Raphidiopsis (previously Cylindrospermopsis) raciborskii is considered as a cylindrospermopsin (CYN) producer in Thailand. There are no reports including molecular information on the genes associated with CYN production of cyanobacteria from Thailand. In the present study, 27 new R. raciborskii strains from Thailand were isolated. Their morphological features and phylogenetic positions based on the partial sequences of 16S rDNA and rbcLX were analyzed for identification. Morphological characteristics of all the strains tested, and the natural samples, were found to be similar to the original description of R. raciborskii. The rbcLX phylogenetic trees revealed that the clade of R. raciborskii is independent from those of R. mediterranea and R. curvata, whereas the 16S rDNA phylogenetic trees failed to discriminate these species. Thus, all the strains tested in the present study were identified as R. raciborskii. Furthermore, the LC/MS/MS analysis confirmed that some of the strains tested produced CYN and 7-deoxy-CYN. The presence of genes associated with CYN biosynthesis (cyrA, cyrB, cyrC, and cyrJ) in Thai Raphidiopsis strains was investigated by PCR and phylogenetic analyses. Results of PCR showed that the PCR amplicons of cyrB and cyrJ were obtained from all toxic strains, while the amplicons of cyrA and cyrC were not obtained in some toxic Thai strains. Our findings supported the observation that cyrB and cyrJ, especially the latter, may be useful for the detection of CYN-producing cyanobacteria in water resources for water supplies and fisheries.

Seasonal variations in environmental variables, chlorophyll a (Chl-a), particulate carbon and nitrogen (PC and PN, respectively), phytoplankton carbon biomass (Ph-C) and primary production were investigated at a neritic station in Sagami Bay, Kanagawa, from January 2008 to December 2013. Size-fractionated Ph-C was converted from cell volume by microscopic observation, adding valuable data for this area. During spring blooms, the micro-size fraction (>20 µm) comprised the majority of the total Chl-a and total Ph-C, whereas during other periods the pico- and nano-size fraction (<20 µm) comprised a larger proportion, indicating that phytoplankton standing crops were affected by sunlight conditions and physicochemical properties of the water. In February–March, phytoplankton biomass increased and formed the first peak of spring blooms under increasing sunlight intensities (>15.7 MJ m⁻² d⁻¹), high nutrient concentrations and balanced molar ratios. From the regression equations of size-fractionated Ph-C-Chl-a relationships, the mean Ph-C/Chl-a ratio was 5.3–7.7, 29.2–32.6 and 22.1–25.1 for the <20 µm, >20 µm and total fraction, respectively. The Ph-C/Chl-a ratio (1.8–128.8) was regulated by irradiance and nutrients. Growth rate (ca. 0–3.7 d⁻¹) was positively correlated with irradiance and assimilation number, and negatively with the Ph-C/Chl-a ratio. The depth-integrated primary production (DIPP) was 0.15–5.43 g C m⁻² d⁻¹. On the basis of the 0–50 m depth-integrated values, the total Ph-C and DIPP accounted for 1.3–34.4% and 1.3–30.9% d⁻¹ of PC, respectively, indicating that PC variations depended on the total Ph-C and DIPP.

The world’s largest Saemangeum Dike was completed on April 21, 2006; however, the water gates were intermittently opened to dilute the polluted water inside the dike with seawater outside the dike. We investigated the macrobenthic faunal change after dike construction in Saemangeum Lake, South Korea, in the summer season. From July 2008 to August 2012, many individuals of bivalve species, such as Arcuatula senhousia, Potamocorbula sp., and Ruditapes philippinarum, were collected from the subtidal zone inside the dike, although the dissolved oxygen content in the bottom water was less than 1 mg/L at many stations, where the water depth was more than 2.7 m. In August 2016 and August 2018, however, most macrobenthic animals disappeared from Saemangeum Lake, because the water became more hypoxic than it was in August 2012. These facts suggest that the Saemangeum Dike negatively affects the water quality and macrobenthic animals inside the dike because of the long closure of sluices.

A new carybdeid species, Carybdea irregularis sp. nov., is described from the Dana Expedition 1928–30 collections of the Natural History Museum of Denmark, Copenhagen, originally sampled from French Polynesia. It can be distinguished from its congeners by the combination of the following morphological features: pedalial canal knee bend rounded, 2 very narrow velarial canal roots/octant with slim, irregularly shaped canals (no canal resembles another one) and bottle tree-like gastric filaments.

Description, photo and and drawings by Bigelow (1909) on three French Polynesian carybdeid medusae, sampled in Rikitea Harbor, Mangareva, Gambier Islands (French Polynesia) during the Albatross Expedition in 1905 and identified by him as “Carybdea rastonii”, fit exactly the anatomical structures of this new species and are therefore designated to it.

Up to now, Carybdea irregularis sp. nov. is the smallest species of the genus Carybdea, maturing with at a bell height of 15 mm.

We investigated the diel variation in motility under different irradiance of the prymnesiophyte Isochrysis galbana Parke. The effectiveness of nutrient uptake by motility relative to molecular diffusion in seawater is quantified by the Sherwood number Sh, which is a function of cell size (µm) and motility rate (µm s⁻¹). We calculated Sh for the prymnesiophyte I. galbana by determining its cellular equivalent spherical diameter (ESD, µm) and motility rate (numbers of ESD s⁻¹). Nutrient-enriched batch cultures were grown at 25°C in 12:12h light:dark cycles at light-limiting or light-saturated irradiances for 48 h. Observed motilities and Sh values were within the range of previously reported values and indicated a significant relationship between motility and Sh with ESD. Diel variations in motility and Sh showed maxima at the middle of the light period and minima at the middle of the dark period under both light conditions. Diel patterns of motility and Sh might be related to those of photosynthesis due to the supply of energy of motility.

Bacteria that pass through a 0.2-µm pore-size filter (0.2-µm-filterable bacteria) have been reported from various aquatic environments. It has been recognized that some 0.2-µm-filterable bacteria are “transiently” small/thin cells, which are considered as starvation forms of heterotrophic bacteria, especially in oligotrophic environments. However, whether these “transiently 0.2-μm-filterable bacteria” have roles in organic matter processing in aquatic environments has not yet been well described. Here we found high potential of the transiently 0.2-µm-filterable bacteria with extracellular protease activity in their natural nutritional condition. We conducted microcosm experiments using 0.2-µm-filtered coastal seawater (FSW) without any nutritional amendment, and monitored changes of the cell numbers trapped on a 0.2-µm filter (not 0.2-µm-filterable cells anymore) and their community structure in the FSW microcosms, with extracellular protease activities as indicators of heterotrophic microbial activity. We observed a rapid increase in the 0.2-µm-trapped cell number in the FSW microcosms that originally included only 0.2-µm-filterable microbes. The regenerating 0.2-µm-trapped cells were typical marine bacteria (Alphaproteobacteria, Gammaproteobacteria, Flavobacteriia). Extracellular aminopeptidase activities increased with increasing numbers of 0.2-µm-trapped cells. These results suggest that transiently 0.2-µm-filterable-form bacteria in the original coastal seawater recovered their size and were producing extracellular proteases, which might catalyze organic matter processing in seawater. Since organic nutrients were not added, the size increase might be caused by the reduction in competition from larger bacteria and/or the absence of grazing pressure. Our results demonstrate the growth potential and extracellular protease activity of “transiently 0.2-μm-filterable” bacteria in seawater, usually obscured due to the coexistence of grazers and other bacteria.

Taxonomic reexamination of Japanese populations of the Perinereis nuntia species group, which constitutes a major polychaete component in intertidal benthic communities, was carried out by analyzing the mitochondrial 16S rDNA and the nuclear ribosomal ITS sequences of 80 specimens, newly collected from 27 sites in Japan, together with 18 museum-preserved specimens collected from southern neighboring countries outside Japan. The Japanese populations of the Perinereis nuntia species group could be divided into four genetically different groups, which corresponded morphologically to four nominal species (P. mictodonta, P. wilsoni, P. shikueii, and P. nuntia), with some exceptions. Perinereis nuntia and P. shikueii were recorded as new to the Japanese fauna, with their distributions restricted to southern Japan. The clade containing P. shikueii was subdivided into two sister clades (forms A and B), indicating that cryptic speciation has occurred within this clade. Our results revealed a remarkable variability in the number of bars in area VI of the proboscis in form B of P. shikueii, which appeared to be caused by breakage of the long bar into short bars during growth from a juvenile to an adult.

Molecular phylogenetic analyses were conducted to clarify how genetically homogeneous the common Japanese species of Marphysa known as “Iwa-mushi” is. This is a well-known polychaete used as a fishing bait that was first described as Marphysa iwamushi Izuka, 1907 (type locality: Japan and Taiwan) and later synonymized to Marphysa sanguinea (Montagu, 1813) (type locality: England). The nucleotide sequences of a nuclear gene (18S rRNA) and two mitochondrial genes (16S rRNA and cytochrome c oxidase subunit I (COI)) were compared between specimens newly collected from 14 localities in Japan including commercially sold fishing baits and DDBJ/ENA/GenBank data for congeneric species. Our results show that the Japanese “Iwa-mushi” is not a single species but a species complex comprising five genetically well-separated clades that were tentatively designated as five undetermined species (Marphysa spp. A, B, C, D, and E). It is unclear whether any of these species corresponds to M. iwamushi. The COI nucleotide sequence of Marphysa sp. A was almost identical to that of M. victori Lavesque, Daffe, Bonifácio & Hutchings, 2017 (type locality: France) and M. bulla Liu, Hutchings & Kupriyanova, 2018 (type locality: China), suggesting that they are conspecific and supporting the hypothesis of Lavesque et al. (2017) that the population in France was introduced from Japan. The COI sequence of Marphysa sp. B corresponds to that of M. maxidenticulata Liu, Hutchings & Kupriyanova, 2018 (type locality: China). Marphysa sp. E was found only in Tokyo Bay and may be an alien species that was introduced by importing live fishing bait. Our results indicate that the European species M. sanguinea is not distributed in Japanese waters.

We investigated inter-species dietary variation and potential cellulose digestion of 12 macroinvertebrate taxa collected from two locations in the estuarine Idoura Lagoon, Sendai Bay. All taxa exhibited cellulase activity (CA), which was higher among surface-deposit feeders (bivalve Macoma contabulata, polychaete Tylorrhynchus osawai, and ocypodid crabs) and obligatory suspension feeders (bivalves Corbicula japonica and Nuttallia japonica) (0.108 to 0.764 µmol min⁻¹ mg-protein⁻¹). In contrast, CA was lower among facultative suspension-feeding and deep-deposit feeding polychaetes (Hediste spp. and Heteromastus sp., respectively), and was lowest in the deep-deposit feeding polychaete Notomastus sp. The stable isotope ratios of the macroinvertebrates differed among feeding groups. A δ¹³C-based isotope mixing model revealed that the major dietary component of the surface-deposit feeders was microphytobenthos (34–50%), regardless of their high CAs. Although CAs of obligatory and facultative suspension feeders were comparable to or lower than those of surface-deposit feeders, they were highly dependent on river-derived materials at the station near the freshwater input (38–59%). These results indicate that CA is a common physiological characteristic of macroinvertebrates in estuarine soft-bottom habitats, but the dietary contribution of riverine detritus is not correlated with enzymatic activity. Our findings indicate that several factors affect the realized dietary components of macroinvertebrates, including feeding mode, the selectivity of ingestion, and digestive enzyme activity.

We surveyed the spatial and temporal distribution of Japanese sea nettle jellyfish Chrysaora pacifica in Omura Bay, Japan, using an environmental DNA (eDNA) method. In 2018, the C. pacifica eDNA concentration increased from March–May at all depths. The seasonal pattern of C. pacifica eDNA was consistent with previous reports based on visual observations along the Japanese coast. Thus, the eDNA method might have advantages to follow the seasonal pattern of C. pacifica while being less time-consuming and less laborious compared with traditional methods. The eDNA concentrations tended to reach a maximum near and/or below the pycnocline throughout this study. Therefore, the vertical distribution of C. pacifica medusae may have been restricted by strong pycnocline formation in July and August 2018. However, even with a weak pycnocline, which C. pacifica should be able to swim across, the apparent distribution of C. pacifica eDNA seems to be restricted by the pycnocline. Therefore, the eDNA method cannot, currently, accurately assess the absolute vertical distribution pattern of C. pacifica, especially when a pycnocline is formed.