Eusociality unveiled: discovery and documentation of two new eusocial shrimp species (Caridea: Alpheidae) from the Western Indian Ocean

The alpheid snapping shrimp genus Synalpheus Spence Bate, 1888, is a prominent component of arthropod diversity found in coral reefs. Notably, Synalpheus is the only genus of marine organisms known to exhibit eusocial behavior. Although eusociality has evolved at least four times independently in Synalpheus, all described eusocial species are from the West Atlantic, with only a single study documenting possibly eusocial species from Indonesia. In 2008, during an expedition to Madagascar organized by the Florida Museum of Natural History (FLMNH), a diverse array of sponge-dwelling species was collected, including two species of Synalpheus exhibiting colonial behavior. Through detailed examination of these specimens, we have confirmed that these two species are eusocial and represent new eusocial species of Synalpheus outside of the West Atlantic. Consequently, we provide the first official documentation of eusocial species from the Western Indian Ocean and present their taxonomic descriptions and their phylogenetic relationships with other species of the genus in this study.