Colony structure of the social snapping shrimp Synalpheus filidigitus in Belize

Several species of sponge-dwelling alpheid shrimps (Synalpheus) exhibit unu sual colony organizations reminiscent of those found in social insects, but few of these shrimp species have been studied in detail. We sampled coloni es of S. filidigitus from its two main hosts, the sponges Oceanapia sp. and Xestospongia spp., at Carrie Bow Gay, 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 m uch greater numbers (maximum = 94 in Oceanapia and 121 in Xestospongia). In all but one of the colonies sampled containing mature female(s), only a si ngle female (the "queen") was present. Distinct cohorts of juveniles, appar ently produced by the resident female, were recognizable in several colonie s, and the total number of individuals in the colony was strongly correlate d with the resident female's size and fecundity. These observations suggest that, as previously shown for the eusocial S. regalis, the colony is a clo se-kin group consisting mainly of the queen's philopatric offspring. Compar ison of females from a range in colony sizes revealed an apparent ontogenet ic transformation in the queen's morphology. Although smaller (presumably y ounger) females were morphologically similar to males, queens from larger c olonies were considerably larger than males, had proportionally longer and wider abdomens, and had the massive major chela of the first pereiopod repl aced with a small chela identical to the normal minor first chela. Colonies of S. filidigitus are similar in organization to those of S. regalis, sugg esting that S. filidigitus may be eusocial according to the traditional def inition. This brings to 3 the number of species of Synalpheus reported to e xhibit this paradoxical social organization.