The freshwater gastropod family Physidae (Pulmonata: Basommatophora) has largely a Holarctic distribution, which extends southward into Central and South America (Burch, 1982, Taylor, 2003). Physids, particularly Physa acuta have been introduced around the world. Physidae diversity is concentrated in the United States and Canada, which has 47 species (Johnson et al. 2013). Wethington and Lydeard (2007) were the first to examine the evolutionary relationships of the family using modern phylogenetic analyses of two mitochondrial genes of 66 specimens representing 28 taxa. Wethington and Lydeard (2007) confirmed the recognition of a number of formal and informal taxonomic groups that had been identified by penial morphology (Te, 1978). Although a complete systematic treatment is still needed for the family, the study at least provided an evolutionary framework that other studies could use as a starting point and/or an hypothesis of relationships that could be tested with additional data.
Recently, an interesting paper entitled “Recognition of a highly restricted freshwater snail lineage (Physidae: Physella) in southeastern Oregon: convergent evolution, historical context, and conservation considerations” was published this year in Conservation Genetics by Alexandria Moore, John Burch and Thomas Duda, Jr. The authors examined the phylogenetic status of the Owyhee wet-rock physa, which is restricted to a series of geothermal springs within the Owyhee River drainage in southeastern Oregon. The Owyhee wet-rock physa has a shell-shape reminiscent of P. zionis rather than the typical physid shape exhibited by members of the gyrina species group, so P. zionis was included in the analysis as well (see photo above showing a) Owyhee wet-rock physa, b) P. zionis and c) P. cf gyrina). Phylogenetic analyses of sequences of mitochondrial cytochrome oxidase I and two nuclear genes (internal transcribed spacer genes I and II), revealed the Owyhee wet-rock physa is genetically distinct and closely related to P. cf gyrina from California. It is distantly related to P. zionis, so the similar shell shape evolved independently. The authors plan to formally describe this unique species and recommend that it be considered critically endangered based on its limited distribution. Clearly, physids offer a wealth of opportunity for those interested in sorting out evolutionary relationships, delimiting species boundaries and discovering taxa.
Burch, J. B. 1982. North American freshwater snails: identification keys, generic synonymy, supplemental notes, glossary, references, index. Walkerana, 1:1-365.
Johnson, P. D., A. E. Bogan, K. M. brown, N. M. Burkhead, J. R. Cordeiro, J. T. Garner, P. D. Hartfield, D. W. Lepitzki, G. L. Mackie, E. Pip, T. A. Tarpley, J. S. Tiemann, N. V. Whelan, and E. E. Strong. 2013. Conservation status of freshwater gastropods of Canada and the United States. Fisheries 38(6):247-282.
Moore, A. C., J. B. Burch, and T. F. Duda, Jr. 2014. Recognition of a highly restricted freshwater snail lineage (Physidae: Physella) in southeastern Oregon: convergent evolution, historical context, and conservation considerations. Conservation Genetics http://link.springer.com/article/10.1007/s10592-014-0645-5
Taylor, D. W. 2003. Introduction to Physidae (Gastropoda: Hygrophila) biogeography, classification, morphology. Revista de Biologia Tropical, Supplement 51:1-287.
Te, G. A. 1978. The systematics of the family Physidae (Basommatophora: Pulmonata). Ph.D. thesis, University of Michigan.
Wethington, A., C. Lydeard. 2007. A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences. J. Molluscan Studies 73:241-257.