On the frequency of eusociality in snapping shrimps (Decapoda : Alpheidae), with description of a second eusocial species

Recently, the Caribbean snapping shrimp Synalpheus regalis was shown to be eusocial by the criteria historically used for honeybees, ants, and termite s, i.e., colonies contain a single reproducing female and a large number of non-breeding "workers." This finding prompted a reexamination of several p reviously puzzling reports of unusual population structures in other Synalp heus species. New collections, and observations made by students of this ge nus over the last century, suggest that several sponge-dwelling Synalpheus species similarly exhibit overlapping generations and monopolization of rep roduction by a few individuals, and thus that these species may also be eus ocial according to classical entomological criteria. The evidence for this conclusion includes reports of several Caribbean and Indo-Pacific species o ccurring in large aggregations of "juvenile" shrimp accompanied by few or n o mature females. Here I describe one of these species as Synalpheus chacei . Like other members of the gambarelloides species group within this genus, S. chacei is an obligate inhabitant of living demosponges, and has been co llected from at least seven host species in Caribbean Panama, Belize, and t he Virgin Islands. Stomach contents comprised primarily detritus and diatom s, suggesting that S. chacei feeds on material inhaled in the host sponge's feeding current. The new species is morphologically similar to S. bousfiel di, and is most reliably distinguished from it (and indeed, apparently from all other species of Synalpheus) by a unique pair of longitudinal setal co mbs on the dactyl of the minor first chela. Like S. regalis, S. chacei live s in colonies of up to several dozen individuals of overlapping generations , in which only a single female breeds, and is thus likely to be eusocial. Interestingly males of S. chacei exhibit an apparent dimorphism in the deve lopment of the major chela (fighting claw) which may reflect a concomitant differentiation in behavior among individuals within a colony.