Size selection by the crab Liocarcinus puber feeding on mussels Mytilus edulis and on shore crabs Carcinus maenas: the importance of mechanical factors

Abstract

Velvet swimming crabs Liocarcinus puber (L.) were observed while they fed on mixtures of different size classes of mussels Mytilus edulis L. or of juvenile shore crabs Carcinus maenas (L.) in the laboratory. Intermediate-sized mussels predominated in the diets. Numbers of contacts with different sized mussels corresponded to the ratios presented, but contacts with smaller individuals frequently evoked no grasping attempts. Due to limited chelal dexterity, small mussels were difficult to grasp and easily dropped. Some large resistant mussels were rejected after unsuccessful breaking attempts. The factors determining size selection also determined profitability, since gleaning of flesh from small prey was inefficient and involvement time with large prey very long. Thus for mussels prey profitability and diet composition were similar. Large shore crabs were eaten most frequently. Being more active, they were contacted more frequently than expected from the ratios presented. Small shore crabs were difficult to grasp and easily dropped. Within the size range used, there was no mechanical upper size limit to predation. For this prey, predation slowed down as more food was ingested, apparently limited by the rate of digestion. Profitability was similar for all prey sizes over the duration of the period of observation, and thus did not correspond to diet composition. Rejection of prey after brief manipulation, a proposed mechanism for optimal foraging in the predator C. maenas, was not observed in the present study. Nevertheless, size selection was marked.