Most burrowing shrimps (infraorder Axiidea and Gebiidea) are characterized by solitary habits, which led many of these species to evolve a remarkable sexual dimorphism both in body size and chelipeds because of sexual selection. Given that monogamous species are known to live in heterosexual pairs and exhibit a low degree of sexual dimorphism, it is expected that burrowing shrimps are not monogamous. We tested this hypothesis using the burrowing shrimp Lepidophthalmus bocourti as a model. Against expectations, shrimps were found living not only individually, but also in homosexual and heterosexual pairs and trios within their respective burrows. The social structure in L. bocourti seemed to be an ontogenetically defined strategy, since most solitary individuals were juveniles of both sexes, while the heterosexual combinations were composed by adult shrimps. Sex distribution of pairs and trios suggested that female-female and male-female-female associations might be stable over time. Only 7% of the inhabited burrows were occupied by a heterosexual pair and ovigerous females were found dwelling either in solitary or in heterosexual combinations, indicating that this species is not monogamous. Most of the male-female associations occurring both in pairs and trios were dominated by females larger than males. We observed sexual dimorphism in the size of the major cheliped, being larger in males than in females. Ontogenetic pattern of burrow occupation allied to considerable sexual dimorphism argue in favour that L. bocourti is not monogamous and suggest that the major cheliped might have an important role during the male-male competition for receptive females.

The specimens of L. bocourti were collected monthly from February 2008 to April 2009 in the mangrove area of Colorado de Abangares, in the inner Gulf of Nicoya, Pacific coast of Costa Rica. We investigate the burrow use pattern, sex ratio, sexual dimorphism and relative growth of L. bocourti from the Pacific coast of Costa Rica, as a potential indicative of the mating system in this species. We used these life history traits to test the hypothesis of polygamy in L. bocourti.