Colony structure of the social snapping shrimp Synalpheus filidigitus in Belize

Several species of sponge-dwelling alpheid shrimps (Synalpheus) exhibit unusual colony organizations reminiscent of those found in social insects, but few of these shrimp species have been studied in detail. We sampled colonies of S. filidigitus from its two main hosts, the sponges Oceanapia sp. and Xestospongia spp., at Carrie Bow Cay, Belize. Colonies contained all size classes of shrimp, from recent hatchlings to mature, brooding females. The majority of colonies consisted of <30 individuals, although a few reached much greater numbers (maximum = 94 in Oceanapia and 121 in Xestospongia). In all but one of the colonies sampled containing mature female(s), only a single female (the "queen") was present. Distinct cohorts of juveniles, apparently produced by the resident female, were recognizable in several colonies, and the total number of individuals in the colony was strongly correlated with the resident female s size and fecundity. These observations suggest that, as previously shown for the eusocial S. regalis, the colony is a close-kin group consisting mainly of the queen s philopatric offspring. Comparison of females from a range in colony sizes revealed an apparent ontogenetic transformation in the queen s morphology. Although smaller (presumably younger) females were morphologically similar to males, queens from larger colonies were considerably larger than males, had proportionally longer and wider abdomens, and had the massive major chela of the first pereiopod replaced with a small chela identical to the normal minor first chela. Colonies of S. filidigitus are similar in organization to those of S. regalis, suggesting that S. filidigitus may be eusocial according to the traditional definition. This brings to 3 the number of species of Synalpheus reported to exhibit this paradoxical social organization.