Redescriptions have consequences: biogeographic and conservation insights from the redescription of a widespread freshwater amphipod (Amphipoda, Gammaridae)

Redescriptions are opportunities to “update” species hypotheses based on modern methodology. As such, they can also lead to new insights into biogeography and evolutionary history. We redescribe Gammarus fasciatus Say, 1818, a freshwater amphipod from the eastern Nearctic, based on material from near its type locality. Our redescription is supported by both molecular phylogenetic and morphometric analyses. Molecular relationships and shallow genetic divergence indicate a recent expansion of G. fasciatus into the interior of the USA, suggesting the potential for anthropogenically aided dispersal. Individuals of G. fasciatus split from their closest congeners during the Miocene, likely promoted by embayments present on the Atlantic coastal plain. While molecular analyses suggest little genetic divergence within the species, especially in the northern part of its range, morphometric analyses suggest that populations at the edges of the range are misidentified and likely belong to other species. Based on trends observed within Gammarus , it seems plausible that other populations transitioned to freshwater and speciated in the southern USA, also utilizing embayments present during the Miocene. We discuss insights afforded by the redescription of G. fasciatus and highlight the utility of such analyses for other taxa and for understanding regional biogeography and evolutionary history.