Adaptive divergence in response to shared environmental pressures: examining gastropod shell exploitation by intertidal hermit crabs.

Gastropods commonly cope with the biotic and abiotic pressures of the intertidal through morphological adaptations of their shells. Snails from wave-exposed areas commonly have more streamlined shells that reduce drag forces with larger openings that favor their attachment to the substrate. In contrast, snails from sites with high predation commonly have a more globose shape that hinders breakage by predators. Hermit crabs use the shells of dead snails to protect their soft abdomen and confront similar biotic and abiotic pressures. Our main goal was to determine whether hermit crabs exploit intraspecific variation in snail shell shape in ways that may be adaptive; we tested two snail species with different shell profiles (Mancinella = globose-like and Stramonita = streamlined) across two intertidal zones. Using geometric morphometrics, we compared the shape and size of shells of snails with those used by hermit crabs in the upper and mid intertidal zones. We found that shell shape differed depending on the type of organism and the intertidal zone they inhabit. Mancinella shell shape was similar between snails and hermit crabs in the mid intertidal, and Stramonita shells were similar between crabs and snails in the upper intertidal. However, the shell shape differed between snails and hermit crabs in Mancinella shells in the upper intertidal and Stramonita shells in the mid intertidal. Furthermore, in the upper intertidal, hermit crabs occupied the most extreme shapes of each snail species, the most globose Mancinella and most streamlined Stramonita shells. Thus, although both taxa are subject to similar pressures on shell shape, different constraints result in some shape similarities and contrasts among species depending on the intertidal zones they inhabit.