The alycaeine Dicharax oshimanus (Pilsbry & Hirase, 1904) from Amami-Oshima Island is redescribed based on newly obtained specimens. Although two distinct species have been confused under the names Chamalycaeus oshimanus and D. oshimanus, true D. oshimanus differs from the other species by its whitish shell and prosocline axial ribs on the body whorl in apertural view. Dicharax kurozumitaijii n. sp. was collected by Mr. Taiji Kurozumi from Iriomote Island and had been reported as an undescribed species. The new species is endemic to Iriomote Island. Because its habitat is restricted to a small area within the limestone region of this island, its conservation is critical.

Two new species of the subgenus Sinica in the genus Diplommatina are described from limestone areas in Kochi Prefecture. Diplommatina (Sinica) torigatadentata n. sp. was obtained from Mt. Torigata, at an altitude of 1,300 m, and is characterized by its slightly large size for the genus (SL 4.5 mm), a strongly stout tooth on the columella, and a thin and flaring peristome. Diplommatina (Sinica) yokoae n. sp. was obtained from Kagami-Shiroiwa, Kochi City, at an altitude of 700 m, and is characterized by the lack of a spiral palatal fold. The latter represents the second species of the subgenus in Japan to lack a spiral palatal fold after D. (S.) nakashimai Minato, 2015, which differs from the present new species by possessing a double peristome, and also its larger size (SL 4.65 mm).

Two new species of camaenid snail belonging to the genus Bradybaena Beck, 1837, are described from Ryusento Island and the Uji Islands off the west and south coast respectively of Kyushu, Japan. Bradybaena toshiyukihamadai n. sp. and Bradybaena kenjioharai n. sp. are taxonomically distinct from all other species. Bradybaena toshiyukihamadai n. sp. has a small thin shell for the genus and major portion of the penial internal surface is covered by pleated diagonal wrinkles. This new species is also morphologically distinct from all other species. Bradybaena kenjioharai n. sp. has a thick shell, and major portion of the penial internal surface is covered by a grid-like micro-sculpture. However, this new species exhibits very similar morphological traits in both shell and genitalia to Bradybaena circulus (Pfeiffer, 1846), making it difficult to distinguish between the two.

A new carditid species of subfamily Carditamerinae, Centrocardita alticostata n. sp., with elevated radial ribs, is described from a sandy gravel bottom at 140 m depth, off Chichi-jima in the Ogasawara Islands, Japan. This is the first record of this genus from the isolated Ogasawara Islands in the Pacific Ocean.

The three large abalone species in the Japanese archipelago (Haliotis discus discus, H. madaka, and H. gigantea) are closely related genetically but exhibit unique ecological characteristics. The shells of the three species also exhibit qualitatively unique morphological characteristics; however, only limited studies have quantitatively evaluated these differences. To address this, we quantitatively evaluated the morphological differences among shells of these three abalone species, collected from the Kyushu coast. Our morphological analyses were based on multiple quantitative features, including overall shell length, breadth, and height, respiratory pore height, apex height, and lip breadth. These analyses reveal that the shell specimens of H. gigantea were quantitatively distinct from those of H. discus discus and H. madaka. However, the morphological characteristics of H. discus discus and H. madaka specimens were not quantitatively distinct, and instead showed substantial overlap. These findings suggest that there are individuals with intermediate quantitative traits between H. discus discus and H. madaka, although the shell phenotypes of these species are typically qualitatively distinct.

A new ungulinid bivalve species, Transkeia sagaensis n. sp., is described from the uppermost Eocene-lowest Oligocene Kishima Formation in Kyūshū, southwestern Japan. The present new species is the oldest member in the genus Transkeia Huber, 2015. In the previous molluscan studies of the formation, no ungulinid species have been recognized. This is probably due to misidentification as the venerid Cyclinella? compressa (Nagao, 1928b).

In the Plio-Pleistocene Omma-Manganji fauna, the carditid genus Cyclocardia comprises four extant species: Cyclocardia (Cyclocardia) rotunda (Tokunaga, 1906), C. (Cy.) isaotakii (Tiba, 1972), C. (Cy.) crebricostata (Krause, 1885), C. (Crassicardia) crassidens (Broderip & Sowerby, 1829), and one extinct species, C. (Cy.) myogadaniensis (Itoigawa, 1958). Among them, C. (Cy.) rotunda is newly designated for the species formerly identified as "C. (Cy.) ferruginea (Clessin, 1888)". This common species likely first appeared in the Early Pliocene in Japan. However, C. (Cy.) rotunda became part of the Omma-Manganji fauna in the Late Pliocene. The extinct species, C. (Cy.) myogadaniensis, first appeared in the Late Pliocene. The other three extant species, C. (Cy.) isaotakii, C. (Cy.) crebricostata and C. (Cr.) crassidens, initially appeared in the Miocene of Kamchatka or Sakhalin and became part of the Omma-Manganji fauna after the climate cooling at Datum A (2.75 Ma). These three species in the Omma-Manganji fauna are smaller than the largest individuals in the present-day fauna, likely due to the warm temperature in the southern limit area.

A new species of cyclophorid gastropod belonging to the genus Cyclophorus Montfort, 1810, is described from the upper Lower Miocene Korematsu Formation (17.0–16.5 Ma) of the Bihoku Group in Kubara, Nishigata, Niimi City, Okayama Prefecture, southwest Japan. Cyclophorus kubarensis n. sp. is morphologically distinct from all other extant Japanese and Vietnamese Miocene fossil species in its weak suture and weakly angulate periphery of the body whorl.

The karyotypes of seven species of the genus Semisulcospira from Lake Biwa, Japan were examined. The observed diploid chromosome number was as follows: Semisulcospira dilatata 2n=20 (14M+2SM+4T), S. rugosa, 2n=22 (16M+4SM+2ST); S. arenicola, 2n=24 (18M+4SM+2ST); S. fuscata, 2n=26 (18M+6SM+2ST); S. ourensis, 2n=28 (20M+6SM+2T), S. reticulata, 2n=28 (1M+6SM+6T), S. sp. cf. multigranosa, 2n=28 (4M+4SM+20T). The number of chromosomes in S. niponica group ranged from 2n=18 to 32, but no case of 2n=30,34 have been reported.

Gari virescens (Psammobiidae) is a near-threatened bivalve species that inhabits tidal flats in Japan. Recently, it has been frequently detected in the gut contents of tidal flat fishes. The purpose of this study is to establish regression formulas for reconstructing the shell length (SL) of predated specimens. Therefore, we clarified the relative growth relationship between body parts of the bivalve and its SL using specimens from the western Seto Inland Sea of Japan. The allometric regression formula for estimating SL (mm) from shell height (SH, mm) was SL = 1.53 × SH^1.0657 (r = 0.998, n = 66, P < 0.01), and the formula for estimating SL (mm) from foot length (FL, mm) was SL = 1.82 × FL^1.0478 (r = 0.969, n = 39, P < 0.01). In addition, we established several other regression formulas for estimating SL. As a case study of applying these formulas, we reconstructed the SL of specimens predated by the fish Acanthopagrus schlegelii. These allometric formulas enable the reconstruction of SL from body parts and shells broken during predation.

The European flat oyster, Ostrea edulis Linnaeus, 1758, a species of edible bivalve native to Europe, was introduced to Japan from the Netherlands in 1952. Seeds collected from specimens were transferred to several areas in Northeastern Japan for aquaculture trials. However, in the 2000s, these trials to introduce O. edulis into Japan were discontinued. Strains of the O. edulis species were preserved in several Japanese laboratories, but since the laboratories were washed away by the 2011 Tohoku Earthquake and Tsunami, the species was considered to be extinct in Japan. However, this study confirmed the survival of the species in several locations in Iwate Prefecture, notably Yamada Bay. This is the first recorded case in Japan of a non-native oyster species, originally introduced through aquaculture, naturalizing in Japan's marine environment. Ostrea edulis has been found in several sites beyond those where it had been introduced previously. This suggests that O. edulis may be expanding into other areas in Japan. Multifaceted discussions on the handling, commercial use, and epidemiological testing of the species will be necessary in the future.