The Recent species of the genus Volutharpa in Japanese and adjacent waters are reviewed based on the morphology of the shell, operculum, penis and radula, as well as their geographical distribution. The genus is provisionally distinguished from the allied genus Buccinum based on the differences in the general morphology of the shell. Five operational taxa are recognized, and compared with each other. The morphology of the penis, especially the gonopore, and of the periostracum were found to be most valuable in distinguishing species and subspecies within the genus. Radula characters, including the number of cusps, were shown to be variable even within species, and not suitable for species-level classification. Three discontinuous morphotypes were recognized in the shape of the gonopore, and these were considered to represent three species groups: 1) papillary type: ampullacea; 2) triangular type: nipponkaiensis species group comprising nipponkaiensis and limnaeformis; and 3) pointed type: perryi species group comprising perryi and ainos. Furthermore, two taxa in each species group were morphologically distinguished by the condition of the periostracum, and distinguished as subspecies. The subspecific distinction of limnaeformis and ainos are also supported by their allopatric distribution with corresponding nominotypical subspecies. As a result, the genus Volutharpa in Japanese waters is revised to comprise three species and two subspecies.

The nudibranch mollusc Trinchesia lenkae Martynov, 2002 is reported in the Japanese fauna for the first time. The newly collected Japanese specimens of T. lenkae are compared with those from the type locality of this species in Peter the Great Bay in the Sea of Japan, Russia, using external and internal morphological data and molecular analyses. The molecular study revealed high genetic homogeneity between Japanese and Russian populations of T. lenkae. The integrative morphological and molecular data thus clearly suggest that the same species occurs in Russia and Japan.

"Calyptogena" marissinica n. sp. is described from the Haima cold seep on the northwestern slope of the South China Sea, China, at depths of 1,372 m and 1,398 m. Phylogenetic reconstruction using the cytochrome oxidase c subunit I (COI) gene shows that it is sister to Calyptogena similaris but distinct from that species by a pairwise distance of 3.9%, larger than the average intraspecific variation within the genus Calyptogena sensu lato. Morphologically, the new species is distinguished from C. similaris in having a less elongate shell, with nearly straight ventral margin, and no apparent anterior lateral tooth.

Cuspidariidae is a family of bizarre heterodont bivalves with a specialized carnivorous lifestyle. An unusual cuspidariid bivalve was discovered from 2,489 m depth in the Snail hydrothermal vent site, Southern Mariana Trough, and is described herein as Thermomya sulcata n. gen. & n. sp. Although cuspidariids are well-known from the deep sea, until now none were known to inhabit chemosynthetic ecosystems. In fact, this is also the first member of the clade Anomalodesmata found in hydrothermal vent ecosystems.

The shape and formation of the radula have traditionally been regarded as important characteristics in classifying nudibranchs. However, we cannot use radular information to aid in the classification of living specimens, because a radula cannot be examined unless dissected. In contrast, the reliability of mantle color patterns as a characteristic for species identification has been open to debate because of the wide variations observed among individuals. When we compared various characteristics of Chromodoris reticulata and Chromodoris tinctoria (Opisthobranchia, Nudibranchia), which have similar color patterns and are sometimes confused with each other, we found considerable differences in the overall body shape, in external and internal morphologies such as the rhinophores and secondary gill, shape of the spines on the surface of the penis, formation style of the embryos and egg capsules, and in mating behavior. A consistent relationship was observed between these characteristics and the color pattern of the mantle. During crossing experiments, although members of the two species tried to mate, they stopped copulation before inserting their penes. Our results support the theory that mantle color pattern is a reliable characteristic for identification of at least some nudibranchs and is applicable to ecological studies.

Multiple neritid species co-occur in tropical and subtropical estuaries. To demonstrate their micro-distribution patterns, the abundances of Neritina turrita, N. plumbea, N. auriculata and Clithon corona were surveyed in an upper mangrove area in Ishigaki Island by setting a 28 m² sampling area. Numbers of individuals were recorded in 25-cm grid positions in January and July 2006. Examinations of Moran's I coefficients showed the presence of positive spatial autocorrelations for all four species in both months. The conditional autoregressive model, a type of generalized linear mixed model, was applied to the abundance data with substrate depth from the high tide level as a fixed effect and spatial random effect. Model evaluation and coefficients of parameters demonstrated interspecific and temporal variations in the distribution. The effects of depth on the patterns of distribution were significant but were overridden by the effects of the spatial autocorrelation except for N. turrita in January and N. auriculata in July. Without considering spatial autocorrelation in the distribution, estimates of the abundances of neritid species in mangrove areas may be erroneous.

Microalgae in fecal pellets of the freshwater pearl mussel Margaritifera laevis were identified to the lowest possible taxon and measured to identify the characteristics of microalgae ingested by the mussels. As a case study, fecal pellets were obtained from adult and juvenile mussels collected from a drainage ditch connected to the Hida River, Gifu Prefecture, central Honshu, Japan in June and October 2015. Ditch water was also collected in October, and microalgae in these samples were analyzed. We identified five microalgal phyla in mussel fecal pellets: Cyanobacteria, Euglenozoa, Ochrophyta, Chlorophyta, and Charophyta. Cells and colonies of Ochrophyta (Bacillariophyceae) accounted for 98–99% of microalgae in the fecal pellets. Only 5–6% of Ochrophyta cells and colonies in fecal pellets had chloroplasts compared to 14–16% in ditch water. These results indicate that Ochrophyta is the major phylum of microalgae ingested by adult and juvenile mussels in the sampling site and that a portion of those cells and colonies were likely assimilated by the mussels. In contrast, 75% or more of the Chlorophyta cells and colonies in fecal pellets had chloroplasts, and thus the Chlorophyta were considered to not be actively assimilated by mussels. Microalgae in mussel fecal pellets ranged from 5 to 690 μm, the suggestion being that organic matter within this size range is potential food for the mussel. The characteristics of microalgae in fecal pellets of both adult and juvenile mussels were the same. The knowledge newly obtained in this study will contribute to clarifying food items of M. laevis in the field.