Geographical subdivision, demographic history and gene Xow in two sympatric species of intertidal snails, Nerita scabricosta and Nerita funiculata, from the tropical eastern PaciWc

The patchy distribution of rocky intertidal communities in the tropical eastern PaciWc (TEP) may impose severe constraints on the genetic connectivity among populations of marine invertebrates associated with this habitat. In this study, we analyzed a portion of the mitochondrial cytochrome c oxidase subunit I (COI) gene in two sympatric species of marine snails, Nerita scabricosta and Nerita funiculata, common inhabitants of the rocky intertidal from the Gulf of California (Sea of Cortez) and outer PaciWc coast of the southern Baja California (Baja) peninsula to northern South America, to assess genetic connectivity among populations of each species. One of our aims was to determine whether the morphological, behavioral, and ecological diVerences observed among populations of both species throughout their range in the TEP corresponded to population genetic diVerences. In addition, we were interested in elucidating the demographic history of both species. We found no evidence of genetic structure throughout the Gulf of California and outer coast of the Baja peninsula region for either species. Comparisons between Gulf of California/Baja and Panama populations, however, showed signiWcant genetic diVerentiation for N. scabricosta, but not for N. funiculata. The genetic diVerences between Mexican and Panamanian populations of N. scabricosta were consistent with previously reported ecological and behavioral diVerences for this species between these two distant regions. However, previously reported size diVerences between northern and central/southern Gulf of California individuals of N. scabricosta do not correspond with our Wndings of genetic connectivity among these populations. Results from neutrality tests (Tajima’s D and Fu’s FS), the mismatch distribution, and Bayesian skyline analyses suggested that both species have experienced dramatic population expansions dating to the Pleistocene.