The geometry of predator avoidance by the blue crab, Callinectes sapidus Rathbun

A model is proposed to explain the pattern of escape orientation of blue crabs ( Callinectes sapidus Rathbun) in the intertidal zone of sandy beaches. The model states that a crab should select the escape heading that maximizes its distance offshore before being intercepted by a pursuing predator. Predictions of the model were evaluated with observations of escape responses elicited by my approach. Fleeing crabs swam offshore, but the heading they chose was influenced by the approach heading of the observer. The distribution of crab escape headings as a function of observer approach heading was symmetrical about the onshore-offshore axis. Crab escape heading was independent of the approach distance (distance between the observer and the crab at the start of the escape response). Finally, crabs did not assess relative speed (crab speed/predator speed) when selecting an escape heading. By testing the predictions of the model it is demonstrated that the escape orientation of blue crabs in the intertidal zone is a compromise between swimming away from a potential enemy and swimming offshore for refuge. The assumptions of the model are discussed and the use of models in understanding the dynamics of predator avoidance is considered.