Synalpheus regalis, new species, a sponge-dwelling shrimp from the Belize Barrier Reef, with comments on host specificity in Synalpheus

Synalpheus regalis (Decapoda: Alpheidae) is described from the demosponges Xestospongia cf. subtriangularis (Petrosiidae) and Hyattella intestinalis (Spongiidae) on the Belize Barrier Reef at Carrie Bow Cay. The new species is a member of Coutiere’s gambarelloides species-group, and more specifically is one of a complex of morphologically very similar species, including S. rathbunae, S. filidigitus, and at least one other undescribed species. Like most members of the gambarelloides group, S. regalis lives exclusively within the internal canals of living sponges, and at the type locality is found in only 2 of the 21 sponge species that harbor commensal shrimps. Such host specificity is typical of Caribbean species of Synalpheus. The pattern of shrimp distribution among sponge species, and among individual sponges, suggests that suitable habitat at this site is saturated and that competition for living space is intense. The resulting restriction of species of Synalpheus to those hosts in which they are competitively superior may thus be an important determinant of their characteristic host specificity.