Molecules and morphology reveal ‘new’ widespread North American freshwater mussel species (Bivalvia: Unionidae)

In the Family Unionidae, the greatest radiation of freshwater mussels, malacologists have been misled by extreme intraspecific shell variation and conversely interspecific conchological stasis or convergence. We characterized the genetic and morphological diversity of two phenotypes of Lampsilis teres from specimens (n = 108) collected across its distribution using geometric and traditional morphometrics and multilocus molecular phylogenetics to test the hypothesis that phenotypes represent separate species. Results from our morphometric and molecular phylogenetic analyses unanimously indicate that L. teres sensu lato is made up of two divergent, widespread species with overlapping distributions. We describe a new species and provide a revised description of L. teres sensu stricto. We use morphometrics and machine-learning classification algorithms to test if shell morphology alone can be used to discriminate between these species. Classification percentages of 97.02% and 93.86% demonstrate that shell morphology is highly informative for species identification. This study highlights our lack of understanding of species diversity of freshwater mussels and the importance of multiple characters and quantitative approaches to species delimitation.