), Lygaeidae (Hemiptera: Heteroptera), Nysius sp. (Hemiptera: Orsilidae), and Thanatus vulgaris Simon (Araneae: Philodromidae). DNA from these specimens was later used as non-target species for sequence alignment during primer design and for primer testing (see below).
Actual prey. To investigate the actual prey of A. amphalodes, spiders were hand-collected with tubes in the strip of short grass over the course of a few days, at different times, at Amanzi Private Game Reserve. A total of 130 spiders were collected and immediately placed in ethanol. Six A. amphalodes Scientific Reports | 5:14013 | DOI: 10.1038/srep14013
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www.nature.com/scientificreports/ individuals were starved to death which occurred after at least one month. The DNA of these individuals and DNA from potential prey was extracted using the salt precipitation method. As the DNA in the gut of spiders is progressively degrading due to digestion51, it was necessary to use primers that amplified fragments < 300 bp27,52 which are still variable enough to distinguish between taxa34,35. In the starved spiders and the potential prey, the barcoding region of the COI gene was first amplified using the LCO and HCO primers53 and Go Taq G2 Flexi DNA Polymerase (Promega) under the following conditions: initial denaturation at 94 °C for 5 min; 35 cycles of 94 °C for 30 s, 48 °C for 30 s as an annealing temperature, 72 °C for 1 min; and a final extension at 72 °C for 7 min. The reaction mixture total volume of 20 μ L consisted of 8.3 μ L nuclease-free water, 4 μ L of 5x Green GoTaq buffer, 2.5 μ L of 25 mM MgCl2, 1 μ L of 10 mM dNTP’s, 1 μ L of 10 μ M forward and 1 μ L of reverse primer, 0.2 μ L of GoTaq G2 Flexi DNA polymerase (5 u/μ L) and 2 μ L of DNA. PCR products were detected by electrophoresis in 2% SafeView-stained agarose gels. Amplified products were sequenced on an ABI Prism 3130 Genetic Analyzer (Applied Biosystems). Sequences were aligned using Mega 5.154. Additional sequences downloaded from the GenBank database, representing another African termite and spiders (accession numbers NC_018122.1, JF302834.1, JF302835.1, JF302833.1, JF302832.1, JF302836.1, JF923296.1, JX023555.1), were included in the alignment. Two primer pairs, which would amplify DNA of potential prey, but not the spiders, were designed using Amplicon.b0855. The primers were tested to determine PCR conditions for successful amplification of all the potential prey. The primer pair AMF1: 5′ - AGCAGGAATAGTAGGAACAT-3′ and AMR1: 5′ -CCWCTTTCWACTATTCTTC-3′ , which amplified a 250 bp fragment, was chosen for subsequent analyses and modified with MID identifiers (10 bp tags) and Ion Torrent adaptors. We used 10 MIDs added to the forward primers and 10 different MIDs added to the reverse primers to assign prey sequences to individual predators. This gave us the capacity to separate sequences derived from the gut contents of up to 100 individual spiders31. DNA was extracted from 94 spider abdomens using the DNeasy Blood & Tissue Kit (Qiagen) following the manufacturer’s protocol (purification of DNA from animal tissues) with a change in a final elution step (only 80 μ L of AE buffer were used). In total, seven juveniles and 87 adults (37 males and 50 females) were screened. PCR reactions were performed using the Multiplex PCR kit (Qiagen) under the following conditions: initial denaturation at 95 °C for 15 min; 43 cycles of 94 °C for 30 s, 47.2 °C for 90 s (annealing temperature), 72 °C for 90 s; and a final extension at 72 °C for 10 minutes. Reaction mixture total volume of 20 μ L consisted of 10.6 μ L of Multiplex PCR Master Mix, 1.8 μ L of Q-Solution, 3 μ L of RNase-free water, 0.8 μ L of 10 μ M forward and 0.8 μ L of reverse primers, and 3 μ L of DNA. PCR products were detected by electrophoresis in 2% SafeView-stained agarose gels. Samples which did not form visible dimers (or only very weakly) were purified using QIAquick PCR Purification Kit (Qiagen). In cases where dimers were strong, DNA was cut from the agarose gel and extracted using the QIAquick Gel Extraction Kit (Qiagen). Both extractions and purifications were performed according to the manufacturer’s protocols. Concentration of all PCR products was assessed by comparison with the 100 bp ladder (BioLabs). Then, 5 μ L of 50 μ g/μ L PCR products was pooled into the same sterile vial and sent for sequencing. Enrichment (emPCR) and one-directional sequencing on an Ion Torrent PGM with a 316 chip was performed at the Centre de Recerca en Agrigenòmica (Bellaterra - Barcelona, Spain). The sequences were processed using the Galaxy platform (https://usegalaxy.org/)56 and BioEdit 7.2.557. Reads were split according to their MIDs (with two mismatches and two deletion thresholds allowed), resulting in files corresponding to individual spiders. Sequences were filtered according to their length (< 200 bp) to remove dimers or too short reads. The sequences were collapsed and rare haplotypes (containing < 2 identical sequences) were removed as well as sequences with stop codons and indels changing the reading frame to eliminate sequencing errors. The remaining haplotypes were clustered into MOTUs (= molecular operational taxonomic units) using jMOTU 4.158 with a 4-bp cut-off (corresponding to 1.6% sequence divergence), following31,32,37. The variation among species is greater than 2% (or even more than 4% in some taxa) when using COI34. We decided to use a lower threshold because H. mossambicus and Odontotermes sp. differed by only 4 bp in the targeted COI region. Each MOTU was compared to the GenBank database (http://blast.ncbi.nlm.nih.gov/Blast.cgi) using megablast, the BOLD database (http://www.boldsystems.org/) and also to sequences obtained from the potential prey specimens. In BOLD, sequences are assigned to species level when their similarity is higher than 99%. Some sequences (0.2%) did not match with any known sequence. One of them (0.06% of all analyzed sequences) did not appear to be a valid COI sequence; another two (0.13%), that looked valid, could not be assigned to any known taxonomic group and were probably caused by sequencing errors.
Predatory behaviour. Forty adult A. amphalodes were collected at Amanzi in order to investigate the prey-capture behaviour of the spider. After transfer to the laboratory, adult spiders of both sexes were placed separately into an arena consisting of 250 ml plastic bottles (8 cm in diameter, 15 cm tall) containing a 3 cm layer of sand in the bottom. They were left for three days to settle down at room temperature (~25 °C) and a natural LD (12:12) regime, during which time they were starved. Spiders usually dug themselves into the sand immediately after being released into the arena. Hodotermes mossambicus termites from one nest were collected in a suburban grassland in Langenhoven Park, Bloemfontein and kept together in plastic containers (40 cm in diameter) filled with soil. Trials began when a termite was introduced to the arena occupied by a spider. The hunting sequence was observed and recorded using handycam Canon Legria HF G10. If the spider did not start hunting within one hour, the prey was Scientific Reports | 5:14013 | DOI: 10.1038/srep14013
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www.nature.com/scientificreports/ removed. We measured the paralysis latency as the time between successful attack and termite immobilization. After the trial the whole body length of termites and prosoma length of the spider were measured with a Nikon SMZ800 stereomicroscope.
Prey acceptance. The acceptance of different prey by A. amphalodes spiders was investigated in the lab. We used only adult females (N = 32) collected in the Amanzi with a mean body size of 5 mm as these are generally more voracious than males. The following prey that occurred syntopically with the spiders were used: termites (H. mossambicus workers, 9 mm body length, T. trinervoides workers, 4 mm, Reticulitermes sp., 3.5 mm), ants (Messor sp. workers, 7 mm; Anoplolepis custodiens (F. Smith) workers, 5 mm), and Tenebrionidae beetles (Zophosis boei Solier, 8 mm). Spiders were placed separately into the arenas (as above) and starved for two days. The prey were collected one day prior to running the trials. The prey was released into the container occupied by the spider and the result was recorded. In each trial we recorded whether the spider attacked and consumed the prey. When the spider did not attack the prey after an hour, the prey was removed from the dish and was replaced with a different prey. Each spider was offered all prey types in a randomised order. When the spider captured the prey the next prey was offered two days later. The design used was a complete block. To control for motivation to hunt the prey, H. mossambicus termites were offered to individuals that refused alternative prey (as a control). Only individuals that subsequently captured H. mossambicus termites were included in the analysis. All analyses were performed within an R environment59. The composition of potential prey (i.e. relative frequency of each prey species in the field plots) was compared with the composition of the actual prey (i.e. relative frequency of spider with DNA of each prey species found in the gut) using the χ 2 goodness of fit test. Smith’s index60 was used to estimate the width of the trophic niche. To compare the paralysis latency, and the relationship between body size ratio and paralysis latency, we used General Linear Model (GLM) with Gamma error structure (GLM-g). GLM with Poisson error structure (GLM-p) was used to compare the number of sequences among juveniles, females and males.
References
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Acknowledgements
We would like to thank S. Korenko and V. Butler for their help to collect spiders and termites in the field, and Beth Clare for her advice with NGS data analysis. Field work for this study was funded through a grant from the National Research Foundation of South Africa through its Knowledge Interchange and Collaboration programme (Grant UID #83231). The lab study was funded by the Czech Science Foundation (GA15-14762S).
Author Contributions
S.P. and W.O.C.S. designed the study; S.P., E.L., L.S. and C.R.H. collected material at the study site; L.P. performed molecular analyses supervised by W.O.C.S.; E.L. and L.S. did behavioural experiments; S.P., E.L., and L.P. analysed data; all authors contributed to writing the manuscript. Scientific Reports | 5:14013 | DOI: 10.1038/srep14013
9
www.nature.com/scientificreports/
Additional Information
Accession codes: The sequences used for primer design are available via GenBank (Accession numbers: KP748184-KP748191). Supplementary information accompanies this paper at http://www.nature.com/srep Competing financial interests: The authors declare no competing financial interests. How to cite this article: Petráková, L. et al. Discovery of a monophagous true predator, a specialist termite-eating spider (Araneae: Ammoxenidae). Sci. Rep. 5, 14013; doi: 10.1038/srep14013 (2015). This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
Scientific Reports | 5:14013 | DOI: 10.1038/srep14013
10
Rukopis G
SUITABILITY OF WOODLICE PREY FOR GENERALIST AND SPECIALIST SPIDER PREDATORS: A COMPARATIVE STUDY Pekár, S., Líznarová, E., Řezáč, M. (2016) Ecological Entomology, 41, 123–130
Dysdera sp.
© Macek
Ecological Entomology (2015), DOI: 10.1111/een.12285
Suitability of woodlice prey for generalist and specialist spider predators: a comparative study S T A N O P E K Á R, 1 E V A LÍ Z N A R O V Á 1 and M I L A N Rˇ E Z Á Cˇ 2
1
Department of Botany and Zoology, Faculty of Sciences, Masaryk University, Brno, Czech Republic and Biodiversity Lab, Crop Research Institute, Prague, Czech Republic 2
Abstract. 1. Predators select a prey according to its energetic and nutritional composition. Generalist predators avoid, whereas specialists often specialise on well-defended prey. The aim of this study was to find the suitability of woodlice prey for generalist and specialist predators by comparing their handling efficiency. 2. Laboratory experiments were performed in which specialist and generalist predators were reared on monotypic diets comprising one or other of two woodlice species that differ in their defensive strategies: rollers (Armadillidium) and clingers (Porcellio). A control group was reared on a mixture of arthropods (excluding woodlice). Three spider predators were used that differ in their adaptations to deal with woodlice prey: a woodlice specialist, Dysdera crocata; an oligophagous generalist, Pholcus phalangioides, that also captures woodlice; and a euryphagous generalist, Tegenaria domestica, that does not feed on woodlice. The frequency of capture was recorded and various fitness parameters were measured, namely survival, growth rate, and ontogenetic development. 3. It was found that the specialist, D. crocata, performed best on the Porcellio diet, and similarly well on Armadillidium and mixed diets. The two generalists, P. phalangioides and T. domestica, had poor performance on both woodlice diets but performed well on the mixed diet. 4. The results show that woodlice are unsuitable prey for both oligophagous and euryphagous generalist predators. Key words. Araneae, fitness, Isopoda, performance, stenophagy.
Introduction Predators, in contrast to parasites, parasitoids, and herbivores, must select and capture a number of prey items they encounter during their lifetime (Begon et al., 1996). Classical optimal foraging theory predicts that predators should select prey based on their energetic gain (Stephens & Krebs, 1986). There is growing evidence, however, that predators also optimise prey selection according to its nutritional quality (e.g. Greenstone, 1979; Mayntz et al., 2005; Jensen et al., 2012). During foraging, generalist predators encounter a variety of prey that are unpalatable or defended and thus do not provide energetic or nutritional gain (Toft & Wise, 1999a,1999b; Bilde & Toft, 2000). And generalist predators capture well-defended prey because they are naive, Correspondence: Stano Pekár, Department of Botany and Zoology, Faculty of Sciences, Masaryk University, Kotláˇrská 2, 611 37 Brno, Czech Republic. E-mail:
[email protected] © 2015 The Royal Entomological Society
in a poor state or optimal prey is not available (e.g. Skelhorn & Rowe, 2006; Halpin et al., 2012). Defended prey is, however, often exploited by specialist predators (Pekár & Toft, 2015). For example, ants, termites, and spiders are well defended and even dangerous to arthropod predators. Yet, there are some specialised myrmecophagous, termitophagous, and araneophagous predators that capture and even specialise on them (e.g. Hölldobler & Wilson, 1990; Pekár & Toft, 2015). Specialist predators must then obtain both energy and nutrients from such prey (e.g. Pekár et al., 2008, 2010). It is not known, however, whether these defended prey types are nutritionally low- or high-quality prey, and whether generalist predators refuse these types of prey because of their behavioural defences alone, or because of their low nutritional quality as well. One type of prey that is well defended is woodlice (Crustacea: Isopoda). They are common arthropods in the epigeon of different habitats, moist or dry (Sutton, 1972). Woodlice are 1
ˇ c 2 Stano Pekár, Eva Líznarová and Milan Rezᡠslow-moving decomposers that are apparently well protected dorsally by armour (sclerites). Besides morphological defence, woodlice also employ behavioural defences; some species even possess chemical defences (Sutton, 1972; Deslippe et al., 1996). Behavioural defences include nocturnal activity and the ability to roll or to cling to the substrate (Schmalfuss, 1984). As a result, woodlice are eaten only rarely by a few generalist predators (Gorvett, 1956). Vertebrate predators that catch woodlice include hedgehogs (Shilova-Krassova, 1952), shrews (Perneta, 1976), moles (Godfrey & Crowcroft, 1960), frogs, toads, lizards, and birds (Sunderland & Sutton, 1980; Bureš & Weidinger, 2003). With respect to arthropods, woodlice are eaten by harvestmen, centipedes, carabid and staphylinid beetles (Sunderland & Sutton, 1980), spiders (Raupach, 2005; Pekár & Toft, 2015), and ants (Dejean & Evraerts, 1997). Among all these predators, only a few spiders and ants are known to speˇ c et al., 2008). cialise on woodlice (Dejean, 1997; RezᡠRepresentatives of the spider family Dysderidae are specialised woodlice eaters (Cooke, 1965a,1965b). Dysdera spiders possess morphological, behavioural, and metabolic adaptations ˇ c that make them very effective at overcoming woodlice (Rezᡠˇ c et al., 2008). Dysdera species have modi& Pekár, 2007; Rezᡠfied chelicera and catch woodlice in different ways: species with elongated chelicerae insert one chelicera into the soft ventral side of the woodlouse; species with dorsally concave chelicerae quickly tuck their chelicerae under the woodlouse and bite the ventral side of the woodlouse’s body; and species with flattened chelicerae insert their chelicerae between the sclerites of the ˇ c et al., 2008). armour shell directly into the woodlouse (RezᡠThe aim of this study was to compare fitness payoffs (energetic and nutritional) by generalist and specialist predators when fed woodlice prey. We used two different woodlice species, Armadillidium and Porcellio, which differ mainly in their behavioural defensive strategies, and three predator species differing in their adaptations to deal with woodlice prey. Armadillidium protects the susceptible underside by rolling, whereas Porcellio clings to the substrate. The three predators use different capture strategies, hunting either by means of a web or without it. We hypothesised that specialist predators are adapted to capture woodlice and obtain all needed nutrients from a single prey type, whereas generalists are less adapted or ineffective in woodlice capture and require a mixture of prey to meet nutritional demands. The elongated chelicerae of the specialist should enable effective capture of both woodlice species, whereas short chelicerae of generalists will not be effective. As a result, specialists should achieve higher fitness on a single prey than generalists and vice versa. We performed laboratory experiments in which we measured various parameters related to handling efficiency during prey capture and food processing. Materials and methods Spider species We used three spider predators: Dysdera crocata C.L. Koch, Pholcus phalangioides (Fuessli), and Tegenaria domestica (Clerck). Dysdera crocata is a cosmopolitan woodlice specialist
(Cooke, 1967). It feeds mainly on woodlice but also takes alternative prey (Pollard et al., 1995). It is a nocturnal hunter (Budiš, 2008) occurring in epigeon, near to human buildings where ˇ c et al., 2008). Dysdera crocata woodlice are common (Rezᡠhas slightly modified chelicera, allowing it to catch woodlice ˇ c et al., 2008). Pholcus phalangioides is using two tactics (Rezᡠalso a cosmopolitan species associated with human buildings (Roberts, 1995). It hunts prey using a web composed of irregular threads. It is a generalist species capturing spiders, dipterans and ants, but also woodlice (Nentwig, 1983). It has a rather moderate diet width and is thus classified as an oligophagous predator (Pekár et al., 2012). In fact, P. phalangioides seems to be adapted to deal with woodlice, as these were found to make up approximately one-third of its diet (Uhlenhaut, 2001). Finally, T. domestica is also a cosmopolitan and synanthropic species (Roberts, 1995). It is a web-building species. The diet of T. domestica is not known; however, we expect it to be similar to that of a closely related species, Malthonica ferruginea (Panzer), which is a generalist that does not feed on woodlice (Nentwig, 1983). This species is assumed to have a wide diet breadth and is thus classified as euryphagous.
Experimental prey Three different diets were used in the experiment: a pure Armadillidium vulgare (Latreille) diet, a pure Porcellio scaber Latreille diet, and a mixed diet composed of a variety of prey types excluding woodlice (flies, mosquitoes, beetles, crickets, termites, spiders, and grasshoppers). Both Armadillidium and Porcellio specimens were collected in a garden in Dubˇnany. Each species of woodlice was reared separately in a plastic box (20 × 10 × 5 cm) filled with moist soil, pieces of bark, Acer platanoides leaves, and dog food (Hornung et al., 1998). The boxes were kept indoors in the dark at approx. 22 ∘ C and moistened once a week. Drosophila melanogaster Meigen fruit flies were raised in the laboratory on a Drosophila medium (Formula 4-2, Carolina, Biological supply, Burlington, North Carolina) enriched with crushed dog food. Tribolium destructor Uyttenboogaart was reared in the laboratory on flour. Acheta domestica Linnaeus was reared in the laboratory on dog food. Reticulitermes sp. was raised in the laboratory on wood. Spiders (Linyphiidae, Araneidae), grasshoppers, flies, and mosquitoes were collected outside and immediately used to feed spiders. Experimental setup In all, 54 juvenile individuals of D. crocata were collected by sieving detritus in Valverde da Mitra, Portugal. Individuals were randomly assigned to one of three diet treatments (N = 18 per treatment) and placed singly in glass tubes (diameter 2.5 cm, length 6 cm) with a piece of moistened filter paper; the tubes were sealed with a punctured lid. Three females of P. phalangioides, each with an egg sac, were collected indoors in Dubˇnany, South Moravia. The females with egg sacs were kept in the laboratory until the spiderlings hatched. After hatching, the spiderlings from each egg sac were split into three groups so
© 2015 The Royal Entomological Society, Ecological Entomology, doi: 10.1111/een.12285
Suitability of woodlice prey 3 Armadillidium
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that there were 20 individuals per treatment. Individual spiderlings were kept in plastic tubes (diameter 2 cm, length 12 cm) sealed with a punctured lid. One female of T. domestica with an egg sac was collected in Dubˇnany. More than 60 spiderlings hatched. These were randomly assigned to three diet treatments, each containing at least 20 individuals. Spiderlings of T. domestica were kept in glass tubes (diameter 1 cm, 5 cm long). The tubes were sealed with a punctured plastic lid. Spiderlings of P. phalangioides and T. domestica built a web inside the tubes. All spiders were kept in a temperature-controlled rearing chamber at 25 ± 1 ∘ C, in ambient humidity, and with an LD 16:8 h photoperiod. The humidity was maintained by providing each individual with a few drops of water (moistening the filter paper inside tubes) regularly at 5-day intervals. The spiderlings were fed with each prey ad libitum in 7-day intervals. The size of prey offered matched the spider’s size, or the prey was smaller. In the mixed diet treatment, the type of prey was randomised among feeding dates. Fruit flies were used as the first offered prey because of the small size of the spiderlings. Remnants of prey were removed from the tubes 2 days after feeding. The experiment continued until spiders reached adulthood or until all the spiders in one diet treatment group died. Thus, it lasted 225 days for D. crocata, 86 days for P. phalangioides, and 40 days for T. domestica. During this time, mortality and moulting were checked daily. Spiders were weighed using a Kern 770 balance with a precision of 0.01 mg at 7-day intervals. At the beginning of the experiment, the body masses of D. crocata spiderlings (anova, F 2,38 = 0.5, P = 0.63), P. phalangioides spiderlings (anova, F 2,51 = 0.2, P = 0.82), and T. domestica spiderlings (anova, F 2,56 = 0.1, P = 0.96) were not significantly different among diets. On the first feeding date only, all spiderling individuals were observed in order to record the frequency of prey capture.
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 D. crocata
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Fig. 1. Comparison of capture frequencies (proportion of individuals that successfully captured prey) achieved by Dysdera crocata, Pholcus phalangioides, and Tegenaria domestica spiderlings on prey offered during the first feeding.
Results Capture frequency During the first feeding there were significant differences between the capture frequencies for Porcellio, Armadillidium, and the prey mixture (Drosophila, in this case) exhibited by the three spider species (GLM-b, X 4 2 = 28.6, P < 0.0001, Fig. 1). Dysdera crocata captured (attacked and consumed) both Porcellio and Armadillidium with a slightly higher frequency than mixed prey. Pholcus phalangioides captured all prey types with a high frequency. Tegenaria domestica very rarely captured Porcellio or Armadillidium, but captured Drosophila with a very high frequency.
Data analyses Survival All analyses were performed in the r environment (R Core Team, 2013). Frequencies of successful attacks and prey capture were compared among species using generalised linear models (GLMs) with a Poisson distribution. The longitudinal data on absolute weight were analysed with generalised least squares (GLS) available from the nlme package (Pinheiro et al., 2013). The ancova model with time as a covariate was improved by the inclusion of continuous autocorrelation (corAR1) and two variance (varPower and varIdent) structures (Pekár & Brabec, 2012). For data on time to death, the semiparametric Cox proportional hazards model from the survival package (Therneau & Grambsch, 2000) was used. Durations of instars were compared among treatments by means of GLM with a Gamma distribution and logarithmic link (GLM-g). The numbers of instars were compared by means of GLM with a Poisson distribution and logarithmic link (GLM-p). The comparison of body mass among treatments at the beginning of the experiment was made by means of Gaussian anova (Pekár & Brabec, 2009).
The survival of D. crocata spiders did not differ significantly among diet treatments (Cox proportional hazards, X22 = 3.4, P = 0.18, Fig. 2a). There was a slightly higher survival on Porcellio than on Armadillidium and on mixed diets. The survival of P. phalangioides spiders differed significantly among diet treatments (Cox proportional hazards, X22 = 32.3, P < 0.0001, Fig. 2b). Survival on Armadillidium was significantly lower (contrasts, P < 0.0001) than on Porcellio and on mixed diets. The survival of T. domestica spiders differed significantly among diet treatments (Cox proportional hazards, X22 = 51.2, P < 0.0001, Fig. 2c). Survival on Armadillidium and Porcellio was similarly low (contrast, P = 0.29) but that on the mixed diet was significantly higher. Development The total number of instars in D. crocata was not signifi2 cantly different among treatments (GLM-p, X51 = 3.4, P = 0.18;
© 2015 The Royal Entomological Society, Ecological Entomology, doi: 10.1111/een.12285
ˇ c 4 Stano Pekár, Eva Líznarová and Milan Rezáˇ
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Table 1). Spiders on all treatments moulted up to five times. The duration of the first instar in D. crocata was not significantly different among diets (GLM-g, F 2,41 = 0.1, P = 0.86; Table 2). Similarly, there was no significant difference for the subsequent instars: on average (± SE) the second instar lasted 44.7 ± 2.2 days (GLM-g, F 2,31 = 0.2, P = 0.31), the third instar lasted 55.5 ± 2.8 days (GLM-g, F 2,18 = 1.1, P = 0.83), and the fourth instar lasted 63.9 ± 3.2 days (GLM-g, F 2,5 = 0.5, P = 0.64). The total number of instars in P. phalangioides was significantly higher on the mixed diet compared with total instar numbers on the Porcellio and Armadillidium diets (GLM-p, 2 X57 = 65.5, P < 0.0001; Table 1). Spiders on Armadillidium did not moult at all, those on Porcellio moulted only once at most, while those on the mixed diet moulted up to three times (then the
Table 1. Comparison of the mean number (SE) of instars in three spider species reared on three diets. Diet Spider species
Armadillidium
Porcellio
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Dysdera crocata Pholcus phalangioides Tegenaria domestica
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3.5 (0.31) 0.3 (0.11) 0.2 (0.09)
2.4 (0.36) 2.0 (0.24) 1.4 (0.17)
experiment was terminated). The duration of the first instar in P. phalangioides was significantly different between Porcellio and the mixed diet (GLM-g, F 1,21 = 2.9, P < 0.0001; Table 2): it was shorter for the mixed diet than for the Porcellio diet.
© 2015 The Royal Entomological Society, Ecological Entomology, doi: 10.1111/een.12285
Suitability of woodlice prey 5 Table 2. Comparison of the mean duration (SE) of the first instar in three spider species reared on three diets. Diet Spider species
Armadillidium
Porcellio
Mixture
Dysdera crocata Pholcus phalangioides Tegenaria domestica
44.9 (4.07) – 20.0 (0.00)
42.3 (1.80) 55.3 (3.74) 21.8 (1.80)
43.8 (4.87) 25.7 (1.86) 21.3 (0.44)
The total number of instars in T. domestica was significantly higher on the mixed diet than on the Armadillidium or Porcellio diets (GLM-p, X22 = 31.5, P < 0.0001; Table 1). Spiders on Armadillidium and Porcellio moulted once at most, while those on the mixed diet moulted twice (then the experiment was terminated). The duration of the first instar in T. domestica was not significantly different among diets (GLM-g, F 2,23 = 1.1, P = 0.34; Table 2).
Body mass The body mass of D. crocata spiderlings changed differently with time (GLS, F 4,769 = 7.8, P = 0.0004; Fig. 3a): the mass of spiderlings on the mixed diet increased more than those on the other two diets (contrasts, P < 0.01), while those on Armadillidium increased the least. The body mass of P. phalangioides spiderlings changed differently with time (GLS, F 4,423 = 82.9, P < 0.0001; Fig. 3b): the mass of spiderlings on the mixed diet dramatically increased, but those on Porcellio and Armadillidium did not change over time. The body mass of T. domestica spiderlings changed differently with time (GLS, F 4,270 = 26.9, P < 0.0001; Fig. 3c): the mass of spiderlings on the mixed diet dramatically increased, but those on Porcellio and Armadillidium decreased.
Discussion The obtained data show that woodlice are not a suitable type of prey for generalist predators. Woodlice were quite difficult to capture and they seem to be of low nutritional quality too. Generalist predators that did not possess adaptations to catch woodlice were markedly less successful at catching and utilising them than the specialist predator. A similar pattern was observed in shell-breaking crabs where generalist species were less successful in prey capture than the specialists (Yamada & Boulding, 1998). The two woodlice species used here employed different defensive strategies, clinging or rolling, that were effective against predators in different ways. Porcellio is a clinger, but due to the round sides of the tubes it may not have been successful with this defensive strategy and was frequently captured by P. phalangioides. Armadillidium, the roller, was rarely captured. The success of this type of defence did not depend on the environment (e.g. shape of tubes). We expected that oligophagous P. phalagioides would be better at capturing
woodlice than euryphagous T. domestica, because the former species has been reported to feed on woodlice. Indeed, detailed observation of the woodlice handling in this study revealed that P. phalangioides captured woodlice with higher frequency than T. domestica. Pholcus phalagioides captured Porcelio usually without difficulty, even though the woodlice sometimes clung to the tube lid. Eventually, the spider was able to detach it from the substrate by repeated attacks. It was far more difficult for P. phalagioides to catch Armadillidium, which often managed to roll completely before P. phalangioides spiders were able to apply the silk strands that could prevent rolling. In the case of T. domestica, the situation was very different. Individuals of T. domestica were observed to capture Porcelio only rarely, when the prey had turned on its back. Tegenaria domestica was observed to attack Armadillidium only once; the spiders usually lost interest after the first contact with the prey. Once captured, P. phalagioides sucked Porcelio usually from the legs or uropods, while Armadillidium was sucked from outside by puncturing the pleura between sclerites. Pholcus phalangioides accepted Armadillidium at the first feeding with a high frequency; however, over the course of experiment, notably after the fourth week, Armadillidium were rejected. In T. domestica the feeding on Porcellio took only a couple of minutes. This indicates that they might have developed an aversion, as was observed in another generalist predator of the genus Schizocosa (Toft & Wise, 1999b). In that spider, the consumption of toxic prey reduced growth by inhibiting the feeding rate, but the apparent aversion disappeared rather quickly, i.e. within a day. For both generalist spiders, the prey mixture (without woodlice) was of superior quality when compared with woodlice diets. This is in agreement with previous results regarding other euryphagous predators (see, e.g., Toft, 1995, 1999; Toft & Wise, 1999a; Bilde & Toft, 2000). For the specialist, it was the other way around. Specialised D. crocata performed best on a monotypic diet composed of Porcellio. This is in agreement with the physiological efficiency hypothesis. The hypothesis argues that the physiological efficiency of food utilization determines the feeding habits of herbivorous insects. Yet, it has gained little support in herbivores (Scriber, 2007) and has also been unsuccessful in explaining the food-mixing habits of individually polyphagous herbivores (Singer, 2008). Unlike for herbivores, the obtained results for woodlice-eaters do not contradict its predictions. The existence of physiological constraints on food utilization as an explanation for dietary specialisation seems to be valid for specialised predators. The hypothesis further assumes that there is a metabolic trade-off in the ability to utilise alternative prey (Singer, 2008). Similarly, other specialised predators, namely myrmecophagous spiders of the genus Zodarion and Euryopis (Pekár & Toft, 2009; Pekár & Cárdenas, 2015), or araneophagous spiders of the genus Portia (Li & Jackson, 1997), had the highest fitness on a monotypic diet made up of the focal prey and low fitness on an alternative or mixed diet treatment. Dietary mixing that included alternative prey was not beneficial to specialised predators. For D. crocata there was a difference between Porcellio and Armadillidium. While the former was of superior quality, the latter diet was of similar suboptimal quality to the mixture of
© 2015 The Royal Entomological Society, Ecological Entomology, doi: 10.1111/een.12285
ˇ c 6 Stano Pekár, Eva Líznarová and Milan Rezáˇ
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Fig. 3. Comparison of mass changes in spiders of Dysdera crocata (a), Pholcus phalangioides (b), and Tegenaria domestica(c) on three diet treatments over the experimental period. Points are means, and whiskers are SE.
arthropods. This shows that woodlice specialists are adapted to overcome both types of behavioural defences in woodlice (rolling and clinging), but are not adapted to deal with the nutritional quality of all woodlice species. The particular woodlice species thus matters to woodlice eaters. The level of prey specificity also varies among other trophic specialists (Pekár & Toft, 2015). Araneophagous species seem to be adapted behaviourally to different spiders (e.g. Pekár et al., 2011). Some myrmecophagous species are adapted to exploit several ant species, while others are adapted to only a few genera (e.g. Cárdenas et al., 2012; Pekár & Cárdenas, 2015; Pekár & Toft, 2015). Similarly, termitophagous species can be specialised only on one type of termite (Dippenaar-Schoeman et al., 1996; Petráková et al., 2015). The three spider species used in this study were selected based on the degree of trophic specialisation. As concerns their phylogenetic relationship, two of them (D. crocata and
P. phalangioides) belong to the basal group (Haplogynae), whereas one (T. domestica) belongs to a higher group (Entelegynae). So the differences observed may be attributed not only to the different degree of trophic specialisation but also to unequal phylogenetic distances. Indeed, higher similarity in performance between D. crocata and P. phalangioides than D. crocata and T. domestica seems to be due to their closer phylogenetic relationship – the presence of traits that are used to handle woodlice. Ideally, for such comparative study, representatives with different degrees of specialisation should be similarly related, e.g. from a single family. This is, however, impossible, as either closely related generalists to Dysdera (e.g. Harpactea) do not ˇ c, unpublished) or there are no speaccept woodlice (M. Rezᡠcialists in Pholcidae and Agelenidae. In our previous experiments involving woodlice, a woodlouseeating spider Dysdera hungarica Kulczyñski was reared on one of three diets: pure woodlice (composed of two species Oniscus
© 2015 The Royal Entomological Society, Ecological Entomology, doi: 10.1111/een.12285
Suitability of woodlice prey 7 asellus Linnaeus and Armadillidium); flies (D. melanogaster); or a mixed woodlouse-fly diet. The spiders developed significantly faster on the woodlice-containing diets (i.e. the pure woodlice and mixed diets) than on the fly diet. In the same study, prey choice with respect to the two woodlice species was investigated. Dysdera hungarica spiders captured flies significantly more often than woodlice. We concluded that D. hungarica is particularly metabolically adapted to deal with ˇ c & Pekár, 2007), which is consistent with woodlice (Rezᡠresults obtained in this study. The results of this study show that two woodlice species, Porcellio and Armadillidium, are unsuitable prey for both oligophagous and euryphagous generalist predators. Armadillidium was captured at markedly lower frequency than Porcellio by both generalists as it used a more effective defensive strategy. The specialist predator, D. crocata, was able to catch and handle woodlice successfully. For the specialist, Armadillidium was of lower nutritional quality than Porcellio. Acknowledgement We would like to thank T. Budiš for performing the experiments as a part of his master’s thesis. Two anonymous reviewers are thanked for useful comments. The study was supported ˇ was by the Czech Science Foundation (GA15-14762S). M.R. supported by the grant MZE 0002700604 from the Ministry of Agriculture of the Czech Republic. References Begon, M., Mortimer, M. & Thompson, D.J. (1996) Population Ecology A Unified Study of Animals and Plants. Blackwell Science, Oxford, U.K. Bilde, T. & Toft, S. (2000) Evaluation of prey for the spider Dicymbium brevisetosum Locket (Araneae: Linyphiidae) in single-species and mixed-species diets. Ekológia, 19 (Suppl. 3), 9–18. Budiš, T., 2008. Adaptations of Dysdera spiders (Araneae: Dysderidae) for the capture of woodlice. Bachelor thesis, Masaryk University, Brno, Czech Republic (in Czech). Bureš, S. & Weidinger, K. (2003) Sources and timing of calcium intake during reproduction in flycatchers. Oecologia, 137, 634–647. Cárdenas, M., Jiroš, P. & Pekár, S. (2012) Selective olfactory attention of a specialised predator to intraspecific chemical signals of its prey. Naturwissenschaften, 99, 597–605. Cooke, J.A.L. (1965a) A contribution to the biology of the British spiders belonging to the genus Dysdera. Oikos, 16, 20–25. Cooke, J.A.L. (1965b) Spider genus Dysdera (Araneae, Dysderidae). Nature, 205, 1027–1028. Cooke, J.A.L. (1967) Factors affecting the distribution of some spiders of the genus Dysdera (Araneae, Dysderidae). Entomologist’s Monthly Magazine, 103, 221–225. Dejean, A. (1997) Distribution of colonies and prey specialization in the ponerine ant genus Leptogenys (Hymenoptera: Formicidae). Sociobiology, 29, 293–299. Dejean, A. & Evraerts, C. (1997) Predatory behavior in the genus Leptogenys: a comparative study. Journal of Insect Behavior, 10, 177–191. Deslippe, R.J., Jelinski, L. & Eisner, T. (1996) Defense by use of a proteinaceous glue: woodlice vs. ants. Zoology, 99, 205–210. Dippenaar-Schoeman, A.S., De Jager, M. & Van den Berg, A. (1996) Ammoxenus species (Araneae: Ammoxenidae)-specialist predators
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ˇ c 8 Stano Pekár, Eva Líznarová and Milan Rezᡠpredator, a specialist termite-eating spider (Araneae: Ammoxenidae). Scientific Reports, 5, 14013. Pinheiro, J., Bates, D., DebRoy, S. & Sarkar, D., R Development Core Team (2013) nlme: linear and nonlinear mixed effects models. R package, version 3.1-109. Pollard, S.D., Jackson, R.R., Van Olphen, A. & Robertson, M.V. (1995) Does Dysdera crocata (Araneae: Dysderidae) prefer woodlice as prey? Ethology Ecology and Evolution, 7, 271–275. Raupach, M.J. (2005) Die Bedeutung von Landasseln als Beutetiere für insekten und andere Arthropoden. Entomologie Heute, 17, 3–12. R Core Team (2013) R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, U.K. URL http://www.R-project.org/. ˇ c, M. & Pekár, S. (2007) Evidence for woodlice-specialization Rezᡠin Dysdera spiders: behavioral versus developmental approaches. Physiological Entomology, 32, 367–371. ˇ c, M., Pekár, S. & Lubin, Y. (2008) How oniscophagous spiders Rezᡠovercome woodlouse armour. Journal of Zoology, 275, 64–71. Roberts, M.J. (1995) Collins Field Guide to Spiders of Britain and Northern Europe. Harper Collins Publishers, London, U.K. Schmalfuss, H. (1984) Eco-morphological strategies in terrestrial isopods. Symposium of the Zoological Society of London, 53, 49–63. Scriber, J.M. (2007) A mini-review of the feeding specialization/physiological efficiency hypothesis: 50 years of difficulties, and strong support from the North American Lauraceae-specialist, Papilio troilus (Papilionidae: Lepidoptera). Trends in Entomology, 4, 1–42. Shilova-Krassova, S.A. (1952) On the feeding habits of hedgehog (Erinaceus europaeus L.) in the southern forests. Zoologichesky Zhurnal, 31, 944–950. Singer, M.S. (2008) Evolutionary ecology of polyphagy. Specialization, Speciation, and Radiation. The Evolutionary Biology of Herbivorous
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7.2 Podíl studenta na jednotlivých rukopisech Rukopis A Líznarová, E., Sentenská, L., García, L. F., Pekár, S. & Viera, C. (2013). Local trophic specialisation in a cosmopolitan spider (Araneae). Zoology, 116, 20–26. Nasbírala jsem data a zvířata v terénu a provedla jsem experimenty v laboratoři. Analyzovala jsem část výsledků a spolupracovala se všemi spoluautory při psaní rukopisu. Můj podíl na tomto rukopisu byl 40 %. Rukopis B Líznarová, E. & Pekár, S. (2013). Dangerous prey is associated with a type 4 functional response in spiders. Animal Behaviour, 85, 1183–1190. Nasbírala jsem zvířata v terénu, provedla experimenty v laboratoři a analyzovala výsledky. Rukopis jsem napsala s připomínkami od spoluautora. Můj podíl na tomto rukopisu byl 70 %. Rukopis C Líznarová, E. & Pekár, S. (2015). Trophic niche of Oecobius maculatus (Araneae: Oecobiidae): evidence based on natural diet, prey capture success , and prey handling. The Journal of Arachnology, 43, 188–193. Nasbírala jsem data a zvířata v terénu, provedla experimenty v laboratoři a analyzovala výsledky. Rukopis jsem napsala s připomínkami od spoluautora. Můj podíl na tomto rukopisu byl 80 %. Rukopis D Líznarová, E. & Pekár, S. (2016). Metabolic specialisation on preferred prey and constraints in the utilisation of alternative prey in an ant-eating spider. Zoology (online). Podílela jsem se na návrhu studie, chovala jsem zvířata v laboratoři a analyzovala jsem výsledky. Rukopis jsem napsala s připomínkami od spoluautora. Můj podíl na tomto rukopisu byl 80 %.
Rukopis E Pekár, S., Šedo, O., Líznarová, E., Korenko, S. & Zdráhal, Z. (2014). David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey. Naturwissenschaften, 101, 533–540. Podílela jsem se na sběru dat a zvířat v terénu, provedla jsem behaviorální experimenty v laboratoři, analyzovala jsem část výsledků a podílela se na psaní rukopisu. Můj podíl na tomto rukopisu byl 20 %. Rukopis F Petráková, L., Líznarová, E., Pekár, S., Haddad, C. R., Sentenská, L. & Symondson, W. O. (2015). Discovery of a monophagous true predator, a specialist termite-eating spider (Araneae: Ammoxenidae). Scientific reports, 5: 14013. Podílela jsem se na sběru dat a zvířat v terénu, provedla jsem behaviorální experimenty a analyzovala jejich výsledky a podílela se na psaní rukopisu. Můj podíl na tomto rukopisu byl 20 %. Rukopis G Pekár, S., Líznarová, E. & Řezáč, M. (2016). Suitability of woodlice prey for generalist and specialist spider predators: a comparative study. Ecological Entomology, 41, 123–130. Podílela jsem se na sběru zvířat v terénu, provedla jsem část behaviorálních experimentů a zanalyzovala získané výsledky, podílela jsem se na psaní rukopisu. Můj podíl na tomto rukopisu byl 30 %.
7.3 Seznam publikací Nedvěd, O., Pekár, S., Bezděčka, P., Líznarová, E., Řezáč, M., Schmitt, M., & Sentenská, L. (2011). Ecology of Arachnida alien to Europe. BioControl, 56(4), 539550. Pekár, S., Šmerda, J., Hrušková, M., Šedo, O., Muster, C., Cardoso, Zdráhal Z., Korenko S., Bureš P., Líznarová E. & Sentenská L (2012). Prey‐race drives differentiation of biotypes in ant‐ eating spiders. Journal of Animal Ecology, 81(4), 838–848. Líznarová, E., & Pekár, S. (2013). Dangerous prey is associated with a type 4 functional response in spiders. Animal Behaviour, 85(6), 1183–1190. Líznarová, E., Sentenská, L., García, L. F., Pekár, S., & Viera, C. (2013). Local trophic specialisation in a cosmopolitan spider (Araneae). Zoology, 116(1), 20–26. Pekár, S., Michalko, R., Korenko, S., Šedo, O., Líznarová, E., Sentenská, L., & Zdráhal, Z. (2013). Phenotypic integration in a series of trophic traits: tracing the evolution of myrmecophagy in spiders (Araneae). Zoology, 116(1), 27–35. Pekár, S., Šedo, O., Líznarová, E., Korenko, S., & Zdráhal, Z. (2014). David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey. Naturwissenschaften, 101(7), 533–540. Leccia, F., Kysilková, K., Kolářová, M., Hamouzová, K., Líznarová, E., & Korenko, S. (2015). Disruption of the chemical communication of the European agrobiont ground‐dwelling spider Pardosa agrestis by pesticides. Journal of Applied Entomology (online). Líznarová, E., & Pekár, S. (2015). Trophic niche of Oecobius maculatus (Araneae: Oecobiidae): evidence based on natural diet, prey capture success, and prey handling. The Journal of Arachnology, 43(2), 188–193. Novakova, M., Bulkova, A., Costa, F. B., Kristin, A., Krist, M., Krause, F., Labruna, M. B., Líznarová, E. & Literak, I. (2015). Molecular characterization of ‘Candidatus Rickettsia vini’in Ixodes arboricola from the Czech Republic and Slovakia. Ticks and tick-borne diseases, 6(3), 330–333.
Pekár, S., Michalko, R., Loverre, P., Líznarová, E., & Černecká, Ľ. (2015). Biological control in winter: novel evidence for the importance of generalist predators. Journal of Applied Ecology, 52(1), 270–279. Petráková, L., Líznarová, E., Pekár, S., Haddad, C., Sentenská, L., & Symondson, W. O. C. (2015). Discovery of a monophagous true predator, a termite-eating spider specialist (Araneae: Ammoxenidae). Scientific reports, 5: 14013. Pekár, S., Líznarová, E., & Řezáč, M. (2015). Suitability of woodlice prey for generalist and specialist spider predators: a comparative study. Ecological Entomology, 41, 123–130. Líznarová, E. & Pekár, S. (2016). Metabolic specialisation on preferred prey and constraints in the utilisation of alternative prey in an ant-eating spider. Zoology (online). Publikace odeslané na recenzi Líznarová, E., Sentenská, L., Šťáhlavský, F. & Pekár, S. Stridulation prevents cannibalism in araneophagous spiders (Science of Nature). Publikace v českém populárně-naučném časopise Sentenská L. & Líznarová E. (2010): Nový řád pavoukovců pro faunu České republiky. Živa, 3, 126–127.
7.4 Příspěvek na mezinárodních konferencích Líznarová E., Sentenská L., Šťáhlavský F., Pekár S.: Stridulation in araneophagic spiders prevents cannibalism, Congress of the International Society for Behavioral Ecology 2016, Exeter (poster). Líznarová E. & Pekár S.: Is prey-capture efficiency innate or gained by experience in a specialised spider? European Congress of Arachnology 2015, Czech Republic (přednáška). Líznarová E., Lubin Y. & Pekár S.: The effect of individual relatedness on colony performance in group-living Cyrtophora citricola, The Association for the Study of Animal Behaviour Winter Meeting, Londýn 2014 (přednáška). Líznarová E., Lubin Y. & Pekár S.: The effect of individual relatedness on colony performance in group-living Cyrtophora citricola, European Congress of Arachnology 2014, Italy (přednáška). Líznarová E. & Pekár S.: Physiological efficiency and trade-offs in adaptations of the ant-eating Euryopis episinoides (Theridiidae), International Congress of Arachnology Taiwan 2013 (přednáška). Líznarová E., Sentenská L., Pekár S., Šťáhlavský F.: Stridulation in araneophagic spiders prevents cannibalism, International Congress of Arachnology Taiwan 2013 (poster). Líznarová E., Sentenská L., Pekár S., Šťáhlavský F.: Stridulation in araneophagic spiders prevents cannibalism, The Association for the Study of Animal Behaviour Winter Meeting, Londýn 2012 (poster). Líznarová E., Pekár S.: Functional response type 4 of myrmecophagous spiders, European congress of arachnology, Slovinia 2012 (přednáška). Líznarová E., Sentenská L., Pekár S.: Predatory versatility enhances local trophic specialisation in a cosmopolitan carnivorous predator, European congress of arachnology, Izrael 2011 (přednáška).
7.5 Příspěvek na domácích konferencích Líznarová E. & Pekár S.: Je efektivita lovu kořisti u specializovaných pavouků vrozená nebo získaná zkušenostmi? Zoologické dny 2016, České Budějovice (přednáška). Líznarová E. & Pekár S.: Fyziologická efektivita zpracování kořisti u myrmekofágního pavouka druhu Euryopis episinoides (Theridiidae), Zoologické dny 2014 Ostrava (poster). Líznarová E. & Pekár S.: Fyziologická efektivita zpracování kořisti u myrmekofágního pavouka druhu Euryopis episinoides (Theridiidae), Ekologie 2013 Brno, (poster). Líznarová E., Sentenská L., Pekár S., Šťáhlavský F.: Stridulation in araneophagic spiders prevents cannibalism, Zoologické dny 2013, Brno (poster). Líznarová E., Sentenská L., Pekár S.: Lokální specializace a kondiční strategie v lovu kořisti u pavouka Oecobius navus, Zoologické dny 2011 (přednáška). Líznarová E., Pekár S.: Funkční odpověď typu 4 u pavouků, Konference České společnosti pro ekologii 2011 (poster). Líznarová E., Sentenská L., Pekár S.: Lokální specializace a kondiční strategie v lovu kořisti u pavouka Oecobius navus, Etologická konference Smolenice 2010 (přednáška). Líznarová E., Pekár S.: Funkční odpověď u myrmekofágních pavouků, Zoologické dny 2010 (přednáška).
