Freshwater Colonization by a Scaleless Scale Worm: Pisione mizuchi sp. nov. (Annelida: Sigalionidae), from Sado Island, Japan

We describe a new species of pisionid annelid, Pisione mizuchi sp. nov., found from rivers on Sado Island, Japan. This species represents the second known member of the genus Pisione Grube, 1857 to inhabit freshwater environments. The new species can be distinguished from its congeners by the absence of elongated ventral cirri on segment 2 and dorsal cirri on segment 3, the absence of bilobed prechaetal lobes, the presence of four chaetae per parapodium, and a single pair of male copulatory organs associated with strongly modified parapodia. A phylogenetic tree of Pisioninae based on four molecular markers (cytochrome c oxidase subunit I, 16S rRNA, 18S rRNA, and 28S rRNA) is also provided.