Why do shore crabs not prefer the most profitable mussels?

We examined the link between handling time (T), adopted feeding techniques, profitability curves and prey size selection to further understand the constraints that influence bivalve prey selection in shore crabs (Carcinus maenas L.). Blue mussels (Mytilus edulis L.) of different sizes were used as prey. Prey size was generalized to the ratio between mussel width and major chela length (MW : MCL) and prey profitability standardized to the ratio between prey mass eaten per unit breaking time and predator mass (1/h T). Crabs either crushed the mussel with their claws or opened the mussel using the more time-consuming cutter technique. The latter technique was employed above a critical MW : MCL ratio (0·24). This threshold appeared to coincide with the ratio where prey profitability is maximal. When a range of prey sizes were offered simultaneously, an observed MW : MCL ratio (0·14-0·16) turned out to be lower than the predicted optimal MW : MCL ratio (0·16-0·22) and lower than the MW : MCL ratio where the first crab switched feeding techniques (0·16). Crabs preferred a MW : MCL ratio of 0·16 over the optimal ratio of 0·24 when given dichotomous choices between different MW : MCL ratios. Although larger and more profitable mussels can be crushed, we suggest that crabs select small crushable mussels to prevent claw wear and tear. Claw damage has a considerable effect on mating success and hence on an individual's fitness. Risk of claw damage may thus be of greater importance to a foraging shore crab than energy maximization.