Gastropod shells or polychaete tubes? The hermit crab Discorsopagurus schmitti' s housing dilemma.

In this study, I address individual and species life-histories with reference to the hermit crab Discorsopagurus schmitti from northern Puget Sound, Washington. In its natural habitat, this crab inhabits only empty, fixed tubes built by the polychaete worm Sabellaria cementarium. In choice experiments where naked individuals were offered both gastropod shells and worm tubes, post-larval stages preferred the former, while sub-adults distributed themselves equally between the two housings and the majority of adults occupied tubes. The behavior of megalopae and juveniles seems to testify to the shell-dwelling ancestry of D. schmitti, while differences in housing preferences among stages suggest the occurrence of the phenomenon of internal maturation. Evidence supports the hypothesis that interspecific competition for shells induced the switch to tubes by D. schmitti. This species is now less agile (and thus less favored in exploitative competition) and smaller (larger hermits are more likely to win agonistic interactions for the possession of shells). However, small size seems to be advantageous, allowing these crabs to occupy the narrow microhabitat provided by worm tubes. The evolution of a tube-dwelling life has taken place as a behavioral response that occurred with the replacement of the habitat selector, then catalyzing modifications at morphological, physiological, and behavioral levels.