Abstract
Marine annelid taxonomy is experiencing a period of rapid revision, with many previously "cosmopolitan" species being split into species with more limited geographic ranges. This is exemplified by the Diopatra genus, which has recently witnessed dozens of new species descriptions rooted in genetic analyses. In the northwestern Atlantic, the name D. cuprea (Bosc 1802) has been applied to populations from Cape Cod through the Gulf of Mexico, Central America, and Brazil. Here, we sequenced mitochondrial cytochrome oxidase I (COI) in D. cuprea populations from the Gulf of Mexico to Massachusetts. We find evidence for several deep mitochondrial lineages, suggesting that cryptic diversity is present in the D. cuprea complex from this coastline.