Plasticity in an extended phenotype and reversed up-down asymmetry of spider orb webs

Abstract

Orb webs built by spiders are an extended phenotype, and spiders can modify their morphology when rebuilding them. Internal and external environmental conditions can affect how spiders rebuild. Most spiders that build vertical orb webs wait for prey at the web hub and orient downwards. Moreover, their webs exhibit up-down size asymmetry; specifically, the area below the hub is larger than the area above it. However, some spiders reverse this asymmetry in their webs. To examine the relationship between phenotypic plasticity and web asymmetry reversal and whether spiders elongate their webs upwards in response to prey capture in the upper web, I manipulated prey capture location in two closely related spider species. Cyclosa octotuberculata build webs with typical asymmetry (larger area below the web hub). Cyclosa argenteoalba build webs with reversed asymmetry, a derived phenotype, and wait in a reversed orientation, facing upwards. I found that, when spiders fed in the lower part of the web, both species elongated their webs downwards and that C. argenteoalba webs lost their asymmetry and became symmetrical. In contrast, webs were not elongated in the upward direction when spiders of either species were fed in the upper part of the web. These results provided evidence that the up-down asymmetry of Cyclosa webs is a plastic trait regardless of whether spiders build webs with typical or reversed asymmetry. However, no significant upward elongation of webs indicates that there is no evidence of a relationship between plasticity in the extended phenotype and web asymmetry reversal.