The Pleistocene and post−Pleistocene evolutionary history of many North Atlantic intertidal invertebrate species is well known, but the evolutionary history of the deep North Atlantic fauna is poorly understood, specifically whether colonization of the deep North Atlantic paralleled the patterns observed in shallow water. Contemporary pan−Atlantic species distributions could result from several colonization pathways that connected different regions of the Atlantic at different times ( e.g. Arctic, Antarctic or Panamanian path− ways). To test potential colonization pathways we quantified geographic variation in nu− clear and mitochondrial markers from Atlantic samples of Nucula atacellana, a pan−Atlantic deep−sea protobranch bivalve, using N. profundorum in the eastern central Pacific as an outgroup. We combined existing 16S data from North and South Atlantic populations of N. atacellana with new sequences of 16S, COI, and an intron of calmodulin from those populations, and newly sampled populations near Iceland. Population genetic analyses indicated a subtropical expansion via the Central American Seaway. We found no evidence for Transarctic migration to the Atlantic in N. atacellana , which suggests that colonization pathways may differ significantly between shallow− and deep−water fauna.