Foraging behavior of Carcinus maenas (L.): Comparisons of size-selective predation on four species of bivalve prey

Abstract

Experiments were designed to investigate size-selective predation by medium (40-55 mm carapace width) and large (55-70 mm) Carcinus maenas when feeding on four bivalves of contrasting shell morphologies, mussel, Mytilus edulis, flat oyster Ostrea edulis, Pacific oyster Crassostrea gigas, and edible cockle Cerastoderma edule. Medium-sized crabs preferred mussels 5-15 mm shell length (maximum shell dimension) and cockles 5-10 mm long, whereas large crabs preferred mussels 15-25 mm and cockles 10-20 mm long. Generally, no preference was shown for any particular size of either species of oyster. Comparisons amongst the preferred size ranges of prey showed that crab preference for a particular size range of prey was more strongly related to the minimum than to the maximum shell dimension, and that the minimum shell dimension was always equivalent to, or smaller than: the maximum cross section of the crabs' chelae. The size ranges of M. edulis and C. edule selected by C. maenas either clearly corresponded to, or were slightly smaller than, the size ranges of prey with the highest profitability (= dry weight consumed per unit of handling time). Profitability values of M. edulis and C. edule, however, showed considerable scatter

whereas those of O. edulis and C. gigas were even more variable. This variation seems to be the result of behavioral strategies by which crabs attack all encountered prey but reject those that remain unbroken after a certain number of opening attempts. Our results emphasize the mechanistic nature of size-selective feeding in C. maenas, and suggest that the differences in the observed patterns of size-selection were mainly determined by the contrasting morphological features of the bivalve shells, and the way these features influence the vulnerability of prey to crab predation.