Recognition of a highly restricted freshwater snail lineage (Physidae: Physella) in southeastern Oregon: convergent evolution, historical context, and conservation considerations

Abstract

Non-marine mollusks have the highest number of documented extinctions of any major taxonomic group. Given their conservation status and the numerous cases of taxonomic uncertainty concerning freshwater mollusks in particular, the recognition of potentially endangered species is critically important. Here, we evaluate the genetic distinctiveness and phylogenetic position of a freshwater snail restricted to a series of geothermal springs within the Owyhee River drainage in Oregon (the ‘Owyhee wet-rock physa’). Because these snails closely resemble Physella (Petrophysa) zionis, a wet-rock physa that occurs in a small area in Zion National Park (Utah), the Owyhee wet-rock physa is presumably either closely related to or represents a disjunct population of P. zionis. However, phylogenetic analyses of sequences of mitochondrial cytochrome oxidase I and nuclear first and second internal transcribed spacer gene regions indicate that the Owyhee wet-rock physa is genetically distinct from other physid species. Despite exhibiting similar morphologies, the Owyhee wet-rock physa and P. zionis are distantly related physids; instead, the Owyhee wet-rock physa is most closely related to a population of physids from central California. These results suggest that convergent evolution may be responsible for the similar shell morphologies of the Owyhee wet-rock physa and P. zionis. Furthermore, the close relationship between physid populations in southeastern Oregon and central California suggests a historical connection between the Owyhee River and river drainages to the south. Finally, we recommend that the Owyhee wet-rock physa be considered critically endangered based on its extremely limited distribution.