A new species of the family Lucinidae, Elliptiolucina ingens, is described from off Amami-Oshima Island. An odd valve of Elliptiolucina magnifica Cosel & Bouchet, 2008 from the Sulu Sea and another of a closely related species from the Manus Basin were examined for comparison. Two other lucinid species were found with the new one: Dulcina musorstomi and Rostrilucina anterostrata, both of which were recently described from the Philippines and are new to the Japanese fauna.
We describe the morphology of a new species of the bivalve family Galeommatidae, Divariscintilla toyohiwakensis n. sp., which lives in the burrows of the mantis shrimp Acanthosquilla acanthocarpus at Oshinden, Nakatsu, Oita Prefecture, Kyushu, Japan. An unpaired median tentacle and three paired tentacles issue from the mid-mantle fold, which partially covers the shell. A single flower-like organ originates on the anterior surface of the visceral mass. A byssal adhesive gland is observed on the most posterior part of the foot. This is the first record of the genus in Japan and the North Pacific Ocean.
Among deep-sea chemosynthesis-based communities with large-scale Calyptogena soyoae/okutanii aggregations in the Off Hatsushima Island seep site, another new vesicomyid species, C. fortunata, was discovered. The analyzed DNA data of these specimens correspond to those from the Off Sanriku coast site, in spite of the fact that shell characters of specimens from both localities show a slight difference, probably representing phenotypic variability.
Three lots of Pacific sepiolid specimens in the collection of National Museum of Nature and Science, Tokyo, were found to contain three new species: Heteroteuthis nordopacifica n. sp., Sepiolina petasus n. sp. and Stoloteuthis japonica n. sp. The second-mentioned taxon is the second species in the genus Sepiolina, and the third species represents the first occurrence of the genus Stoloteuthis in the Pacific Ocean. The descriptions of each species and comparison with related species are given herewith.
The fossil record of extant vesicomyid bivalve species from Japan is re-evaluated, based on newly collected material and a literature survey. Calyptogena pacifica Dall was newly collected from the upper Miocene Akaishi Formation in Aomori Prefecture. The new data support previous observations that the extant C. pacifica has been distributed along the Japan Sea borderland since the late Miocene, although it is not living in the Japan Sea today. It is shown that the shell described by Yokoyama (1925) from the Pliocene Hitachi Formation in Ibaraki Prefecture as Tapes undulatus (Born) is another Recent species, Archivesica kawamurai (Kuroda). On the other hand, it is clear that Akebiconcha cf. kawamurai illustrated by Katto & Masuda (1978) from the lower Miocene Shikiya Formation in Wakayama Prefecture is not Archivesica kawamurai. In contrast to C. pacifica, A. kawamurai appeared in the Pliocene and was distributed along the Pacific side of southwestern and central Japan, where the species is still living today. Consequently, among sixteen Recent species of Pliocardiinae, only two have a fossil record.
In Yamaguchi Prefecture, there are found Trishoplita eumenes eumenes and Trishoplita eumenes cretacea, which both belong to the "Trishoplita eumenes" complex. These two subspecies have four forms that were described by the differences in shell coloration. There are some opinions concerning the classification of this complex. In this study, the taxonomic relationship of these two subspecies is investigated based on various morphological characters of the hard parts and on habitat preference. As a result, the distribution areas of the two subspecies have been clearly separated. Both subspecies are arboreal, but Trishoplita eumenes eumenes lives on parts of trees that are up to 1 m in height from the ground, and in meadows. On the other hand, Trishoplita eumenes cretacea lives on trees at from 1 to 3 m from the ground, and in forests. It was revealed that significant differences were found in the t-test for the diameter of the shell, the height of the shell, the angle of the apex, and the thickness of the shell. Significant differences were also found in the Mann-Whitney test for height/diameter and diameter of the umbilicus/shell diameter. In addition, these two subspecies were distinguishable in coloration and the surface quality of the shells. These results strongly suggest that these two subspecies are distinct species. So we think Trishoplita eumenes eumenes and Trishoplita eumenes cretacea are two independent species.
A fossil pearl from the Upper Miocene Kubota Formation in the Higashitanagura area, Fukushima Prefecture, northeastern Japan is described. The pearl is spherical in shape and made up of layers of regularly foliated or crossed foliated structure composed of thin lamellar crystallites. It also contains a lenticular sublayer of prismatic structure composed of needle-like crystallites. These correspond to the shell microstructures of Crassostrea gigas, which occurs commonly in the same stratigraphic horizon, indicating that the pearl was formed by this bivalve species. The prismatic sublayer might have been formed as a result of abnormal secretion by the mantle epithelium of the mother shell.
The diel rhythm of glochidial release from gravid mussels of the species Pronodularia japanensis was investigated in the laboratory. Gravid mussels discharged glochidia principally at night, with a peak in the early part of the night. Glochidia formed a mass by entangling their larval threads.