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 termites, i.e., colonies contain a single reproducing female and a large number of non-breeding "workers." This finding prompted a reexamination of several previously puzzling reports of unusual population structures in other Synalpheus species. New collections, and observations made by students of this genus over the last century, suggest that several sponge-dwelling Synalpheus species similarly exhibit overlapping generations and monopolization of reproduction by a few individuals, and thus that these species may also be eusocial according to classical entomological criteria. The evidence for this conclusion includes reports of several Caribbean and Indo-Pacific species occurring in large aggregations of "juvenile" shrimp accompanied by few or no 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 collected from at least seven host species in Caribbean Panama, Belize, and the Virgin Islands. Stomach contents comprised primarily detritus and diatoms, suggesting that S. chacei feeds on material inhaled in the host sponge's feeding current. The new species is morphologically similar to S. bousfieldi, and is most reliably distinguished from it (and indeed, apparently from all other species of Synalpheus) by a unique pair of longitudinal setal combs on the dactyl of the minor first chela. Like S. regalis, S. chacei lives 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 development of the major chela (fighting claw) which may reflect a concomitant differentiation in behavior among individuals within a colony.