SOWERBY I, 1825 (Gastropoda, Calyptraeidae) are suspension-feeding snails that engage in long-term epibiotic associations. Therefore, it is often to the epibiont's advantage, especially for one with limited movement, to reside on a host species with a low predation rate and to avoid damaging the host in a manner that would dramatically increase that rate. Even if the epibiont survives direct, immediate mortality caused by the host's predator, the attached organism may eventually die if it cannot readily move to a new suitable host. “One of the greatest hazards in an epibiont's life is the danger of falling victim to predators of the substrate organism” (p. Residing on the wrong host may leave an associate at risk of death by shared doom (Wahl and Hay, 1995), wherein both the epibiont and host are killed by the host's predator. 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. Finally, we found that Calliostoma ligatum used both chemical and physical defenses to effectively repel attacks by Leptasterias hexactis, while defenses by M. pupillus to the presence of a common predator ( Leptasterias hexactis) in such a way that likely elevates the snails' chance of escape. Thus Calliostoma ligatum, and especially large individuals of this species, react more than M. 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. 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. 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. pupillus than Calliostoma ligatum in laboratory experiments. The small but voracious sea star Leptasterias hexactis was common across all our intertidal survey sites and consumed significantly more M. We examined the reasons for host fidelity by Crepidula adunca in the San Juan Islands. pupillus are almost never utilized as a host by Crepidula adunca. 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. 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.
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