Selective Predation by Blue Crabs on the Gastropod, Bittium varium: Confirmation From Opercula Found in the Sediments
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Citation:Wright, Russell A.; Crowder, Larry B.; Martin, Thomas H. "Selective Predation by Blue Crabs on the Gastropod, Bittium varium: Confirmation From Opercula Found in the Sediments," Estuaries, v. 19, no. 1, 1996, pp. 75-81.
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 field enclosures. We test the hypothesis that these size-dependent effects are due to size-selective predation rather than size-dependent emigration from the field enclosures. In laboratory feeding trials, blue crabs showed negative selectivity for snails <2.5 mm and positive selectivity for snails >3.3 mm. When feeding, blue crabs crush Bittium shells, but the opercula are deposited undamaged in the sediment. Sediment from a field enclosure experiment contained 6.5 X more Bittium opercula from enclosures with blue crabs than from enclosures without blue crabs. We reconstructed the size distribution of Bittium killed by blue crabs from the opercula recovered from sediment. This distribution qualitatively matched those "missing" from the size frequency distribution of surviving snails. Estimates of selectivity from laboratory feeding trials predicted the pattern of size-selection from Bittium killed in the enclosures. We also estimated Strauss's linear index of selectivity and Chesson's α based on the size distribution of snails available 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 size distributions.