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Predator Perception of Detritus and Eggsac Decorations Spun by Orb-Web Spiders Cyclosa Octotuberculata: Do They Function to Camouflage the Spiders?

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Predator Perception of Detritus and Eggsac Decorations Spun by Orb-Web Spiders Cyclosa Octotuberculata: Do They Function to Camouflage the Spiders? Current Zoology 56 (3): 379−387, 2010 Predator perception of detritus and eggsac decorations spun by orb-web spiders Cyclosa octotuberculata: Do they function to camouflage the spiders? Wenjin GAN 1, Fengxiang LIU 1, Zengtao ZHANG 1, Daiqin LI 1, 2* 1 College of Life Sciences, Hubei University, Hubei, Wuhan 430062, China 2 Department of Biological Sciences, National University of Singapore, Singapore 117543 Abstract Camouflage is one of the most widespread and powerful strategies that animals use to make detection/recognition more difficult. Many orb-web spiders of the genus Cyclosa add prey remains, plant debris, moults, and/or eggsacs to their webs called web decorations. Web decorations resembling spider body colour pattern have been considered to camouflage the spider from predators. While this camouflage is obvious from a human’s perspective, it has rarely been investigated from a predator’s perspective. In this study, we tested the visibility of web decorations by calculating chromatic and achromatic contrasts of detritus and eggsac decorations built by Cyclosa octotuberculata, against four different backgrounds viewed by both bird (e.g., blue tits) and hymenopteran (e.g. wasps) predators. We showed that both juvenile and adult spiders on webs with detritus or egg-sac deco- rations were undetectable by both hymenopteran and bird predators over short and long distances. Our results thus suggest that decorating webs with detritus or eggsacs by C. octotuberculata may camouflage the spiders from both hymenopteran and bird predators in their common habitats [Current Zoology 56 (3): 379–387, 2010]. Key words Spider, Web decorations, Cyclosa octotuberculata, Camouflage, Chromatic contrast, Achromatic contrast Most animals are under predation pressure from largely untested from a predator’s perspectives. visually hunting predators, and they have evolved a Many orb-web spiders add a variety of materials such suite of protective strategies, including camouflage, as silk ribbons, silk tufts, prey remains, eggsacs and warning signals and mimicry (Poulton, 1890; Cott, 1940; plant debris to their webs, called ‘web decorations’ Edmunds, 1974; Ruxton et al., 2004). Camouflage is a (Herberstein et al., 2000; Starks, 2002; Bruce, 2006). classical example of evolution; Darwin and Wallace However, in spite of considerable interest, widespread used it to illustrate and defend their ideas of natural se- occurrence, and being subjected to extensive research, lection (Stevens and Merilaita, 2009). It can be achieved there has been no consensus on the function of these via physical appearance (e.g. colour patterns or protec- intriguing decorations (reviewed in Herberstein et al., tive coloration), which prevents animals from being 2000; Starks, 2002; Bruce, 2006; Théry and Casas, detected and/or recognised (Stevens, 2007; Stevens and 2009). Nevertheless, web decorations are commonly Merilaita, 2009). However, while camouflage is well considered as deceptive visual signals. However, there accepted, it has only recently received significant atten- is controversy surrounding the questions of who is the tion and empirical investigations, partly because of a receiver, prey, predator or both, and whether the re- growing body of research into mimicry and warning ceiver can detect these signals (Craig and Bernard, 1990; coloration, and also increased knowledge of visual per- Blackledge and Wenzel, 2000; Bruce et al., 2005; Chou ception (Stevens and Merilaita, 2009). More importantly, et al., 2005; Cheng and Tso, 2007; Tan and Li, 2009; there has long been a common flaw in research on Tan et al., 2010). camouflage: that it is often erroneously regarded as in- Unlike Argiope spiders that decorate their webs with tuitively obvious because human perception has often only silk (Herberstein et al., 2000; Bruce, 2006), many been used to subjectively evaluate its effectiveness. orb-web spiders of the genus Cyclosa (Araneae: Ara- Thus some forms of camouflage, e.g., detritus and egg- neidae) often add prey remains, moults, plant debris or sac decorations added by orb-web spiders, remain egg sacs to their webs and place such detritus and/or Received Nov. 17, 2009; accepted Mar. 08, 2010 ∗ Corresponding author. E-mail: [email protected] © 2010 Current Zoology 380 Current Zoology Vol. 56 No. 3 eggsacs in a vertical row of material above and below tions from spiders from a predator’s perception. the hub where the spider sits. Body size and body colo- 1 Materials and Methods ration often matches that of detritus and eggsac decora- tions (Marson, 1947; Rovner, 1976; Neet, 1990). It has 1.1 Study species and specimen collection long been speculated that detritus and eggsac decora- Cyclosa octotuberculata (Fig. 1A) is diurnal species tions are built to camouflage the spider against predators that spins vertical orb webs (Fig. 1B) at sunrise and (e.g., Marson, 1947; Marples, 1969; Eberhard, 1973, consumes spirals and most radial threads during the 2003, 2006; Rovner, 1976; Horton, 1980; Edmunds, following night (Tanikawa, 1992; Nakata and Ushimara, 1986; Neet, 1990), and a few observations suggest such 2004). In nature, both juveniles and adult females are camouflage properties. Eberhard (2003) observed the found to add prey remains, plant detritus (e.g., leaf component of detritus and eggsac decorations by Allo- fragments, flower pedals or small stems) and sometimes cyclosa bifurca and C. monteverde of different life moults to their webs (Fig. 1C, D). When reaching ma- stages, and found that the way the eggsacs were turity and after mating, females incorporate subse- wrapped by silk resembled the appearance of the spi- quently laid egg sacs into a string of pellets (Fig. 1E). der’s abdomen, and concluded that the decoration of A. Since spiders build their webs and decorations in wood- bifurca might function as a camouflaging device. Using lands and shrubs (W.J. Gan, personal observations), artificial webs and clay model spiders with detritus potential predators may view them against different decorations of two species of Cyclosa (C. morettes and backgrounds, e.g., green foliage, dry leaves, bark or a C. fililineata), Gonzaga and Vasconcellos-Neto (2005) blend of these three backgrounds. found that clay model spiders on detritus-decorated We collected C. octotuberculata spiders and their webs had fewer bite marks than those without, thus ar- decorations between 09:00 h and 17:00 h from April guing against the prey-attraction hypothesis and suggest 2007 to October 2008 at two study sites, Huazhong Ag- that decorating webs with detritus may reduce predation ricultural University Campus (HZAU) (30°28′48.75″N, by changing the outline image of the spiders by disrup- 114°21′03.46″E, 57 m a.s.l.), and Mt. Mo or Millstone tive coloration. A similar result was also reported in C. Hill (30°33′11.91″N, 114°24′38.14″E, 92 m a.s.l.) in octotuberculata by Baba (2003), who argued against the Wuhan, Hubei Province, China. Both juvenile and prey-attraction hypothesis, suggesting that decorating adult female C. octotuberculata as well as their decora- webs with detritus might actually reduce the predation tions were collected. For measuring reflectance spectra risk. A few recent studies indicate that predators cannot of vegetation backgrounds, we collected ten pieces of distinguish the colour signal of nonsilk decorations from green leaves for each of six plant species (Acer buer- that of Cyclosa spiders over a short and/or long distance gerianum, Broussonetia papyrifera, Hypericum ascyron, (Chou et al., 2005; Tan and Li, 2009; Tseng and Tso, Nerium indicum, Pyracantha fortuneana, Senecio scan- 2009; Tan et al., 2010). dens), 10 pieces of dry leaves for each of three plants All these results suggested that, at least in certain (Cinnamomum camphora, Nerium indicum, Sabina species, the nonsilk decorations may make the spiders chinensis cv. kaizuka) and 10 pieces of bark for each of more difficult to detect by predators. However, there is three species (Paulownia sp., Pinus massoniana, Sabina empirical evidence that the nonsilk decorations of Cyc- chinensis cv. kaizuka) from the same sites where spiders losa render the spiders webs more conspicuous, and and their decorations were collected. redirect the attacks of predators to the decorations in- 1.2 Spectral reflectance measurements stead of spiders (Chou et al., 2005; Tseng and Tso, We measured the spectral reflectance of both juvenile 2009). Thus, the camouflage function and underlying and adult female C. octotuberculata spiders and their mechanisms of nonsilk decorations built by Cyclosa decorations following standard protocols (Lim and Li, need further investigation. Moreover, while this camou- 2006; Tan and Li, 2009), and only the essential details flage is intuitively obvious to human observers, whether are given here. To collect the spectral reflectance data, a predator detects but ignores the spider or is simply we used an Ocean Optic USB2000 spectrometer (Ocean unable to detect it remains to be tested. Optics Inc., Dunedin, Florida, USA) with a DH-2000 In this study, using Cyclosa octotuberculata (Fig. 1A), deuterium and tungsten halogen light source (Ocean a common detritus-decorating species in East Asia in- Optics Inc., Dunedin, USA). The reflectance reading cluding China, Japan and Korea (Platnick, 2010), we (300–700 nm) was recorded from a circular spot (di- assessed whether predators can distinguish web decora- ameter 1 mm) on the sample (spider or decoration), W. GAN et al.: Function of detritus decorations 381 Fig. 1 Cyclosa octotuberculata and its web decorations A. Close-up of a female sitting at hub and facing down. B. Spider on a web without decorations. C. Spider at hub with a linear decoration of prey remains. D. Spider with a linear decoration consisting of moult, plant detritus and prey remains. E. Spider at hub with a linear eggsac (arrows) deco- ration. perpendicular to and 2 mm above the sample. Eight can only be detected by a visual system with at least two spots were randomly chosen from each spider (four on types of photoreceptors.
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