Meeting Abstract
The gastropod Crepidula onyx is common in Alamitos Bay in southern California, where it is usually found as an epibiont on the bivalve Mytilus galloprovincialis on docks, or the gastropod Conus californicus in soft sediments. The two hosts differ in maximum shell length (~10 vs. 4 cm, respectively), potentially affecting the sizes of epibionts. We hypothesized that host species would have large effects on body size and fecundity of C. onyx. To test this hypothesis, we quantified body size, sex, and fecundity of C. onyx from both hosts. Females of C. onyx on Mytilus reached larger sizes (mean 38.4 mm length and 300.4 mg dry tissue mass) than those on Conus (19.9 mm and 66.9 mg); on average, then, C. onyx on Mytilus grow to more than four times the tissue mass of those on Conus. The body size of females of C. onyx had no effect on the number of egg capsules produced, but was positively correlated with capsule size and the number of embryos per capsule. Females of C. onyx on Mytilus produced broods containing a mean of 16318 embryos, ~2.4 times the number of embryos produced by those on Conus (6811). These results are consistent with our initial hypothesis. The two hosts may also differ in quality in other ways. Snails on Mytilus can suspension feed at all times, but Conus is often found buried in sediment, potentially restricting time available for suspension feeding by epibiotic C. onyx. Further, predators of Mytilus and C. onyx are rare on docks in Alamitos Bay, but predatory gastropods are common in soft sediments, where we have observed them feeding on C. onyx on Conus. In Alamitos Bay, Mytilus thus seems to be far superior to Conus as a host for C. onyx, and settlement choices by the planktonic larvae of C. onyx have large effects on their fitness. It is not yet clear if larvae discriminate among potential hosts at settlement.
