Can hermit crabs recognize social partners by odor? And why?

Individual recognition, i.e., the ability of an animal to recognize the identity of a conspecific, is a key element in the social life of many vertebrates. This phenomenon has rarely been investigated in invertebrates, particularly in pre-social arthropods, and is generally poorly understood for several reasons, including (1) the disparity between what the experiments actually demonstrated and what we implicitly mean by individual recognition, (2) the limited knowledge of the sensory channels used to recognize the social partner (particularly in the case of pre-social arthropods), and (3) the general disregard for the nature of representations that an animal constructs concerning the recognized conspecific. This article aims to review what is currently known about individual recognition mediated by pheromones in the shallow-water hermit crab Pagurus longicarpus. Methodological biases, on one hand, and promises of future success, on the other, are pinpointed while explaining the results obtained so far. Finally, an adaptive explanation is offered that attempts to solve the observed hiatus between the ‘‘virtuosism’’ shown by P. longicarpus in the laboratory and the apparent simplistic way of shell recruitment used in the field. Individual recognition, even if its expression is very rare in nature, might be a matter of life or death for the hermit crab that makes recourse to it.