Fecundity increase supports adaptive radiation hypothesis in spider web evolution

Abstract

Identifying the mechanisms driving adaptive radiations is key to explaining the diversity of life. The extreme reliance of spiders upon silk for survival provides an exceptional system in which to link patterns of diversification to adaptive changes in silk use. Most of the world’s 41,000 species of spiders belong to two apical lineages of spiders that exhibit quite different silk ecologies, distinct from their ancestors. Orb spiders spin highly stereotyped webs that are suspended in air and utilize a chemical glue to make them adhesive. RTA clade spiders mostly abandoned silk capture webs altogether. We recently proposed that these two clades present very different evolutionary routes of achieving the same key innovation – escape from the constraints imposed by spinning webs that contain a relatively costly type of physically adhesive cribellate silk. Here, we test the prediction that orb and RTA clade spiders are not only more diverse, but also have higher fecundity than other spiders. We show that RTA clade spiders average 23% higher fecundity and orb spiders average 123% higher fecundity than their ancestors. This supports a functional link between the adaptive escape from cribellate silk and increased resource allocation to reproduction in spiders.

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Acknowledgements

This research was supported by National Science Foundation awards #DEB-0516038 and IOS-0745379 to T.A.B. and EAR-0228699 to J.A.C., as well as by Slovenian Research Agency fellowship ARRS Z1-9799-0618-07 to I.A.

Figures and Tables

Figure 1 Spiders use silk in three broadly different strategies for prey capture. (A) Many spiders spin a variety of sheet webs with relatively amorphous architectures. These webs lack stereotyped major ampullate supporting threads and utilize cribellate adhesive threads (shown here in darkfield at the bottom of the panel). Cribellate silk consists of one or more pairs of core fibers surrounded by a sheath of nanoscale fibrils physically combed into puffs by the spider. (B) Nearly all orb weaving spiders and their relatives use stereotyped web spinning behaviors and defined frameworks of dragline silk to suspend webs relatively far from substrate. Thus, the form of the web is taxonomically rather than substrate specific. Most also utilize aggregate capture silk (see text). (C) RTA clade spiders tend to stalk or ambush prey, having abandoned the use of capture silks altogether.

Figure 2 Log-log plot of clutch size versus body size in spiders.

Figure 3 Relationship between fecundity and silk use by spiders. Residuals for egg production were calculated from the regression of individual clutch size on spider body length in figure 2. The graphs show histograms of residual egg production for species within each of two highly successful, apical clades of spiders compared to all other taxa of spiders. The phylogeny is summarized from Blackledge et al. (2009) and illustrates the relationship between species diversity (total size of pie charts) and silk use ecologies (colors) among spiders.

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