Functional significance of shell sculpture in gastropod molluscs: test of a predator-deterrent hypothesis in Ceratostoma foliatum (Gmelin)

Abstract

A predator-deterrence function for the varices of the muricid gastropod Ceratostoma foliatum was tested. In the first set of experiments, snails of five treatment groups were presented over a 10-week period to one of three predators: sunflower stars ( Pycnopodia helianthoides Brandt), red rock crabs ( Cancer productus Randall), and kelp-greenling fish ( Hexagrammus decagrammus Pallas). The five treatment groups were: snails with all varices removed, snails with one only of the right, middle, or left varix removed, and snails with all varices intact. The kelp-greenling fish ate no snails and were discarded from subsequent experiments. Seastars consumed significantly more snails with all varices removed than any of the other treatment groups. Crabs ate significantly more snails with all varices- and right varix-removed compared with the other three groups. Thus, snails with all varices intact were less likely to be eaten by either crab or seastar predators. Scoring the shells of Ceratostoma, but leaving the varices intact had no effect on consumption rates by either seastars or crabs. In a second set of experiments, the smooth-shelled Nucella lamellosa (Gmelin) was presented as a food item along with Ceratostoma with all varices removed and Ceratostoma with all varices intact for an 8-week period. Seastars clearly preferred the smooth-shelled Nucella over the other two groups. In contrast, crabs ate Ceratostoma with all varices removed at a greater rate than control Ceratostoma and Nucella. Energy content of dry flesh of the two snail species was almost identical, suggesting that other factors were involved in governing selection of one prey species over the other. In both sets of experiments, ingestive conditioning learning was not apparent for either the seastars or the crabs. Crabs were videotaped while feeding on Ceratostoma with all, one only, or no varices removed. The videotapes revealed that a combination of chipping the shell around the aperture with the chelipeds, then snapping it in half, was the method most often used to gain access to the soft parts of the shell. However, snails with all varices removed were more often snapped in two without any preliminary chipping, presumably because the lack of varices made them easier to break apart. There was no relationship between snail treatment and the time it took to break into the shell.