The shell selection behaviour of two closely related hermit crabs

Shell selection behaviour in response to shell features or stimuli that might contribute to shell detection, investigation and ultimately shell choice were examined in two inter-tidal hermit crabs, Pagurus samuelis and P. hirsutiusculus. Each species displayed a different preference for either Tegula funebralis shells (P. samuelis) or Nucella canaliculata shells (P. hirsutiusculus), shells that differ greatly in their external appearance and internal configuration. Visual and chemical shell stimuli were found to have a different effect on each of the two closely related species. Pagurus samuelis relied on visual cues for shell detection. It also displayed visually-mediated tracking of objects resembling T. funebralis shells in colour, shape and size. Pagurus hirsutiusculus did not display tracking behaviour. Both species responded to the chemical composition of shells, specifically their calcium content. Various coatings were applied to the shell surfaces to remove (cover) the calcium cue. For P. hirsutiusculus only, shell exploratory behaviour was not initiated if this one stimulus was absent. Both species were found to rely on the chemical cues alone to locate and uncover partially buried shells. Each species apparently relies on shell cues that are distinguishing features of their preferred shells. Thus, underlying differences in sensory and other neural mechanisms enable each species to detect quickly and respond to its preferred shell type. The stimulus value of the aperture and possible importance of the shell's internal configuration, for shell selection, were also examined. Lastly, the role of learning in shell selection is discussed.