Subjective resource value and shell abandoning behavior in hermit crabs

Understanding the factors that motivate animals to hold or abandon a valuable resource is a central goal of behavioral ecology, the study of which will grow more important in the face of increasingly frequent extreme events. We compared the shell-abandoning behavior of the sympatric hermit crabs Clibanarius antillensis and Pagurus criniticornis in response to simulated burial and entrapment by rocks and other debris. While these hazards are relatively common in dynamic intertidal habitats, the frequency and severity of such disturbance are increasing due to human activity. While both species exhibited shell-abandoning behavior in response to experimental burial, it was far more prevalent for the soft-bottom dwelling species P. criniticornis (90%) when compared to the rocky bottom inhabitant C. antillensis (55%). Simulated entrapment experiments highlighted further differences in species response, with the decision to abandon domicile shells again far more common for P. criniticornis (80%) than it was for C. antillensis (10%). Given the tendency for P. criniticornis to abandon its shell, we subsequently focused on this species to test specific hypothesis about subjective resource value in hermit crabs. There was no difference in the tendency for this species to abandon optimal or sub-optimal (poorly-fitting) shells when faced with the immediate and potentially fatal risk of burial. This contrasted the response seen under conditions of entrapment, whereby individuals inhabiting sub-optimal shells abandoned them more rapidly and in greater numbers than those inhabiting optimal shells. Combining these two outcomes, we suggest that hermit crabs subjectively assess shell-value and respond according to the nature of the disturbance and its associated risks (i.e., high-value shells are abandoned in minutes under conditions of burial; but are held for hours under the less severe conditions of entrapment). In effect, the results show that extrinsic environmental cues can be just as important to decision making processes as those intrinsic cues that govern an animal's physiological condition. The contrasting behavior of the two hermit crabs to the movement of mud and other debris may lead to a higher survivorship of P. criniticornis than C. antillensis where activities such as dredging and harbor construction are intensifying along developing coastlines.