Review of the North American aquatic snail genus Probythinella (Rissooidea: Hydrobiidae)

The aquatic gastropod genus Probythinella THIELE is redefined and proposed as a monophyletic subunit (sister to Cincinnatia) within the North American component of the family Hydrobiidae (Prosobranchia: Rissooidea) on the basis of synapomorphies of blunt shell apex, presence of two seminal receptacles, short albumen gland, and blunt terminus of penis. Probythinella is composed of two parapatric species that are distinguished by derived modifications of the outer edge of the operculum and pigmentation of the pallial roof and cephalic tentacles, and also differ in shell shape, thickness of outer shell lip, size of buccal mass (relative to shell height), and number of cusps on the lateral and marginal radular teeth. Species recognized are Probythinella emarginata (KUSTER), distributed throughout the Mississippi River drainage and drainages of Great Lakes, Hudson Bay, and Mackenzie River; and P. protera PILSBRY, found in brackish waters along the eastern Gulf of Mexico. Additional key words: systematic malacology, Prosobranchia, Mollusca, North America Aquatic snails of the genus Probythinella THIELE 1928 comprise a small group of North American hydrobiids recognizable by the cylindrical spire and extremely blunt apex of their shells. Members of the genus are found throughout much of eastern North America in both inland freshwater habitats and brackish waters along the Gulf of Mexico. Despite the small size of the genus, its systematics remains confused and, in addition, neither monophyly of Probythinella nor its relationships to other hydrobiid genera have been established. This study was prompted by my long-term interest in the systematics of Hydrobiidae in North America. These small prosobranch snails are ubiquitous members of freshand brackish-water communities, in which they often are among the more abundant benthic invertebrates (e.g., Noel 1954; Wells 1978), and comprise a significant element of taxonomic diversity in the region, with more than 45 genera (Kabat & Hershler 1993) and 170 Recent species (Turgeon et al. 1988) in current usage. This old (early Mesozoic; Kabat & Hershler 1993) group of obligately aquatic, poorly dispersing snails is tightly linked with drainage history (Taylor & Bright 1987), yet has not been used for studies of vicariance biogeography because of an absence of rigorous phylogenetic hypotheses. The inland hydrobiid fauna of North America, which includes many narrowly localized species, also is assuming importance in concert with efforts to protect and manage aquatic ecosystems: several species have already been added to the Federal List of Threatened and Endangered Wildlife, and more than 100 others currently are candidates for addition to this list (USDI 1994). The purposes of this study were to determine, describe, and assess the distribution of the recognizable species of Probythinella; and to delineate character states that help define the genus as a monophyletic unit. The latter is proposed on the basis of a cladistic analysis of Probythinella and several possibly allied North American taxa. The resulting hypothesis of evolutionary relationships is tentative, as much of the North American hydrobiid fauna is poorly studied and the state of systematics of this huge family is chaotic (Kabat & Hershler 1993). Thiele (1928) introduced Probythinella and designated the type species as Paludina emarginata KUSTER 1852. The validity of this species name has proven controversial as some workers have argued that Kuster did not describe a new taxon, but instead misidentified or transferred Lymnaea emarginata SAY 1821. Opinion also has been divided as to whether emarginata sensu KUSTER is conspecific with an earlier described snail, Paludina obtusa LEA 1841. Although the type species of Probythinella was described during the middle of the 19th century and is one of the most widespread Taxonomic review of Probythinella 121 hydrobiids in North America, its anatomy still has not been thoroughly depicted. Although Baker (1928) and others illustrated the radula, while Berry (1943) figured the penis, even these few details have not been reported without errors, to wit, the assertion that the outer marginal radular teeth of the type species lack cusps (Berry 1943:38, plate III, fig. 5; Clarke 1981: 58). A second, distinctive species, Probythinella protera PILSBRY 1953, was later described from putative Pliocene beds in St. Petersburg, Florida. Solem (1961) documented an extant population in Lake Pontchartrain which he referred to this species, whereas Morrison (1965) named a new genus and species for this population, Vioscalba louisianae. The possible conspecificity of the extant coastal populations and fossil protera remains unresolved (Heard 1979).