A new eusocial species of snapping shrimp, Synalpheus idelisae n. sp. (Decapoda, Alpheidae), with a review of caste differentiation in eusocial shrimp

Snapping shrimp in the genus Synalpheus Spence Bate, 1888 are the only marine organisms known to exhibit eusociality, a social structure characterized by colony-living and a reproductive division of labor. Here we describe a new species of eusocial sponge-dwelling snapping shrimp, Synalpheus idelisae n. sp., based on material collected from Puerto Rico and the Dominican Republic. Phylogenetic analyses (based on mitochondrial 16S and COI loci) and morphological traits place S. idelisae n. sp. within the Synalpheus rathbunae Coutière, 1909 species complex, which includes several other eusocial species. Synalpheus idelisae n. sp. can be distinguished from its close relatives by the shape of the anterodorsal spine on the major chela, the morphology of the pleura, and the number of setal combs on the fifth pereopod. Like many other eusocial species in the genus Synalpheus, S. idelisae n. sp. exhibits traits such as direct larval development and a second pereopod with a four-segmented carpus. Notably, we document several colonies of S. idelisae n. sp. in which queens replace the major snapping chela with a second minor chela, a pattern linked to morphological caste differentiation in Synalpheus shrimps. Finally, we review the occurrence of this pattern across other eusocial and putatively eusocial Synalpheus species. With the addition of S. idelisae n. sp., there are now a total of seven eusocial species in the West-Atlantic S. rathbunae species complex.