A species of marine limnodriloidine oligochaete, Thalassodrilides cf. briani Erséus, 1992 , is recorded from gravelly sand sediments of the subtidal zone in Ehime Prefecture, Japan. The present material agrees with the original description of T. briani, which was first found at Hong Kong, with the exception that the copulatory sacs are oval; not slender. Despite the lack of genetic data for the Hong Kong population, we conclude that the Japanese specimens are conspecific with it, or at least very closely related, based on morphological considerations. This is the first record of the genus Thalassodrilides Brinkhurst and Baker, 1979 in Japan. The phylogenetic relationships between T. cf. briani and three other species of Thalassodrilides are estimated, based on partial DNA sequences of the mitochondrial cytochrome c oxidase subunit I (COI) gene and the complete nuclear ribosomal Internal Transcribed Spacer (ITS) region, using two members of the closely related genus Doliodrilus Erséus, 1984 as outgroups. The genetic analysis shows that T. cf. briani is a species delimited by both mitochondrial and nuclear data, and clearly separated from at least its closely related congeners in the Northwest Atlantic (Caribbean and adjacent areas).
We are grateful to Dr Akifumi Ohtaka (Hirosaki University, Japan), Dr Yoshikazu Takashima (Hokkaido), Steven V. Fend (U.S. Geological Survey), and Dr Takafumi Nakano (Hiroshima University, Japan) for making suggestions about the taxonomic status and this manuscript. Also thanks to Katustoshi Ito (National Research Institute of Fisheries and Environment of Inland Sea) for providing us the material in this study, and to Anna Ansebo, Per Hjelmstedt, and Marcus Svensson for assistance in the molecular lab. Financial support to CE was provided by the Swedish Science Research Council.