Phylogeny of the critically endangered North American spinymussels (Unionidae: Elliptio and Pleurobema)

Lateral spines are uncommon among freshwater bivalves. The North American freshwater mussel fauna includes three taxa that exhibit spines: Elliptio spinosa, Elliptio steinstansana, and Pleurobema collina. All three taxa are endemic to the Southeastern US, and critically endangered. Currently, these species are recognized in two genera and are a source of taxonomic confusion within the unionid tribe Pleurobemini (Elliptio and Pleurobema). Because freshwater mussels exhibit phenotypically plastic shell morphology, morphologically-based diagnoses are problematic. I sequenced two mtDNA gene fragments and a fragment of the nuclear ITS-1 locus from >70 spinymussel specimens. Bayesian phylogenetic reconstructions suggest that the spinymussels do not comprise a monophyletic group. Elliptio steinstansana is sister to P. collina and these taxa form a monophyletic clade that possibly diverged from its nearest ancestor in the late Miocene. Additionally, E. spinosa forms a monophyletic clade that diverged from members of the core Elliptio lineage in the mid Pliocene, >1.5 million years before multiple radiations within the Elliptio clade. Furthermore, E. spinosa is divergent from the other spinymussels, suggesting that spines have evolved separately in two distinct bivalve lineages endemic to this region. These findings suggest a need to revise the taxonomy of this highly imperiled mussel group.