The roles of profit and claw strength in determining mussel size selection by crabs

Earlier research indicated crabs select small bivalve prey that are not always energetically optimal. We test whether patterns of bivalve prey selection differ between relatively strong-clawed stone crabs (Menippe adina) and two size classes of blue crabs (Callinectes sapidus) on Ischadium recurvum. Handling times were highest for small blue crabs, and lowest for the small mussels, but clear differences among mussel sizes occurred only for C. sapidus. Two analyses suggested profit (dry tissue mass/handling time) did not differ among mussel sizes for any predator treatment. M.adina was not size selective, but both sizes of C. sapidus preferred smaller mussels. Because i. blue crabs select smaller mussels than they can consume, ii. profit does not differ with mussel size, and iii. large mussels require force generation near levels that can damage claws, we conclude blue crabs consume small mussels to reduce risks of claw damage, or select prey with small handling times to limit their own predation risk. Stone crabs instead show no size preference because their stronger claws are less prone to damage, or allow them to mechanically handle larger prey.