SELECTIVE PREDATION BY BLUE CRABS ON THE GASTROPOD, BITTIUM-VARIUM - CONFIRMATION FROM OPERCULA FOUND IN THE SEDIMENTS

Small blue crabs (Callinectes sapidus Rathbun, 43-70 mm carapace width ) can influence the size-distribution of the gastropod, Bittium varium , strongly reducing the contribution of snails >3 mm shell length in f ield enclosures, We test the hypothesis that these size-dependent effe cts are due to size-selective predation rather than size-dependent emi gration from the field enclosures. In laboratory feeding trials, blue crabs showed negative selectivity for snails <2.5 mm and positive sele ctivity for snails >3.3 mm. When feeding, blue crabs crush Bittium she lls, but the opercula are deposited undamaged in the sediment, Sedimen t from a field enclosure experiment contained 6.5x more Bittium opercu la from enclosures with blue crabs than from enclosures without blue c rabs. We reconstructed the size distribution of Bittium killed by blue crabs from the opercula recovered from sediment. This distribution qu alitatively matched those ''missing'' from the size frequency distribu tion of surviving snails. Estimates of selectivity from laboratory fee ding trials predicted the pattern of size-selection from Bittium kille d in the enclosures. We also estimated Strauss's linear index of selec tivity and Chesson's alpha based on the size distribution of snails av ailable during the field enclosure experiment. These indices predicted both the pattern of selectivity and the size distribution of Bittium killed in enclosures with blue crabs. We conclude that size-selective predation by blue crabs can explain the observed shifts in Bittium siz e distributions.