Since 2015, occurrences of Karenia spp. blooms have been documented in Hakodate Bay, potentially expanding to the Pacific coastal area of Hokkaido via the Tsugaru Warm Current. However, available information on the expanding distribution remains limited. This study aims to elucidate the distribution of Karenia spp. in Hidaka Bay, which is an area downstream of the Tsugaru Warm current from Hakodate Bay, using image analysis technology, specifically the FlowCam, following a comprehensive quantitative assessment to facilitate cell detection. The FlowCam could detect Karenia spp., with assured quantitative precision when the cell density exceeds 1 cell mL⁻¹. Counting resulted in lower calculated densities that varied widely, suggesting potential subsampling biases. Using the FlowCam’s software enabled image filtration of Karenia spp. from other plankton species with a maximum accuracy of 96%. Optimal filtering performance in the software was achieved through a combination of Equivalent Spherical Diameter, Aspect Ratio, and Average Green values. Despite a loss of ～10% of Karenia spp. images using the filter function, the application of this function significantly reduces the effort required for the cell sorting-out process. Field observations conducted in September 2022 revealed the presence of Karenia spp. across all stations in Hidaka Bay and Hakodate Bay. Furthermore, the observed gradient in cell density from the warmer waters of Hakodate Bay towards Hidaka Bay suggests the potential role of the Tsugaru Warm Current in facilitating transportation of Karenia spp. cells into Hidaka Bay.

While most polyclad flatworms are free-living, a handful of species exhibit intriguing associations with other invertebrates. One such case is Euilyoida takewakii (Kato, 1935), which inhabits the genital bursa of the ophiuroid species Ophioplocus japonicus H.L. Clark, 1911, found in Japan. This study presents the second report of E. takewakii from the Japan Sea coast, based on specimens collected in Fukuoka, northern Kyushu, Japan. We additionally identified a 4.1% infestation rate of the ophiuroid O. japanicus by E. takewakii. We further elucidated the phylogenetic position of E. takewakii within acotyleans using partial 18S and 28S rDNA sequences for molecular phylogenetic analysis. Despite previous confusion regarding its genus assignment between Discoplana Bock, 1913 and Euilyoida Faubel, 1983, our results support the classification as Euilyoida. This conclusion is based on the finding that E. takewakii did not form a clade with Discoplana gigas (Schmarda, 1859), the only species whose sequences are available in public databases currently among Discoplana.

To investigate seasonal changes in the abundance and species composition of Skeletonema cells in the innermost area of Tokyo Bay, we examined the species composition of Skeletonema in the surface sediment using real-time PCR and enumerated the resting cell density by the MPN method. The process of the decrease in Skeletonema cells in the sediment was also investigated using incubation experiments. Six species of Skeletonema (S. ardens, S. costatum s.s., S. japonicum, the S. marinoi–dohrnii complex, S. menzelii, and S. potamos) were detected in the sediment. The light intensity at the bottom, estimated based on the Secchi depth, was very low, suggesting that the possibility of Skeletonema rejuvenation was not high for most of the year. In the incubation experiment, the cell density of Skeletonema decreased by 40% on average after 30 days of incubation. In August and October 2019, S. potamos was the only species to be detected in the water column; however, its cell density in the sediment remained quite low (<10³ cells g⁻¹). The result suggests that not all sinking Skeletonema cells form resting cells, and the resting cell formation differed among the species.

Melinnopsis shinkaiae sp. nov. represents the fifth species of the genus Melinnopsis to be discovered in Japan. The new species was collected using the submersible Shinkai 6500 at 3623 m depth of Daiichi-Kashima Seamount, a locality with relatively unexplored biodiversity. Our in-situ observation revealed that the species inhabits the upper end of a vertical tube built on the seafloor. The species is differentiated from its congeners by combination of the following morphological characteristics: i) the presence of 12 thoracic uncinigers; ii) the absence of a postbranchial dorsal membrane; iii) the arrangement and shape of four pairs of branchiae; iv) thoracic uncini with a small basal tooth on each three teeth above the rostral tooth; v) absence of longitudinal groove in the large buccal tentacle.

The giant Pacific octopus (Enteroctopus dofleini) is the world’s largest octopus and the most abundant species in coastal waters of the North Pacific. Despite the commercial importance of this species in Japan, studies of its genetic population structure have been limited to the Gulf of Alaska. This study was conducted to clarify the genetic diversity and population structure of the giant Pacific octopus (Enteroctopus dofleini) in Japan and evaluate genetic differences in this species between the northwest Pacific (NWP) and northeast Pacific (NEP) based on analyses of the mitochondrial cytochrome c oxidase subunit I (CO1) and cytochrome b (Cytb) genes. A total of 263 specimens were collected in Japan (NWP) during June 2020–August 2022, and the CO1 and Cytb genes were successfully sequenced and analyzed in 230 and 229 specimens, respectively. Diversity indices were also calculated for the NEP using previously published sequence data, and genetic variance was compared between the NEP and NWP. In Japan, ten CO1 haplotypes and nine Cytb haplotypes were detected, and genetic variation among sites was not significant. Comparisons of genetic diversity between the NWP and NEP were inconclusive, but clear genetic differentiation was found between the two regions. The genetic population structure of E. dofleini has likely been shaped by dispersal during the planktonic paralarval stage and plausibly by changes in climate that occurred during the Last Glacial Maximum.

Benthic invertebrates are an important biome in ecological processes and are useful indicators for assessing benthic environments. The maximum entropy model (Maxent) is a common species distribution modeling tool for predicting species distribution from appearance records and environmental predictors. In this study, to overcome the requirement for a large amount of benthic environmental data with sediment collection to apply the Maxent to the marine benthos, the use of simplified estimable environmental variables such as water depth and grain size was evaluated. The habitat suitability of 10 representative species of macrobenthos in the 1990s, 2000s, and 2010s were successfully estimated by the Maxent using seven physicochemical parameters of the sediment in the sea. The parameter with the highest contribution rate in Ampelisca naikaiensis was total organic carbon content (44%), while it was mud content for Leptochela gracilis (57%), water depth for Glycela sp. (30%) and Chaetozone sp. (32%), and gravel content for the other six species (29–63%). The mean values of habitat suitability in the whole sea increased for five species, whereas those of the remaining five showed a flat trend from the 1990s to 2010s. The AUC values were >0.70 even when only water depth and grain size were used for the Maxent as simplified estimable environmental variables, and the analysis was sufficiently accurate.

Four species belonging to the genus Orchistoma in the monotypic family Orchistomatidae are currently recognized from several localities around the world, but none of these have been reported from the Northwest Pacific, including Japan. In this study, juvenile medusae collected in Tanabe Bay, Wakayama Prefecture were raised to full maturity and used to describe a new species, Orchistoma integrale sp. nov., based on integrated analysis of morphological and molecular data. Orchistoma integrale can be distinguished from its congeners by several morphological features, including prominently curled gonads and scattered nematocysts on the umbrella. Our molecular characterization of this species, connected to its type material, will allow the species to be detected through emerging genetic techniques involving eDNA derived from environmental samples. This is the first record of this family in Japan. We also describe medusa development from juvenile to adult.

The Japanese sea cucumber Apostichopus japonicus is a major target for fisheries, aquaculture, and stock enhancement because of its high commercial value. Knowledge of their survival strategies is crucial for appropriate management of this species. Therefore, this study aimed to determine the behavioral response of A. japonicus juveniles upon contact with a predator or its chemical stimuli. When hatchery-reared A. japonicus juveniles had direct contact with the predatory sea star, their movement speed increased, and the direction of their body axis changed compared with those before contact. The body lengths of the A. japonicus juveniles were longer when they were closer to the predator, suggesting that stretching their body is among their strategies to avoid predation. When the chemical stimuli from the predator was provided, the A. japonicus juveniles changed their movement similar to how they moved upon direct contact with the predator. These results suggest that the predator avoidance behavior of A. japonicus juveniles is elicited by the chemical and/or tactile stimuli from predators. Considering that we used hatchery-reared seedlings, these responses were innate rather than an acquired behavior. An extension of the present study, including the comparison to wild individuals, should substantially contribute to the management of A. japonicus population.

The aetideid copepod Bradyidius angustus was originally described 67 years ago based on two males collected from the Pacific coast of central Japan. This species has been the only one with the female unknown in the genus. Based on specimens from the Seto Inland Sea, Japan, this study describes the female of B. angustus for the first time, together with a complementary description of the male and a brief description of the later copepodids. The female has no seta on the medial margin of the exopod terminal segment of the antenna, which is a unique character within the genus. The distinction of the female from the currently known 18 congeners is also possible by a combination of a body length less than 2.0 mm and ornamentation on the legs, the antenna exopod or the mandible basis. The male is distinct from all congeners with regards to leg 5, the ornamentation of the endopod of legs 3 and 4, or the rostrum. The male specimens have some minor differences from the figures of the original description. The later copepodids (CIV and CV) have a stockier prosome than the adult, but are similar to the adult in some easily observable morphologies, by which they could be possibly identified to the species. This species is considered to be an epipelagic form. Discussion is also made on a past description of B. armatus from Japan.

Quorum sensing (QS) is a term referring to the language of bacteria, or intercellular communication. In recent years, the details of QS mechanisms have been rapidly elucidated in the field of marine microbiology. We investigated QS in bacteria that inhibit the growth of the paralytic shellfish toxin-producing dinoflagellate Alexandrium catenella (Group I, formerly A. tamarense) isolated from Akkeshi-ko Estuary and Akkeshi Bay. There were bacterial strains that showed strong growth inhibitory effects against A. catenella (Group I) when the colonies of bacterial cells were added to the alga. On the other hand, the same bacterial strain showed no inhibitory activity when added in liquid form (cell suspension) to A. catenella (Group I). We explored whether the growth inhibitory activity of these bacterial strains were due to QS or not using the QS inhibitor β-cyclodextrin. The algal cultures with the bacterial strains AK12 and AK24 exhibited no growth inhibition in the presence of β-cyclodextrin, showing the same results to the control without the bacterial addition. It was considered that these strains inhibited the growth of microalgae through QS using N-acyl-homoserine lactones (AHLs) as autoinducers. The strain AK12 belonged to the genus Kordia, while the strain AK24 was closely related to the genus Vibrio. This study reinforces the significance of QS as a crucial regulatory factor controlling ecosystem functions of plankton in marine environments. Considering QS will be important to elucidate the interactions between algae and bacteria in the marine environment.

Eighteen named species of acoels have been reported from Japanese waters, though none from Hokkaido. Our faunal survey in Oshoro Bay, Hokkaido, revealed the presence of Oxyposthia praedator Ivanov, 1952, originally described from material collected in southwestern Sakhalin and subsequently recorded in the Gulf of Peter the Great. Molecular phylogenetic analysis based on the cytochrome c oxidase subunit I, 18S rRNA, and 28S rRNA genes indicates that O. praedator belongs to the family Convolutidae. However, its sister taxon could not be identified with high confidence. This is the first species-level identification of Acoela from Hokkaido and a new record of the species in Japan.