Resource partitioning between sexes in the “unconventional” hermit crab, Calcinus tubularis.

Together with Calcinus verrilli from Bermuda, the Mediterranean hermit crab C. tubularis is unique in that it exhibits a sharp sexual dimorphism in resource use in which one sex, the male, occupies loose shells and the other, the female, occupies attached tubes. Field surveys and laboratory experiments addressed two issues that help understand male-female resource partitioning in this species. First, the value of shells as shelters differs between sexes. Shells furnish mobility to their inhabitants, but also greater opportunities to grow because they can provide larger size than can the tubes, whereas both mobility and large dimensions are crucial selective factors for the reproductive outcomes of males only. In fact, egg production seemed not to be affected by females’ sedentary life and the number of eggs was not related to the female size. Second, previous shelter experience plays a role in reducing male-female competition. I found that individual crabs, once presented simultaneously with a shell and a tube, more likely selected the type of shelter that they were collected in. Then, in male-female competition experiments, pairs composed of one crab found in a shell and one crab found in a tube and offered a shell and a tube in conjunction occupied the housing in accordance to the shelter previously occupied without any influence of their sex or size. The question remains as to how, why, and when such a sexual dimorphism in the distribution of C. tubularis between microhabitats did occur during a hermit crab’s life.