Molecular species delimitation reveals hidden endemism in Faxonius placidus (Hagen 1870) (Decapoda: Astacidea: Cambaridae) in southeastern USA

Molecular surveys are critical for understanding species boundaries and evolutionary relationships of North American crayfishes, as traditional morphological characters used for taxonomy frequently misrepresent crayfish diversity. The bigclaw crayfish, Faxonius placidus (Hagen, 1870), is currently described as a widely distributed crayfish that is common throughout the Cumberland, Tennessee, and Lower Ohio river drainages. The geographic distribution of F. placidus is based on identifications using traditional morphological characters including the first-form male gonopod, chelae, and rostrum. Within F. placidus, color pattern variation is specific to populations separated by hydrogeographic barriers. We used DNA barcoding data (mtDNA-COI) and genome-wide molecular markers (nuclear SNPs) to examine geographic patterns of genetic variation in F. placidus within the Cumberland and Tennessee river drainages. The federally endangered Nashville crayfish, F. shoupi (Hobbs, 1948), was included in our analyses, as previous phylogenetic reconstructions suggest F. placidus is paraphyletic with respect to F. shoupi. Phylogenetic reconstructions and molecular species delimitation identified four genetically distinct lineages within F. placidus that are paraphyletic with respect to F. shoupi. Our results add to numerous studies demonstrating the utility of robust molecular analyses for understanding the biodiversity of North American crayfishes.