Microbial symbionts play important roles for zooplankton by promoting nutrition, growth, and reproduction. We report bacterial communities from bodies and feces of three zooplankton taxa (Eodiaptomus japonicus, Daphnia pulicaria, Cyclopoida spp.) from Lake Biwa using meta-barcoding analysis targeting the prokaryote-specific V4 region of 16S rDNA. A total of 669 bacterial OTUs were generated. Bacteria associated with the body and feces of E. japonicus were most divergent. The bacterial compositions of the bodies and feces in all three host taxa differed significantly. The bacterial phyla in the zooplankton bodies and feces were dominated by Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria. At lower taxonomic levels, the bacteria of bodies (in particular) and feces differed among zooplankton taxa. Prevalent bacteria of D. pulicaria and Cyclopoida spp. bodies were Aeromonadales (Gammaproteobacteria), and those of E. japonicus were dominated by Burkholderiales (Betaproteobacteria). Aeromonadaceae was the most dominant bacterial family in zooplankton bodies, while Pelagibacteraceae was dominant in feces. The bacterial communities present in the feces were similar among the taxa but differed from those found in the bodies of the animals themselves, implying that the feces contained transient bacteria passing through the digestive system. The bacterial composition associated with the zooplankton bodies may be related to the host’s feeding habits, habitat selection, and behavior.

Three distinct species of fauveliopsid polychaetes were collected from deep-sea sediments in the northwestern Pacific Ocean off Japan. Two of these species were obtained from bathyal and abyssal depths in the Shikoku Basin and were identified as Fauveliopsis levensteinae and Laubieriopsis hartmanae, which have been previously reported from the abyssal plains of the North Pacific Ocean. The third species collected from bathyal depths off Yakushima is new to science and described as Laubieriopsis verrucosa sp. nov. This new species can be distinguished from its congeners by having numerous body papillae across the whole body, 21 chaetigers, and an unpaired genital papilla on the right side of the posterior margin of chaetiger 8. A key to identify Japanese species of Fauveliopsidae is also included.

The life expectancy (i.e., an average lifespan in a population) of Tripneustes gratilla has been reported as short as 1 or 2 years in natural environments, which is relatively short among sea urchins. However, some T. gratilla individuals can live for up to 5 years under rearing and 6.85 years in the field, according to previous reports. Until present, the maximum longevity that a certain individual of T. gratilla can physiologically live remains unknown. The present study conducted long-term rearing during 12.2 years of hatchery-produced T. gratilla in tanks to investigate its longevity. We especially focused on whether the maximum longevity among T. gratilla individuals exceeds a decade, as in the case of most sea urchin species. At the end of the rearing period, eight T. gratilla individuals were still alive. In last 9.5 years of the rearing period, during which dead individuals were counted, the mortality was 41% (7 out of 17 individuals). During growth, the body weight was related to the transverse test diameter to a power of 2.84. The asymptotic transverse test diameter and body weight, determined by fitting to the von Bertalanffy growth function, were 95.9 mm and 344 g, respectively. The results of the present study suggest that T. gratilla has the maximum longevity of more than a decade under rearing conditions, and the short-lived characteristics of this species in natural environments are mainly attributed to ecological factors.

During an ROV Hyper-Dolphin dive to bathyal depths in the Kamaishi Marine Canyon, off Tohoku District, the north Pacific, the association of a large specimen of Caprella (Crustacea: Malacostraca: Amphipoda) was observed, surrounded by juveniles, clinging to the dorsal tube feet of the deep-sea holothurian Scotoplanes globosa (Théel, 1879) at 1142 m depth. Scotoplanes globosa “walks” on the seafloor using long tube-like limbs extending from its body. The present observation indicated that “walking” deep-sea holothurians are an underrated substrate for Caprella spp. in the bathyal depths. Approximately 25 juveniles were collected with the mature female specimen from the same holothurian. These individuals were identified as Caprella fimbrillata Vassilenko, 1993 which has previously been reported from the eastern seaboard of Etorofu/Iturup Island, Chishima/Kuril Islands, in 605 to 1200 m water depth; these sites are located approximately 600 km northeast from the present site off Tohoku in the northwest Pacific. The mature female was closely surrounded by juveniles on the same deep-sea holothurian, suggesting maternal care by the mother caprellid in the bathyal depth environment.

Pterotrachea coronata is a carnivorous gastropod slowly swimming in the open sea all its life. Since P. coronata ingests prey without mastication, the whole bodies of the ingested organisms are often recognizable through the transparent body. The present study described fine structures of epithelial cells along the alimentary canal to elucidate functional adaptations to this digestion mode. The alimentary canal is a simple tube; the epithelial cells lining the tube always have microvilli and are basally underlain by a muscular layer. The structure of the epidermis was different in each organ, i.e., buccal mass, esophagus, intestine, and visceral nucleus, with differences in cell shape, microvilli length and density, and muscle layer thickness. Additionally, a cuticular layer overlies the epithelia in the buccal cavity and the alimentary epithelial cells are multi-ciliated in the visceral nucleus where absorption of digested material and formation of fecal pellets proceed. The long esophagus and intestine of P. coronata are probably suitable to retain ingested materials for the slow digestion process, without mastication. While the intestine occupied most of the alimentary canal, the epithelium was thin and had short, sparse microvilli. This poorly developed but large, transparent intestine in P. coronata is one of the major characteristics of this animal. Slow digestion with retention of prey in the large intestine would be an adaptation for the floating life of this transparent carnivore, allowing it to survive in the epipelagic layers where there are limited chances to obtain prey organisms, especially during the day.

An environmental DNA (eDNA) detection technique using TaqMan qPCR and LAMP analysis was developed for the Asian clam Corbicula fluminea, which is causing alarm in Japan and worldwide as an invasive alien species. The qPCR primer and probe designed in this study specifically amplified C. fluminea DNA, but did not amplify C. sandai or C. japonica DNA, both native Japanese species. In contrast, LAMP analysis showed that C. sandai DNA also reacted, but with a delay after the amplification of C. fluminea DNA, and it was possible to specifically detect C. fluminea DNA by setting the LAMP analysis time to 30 min. Field surveys at three study sites with different habitat conditions for C. fluminea showed that eDNA detection using qPCR/LAMP analyses were consistent with C. fluminea distribution. The applicability of a simple filtration and concentration method for eDNA using glass fibers in suspension (SGF method) was verified, and the SGF method had a higher eDNA recovery capacity than the conventional aspiration method using glass fiber filter paper. eDNA extraction using the SGF method and eDNA analysis using qPCR/LAMP may be useful for qualitative and simple habitat estimation of C. fluminea in poor water quality environments.

We measured the concentrations of photoprotective compounds, carotenoids and mycosporine-like amino acids (MAAs), in surface-dwelling copepods in relation to their vertical distribution patterns along a zonal transect in the subtropical North Pacific to better understand their adaptation to strong ultraviolet (UV) radiation. All the dominant copepod groups analysed had detectable levels of both photoprotective compounds, while the concentrations differed among taxonomic groups. Farranula spp. had high carotenoid and MAA content (mean carotenoids: 0.0656; mean MAAs: 0.0230 µg mg dry wt⁻¹) and the accumulation of MAAs correlated positively with in-situ UV penetration, explaining their constant occurrence in the surface layer throughout the transect. Oncaea spp. and Acrocalanus spp. had higher levels of carotenoids than MAAs and showed variable vertical distribution patterns regardless of UV penetration, suggesting that carotenoids are used only temporally to mitigate the effect of UV radiation. In contrast, Clausocalanus spp. and Calocalanus spp. had the lowest concentrations of both photoprotective compounds (mean carotenoids: 0.0148 and 0.0209; mean MAAs: 0.0015 and 0.0029 µg mg dry wt⁻¹). The mean weighted depth of Clausocalanus spp. showed a significant negative relationship with UV penetration, suggesting that they mitigated the harmful effects of UV radiation through behavioural responses. Calocalanus spp. may have adapted to subtropical surface waters with relatively low photoprotective compound concentrations. Our study demonstrated that surface-dwelling subtropical copepods exhibit taxon-specific patterns in photoprotective compound concentrations that are adaptations to intense UV radiation, which is crucial for understanding the high species diversity of copepod communities in subtropical open waters.

We found Aulacoseira pusilla individuals with valves with a high ratio of mantle height to valve diameter, making long filamentous colonies in Lake Kasumigaura. Light microscopy (LM) and scanning electoron microscopy (SEM) observation confirmed that these individuals belong to A. pusilla. Such a high ratio of mantle height to valve diameter has never been reported in previous studies. However, it was clarified that the relationship between these individuals' mantle height and valve diameter generally fits the same allometric relationship as with typical A. pusilla observed from the same sample.