Social conquest of land: Sea-to-land changes in shell architecture and body morphology, with consequences for social evolution.

Architecture, like nests, burrows, and other types of fortresses, may have played an important role in the evolution of social life on land. However, few studies have examined architecture in organisms that transitioned from sea to land to test how and why architectural and morphological changes might have jointly impacted social evolution. Here I contrasted the shell architecture and body morphology of two of the phylogenetically most closely-related land versus sea species of hermit crab (the terrestrial hermit crab, Coenobita compressus, and the marine hermit crab, Calcinus obscurus), as well as the original builder of their shells (the gastropod, Nerita scabricosta). In contrast to the shells of gastropods and marine hermit crabs, only the shells of terrestrial hermit crabs were architecturally remodeled, with no columella inside for the occupants to grip upon to resist eviction. The bodies of terrestrial hermit crabs were also significantly more exposed outside the enlarged openings of their remodeled shells, whereas the substantially smaller-bodied marine hermit crabs were safeguarded deep within the recesses of their unremodeled shells. Ultimately, these changes in shell architecture and body morphology likely had consequences for social evolution on land, making conspecifics not only more dependent upon one another for homes, but also potentially easier to evict. Further changes in claw shape on land (with the claws of terrestrial hermit crabs becoming shorter, wider, and thicker) may have evolved to help offset their heightened danger of social eviction, acting as a more effective door against conspecifics.