This thesis examines the foraging behaviour of Carcinus maenas and Cancerpagurus when presented with bivalves of contrasting morphology: Mytilus edulis, Ostrea edulis, Crassostrea gigas and Cerastoderma edule. Because foraging may be influenced by the relative abundance and morphological characteristics of both predator and prey, these aspects are also considered. Chelal size and strength of these crabs and prey shell shape largely determined handling techniques. When offered a size range of these bivalves individually, crabs attacked all encountered prey but rejected those that remained unbroken after several opening attempts, thus, emphasising the passive nature of their size-selective feeding. When offered paired combinations of mussels, oysters and cockles, larger crabs selected species in the ranked order of their profitability. Species-related preferences exhibited by crabs feeding on prey at or near the optimal size suggest that foraging decisions are partly based on evaluations of overall prey shape and volume, and that shell width constitutes an important feature which crabs recognise and associate with prey value. Variations in crab strength relative to size accounted for most intra-specific differences in foraging behaviour. Juvenile C maenas are limited in their choice of prey size, and are thus less species-selective. Adult C maenas are not so constrained, and exhibit a higher degree of species-selectivity. C. pagurus possesses powerful monomorphic chelae that operate at higher mechanical advantage than the cbelae of C maenas, and readily crushed larger mussels relative to their size. Differences in prey size selection between crab species varied with the species of prey offered, suggesting that certain shell features of these bivalves constitute effective barriers to even the powerful chelae of C pagurus. These results are relevant in the context of aquaculture, since predatory impact on commercially reared bivalves might be reduced by combining different prey species that offer predators alternative or preferred sources of food.