In the fourth paper about Ampharetidae from Japan, we describe 12 species in seven genera. The genus Orochi gen. nov. and the eight species Amage ehlersi sp. nov., A. longitorus sp. nov., Glyphanostomum hesslei sp. nov., Lysippe nipponica sp. nov., Melinnopsis augeneri sp. nov., M. mcintoshi sp. nov., Orochi palacephalus gen. et sp. nov., and Samytha annenkovae sp. nov. are newly described. Orochi gen. nov. is characterized by the shovel-like prostomium, the high membrane, from which all four pairs of branchiae emerge and the presence of neuropodia of the abdominal type in the last thoracic unciniger. In the discussions of the remaining genera, we comment on the taxonomic changes suggested by Jirkov (2011). Furthermore, we provide a list of all Ampharetidae species that have been recorded from Japan.

During the Capbreton cruises (1987–1990), samples of bathyal benthic macrofauna were taken at 37 stations situated along the continental slope (480–1113 m depth) of the Capbreton Canyon (Bay of Biscay, north-east Atlantic). Ten species of polychaetes belonging to six genera (Melinna, Eclysippe, Amphicteis, Anobothrus, Glyphanostomum and Tanseimaruana) of the family Ampharetidae Malmgren, 1866 were collected at 13 of these stations. Most of these species are reported for the first time in the area. Amphicteis aff. wesenbergae is reported for the first time after its recent description and a new species belonging to the genus Glyphanostomum is here described. Moreover, the finding of Melinna monoceroides represents the northernmost record of the species; Tanseimaruana vestis is recorded for the second time from the East Atlantic after its recent finding in Icelandic waters; Anobothrus aff. gracilis is reported for the second time from the Iberian Peninsula, representing, the southernmost record of the species in the Atlantic Ocean. The potential identification of two fragments of an Ampharetinae indet as Anobothrus laubieri, a deep water Arctic species, is discussed.