Individuals of Crepidula adunca (Mollusca, Gastropoda) avoid shared doom through host specificity

Abstract

Individuals of the epibiotic gastropod Crepidula adunca predominantly utilize the gastropod Calliostoma ligatum as their hosts in the San Juan Islands, Washington, rather than exploiting other potential host species. Snails of Margarites pupillus would appear to be particularly good alternate hosts, given their similarities in microhabitat choice, shell shape, and taxonomic closeness with Calliostoma ligatum. However, M. pupillus are almost never utilized as a host by Crepidula adunca. We examined the reasons for host fidelity by Crepidula adunca in the San Juan Islands. The small but voracious sea star Leptasterias hexactis was common across all our intertidal survey sites and consumed significantly more M. pupillus than Calliostoma ligatum in laboratory experiments. Individuals of Calliostoma ligatum stimulated by tube feet from Leptasterias hexactis moved significantly more quickly than non-stimulated individuals, but snails of this species stimulated by Pycnopodia helianthoides did not move at a significantly different rate than non-stimulated snails. There was a significant positive correlation between gastropod size and the rate of movement for Calliostoma ligatum stimulated by Leptasterias hexactis, but this correlation was not significant for non-stimulated individuals. Neither stimulation by asteroids nor gastropod size (whether for stimulated or non-stimulated snails) had a significant effect on the rate of movement for M. pupillus. Thus Calliostoma ligatum, and especially large individuals of this species, react more than M. pupillus to the presence of a common predator ( Leptasterias hexactis) in such a way that likely elevates the snails' chance of escape. Finally, we found that Calliostoma ligatum used both chemical and physical defenses to effectively repel attacks by Leptasterias hexactis, while defenses by M. pupillus were less successful. We suggest that epibiotic Crepidula adunca specialize on Calliostoma ligatum in the San Juan Islands because this particular host species is more likely to protect the epibiont from shared doom due to its effective defenses against potential predators.