Criteria for prey size-selection in molluscivorous crabs with contrasting claw morphologies

This paper briefly reviews our current understanding of prey size-selection in several families of tropical and temperate water molluscivorous crabs. Differences in the morphological and mechanical features of crab claws reflect their function and account for many of the observed differences in prey handling techniques and foraging behaviour. Claws, however, have functions other than feeding and these can confound simple correlations between form, strength and feeding habits. Species feeding predominantly on rapidly moving prey typically have fast, weaker acting claws whereas those specialising on heavily-armoured molluscan prey have claws that are slower but more powerful. Complex shell opening behaviour or specialised claw morphology can sometimes compensate for limited crushing power. Claw characteristics and foraging behaviour also vary geographically; thus, compared with their temperate water counterparts, tropical crabs generally are considered to be more specialised for attacking hard-shelled molluscan prey, possibly as a result of prolonged and intense coevolution between these predators and their prey. Most crabs forage selectively on small-sized molluscan prey well below the critical size that can be opened. The reasons for such size-selection are not entirely clear but energy maximisation, rime minimisation, or the risk of claw damage associated with attacks on larger more resistant prey have been suggested as causal factors. The structural and ecological features evolved by molluscan prey that effectively reduce the risk of predation and thus facilitate the continued coexistence of predator and prey are also briefly considered.