Sexual and size dimorphism in two deep-water hermit crabs (Decapoda: Parapaguridae) from the Western Atlantic Ocean

Abstract

The Parapaguridae comprises hermit crabs that inhabit deep-waterenvironments. In these environments, shell availability can be limited, mostlyconsisting of small and fragile-shelled gastropods. Thus, different strategieshave evolved to mitigate this limited shell supply. Sympagurus dimorphus(Studer, 1883) lives in association with a zoanthid cnidarian that creates apseudo-shell that grows with the hermit crab. In contrast, Oncopagurus gracilis(Henderson, 1888) inhabits small, calcified gastropod shells. Therefore, weselected these two species as models to test sexual dimorphism and shapepatterns of their chelipeds and cephalothoracic shield, due to their differentshelter acquisition methods. We photographed the animals and digitizedthe images to employ comparative geometric morphometric techniques. Wetested the differences in shape between the sexes within each species, andalso tested sexual size dimorphism based on centroid size. For O. gracilis,we found shape differences for the chelipeds and cephalothoracic shield,however, we only observed sexual size dimorphism for the chelipeds. For S. dimorphus, an inverse pattern was found, in which females presented more robust chelipeds, and sexual sizedimorphism was present in which males were larger. These differences can be reasonably explained by theirshelter acquisition methods, in which O. gracilis depends on small shells that limit growth, while S. dimorphusgrows with its cnidarian pseudo-shell. The robustness found in the shape patterns may also be related to theirbehaviors, e.g., in addition to competition for shells, they also fight during mating. However, we emphasizethat future studies with other populations of these species are needed for comparative purposes.