Nautiliniellid Polychaetes Living in the Mantle Cavity of Bivalve Mollusks from Cold Seeps and Hydrothermal Vents around Japan

摘要

Two new genera and four new species of nautiliniellid polychaetes living in the mantle cavity of bivalve mollusks are described from cold seeps and hydrothermal vents around Japan. Iheyomytilidicola tridentatus, n. gen., n. sp. from the ni.eya Ridge of the Okinawa Trough, has three kinds of setae and differs from other nautiliniellid polychaetes that have one or two kinds of setae. Thyasiridicola branchiatus, n. gen., n. sp. found on gill filaments of a deep-sea thyasirid clam from the Hatsushima cold seep in Sagami Bay, has elongated notopodia and less developed tentacular segment. This new genus is unique in the family in having an achaetous tentacular segment with both dorsal and ventral cirri. Shinkai semilonga, n. sp. from the Minami-Ensei Knoll of the Okinawa Trough, differs from other congeneric species in having more than five simple hooks on each para podium. Natsushima graciliceps, n. sp. from Kagoshima Bay differs from N. bijurcata in having enlarged notopodia in the mid-body region instead of lacking them. In addition, S. sagamiensis, S. longipedata, and N. bifurcata are reported with new records, remarks, and biological notes.