Fifteen species of Enixotrophon collected during expeditions of the Tropical Deep-Sea Benthos programme are reported from the Indo-West Pacific. All are illustrated using the holotypes and material collected during more than 30 years. The species were collected in the bathyal zone, living specimens occurring in depths from 198 to 1,280 m, with a majority between 250 and 800 m. One new combination, Enixotrophon pistillum (Barnard, 1959), is introduced; Trophon johannthielei Barnard, 1959 and Trophonopsis ziczac Tiba, 1981 are synonymised with E. pulchellus (Schepman, 1911). New geographical distribution data are presented for E. lochi (Marshall & Houart, 2011), E. pistillum, E. planispinus (E.A. Smith, 1906), E. plicilaminatus (Verco, 1909), E. pulchellus (Schepman, 1911) and E. sansibaricus (Thiele, 1925). Six species are recorded from the New Caledonia region, E. lochi, E. multigradus (Houart, 1990), E. obtuseliratus (Schepman, 1911), E. plicilaminatus (Verco, 1909), E. procerus (Houart, 2001) and E. pulchellus (Schepman, 1911). E. karubar n. sp. is described from Indonesia and one subadult specimen collected in the Mozambique Channel during the MAINBAZA expedition remains unidentified (Enixotrophon sp. cf. E. sansibaricus).

Two new species of the family Epitoniidae are described from the continental slope of the Sea of Japan, which is a marginal sea of the northwestern Pacific that has undergone major geographical events since the Lower Pleistocene, and has a rather poor but unique fauna. Papuliscala acus n. sp. was recorded not only in the Sea of Japan but also on several parts of the Pacific coast of Japan, e.g. off Wakayama and Kochi Prefectures, and shows considerable intraspecific variation in size and sculpture. Cirsotrema sugimurai n. sp. is considered to be endemic to the Sea of Japan, and thus one of the survivors of the mass extinction that occurred in the sea during the Last Glacial Maximum, ca. 20,000 years ago.

Genetic analysis revealed that Pronodularia japanensis can be divided into three groups. But it was not possible to determine which group corresponds to P. japanensis, because the exact type locality of this species is unknown. By comparing the ratios of shell height and shell width to shell length, the group (P. cf. japanensis 3) distributed in northeastern Honshu was regarded as that corresponding to P. japanensis. On this basis it was estimated where the lectotype of this species was collected.

A new thyasirid bivalve, Thyasira inadai n. sp., is described from the Japan Sea. This species lives in the Japan Sea and Yellow Sea and can be separated from the relatively shallower-water dwelling T. tokunagai Kuroda & Habe, 1951 by having a larger shell, a long and narrow auricle, a medial flattened area, an excavated lunule demarcated by an incision, a narrow apical angle and a larger prodissoconch. Based on the size of the prodissoconch and on molecular data, T. inadai n. sp. is related to the north Atlantic species, T. gouldii (Philippi, 1845). This phylogenic relationship is likely due to migration through the Bering Strait, which has opened since the latest Miocene.

A semi-enclosed marginal sea such as the Japan Sea was an important locus for species diversification in the benthos, along with climate change. The genus Scapharca Gray, 1847 represents an example of such diversification after its first appearance in the Japan Sea in the Late Pliocene (Piacenzian) via the inflow of the Tsushima warm current. In this paper, S. satowi Dunker, 1882 is newly recorded and described from the uppermost Lower Pleistocene (Calabrian) Omma Formation in Ishikawa Prefecture. S. pseudosubcrenata (Ogasawara, 1977) is redescribed based on newly obtained specimens from the type locality and the Lower Pleistocene (Calabrian) Uonuma Group (Middle Formation) in Niigata Prefecture. By summarizing the geologic range of all Plio-Pleistocene species of Scapharca in the Japan Sea, it has been revealed that two latest Early Pleistocene glacial stages (MIS 22 and MIS 20) caused the extinction of S. ommaensis (Otuka, 1936) and S. pseudosubcrenata respectively. In the glacial-interglacial age of the Middle Pleistocene (Chibanian), S. akitaensis (Noda, 1966) appeared and S. broughtonii (Schrenck, 1867) also adapted itself to colder conditions. Four Recent species, S. satowi, S. broughtonii, S. kagoshimensis and S. akitaensis, also survived the glacial ages since the latest Early Pleistocene (Calabrian) because fossils occur on both the Japan Sea side and the Pacific Ocean side.

Salinity tolerances of unionoid mussels were examined in experimental tanks, and their habitats were investigated in a brackish lake. In an experimental study involving adults of three mussel species, Margaritifera sp., Lanceolaria oxyrhyncha and Nodularia douglasiae, water salinity was gradually increased from 3 to 12 psu. At 3 psu, all the specimens of Margaritifera sp. died. All the specimens of L. oxyrhyncha died in the range 6–9 psu, but N. douglasiae survived. At 9–12 psu, all the subject specimens died. In subsequent experiments on Margaritifera sp. and N. douglasiae, individuals were kept at lethal salinity (3 and 9 psu respectively) for 3 h to 2 d and then moved to safe salinity (0 psu), and almost all survived. In experiments that manipulated salinity at 1–2 d intervals from a safe level (0 psu) to a lethal level (9 psu) to safe level, and many specimens of N. douglasiae survived. However, in experiments that manipulated salinity at 1–2 d intervals from a safe level (3 psu) to a lethal level (9 psu) to safe level, many specimens of N. douglasiae died. For the experimental study on juveniles, we manipulated the salinity levels from 0 to 22–33 psu then lowered them to 0 psu within a few days. We confirmed that several glochidia of N. douglasiae and Buldowskia spp. successfully detached from their hosts and metamorphosed. During these experiments, five euryhaline fish species, Plectorhinchus cinctus and Scatophagus argus for N. douglasiae and Terapon jarbua, Paralichthys olivaceus and Paraplagusia japonica for Buldowskia sp., were newly confirmed as host species. Field research on the host species of Glochidia was conducted in the brackish Lake Ogawara in Aomori Prefecture. The number of glochidia that parasitized the fish was determined at the infected sites on the fish, as well as the glochidial encystment rate, using formalin-fixed specimens collected from the lake. Two types of glochidia were found to be parasitic in several fish species examined, both from the known host species Hypomesus nipponensis, Pseudaspius hakonensis, and Gymnogobius castaneus, and from species not previously known to be hosts, namely Platichthys stellatus, and Mugil cephalus. Taken together, these results show that adult mussels could not survive at water salinity levels above 3–6 psu but that glochidia on a host were able to survive at high salinities. This suggests that these mussel species might expand their populations to other geographical areas through brackish and sea environments via host fish migration during the glochidium stage.