Two species of bivalve of the genus Vesicomya were collected by the submersible Shinkai 6500 and the ROV Kaiko from chemosynthetic environments in upper abyssal depths in the western and central sectors of the Nankai Trough, extending from off Shikoku to off western Honshu, Japan. They are Vesicomya crenulomarginata n. sp. and large specimens of V. kaikoae Okutani, Fujikura & Kojima, 2000. Besides five known species of Calyptogena collected, another new species of the same genus Calyptogena (Archivesica) magnocultellus n. sp. was found and is described here. Ontogenic and individual variabilities in C. (Ectenagena) nautilei are discussed.
Conchocele novaeguinensis n. sp. is described from a thanatocoenosis found on the continental slope of northern coast of New Guinea. The occurrence of this species together with other animals suggests the existence of cold seep activity in the area.
Monstrotyphis singularis n. sp. is described from New Caledonia. A revised classification of all the Recent species included in Monstrotyphis is provided. Talityphis Jousseaume, 1882 and Typhina Jousseaume, 1880 are considered to be congeneric and Talityphis bengalensis Radwin & D'Attilio, 1976 is regarded as a junior synonym of Typhis nitens Hinds, 1843.
Euprymna megaspadicea n. sp. collected from Nago Bight, Okinawa Island, Japan, is described. This species is characterized by its small size compared with its congeners, and by having four rows of suckers on all arms, arm II longer than the other arms and a hectocotylized left arm I in males. Modification of the hectocotylus affects almost the whole length of the arm, and is characterized by the fusion of all sucker pedicels giving the arm a peculiar massive, wavy and twisted appearance.
Taxonomic relationships among the four corbulid species, Corbula peregrina, C. tanabensis, C. tsukaharai and C. succincta, all of which were described based on the specimens from the Miocene Tanabe Group, are discussed. C. peregrina is a member of the genus Solidicorbula, and C. tanabensis and C. tsukahara are synonyms ; C. succincta is a distinct species and is reassigned to the genus Varicorbula ; Solidicorbula succincta (Yokoyama) auct. reported from the upper lower-lower middle Miocene in southwest Japan is synonymous with C. peregrina, not C. succincta. Two valid species, Solidicorbula peregrina, n. comb. and Varicorbula succincta, n. comb., are redescribed.
The mode of feeding by Conus tulipa on fishes was observed to differ markedly from that of other piscivorous congeners such as C. striatus, C. fulmen and C. geographus. Conus tulipa swallows the prey fish into its rostrum together with a large quantity of water, but the present observations did not determine whether or not a radular tooth is injected. The role of a set of unusual structures, here termed 'coronary rostral tentacles', in C. tulipa feeding is described. Another unusual habit of this species is group feeding. In the laboratory, several individuals attacked a single prey animal, which was eventually swallowed by one of them.
The symbiotic association of the myid bivalve Cryptomya (Venatomya) truncata with thalassinidean shrimps was investigated together with morphometric variation on tidal flats in western Japan, including the Nansei Islands. Several specimens of C. truncata were found buried very close to the burrows of Upogebia major, U. yokoyai, U. carinicauda and Callianassa japonica. In each case the bivalve was found with its posterior end facing the burrow tunnel at about 5-20 cm deep in the sediment, suggesting that the bivalve was a filter feeder using the shrimp burrow as an extended siphon. Cryptomya truncata was also found in sediments with abundant burrows of U. issaeffi, U. narutensis and U. pugnax, although direct evidence of a symbiotic association was not obtained. Morphometric analyses showed that C. truncata grew taller and thicker in shape ontogenetically, probably as a result of its symbiotic lifestyle with a reduced need to use its long siphons. Cryptomya truncata from Amami-Oshima Is. were smaller and thicker than those from Honshu, Okinawa Is. and Ishigaki Is.
Bivalves on subtropical sandy beaches are potentially important food resources but, in general, estimating their abundance has been thought to be difficult due to a high variability in their density. This study quantitatively demonstrates the vertical distribution of Atactodea striata and Latona (=Donax) faba on a sandy beach in Ishigaki Island in southwestern Japan in the summer. Both species occurred over a wide vertical range in the intertidal zone with their peaks around the mean tide level. Associated bivalve fauna consisted of Donacilla picta, Davila plana, and Katelysia hiantina, but all were very rare. The vertical distribution patterns of A. striata and L. faba did not change between the low and high tides due to the small range of migration, which was revealed by the recapture of marked bivalves during 6 hrs and 24 hrs on very fine and calm days in the summer. Size-frequency histograms showed that small individuals (probably juveniles) were predominant in the populations of both species. The low efficiency of sampling at high tide and the intraspecific zonation by size suggested that the sampling should be carried out over the entire range of the intertidal zone at low tide.
Tree climbing behavior of the snail Cerithidea rhizophorarum A. Adams, 1855 (Gastropoda : Potamididae) was observed in a mangrove forest at the mouth of the Atago River, southern Kyushu, Japan. The forest is dominated by Kandelia candel (L.) Druce and Hibiscus hamabo Sieb. et Zucc. It is the northern limit of distribution of mangroves in the west Pacific. C. rhizophorarum was in the intertidal zone of the tidal flat most of the time but occasionally climbed up the trunks of mangrove trees The number of snails that climbed up trees in 100 m^2 decreased between June and August, but suddenly increased in late August and fluctuated between 1000 and 2000 through January. The number of snails inhabiting the tidal flat kept decreasing from June to December and increased from January through April. A larger proportion of snails moved off the trees in summer than in winter. There was a clear ebb and flow rhythm in the locomotion activity which synchronized with the tidal shift in the summer. Snails tended to fall onto the tidal flat at low tide and to climb up trees at the high tide. Snails aggregated on particular trees regardless of the physical traits of the trees. The climbing behavior of C. rhizophorarum seems to be dependent on both seasons and the ebb and flow rhythm of locomotion.