During recent dives by the crewed submersible Shinkai 6500 in abyssal depths on the landward slope of the Japan Trench (5343-5379 m), five gastropod and four bivalve species were collected from seep environments. The gastropods include a recently described species in the Neomphalidae, two new species in the Provannidae, a single species in the Buccinidae, and a new species in the Cancellariidae. Bivalves are represented by a single species of the Solemyidae, a single species of the Thyasiridae and two species of the Vesicomyidae, the most of which were previously described from greater depths in the same trench.
The distribution of molluscan species at six cold-seep sites at between 5291 and 6290 m on the landward slope of the Japan Trench was studied using the crewed submersible Shinkai 6500 and the ROV Kaiko. At least four species of bivalve (Acharax johnsoni, Calyptogena phaseoliformis, C. fossajaponica and Parathyasira kaireiae) and three species of gastropod (Retiskenea diploura, Provanna abyssalis, and Provanna shinkaiae) were determined to be endemic to chemosynthesis-based communities. Two gastropod species (Bayerius arnoldi and Admete tenuissima) were confirmed to be colonists in chemosynthesis-based communities. The molluscan assemblages were of differing composition at each site. The two dominant bivalves, C. phaseoliformis and C. fossajaponica, showed specific differences in their distributions. C. phaseoliformis was mainly associated with silt sediments, whereas C. fossajaponica dominated the aggregations on pebble substrates. However, in a single case of co-dominance by both Calyptogena species, almost all individuals were of small size. The small gastropod Retiskenea diploura was found in clusters on the shell surface of C. phaseoliformis and on the silt sediments within the aggregations. This is the first record of a species of the family Neomphalidae from a chemosynthesis-based community in Japanese waters. The Molluscan faunal assemblages of cold-seep communities in the Japan Trench are similar at the familial and generic levels to those of cold-seep communities in the eastern Pacific, especially the Aleutian Trench subduction zone.
Eucleoteuthis luminosa paralarvae ranging from 1.4 to 12.4 mm in mantle length (ML) and Ornithoteuthis volatilis paralarvae ranging from 2.4 to 5.4 mm ML are described based on specimens collected from waters off the Ogasawara Islands and from off Shikoku to the Philippine Sea. E. luminosa paralarvae are characterized by having subocular and intestinal photophores, a relatively long proboscis with a mean proboscis index (ratio of proboscis length to mantle length) of 0.70, and moderately enlarged lateral proboscis suckers with a mean ratio of lateral sucker diameter to medial sucker diameter ratio of 1.57. O. volatilis paralarvae are characterized by having subocular and intestinal photophores, a relatively short proboscis with a mean proboscis index of 0.49, and considerably enlarged lateral proboscis suckers, with a mean ratio of lateral sucker diameter to medial sucker diameter of 2.09.
Spatio-temporal patterns of the populations of two congeneric buccinid gastropods Japeuthria ferrea and J. cingulata on an intertidal stony shore in the western Pacific were surveyed. The two species had similar patterns of life history and maintained stable population densities from July 1994 to October 1995. The smallest cohort appeared in August and rapid shell growth was observed in both species during the warm months from May to October. The data suggested that both species live at least four years. J. ferrea reached 11 mm in shell length in the first year, 17 mm in the second and 22 mm by the third year. J. cingulata reached 13 mm in the first year, 20 mm in the second year and 26.5 mm in the third. Recruitment occurred mainly at the mid-tidal level and there was a clear downward shift in the mode of distribution in both species as body size increased. Dispersion analysis revealed that Japeuthria individuals were more strongly aggregated towards the edge of their vertical distribution.
Copulatory behavior of the simultaneously hermaphroditic land snail, Achatina fulica Bowdich, 1822 (Gastropoda : Pulmonata : Achatinidae) was observed by radio-tracking. Snails with attached radio-tags were in the latter two of the three age groups here established ; juveniles, young adults and old adults. The home ranges of young adults were larger in area than those of old adults. The cumulative dispersal distances from release points of young adults were greater than those of old adults. The courtship initiator and courtship acceptor showed very different mating behaviors. Young adults copulated more frequently than old adults. In many cases, young adults held the courtship initiator role when they copulated, while old adults were courtship acceptors. Young adults initiated courtship with old adults more frequently than old adults did with young adults. Multiple copulation was confirmed for every individual observed in the field. Young adults moved around wide areas searching for their mating partners and positively chose old adults as their mating partners. On the other hand, old adults stayed within a narrow area and showed passive mating bahviour. The mating manners of A. fulica clearly correspond to protandrous maturation. Since young adults are able to produce only sperm, it would be advantageous for them to copulate with older adults that can produce both eggs and sperm.
The vertical distribution of Japeuthria cingulata (Reeve, 1847) was studied on Sakura-jima Island, Japan. Frequency distributions of shell size showed that newly hatched juveniles appeared in April, but their recruitment did not seem to be concentrated in any specific period over a year. In the intertidal area, the vertical distribution range of J. cingulata was larger than that of Cronia margariticola (Broderip, 1833) and Morula musiva (Kiener, 1836). Juvenile J. cingulata occurred in the lower zone, while adults occurred from the lower zone to the middle zone (broadly defined). No seasonal change of distribution in the intertidal area was observed.
The ancient Nakasuji sand dune on Taramajima Island in the Sakishima Archipelago features two layers of intercalated eolian dust paleosol. The eolian dust layers have yielded terrestrial molluscan fossils in large numbers. There are in all 21 species of land mollusc, including six now extinct on the island. The fossils are predominantly minute fossils, but larger specimens are also preserved well without severe damage. This indicates that these assemblages are autochthonous. The species composition of the minute fossils differs from layer to layer. Gastrocopta (Sinalubinula) armigerella and Tornatellides boeningi are dominant in the lower paleosol layer and the underlying sand layer, while Georissa japonica, Carychium cymatoplax, Platyrhaphe hirasei nudus and Aphanoconia yaeyamaensis dominate the upper layer. Judging from the habitats of their Recent counterparts, it is inferred that the fossil assemblages in the lower layers were formed near the coast lines and that those in the upper layer were formed in and around coastal forests. This could be due to a fall in sea level associated with glacial eustasy in the last ice age.