The molding hypothesis: linking shell use with hermit crab growth, morphology, and shell-species selection

Abstract

The 'molding hypothesis' (Elwood et al. 1979. Anim Behav 27:940-946) predicts that shell-species selection by hermit crabs may be influenced by past experience in shell use through shell-imposed alterations in crab morphology. The present study was designed to test this hypothesis and to further the understanding of plasticity in the shell-species selection in hermit crabs. Shell use was demonstrated to influence crab growth and morphology. Individuals reared in shells of Tegula viridula attained larger sizes than individuals in shells of Morula nodulosa. Crab growth was also dependent on crab sex, since males reached larger sizes and presented longer intermolt periods than females. The most conspicuous influence of shell utilization on crab morphology was in dorso-ventral flattening, which occurred on a decreasing scale with the shell species, as follows: M. nodulosa > Cerithium atratum > T. viridula. Crabs from all treatments and those collected in nature in shells of Olivella minuta chose mainly shells of C atratum, but the previous experience of crabs strongly influ- enced their shell-species selection pattern. Individuals maintained in M. nodulosa shells selected M. nodulosa shells more frequently than individuals reared in T. viridula or C. atratum. The influence of past experience was demonstrated to depend on shell type, since patterns of shell-species selection of individuals reared in T. viridula or C. atratum were identical. These differences in the shell-species selection pattern directly reflected the differences in crab dorso-ventral flattening, i.e. dorso-ven- trally compressed individuals (those in shells of M. nodulosa or O. minuta) showed a higher proba- bility of selecting narrow-aperture shells than did 'rounded' individuals (those in shells of T. viridula or C. atratum). The weight of the selected shells at the end of the experiment depended on crab size instead of previous experience with different shell architectures.