Prey Size Selection in Dungeness Crabs: The Effect of Claw Damage

Abstract

We investigate prey size selection by Cancer magister, a decapod crustacean, feeding on Protothaca staminea, a hard—shelled venerid clam, and test predictions of the optimal diet model using three different currencies. We also evaluate the effect of claw damage on crab feeding efficiency. To model parameters measured were: energy content of the prey, energy cost of predation, and handling time. All of these variables were positive exponential functions of clam size. The different measures of prey value (or profitability) provided contrasting prey rankings. Both net and gross rates of energy intake [(Benefit–Cost)/Time and Benefit/Time, respectively] predicted that the largest clams were the most profitable. Energetic efficiency (Benefit/Cost) predicted that the smallest claim sizes were the most profitable. Prey—size selection experiments, offering crabs mixtures of two or three clam size classes, showed that crabs always ate more clams from the smallest clam size class offered. Thus the predictions from the energetic efficiency model provide the best fit to the results. The use of energetic efficiency as a currency requires energy cost to be limiting over the animal's lifetime. In this study we propose claw damage, defined as chela breakage and claw—tooth wear, as a consequence of fatigue failure (breakage of a structure due to repeated loading), to be a limiting cost. A field study of a natural population of C. magister indicated that levels of wear and breakage were significant, and were related to sex and molting state. Laboratory experiments showed that crabs with broken claws were unable to crack clams. Crabs with artificially worn claw teeth had significantly longer handling times than undamaged crabs. Finally, the ecological effects of claw breakage in decapods are evaluated with reference to growth, molting ability, regeneration load, and reproductive success.