Species boundaries, specialization, and the radiation of sponge- dwelling alpheid shrimp

Abstract

Microevolutionary studies and natural history suggest that host- specialization has promoted the high diversity of tropical sponge- dwelling snapping shrimps (Decapoda, Alpheidae, Synalpheus). Yet the taxonomic difficulty of this genus precluded rigorous tests of this hypothesis, S. rathbunae Coutiere is among the most abundant invertebrates inhibiting the framework of sponges and dead coral that forms the floor of CAribbean coral reefs. Even within a small area S. rathbunae exhibits the apparently wide variation in size, color, and morphology that has long frustrated efforts to identify and define species boundaries within those large (> 100 described species) genus. Here I show that sympatric populations of this nominal species occupying different sponge hosts display clear, concordant difference in allozyne genotypes and in multivariate morphometries, confirming that the populations represent three distinct biological species. Moreover, careful field sampling revealed that the three S rathbunae taxa and the closely related S. flidigitus Armstrong showed almost no overlap in the species of hosts occupied. Interestingly, while there was significant differentiation between Belizeab and Panamanian populations of the one taxon that occurred at both sites (~1500 km apart) these populations were recognizable as conspecific using both genetic and morphological characters. These results show that (1) diversity of Synalpheus, which is already among the most species-rich crustacean genera, is probably several times higher than currently recognized, and (2) species of sponge-dwelling Synalpheus are highly host specific, with related species distinctly segregated among the hosts. Together with previous evidence of host race differentiation within shrimp species, these results suggest a primary role for the resource specialization in the origin and/or maintenance of this group's characteristically high diversity.