Foraging behavior of Carcinus maenas (L.): Species-selective predation among four bivalve prey

Species-selective predation by medium (40-50 mm carapace width) and large (55-70 mm) Carcinus maenas was investigated by presenting individual crabs with paired combinations of mussels Mytilus edulis, flat oysters Ostrea edulis, Pacific oysters Crassostrea gigas, and cockles Cerastoderma edule in various proportions. When offered mussels and either oyster species simultaneously, both size categories of crabs consistently selected mussels, and food choice was independent of prey relative abundance. By contrast, C. maenas selected mussels and cockles as expected by the frequency in which each size category of crab encountered the preferred size ranges of prey. Crab preference clearly paralleled the rank order of prey profitability, which, in turn, was mainly determined by prey biomass, suggesting that active selection takes place at some point of the predation cycle. Decisions by crabs on whether to attack oysters and mussels initially were not influenced by the flavor or odor of their flesh. Both mussel and oyster filtrates had a similar reinforcing effect on crab perseverance to open prey. However, species-related preferences exhibited by crabs feeding at or near the optimal size suggest that foraging decisions are partly based on evaluations of over-all prey shape and volume, and that the minimum dimension of the shell constitutes an important feature that crabs recognize and associate with prey value