The phylogeny of the Rhopaliophora (Scyphozoa, Cubozoa and Staurozoa) has been discussed and modified extensively up to the present day. Attempting to finally solve this question, the morphology and development of the different early life history stages stages of 17 scyphozoan, 2 staurozoan and 8 cubozoan species were investigated. Due to the magnitude of the study, the findings will be published in two parts. This first part will focus on the comparison of early life cycle stages, the second part (to be published in the following issue of Plankton & Benthos Research) will focus on the evolutionary connection and systematics. The present part of the study focused on the morphology of different types of sessile stages (polyps), the processes of strobilation and metamorphosis, and the resulting products (residues and ephyrae/young medusa) and their further development. The different morphological characters in the polyps helped to categorize them into types. By using the strobilation rates and information on the way morphological characters changed during the process of strobilation/metamorphosis, the strobilae were able to be separately categorized.

Four mantis shrimp species are recorded from the Ryukyu and Ogasawara islands, southern Japan: Gonodactylaceus ternatensis (De Man, 1902) (Gonodactylidae); Lysiosquilloides siamensis (Naiyanetr, 1980) (Lysiosquillidae); Alimopsoides tuberculatus Moosa, 1991 (Squillidae); and Busquilla plantei Manning, 1978 (Squillidae). This is the first specimen-based record of G. ternatensis and the first distributional records of the latter three species from Japan. The genus Lysiosquilloides Manning, 1977 is recorded for the first time from Japanese waters, and the genus Alimopsoides Moosa, 1991 is newly found from the northern hemisphere. Diagnoses, taxonomic remarks, ecological notes and new standard Japanese names are provided.

Estuaries are considered to be among the most biodynamic locations on the globe, where materials are heavily processed by microbes, resulting in high elemental exchange. However, the precise contribution of bacteria in transforming estuarine organic matter has rarely been clarified. In this study, the temporal and spatial distributions of bacterial communities in the surface sediments were investigated at four sites in the Pearl River estuary over a 10-month period. The bacterial communities were affected significantly by salinity and shifted strongly from freshwater sediments (dominated by Alphaproteobacteria, Betaproteobacteria, Chloroflexi and Actinobacteria) to saline sediments (dominated by Deltaproteobacteria and Gammaproteobacteria). The abundance of bacteria was best explained by phototroph abundance in sediments compared to total organic carbon (TOC), carbon stable isotope ratios of TOC (δ¹³C-TOC) and phototroph abundance in bottom water. Bacteria presented higher utilization efficiency for sedimental phototroph-derived organic matter in high-salinity (10.6–33.0‰) sediments. Proteobacteria, the most abundant taxa, were considered the main consumer for phototroph-derived organic matter. In conclusion, bacterial communities perform the niche specificities and play a major role in carbon cycling by processing phototroph-derived organic matter in different habitats.

Seasonal occurrences of marine cladocerans in offshore Suruga Bay, Japan, were studied from 2014 to 2019. Zooplankton samples were collected monthly from a station (depth: ca. 1000 m) located in the offshore area of the bay. Cladocerans appeared each year from February to December, and disappeared in January. Their abundance increased drastically from April to September and comprised a substantial portion of the offshore mesozooplankton community during this period. Maximum cladoceran abundance occurred from June to August, ranging from 65.9–1341.9 individuals m⁻³. These patterns in cladoceran abundances were basically repeated each year throughout the study period. This result suggests that mass occurrences of cladocerans in offshore regions of the bay during the spring-summer are regular events rather than sporadic. Seven species, which have previously been reported from Japanese waters, were identified, and successional changes in the dominant species were observed. The most abundant species, Penilia avirostris, carried parthenogenetic embryos in brood pouches, suggesting that they were not simply transported from coastal areas, but that they also reproduce in the offshore waters. Regular mass occurrences of marine cladocerans in offshore Suruga Bay could have an important impact on the offshore ecosystem of the bay, and factors enabling these population cycles need to be determined.

Ellobium chinense is a red-listed snail species of the family Ellobiidae with a geographic distribution from Vietnam and south coast of China to South Korea and mainland Japan. This species is restricted to specialized habitats in a narrow upper-intertidal to lower-supratidal zone of salt marshes and thus particularly sensitive to environmental degradation through land reclamation and other human activities. Here, we first report the genetic diversity and population structure of E. chinense in Japan to evaluate the connectivity and conservation value of its local populations. Specimens were collected from seven localities (Tsu, Okayama, Yamaguchi, Usa, Imari, Saga and Izumi) that cover the species’ present distribution in the country. Analyses of 612-bp sequences of the mitochondrial cytochrome c oxidase subunit I gene showed high genetic diversity within populations and a reasonable level of connectivity among populations. However, significant genetic differentiation was detected among distant geographic regions in Japan and South Korea, due potentially to the disjunct distribution of habitable salt marshes and a short pelagic larval period of the species. The population of the Ise–Mikawa Bay area, representing the eastern limit of the current distribution range, showed the highest level of genetic differentiation and deserve particular conservation efforts to avoid local extinction, which occurred in Tokyo Bay area in the last century.

Individuals of a deep-sea snailfish, Careproctus trachysoma, endemic to the Japan Sea have been classified into the lighter and darker color morphotypes, which are sympatrically distributed. These morphotypes have only been described qualitatively, with individuals that could not be classified as either morphotype. Therefore, in this study, we quantitatively evaluated individuals of this species using a colorimeter. Based on principal component analysis, all examined individuals were classified into two groups containing all typical individuals of the two original color morphotypes. We redefined these groups as “Red” and “Black” morphotypes. Stomach contents, sex, body length, and sampling depth did not clarify the color differences between Red and Black individuals. No genetic differences were detected between morphotypes using microsatellites, although the frequency of two mitochondrial DNA (mtDNA) groups, SOJ1 and SOJ2, was found to be significantly different. Individuals with SOJ1-type mtDNAs, which are thought to have migrated into the Japan Sea more recently than those with SOJ2-type mtDNA, showed body coloration similar to that of the Pacific species, C. pellucidus. Although further studies using more specimens are required to confirm these findings, our results indicate that C. trachysoma body color is related to mtDNA lineages.

In the Sea of Japan, changes in the oceanographic conditions affect fisheries, and new phenomena such as blooms of the harmful dinoflagellate Margalefidinium polykrikoides and mass occurrences of the giant jellyfish Nemopilema nomurai have been observed since the 2000s. In order to elucidate and formulate countermeasures to such new issues, it is essential to enhance the oceanographic/biological data. However, data especially on the phytoplankton assemblage are limited. In the present study, we investigated the population dynamics of the phytoplankton community together with environmental factors at two stations off Tajima, southwestern Sea of Japan for seven fiscal years from September 2009 to March 2016. The oceanographic observations revealed the general pattern in the seasonal and annual abundance of phytoplankton off Tajima. The major component of size-fractionated chlorophyll a concentrations were micro-size (filtered pore size: >10 µm). The results indicated that there was usually a low abundance of phytoplankton in this area. Relatively high abundances (cell densities over 100 cells mL⁻¹) of micro-sized phytoplankton were observed only in spring, but total cell densities were less than 10 cells mL⁻¹ in most months of the other three seasons. The major phytoplankton component was diatoms. Based on the abundance and frequency, the dominant seven diatom taxa were classified into three groups: Skeletonema spp., Thalassiosira spp. and Eucampia zodiacus were dominant in spring, Chaetoceros spp. and Pseudo-nitzschia spp. were dominant in spring and autumn, and Rhizosolenia spp. and Leptocylindrus danicus were dominant in autumn.

Image analyses based on deep learning techniques have been expanding rapidly in various fields, and the development environment has become more accessible. In our previous study, numerous time-lapse images (approximately 9000 images) of juvenile Yesso scallops reared in lantern nets were acquired, but only a few have been used for data analysis owing to the difficulty associated with the automatic processing of such vast numbers of images. In this study, an algorithm was developed for the automatic detection of scallops from training and test images selected from the time-lapse images (195 images) based on the use of a deep learning technique referred to as “semantic segmentation.” The developed algorithm recognized the juvenile scallops with high accuracy in the test images. The algorithm was also applied to the other time-lapse images, and high accuracy was confirmed by visual inspection with the exception of a few cases. Data analysis was conducted within the automatically recognized areas of the scallops to explain their growth and behavioral changes owing to stormy weather. Thus, numerical analyses conducted based on semi-automatic processing of massive time-lapse image data may be useful for ecological studies of cage-cultured scallops.

We focus on the ichthyofauna on continental shelves and slopes of the Enshu-nada Sea—an area of the northwest Pacific Ocean off the coast of Southern Japan as one of the essential trawl fishery grounds in Japan. We conducted market samplings of the trawl catches from the Enshu-nada Sea, and trawl surveys of bycatch species by commercial trawlers. As a result, 208 deep-sea fish species (92 families) were recorded, with 46 species first recorded from the Enshu-nada Sea, of which five species were the northernmost record for these species. The results reveal the ichthyofaunal diversity in the continental shelf and upper continental slope of the Enshu-nada Sea, providing biogeographic information on various species based on the vouchers.

This study investigated the temporal and spatial variations of Takayama blooms in Uranouchi Inlet, Japan. From July to September 2017, Takayama cells appeared (≥1 cell mL⁻¹) at depths of 0–10 m, temperatures of 25.4–31.7°C, and salinities of 23.1–34.2. The maximum cell density was 1.37×10³ cells mL⁻¹. By analyzing partial rDNA sequences of Takayama strains collected at the bloom initiation, we found Takayama acrotrocha/Takayama xiamenensis and Takayama tasmanica in the Uranouchi Inlet. This study contributes to understanding of the dynamics of and risk to aquaculture of Takayama blooms in the coastal waters of Japan.