Systematics, molecular genetics and historical zoogeography of the viviparous freshwater gastropod Pseudopotamis (Cerithioidea, Pachychilidae): a relic on the Torres Strait Islands, Australia

Freshwater snails of the genus Pseudopotamis Martens, 1894 exemplify one of the most enigmatic distributional patterns among limnic organisms in Australasia. These viviparous cerithioidean gastropods are endemic to the Torres Strait Islands, located on the Sahul shelf corridor between Australia and New Guinea. The occurrence of the two constituent species is highly restricted: P. supralirata (Smith, 1887) on Prince of Wales Island, and P. semoni Martens, 1894 on Hammond Island. Long recognized erroneously as members of the heterogeneous, polyphyletic and widespread Thiaridae, morphological and molecular data presented here reveal that the taxon has to be placed within Pachychilidae, a family otherwise absent from Australia. Comparison with other South East Asian pachychilids, in particular Brotia H. Adams, 1866, Tylomelania Sarasin & Sarasin, 1897, and Jagora Köhler & Glaubrecht, 2003 suggests that Pseudopotamis is most closely related to taxa endemic to Sulawesi (Celebes), thus occurring disjunctly about 2000 km to the west of the Torres Strait. It is only with the Sulawesi pachychilids that Pseudopotamis shares its peculiar uterine brood pouch. Consequently, all pachychilid taxa east of the much-debated Wallace's line (east of Bali, Borneo and the Philippines) possess this uterine incubatory structure. In this paper the historical background of the discovery, the taxonomic history, and an evaluation of the systematic position of Pseudopotamis is provided, including molecular genetic data (genes of cytochrome c oxidase subunit I and 16S ribosomal DNA). Finally, based on the phylogenetic assignment and the recent distribution, implications for a historical biogeography are discussed utilizing a recently developed model for the palaeogeography in Australasia. According to these available data, we favour early vicariant event(s) over recent dispersal.