Dicharax amamiensis n. sp. is described based on specimens from Amami-Oshima Island, Japan. The new species has hitherto been identified in the Japanese literature as Dicharax purus (as Chamalycaeus purus), which was originally described from Tokunoshima Island. However, a molecular phylogenetic analysis using mitochondrial cytochrome oxidase subunit I gene and 16S rRNA region sequences of 10 species of Alycaeinae, as well as those of D. amamiensis n. sp., revealed that D. amamiensis n. sp. is distinct and separated from the congener D. purus. On the basis of morphological observations, D. amamiensis n. sp. differs from D. purus in possessing a relatively taller shell, a thick, slightly reflexed peristome, a proportionally broader penultimate whorl, and a narrower umbilicus.

Two new species of Benimakia Habe, 1958 are described from Japan: Benimakia ogasawarana n. sp. (Ogasawara Islands, Japan) and Benimakia tengusacola n. sp. (central Pacific coasts of Japan). The holotype of Turbinella rhodostoma Dunker, 1860 and a syntype of Turbinella fastigium Reeve, 1847 are figured for comparison.

Culmenella prashadi (Clench, 1931) is a critically endangered freshwater snail whose habitats have significantly declined despite its wide recorded distribution. The authors discovered a new population of Cu. prashadi in a small mountain pond in Fukushima Prefecture, northern Honshu, Japan, from where the species had not been previously recorded. Morphological examination of the shell, head-foot, and digestive, reproductive, and central nervous systems revealed that the examined population had similar features to previously dissected snails from Ishikawa Prefecture, while exhibiting intraspecific variations in the accessory preputial organs and radulae. The present anatomical observations confirmed that Cu. prashadi can be distinguished from its congeners and the closely related Camptoceras hirasei Walker, 1919 based on morphological differences in the mantle flap, male reproductive organs, and central nervous system. Base substitutions were also identified between the haplotypes of the populations in Fukushima and Aomori Prefectures.

Genetic analyses have detected five distinct lineages of Sinanodonta, including two potentially cryptic species, native to Japan. For the formal description of two new species (S. plana n. sp. and S. kyushuensis n. sp.), their shell morphological diversity must be examined. The linear morphometric shell indices were analyzed by performing linear discriminant analysis of these species. Sinanodonta tumens exhibited distinct morphological features, with a roundly ovate and inflated shell, and S. plana n. sp. exhibited an elongated ovate and flat shell. Sinanodonta calipygos was distinguished from S. lauta and S. kyushuensis n. sp. by the standardized convexity index. Although marginal differences in shell shape were observed between S. lauta and S. kyushuensis n. sp., young shells displayed contrasting colors, with a brownish-green periostracum in S. lauta and a greenish-brown hue in S. kyushuensis n. sp. Our research adds to the information available for the genus Sinanodonta, and we describe two new Sinanodonta species.

A new cardiid species of the subfamily Fraginae, Ctenocardia excavata n. sp., is described from the southern parts of Japan, from the Kii Peninsula to Okinawa Islands. It represents the sole Ctenocardia whose northern distribution limit reaches southern Honshu. It is characterized by a deeply excavated posterior margin, a relatively high rib number (44–49), and a rib sculpture consisting of knobby scales on the umbonal keel.

Pholadid bivalves in the genus Penitella Valenciennes are well represented in Japanese molluscan literature and have most often been cited as either Pholadidea or Penitella kamakurensis (Yokoyama) (i.e., Jouannetia kamakurensis Yokoyama, 1922). However, the binomen Penitella kamakurensis has most commonly been applied to two different species, one being the large Penitella gabbii (Tryon, 1863), and the other of a quite distinct smaller species, here described as Penitella aikoae n. sp. Penitella aikoae n. sp. differs from all other species in the genus by its short, almost straight (not distinctly concave) anterior-dorsal margin, a rounded, not posteriorly pointed, mesoplax (umbonal accessory plate), a short, thickened, leafy periostracal fringe on the posterior margin of the shell, and an incomplete callum that only partially closes the anterior pedal gape in the adult stage.

Megacardita Sacco, 1899 in the Plio-Pleistocene Omma-Manganji fauna contains two species: M. ferruginosa (A. Adams & Reeve, 1850) and M. ommaensis (Ogasawara, 1977). Megacardita ferruginosa first appeared in the latest Middle Miocene in the Pacific side of central Honshu and expanded into the semi-enclosed Japan Sea via the Tsushima Warm Current in the Late Pliocene. The extinct species M. ommaensis first appeared on the Japan Sea side of central Honshu and survived until the Middle Pleistocene. This species can be discriminated from M. ferruginosa by having no nodes on its radial ribs, which become low and flat-topped toward the ventral margin and are separated by narrower interspaces. Moreover, M. ommaensis has more numerous radial ribs (16 to 21; commonly 17 to 19) than M. ferruginosa (13 to 19; commonly 14 to 16). It has become clear that M. ommaensis also occurred in the uppermost Lower Pleistocene Umegase Formation in Chiba Prefecture, which is the only record of this species on the Pacific side. It might have migrated from the Japan Sea to the Pacific side of central Honshu via the Tsugaru Warm Current in the late Early Pleistocene.

Genetic variation is often determined by species' specific life history traits, but natural disturbances are also expected to affect it. However, studies of the latter are limited due to the contingency of disturbances. Here, we focus on six rocky intertidal snails, including Nucella heyseana which is suitable model species for a comparison of genetic diversity and structure before and after the Great East Japan Earthquake and tsunami in 2011. They are distributed on the Sanriku coast, where the marine ecosystems have been repeatedly affected by tsunamis. Two years after the 2011 tsunami, we evaluated genetic diversity and the levels of genetic connectivity of snails representing different developmental types: five planktonic larval developers and one direct developer (N. heyseana). There was no significant genetic differentiation among populations in the planktonic larval developers, while N. heyseana exhibited significant genetic differentiation. Comparing the genetic diversity before and after the 2011 tsunami, we detected changes in haplotype frequencies and a decline in genetic diversity (Hachinohe population). Given its low dispersal ability, the effects of a massive disturbance can vary among populations. Comparative genetic studies can contribute to understanding the influence of massive natural disturbances on the genetic structure and diversity of marine species.

The epizoic lottiid limpet Lottia tenuisculpta uses tegulid and muricid snails as hosts in its juvenile period. However, it is unclear when and how it finds its host species. To clarify the timing of attachment to the host shells, larval settlement was experimentally induced under controlled conditions. Between January and February 2017, mature L. tenuisculpta were collected from rocky shores around the Seto Marine Biological Laboratory, Japan. Planktonic larvae were prepared using artificial fertilization methods. Four small plastic plates, each covered with benthic diatoms and treated with tegulid host mucus, muricid host mucus, conspecific adult mucus, and a control were hung in beakers as settlement substrates with a certain amount of veliger larvae and filtered seawater. After 24 h, the metamorphosed larvae were counted on each plate. The number of settled larvae was significantly higher on the mucus of the tegulid host than on the control. This result suggests that the larvae of L. tenuisculpta are attracted to their host species by their mucus and may settle on the host shells directly.