EPIFAUNAL BIVALVE SHELL MORPHOLOGY AND ITS RELATION TO EPIBIONTS AND PREDATOR PROTECTION

Spines are widely considered to be antipredatory structures, particularly for marine epifaunal organisms. However, spines may protect epifaunal species by supplying a substrate for encrusting organisms (epibionts), which would potentially assist a host by providing camouflage from predators. This may be the case for some bivalves, since their spines can be delicate, widely spaced and blunt (i.e. not ideal protective structures).

To determine empirically whether spines and/or shell ornaments can increase the species diversity or percent coverage of epibionts, an experiment was conducted at the Keys Marine Laboratory, Florida Keys over a period of one year. Forty plates, with four shell surfaces, were anchored and deployed in shallow (3 m) water in June 1999. The surfaces tested were: 1) spinose (Spondylus regis), 2) ribbed (S. regis, with spines removed and Pectin sp.), 3) rough (Crassostrea sp.), and 4) smooth (Mytilus edulis). Ten plates were retrieved after 2, 5, 26 and 51 weeks. Average epibiont species richness and coverage were then compared statistically among the surface types using the Kruskal-Wallis test.

After even the earliest sampling, epibiont species richness varied significantly among surfaces (p < 0.0005). Spinose shells consistently bore the greatest quantity, followed by S. regis, with spines removed (p £ 0.05). Species richness on other surface types was not significantly different on any sampling occasion.

The average abundance of epibiont species (measured as the percent of shell area encrusted) was also not uniform. After 2 and 5 weeks there were highly significant differences among surfaces (p < 0.0005 and p=0.001, respectively), with slightly less distinction after 26 and 51 weeks (p=0.025 and p=0.10, respectively). As with species richness, spinose shells had the greatest epibiont abundance at each sampling interval. In all but one case, there were no significant differences among non-spinose shells.

The experimental results indicate that spines may indeed provide epibiont-laden camouflage for some epifaunal bivalves. It does not appear that shell ornamentation other than spines (e.g. coarse ribbing) significantly increases encrustation.

This analysis is part of a broader, multifaceted investigation into the evolution of spine morphology and function in epifaunal bivalves.