Clustering behaviour in a mediterranean population of the hermit crab, Clibanarius erythropus

The habit of clustering was studied in a population of the hermit crab, Clibanarius erythropus, inhabiting a Tyrrhenian rocky shore. Aims were to (1) analyse both the structure of clusters and the dynamics of the clustering behaviour and (2) contribute to an understanding of the adaptive function of clusters. A number of clues make it reasonable to hypothesise that clustering behaviour in this species has an anti-dispersing function, facilitating shell exchanges. On the one hand, clumping behaviour was found in those categories of the population most likely to be wearing low-quality shells. In contrast, clustered and scattered hermit crabs occupied different shell arrays as far as we analysed variety, species and architecture. Isolated crabs seemed to inhabit better-fitting shells and particularly globose ones. These were easily occupied by hermit crabs when introduced into a cluster in the presence of elongate shells. And also, hermit crabs occupying these new shells rapidly abandoned the clusters of origin and were found isolated in the habitat. That is, they switched from a gregarious to a solitary life-style, thus avoiding any risk of being deprived of their housing. The conclusive remark is that clustering constitutes a further example of behavioural plasticity in hermit crabs.