In the gnathiid isopod, Gnathia hayashiae, male adults attract female larvae to mate by releasing sex pheromones. Gnathiid females are thought to mate only once in their lifetime, immediately after metamorphosis. Therefore, this attraction should be an effective adaptation strategy to prevent females from missing mating opportunities. Attraction by adult males is reasonable because larvae are more motile than adults are. In contrast, male adults are potential reproductive competitors for male larvae. How do the male larvae respond to adult males? The present attraction test showed that neither adult male nor adult male extracts provoked a specific reaction in male larvae. In addition, the adult female extracts did not exhibit larval attractant/avoidant activity. We hypothesized that male larvae are not required to avoid male adults, because G. hayashiae adults have low motility and rarely encounter conspecific males in the wild.

The present study documents the first record of the fanworm Branchiomma luctuosum [new Japanese name: Koukai-murakumo-keyarimushi] (Annelida: Sabellidae) from the Pacific, based on a specimen collected from the tidal flat at Torinosu, Wakayama, Japan. The species was originally described from the Red Sea and has been reported as a non-indigenous species in the Mediterranean, Brazil, and Angola. It has also been recorded from Pakistan, although its status there, whether native or introduced, remains uncertain. Mitochondrial COI and cytb gene sequences of the specimen exhibited 99.9–100% identity with sequences from Italy, France, and Pakistan deposited in GenBank. Although the taxonomic validity of diagnostic characters within the genus Branchiomma has been questioned, B. luctuosum can be morphologically distinguished from other congeneric species recorded in Japan—Branchiomma cingulatum, Branchiomma sp. B sensu Capa et al. (2013), and Branchiomma sp. I sensu Kobayashi et al. (2024)—by the combination of its large size, the absence of macrostylodes, the presence of slender stylodes, including basal paired ones, and uniformly colored radioles. Given its type locality in the Red Sea and the absence of prior records from the Pacific, Branchiomma luctuosum is considered non-indigenous in this region.

A diet switching experiment was conducted to clarify the trophic discrimination factors (TDF) for nitrogen and carbon stable isotope ratios in Japanese abalone Haliotis discus discus. The experiment was carried out in complete darkness to prevent contamination by microalgae growth. After the experiment, the weight of the abalone increased three-fold. Nitrogen and carbon stable isotope ratios in the control group showed little change, while the treatment group showed large changes in both values. As a result of this study, the turnover (half-life) for both δ¹³C and δ¹⁵N were estimated to be 1.8. The results indicate that a 1.8-fold increase in the soft body of Japanese abalone results in a half-replacement of the stable isotope ratio. Therefore, the stable isotope ratios of Japanese abalone in natural environment may reflect a more recent diet in young abalone, whereas in older abalone they may represent a long-term average of a diet of more than one year. And TDF for δ¹³C and δ¹⁵N were estimated to be 2.97‰ and 1.13‰, respectively. Although our results for the TDF of Japanese abalone differed significantly from empirical principles, they were in broad agreement with previously reported TDF values for abalone species. Then, application of the results of this study may contribute to an accurate understanding of the feeding habits of Japanese abalone.

The sudden coastal uplift resulting from the 2024 Noto Peninsula earthquake exposed the previously subtidal and intertidal habitats, providing a unique opportunity to document the biotic community and immediate effects of geological disturbances on it. This study describes the characteristics of the benthic death assemblages around the Kaiso fishing port, which experienced approximately 3.6–3.9 m of uplift, with special attention to the vertical distribution of coastal benthos inhabiting Kaiso Reef. Our field surveys were initiated 66 days after the earthquake, integrating quadrat-based sampling with direct observation of emergent marine habitats. 92 molluscan and 5 echinoderm species were found in our quadrat investigation. The soft tissues of most molluscan individuals remained, indicating that their in situ life postures were undisturbed. Given this premise, species richness was estimated to have peaked between the pre-disaster middle to lower intertidal and uppermost subtidal zones, particularly in areas with reduced wave exposure. Additionally, the surveys identified several notable species at the Kaiso fishing port, including the Near-Threatened coral Oulastrea crispata and bivalve Pinna bicolor, both of which have their northernmost records in the Sea of Japan from the Noto area. The uplifted zone presents a rare opportunity to examine the subtidal fauna from terrestrial vantage points, yielding novel perspectives on marine biodiversity in the Sea of Japan. Also, these findings provide critical baseline data for monitoring post-uplift ecological succession against a backdrop of ongoing climate change.

The goby fish Eutaeniichthys gilli, distributed in eastern Asia, utilizes shrimp burrows as shelters and spawning nests in tidal flats. Despite previous research, comprehensive understanding of the distribution and morphological development of larvae and juveniles remains limited. This study aimed to characterize the morphological features of E. gilli larvae and juveniles and to investigate how these traits change throughout ontogeny, thereby offering insights into larval dispersal within inner bays and the mechanisms facilitating successful settlement. To clarify these ontogenetic changes of larval distribution, monthly sampling and morphological observations were conducted at the Susaki Inlet of Tosa Bay, southwestern Japan, in 2015–2016. Sampling was carried out monthly in the inner, middle, and outer areas of the inlet using larva and beam trawl nets. Eutaeniichthys gilli specimens were only found in June 2016 and only from the inner inlet. A total of 69 specimens were collected, and morphological measurements were performed. Morphological data were then compared with those of reared individuals previously reported in the literature. While the overall morphology was largely consistent with earlier descriptions, differences were evident in pigmentation patterns. A comparison of their eye diameter-to-body length ratio, which is particularly relevant to larval development, revealed a marked decrease during ontogeny. Because no larvae or juveniles were collected from the middle and outer inlets, most E. gilli individuals were assumed to be vertically dispersed after hatching, remaining in the inner part of the inlet.

Sargassum beds provide important habitats for diverse epifaunal organisms including gammarid amphipods that are closely associated with macroalgae. This study aimed to describe the seasonal dynamics of gammarid assemblages associated with two predominant temperate seaweeds, Sargassum macrocarpum and S. patens in Tsukumo Bay, Noto Peninsula, central Sea of Japan. Because tidal amplitude in the Sea of Japan is minimal, this region offers an ideal setting for evaluating seasonal biological patterns with limited tidal influence. Seaweed samples were collected seasonally from winter to autumn in 2023 and associated gammarids were identified to the family level to assess abundance, richness, and Shannon diversity. Statistical analyses revealed significant seasonal variation in gammarid assemblage composition, whereas no significant differences were detected between host seaweed species. Gammarid richness, diversity, and abundance were highest in winter and spring, while lowest in summer and autumn. Ampithoidae consistently dominated the assemblages across seasons. These seasonal shifts closely corresponded with the phenological changes of the host seaweeds, suggesting that seasonal variation in habitat availability and food resources is primary driver of gammarid assemblage structure. This study provides baseline information on gammarid phenology in temperate Sargassum beds of the Sea of Japan.

To clarify the community compositions and three-dimensional (3D) structure of microbial biofilms on marine plastics, five types of plastic and glass plates were incubated in temperate coastal waters (Sagami Bay, Japan). Metabarcoding analysis showed that both prokaryotic and eukaryotic community structures on plastics and glass were largely different compared to ambient seawater, although similar communities were observed on different types of substrates except for polyethylene. Among the plastic-associated communities, Flavobacteriales (33.5–54.7%) were the most dominant prokaryotes, and microbial eukaryotes were dominated by Exogenida (suctorian ciliates) (74.5–80.3%), particularly the genus Ephelota, followed by Naviculales (pennate diatoms) (17.7–23.3%). Confocal fluorescence microscopy visualized the 3D layered structure of plastic biofilms mainly composed of bacteria and Navicula-like diatoms with the abundant and spatially heterogeneous distribution of the stalked suctorian ciliates. Bacteria and diatoms were found from the inner to outer layers of biofilms with abundant bacterial cells on the plastic surface, whereas the cell bodies of suctorian ciliates were observed exclusively in the outermost layer. Furthermore, chlorophyll autofluorescence was detected abundantly in the cell bodies of the ciliates, indicating their active grazing on diatoms. These combined taxonomic and structural results suggest the formation and developmental processes of microbial biofilms and biological interactions (synergistic and predatory relationships) on marine plastics. The present study indicates the proliferation of suctorian ciliates, which have been reported to cause serious damage to aquaculture, on plastics, implicating the potential ecological roles of marine plastic debris as a dispersal vector transporting harmful hitchhiking microorganisms.

Undescribed cubomedusae of the family Alatinidae were observed off Kume Island, Okinawa Prefecture, Japan and Koror Island, Palau. Taxonomic investigations, including morphological observations and molecular phylogenetic analyses of the collected specimens and analyses of photographs from Japan and Palau, were conducted. The specimens from Japan had the following characteristics: a rectangular bell, taller than wide with an apical projection, and four pedalia bearing a total of two short and two long tentacles. The short tentacles are very short and club-shaped, and the long tentacles have copepod-shaped lateral branches. Based on these findings, we describe this specimen as a new species, Manokia bandiera sp. n.. The specimen from Palau resembled M. bandiera sp. n.; however, it was distinguished by a rounded bell apex and krill-shaped lateral branches on the two long tentacles. This study provides additional information on the morphology and ecology of these two Manokia species in the family Alatinidae, constituting new records for Japanese and Palauan waters.

Opine dehydrogenase (OpDH) activities in ten benthos taxa at deep-sea hydrothermal vents were analyzed. Although the type and strength of the activity detected differed among taxa, OpDH activity was observed in all specimens. Particularly high activities were detected in Aphroditoidea sp. (strombine dehydrogenase 24.07 units g⁻¹; alanopine dehydrogenase 15.08 units g⁻¹). Furthermore, high OpDH activities were detected from two crustacean species. Activities of tauropine dehydrogenase (3.08 units g⁻¹), β-alanopine dehydrogenase (0.97 units g⁻¹), and strombine dehydrogenase (0.69 units g⁻¹) were detected in neoverrucid barnacle Neoverruca intermedia Sha & Ren. In caridean shrimp Opaepele loihi Williams and Dobbs, strombine dehydrogenase (7.21 units g⁻¹), alanopine dehydrogenase (1.37 units g⁻¹), and tauropine dehydrogenase (3.03 units g⁻¹) activities were detected. A previous report has shown that crustacean species have no or very low OpDH activity levels. Therefore, this is the first report of the detection of high OpDH activities in Crustacea.

Pen shell Atrina spp. in the Sea of Ariake, have been reported to show a high survival rate when reared while suspended in the water column (e.g., using baskets) compared to the muddy bottom of its natural habitat. We inferred that Atrina spp. take in water with different properties, such as suspended sediment composition, depending on their vertical position in the water column. Such an inference would become more likely, if Atrina spp. can only take in a very thin layer of water near the inhalant opening regardless of their vertical position, as suggested from hydrodynamic studies of other bivalves. To test this hypothesis, a preliminary experiment was conducted by fixing an Atrina-shaped resin model in various vertical positions in a circular flow tank and recording the streamlines of water that entered the quasi-inhalant opening of the model. Results supported our hypothesis: the resin model, whether embedded in the sand at the bottom of the tank, positioned slightly protruding from the sand or suspended in the water column, took in a very thin layer of water near the inhalant opening (distance between the upper edge of inhalant opening and the top of the incoming water < 1.5 cm). This layer became thinner as the mid-layer current speed increased in the tank. Future research should investigate the water streamlines actually taken in by live Atrina spp. and the characteristics of the water such as concentration of particulates.