An individual narcomedusa assignable to Solmissus incisa sensu lato was observed having ingested a fish at 573 m depth near the southeast slope of the Kaikata seamount, Japan. Solmissus is a very common deep-sea narcomedusan genus that is widely considered to be a predator specializing on gelatinous plankton. Several cryptic species, with differences in the number of tentacles and form of manubrial pouches, are thought to be included in the nominal species Solmissus incisa. Therefore, the present study gives a short description of the morphotype of Solmissus incisa s.l. observed with a fish in its stomach, as well as several individuals of the same morphotype that had ingested gelatinous prey.

Ciguatera fish poisoning (CFP) is caused by toxins originating from an epiphytic/benthic dinoflagellate of the genus Gambierdiscus. In Japan, CFP cases have been increasingly reported not only in subtropical areas but also in temperate areas. It is therefore important to study Gambierdiscus cell occurrences, cell densities, and population dynamics to address CFP outbreaks in Japan. This study assessed the densities in Japanese shallow waters (0.1–3 m depths) and revealed that the densities were lower than those in tropical and subtropical areas worldwide. In the shallow waters of Tosa Bay, a Japanese temperate area, population dynamics of Gambierdiscus cells were assessed monthly between 2007 and 2013. Gambierdiscus did not show substrate preferences for macroalgal species. The cell densities in the area ranged from 0 to 232.2 cells g⁻¹ wet weight algae. The average cell densities in spring, summer, autumn, and winter were 0.1±0.4, 0.9±2.6, 4.0±20.6, and 0.4±1.4 cells g⁻¹ wet weight algae, respectively. The cell densities in summer and autumn were not significantly different (p>0.05), whereas those in summer and autumn were significantly higher than those in spring and winter (p<0.01). A significant positive correlation between cell densities and sea surface temperatures (SSTs) was observed (r_s=0.21, p<0.001), while a significant negative correlation between cell densities and salinity was recognized (r_s=−0.18, p<0.001). These results suggest that cell densities of Gambierdiscus in Japanese temperate shallow waters increase in summer and autumn when the SST is high and salinity is moderately low.

Hypoxia in bottom environments of coastal marine ecosystems is a serious problem adversely affecting both benthic life and local fisheries. In this study, we monitored abundance, composition, and feeding types of nematode communities under pre-, mid-, and post-hypoxic conditions in Omura Bay, Nagasaki, Japan, for three consecutive years (2013–2015). The bay is almost completely enclosed, and experiences hypoxia at the bottom every summer. A positive correlation was found between dissolved oxygen (DO) concentration and nematode abundance over the entire sampling period (p<0.05, r=0.61). The nematode community compositions among the pre-, mid-, and post-hypoxic conditions were significantly different (one-way analysis of similarities (ANOSIM), p<0.05), which suggests that DO in the bottom water acts as a major driver for the community shift. The increases in abundance of nematodes with toothless feeding apparatus in hypoxic periods, relative to normoxic periods, further suggested that the transfer of organic matter from bacteria through nematodes became more important in the bay under hypoxia than normoxia. It was also demonstrated that full recovery of nematode populations from hypoxic to normoxic conditions would require more than two weeks of continuous normoxic DO levels (>3 mg L⁻¹). These findings will help us to understand how global trends of ocean deoxygenation could shape the meiobenthic community and alter benthic ecosystem functioning in coastal areas.

In this study, we investigated the trophic interactions between the Japanese giant box jellyfish, Morbakka virulenta, and fish in the central part of the Seto Inland Sea, western Japan, in autumn and winter. Occurring in the surface waters at nighttime, these cubomedusae, regardless of their size (1.5 to 22.5 cm in bell height), were found to be piscivorous, feeding mainly on the Japanese anchovy, as shown by their stomach contents analysis. This finding was supported by a stable isotopic analysis and by an unchanged cnidome, irrespective of the bell height of medusae. Their nocturnal occurrence near the surface often took place around the slack tide, during which the medusae were foraging with tentacles fully extended. Other associations between the medusae and fish were also observed at this time: presumed commensalism with juvenile Japanese horse mackerel, and predation by black scraper.

Metagenetic diet analyses of the 18S V9 region were conducted in 40 adult female Calanus sinicus during winter in Tosa Bay (Japan). The majority of prey items were small crustaceans (of Copepoda and Cirripedia) and diatoms, taxa that are dominant in the environment and have been previously reported as important prey items of Calanus. The abundance of sequences attributable to Dinophyta and Chlorophyta was significantly lower in C. sinicus gut contents than in environmental plankton communities, suggesting that C. sinicus avoids prey from these groups. Hydrozoans were also observed, and aplanochytrids (Labyrinthulea) were detected for the first time as a major prey of C. sinicus. Additionally, high proportions of unclassified eukaryote material were observed, suggesting undetected predator–prey relationships in key copepod species in marine ecosystems. The dietary importance of aplanochytrids, heterotrophic protists that accumulate unsaturated fatty acids such as docosahexaenoic acid, has been overlooked in previous research. Calanus sinicus is a key copepod species in the subtropical coastal regions of the western North Pacific, and a major food source for the larvae of commercially important fish; therefore, further investigation into novel prey items such as aplanochytrids is recommended to understand the complex food web structures in marine ecosystems.

We describe the complete life cycle of the ostracod Euphilomedes nipponica Hiruta, 1976. This was completed by collecting individuals from the sea once a month over the course of a year and through the parallel rearing of individuals under laboratory conditions. Females developed their first brood in mid-April, with 20–40 first instar offspring being released in early May. Because adult males did not live long, adult females produced subsequent broods without re-exposure to males, producing up to four broods in total by releasing juveniles at 1-month intervals. First instar individuals reached the fourth instar stage in less than 2 months during which time they molted three times. Molting paused at the fourth instar stage; thus, all offspring from the first to fourth broods reached the fourth instar stage by October. All individuals that have reached the fourth instar stage resume molting to the fifth instar stage in mid-January in response to unknown environmental cues, such as photoperiod and water temperature. Adult males appeared in March slightly earlier than females, with females initiating brooding again in April. This detailed information is expected to contribute toward improving our understanding of the life history strategies and reproductive modes of ostracod crustaceans.

The marine dinoflagellates species belonging to the genera Dinophysis and Phalacroma have been reported as mixotrophic or heterotrophic actively feeding on planktonic ciliates. However, limited information is available on the identification of ciliate species preyed on by Phalacroma mitra. An effective method is introduced for detecting prey DNA of dinoflagellates belonging to these two genera. Three cells of P. mitra possessing food vacuoles were isolated from natural seawater, and the prey DNA within the vacuoles was analyzed. After the polymerase chain reaction (PCR)-based amplification, a restriction enzyme specific to the DNA of Dinophysis and Phalacroma was used to concentrate the prey DNA. Gene cloning revealed that the undigested PCR products contained DNA of ciliate species (Euplotes sp., Mesodinium rubrum, Spirostrombidium sp., etc.), as well as that of dinoflagellates, cryptophytes, and radiolarians. These results imply that the prey diversity of P. mitra can be traced. Furthermore, this method can provide useful data to reveal novel insights into food webs in the planktonic ecosystem.