Molecular evidence of the evolutionary origin of a Bonin Islands endemic, Stenomelania boninensis

Oceanic islands, which have never been connected to continental land masses, often contain many endemic nonmarine species (e.g. Darwin, 1859; Freed, Conast & Fleischer, 1987; Shaw, 1996). The Bonin Islands (Ogasawara Islands), located approxi- mately 1,000 km from the Japanese mainland, were formed by volcanic activity and have never been connected to Eurasia (Asami, 1970). Several endemic species have been recorded from the Bonin Islands (e.g. Kobayashi, 1978; Chiba, 2002) and some of these have undergone adaptive radiations within the archipe- lago (Chiba, 1999). The Bonin Islands rose above sea level, pre- sumably between the Pliocene and the early Pleistocene (Asami, 1970; Kaizuka, 1977; Imaizumi & Tamura, 1984), and were subsequently colonized by nonmarine organisms. The geo- graphic isolation of the Bonin Islands in the western Pacific Ocean provides an ideal setting for allopatric speciation (Takayama et al., 2005). Historical colonization of these islands from the Japanese mainland, Nansei Islands or other proximal regions presumably has led to the evolution of endemic species. Stenomelania boninensis (Lea, 1850) (Gastropoda: Thiaridae) is endemic to the Bonin Islands, where it typically lives in fresh- water rivers. It is rarely found in higher salinity habitats where other congeners commonly occur. We hypothesize that S. boni- nensis was derived from a congener that dispersed from a proxi- mal region. To test this hypothesis, we evaluated phylogenetic relationships among this species and congeners distributed in geographically proximal regions using molecular data. We also estimated the divergence time of S. boninensis relative to its puta- tive ancestor to help elucidate the evolution of S. boninensis. We collected six Stenomelania species (S. boninensis, S. costellaris