The intertidal hermit crab Pagurus nigrofascia reproduces in March in Fukuoka, Japan. The embryos remain in diapause until October, begin to develop in November, and most broods hatch in December. This study examined the effects of photoperiod and temperature on embryonic diapause termination (EDT) in both laboratory and field conditions. Females with diapause eggs were maintained in the laboratory under three different photoperiod/temperature regimes: constant (14L/10D, 22°C), decreasing photoperiod condition (from 12L/12D to 9.8L/14.2D, 22°C), and decreasing temperature condition (14L/10D, from 22°C to 10°C). The EDT date was compared among four groups: the above three experimental groups and the field group. The dates at which >50% of females possessed eggs after the EDT were November 10 (constant), October 30 (decreasing photoperiod), September 27 (decreasing temperature), and November 4 (field). The mean duration until EDT was 82 days (constant), 67 days (decreasing photoperiod), and 36 days (decreasing temperature). Diapause was significantly shorter under both the decreasing photoperiod and the decreasing temperature conditions compared to the constant. Thus, EDT was strongly affected by temperature but weakly altered by photoperiod. This study is the first to report the influence of photoperiod and temperature on EDT in decapod crustaceans.

Kamaka corophina Ren & Sha, 2013 (Crustacea: Amphipoda: Kamakidae) individuals were collected from the Ryukyu Archipelago in Southwest Japan. This is the first record of the amphipod in Japan and the fifth species of the genus in Japan. Kamaka corophina is characterized by a swollen peduncular article 4 of the male antenna 2 with a small process at the posterodistal corner, the male gnathopod 2 propodus with an oblique or transverse palm, and the bases of pereopods 5–7 each bearing 5–9 posterior plumose setae. Although the male gnathopod 2 in the original K. corophina from China has only an oblique palm, those in the Japanese specimens have two types of palm.

Declining oxygen concentrations in aquatic habitats represent extreme conditions that threaten benthic life. Hypoxia has recently become an important research topic, as areas affected by these phenomena are spreading globally. Omura Bay is one of the most highly enclosed seas in Japan, and severely hypoxic conditions occur at the bottom every summer. We conducted a preliminary study in the center of the bay to evaluate how seasonal hypoxia affects the abundance and community composition of benthic meiofauna, with particular reference to copepods. The copepod densities and their nauplii differed significantly among seasonal categories (before, during, and after hypoxia). Furthermore, the degree of the seasonal decline in copepods during hypoxia seemed much more severe than that in nematodes, the most abundant meiofauna. The assemblages of adult copepods had the simplest composition during hypoxia, when harpacticoid copepods in the family Cletodidae, which have smaller and slender bodies, occurred at significantly higher frequencies (a contribution of 84% to the mean similarity among seasons). After hypoxia, the relative abundance of copepods in the families Ectinosomatidae and Longipediidae increased, which may likely be attributed to their higher swimming abilities and rapid recruitment via specific planktonic nauplius stages, respectively. High frequencies of copepods in the Cletodidae family have also been observed under hypoxic conditions in the Mediterranean Sea, suggesting that similar processes affect benthic copepod communities, which work to the advantage of cletodid species with small and slender forms in the subtidal sediment bottom under severe hypoxia in Omura Bay and other regions.

The golden mussel Limnoperna fortunei (Dunker, 1857) is an invasive freshwater bivalve species that exerts harmful effects on the environment, as well as man-made structures, such as water-treatment systems. By using conventional sampling methods, it is difficult to detect mussels under low-density conditions; however, environmental DNA (eDNA) analysis may be a rapid and efficient method for monitoring this aquatic organism. In this study, we conducted surveys based on the eDNA analysis of L. fortunei in 15 farm ponds in Japan and compared the results with those of two conventional survey methods, visual census and plankton larval survey, to clarify the effectiveness of eDNA analysis for field surveys of L. fortunei. Primers and a probe specific to L. fortunei were developed, and a method for analysis was established. In the laboratory experiments, the species eDNA was detected in all water tanks containing the mussels, and the concentration of eDNA was high in the experimental tank that had high density of L. fortunei. In the field survey, L. fortunei eDNA was detected in all ponds where the mussels were found by conventional survey, and low concentrations of eDNA were also detected in several ponds where no L. fortunei were found by traditional methods. These results suggest that eDNA analysis has greater sensitivity for the detection of L. fortunei in farm ponds than that of conventional methods. Environmental DNA surveys have little impact on water management and are suitable for surveys at water facilities that have not yet been damaged by the mussels.

Many plastics cause pollution in the marine environment, with microplastics (0.1 µm–5 mm) representing a key research focus. The number of microplastics in sediments may increase rapidly, affecting organisms inhabiting marine sediments. The aim of this study was to determine how microplastics affect nematodes in intertidal sand. We assessed: (1) intake of microplastic particles (10 µm, 5 µm, 1 µm, or 0.5 µm) by Enoplolaimus spp. over 48 h; (2) microplastic intake by nematodes depending on feeding type (selective deposit feeders, non-selective deposit feeders, epistrate feeders, or predators/omnivores) over 48 h; and (3) microplastic egestion by Enoplolaimus spp. The proportion of Enoplolaimus spp. individuals containing microplastics was significantly less in the 10-µm microplastic treatment than in the treatments where Enoplolaimus spp. were exposed to microplastic particles of smaller sizes (5 µm, 1 µm, or 0.5 µm). The ingestion rates of microplastics by predators/omnivores, non-selective deposit feeders, and selective deposit feeders increased as the size of the microplastic decreased. After transferring Enoplolaimus spp. to filtered seawater following microplastic ingestion, the proportion of Enoplolaimus spp. individuals containing the smallest size microplastic (0.5 µm) decreased by 15% of the ingested amount in 3 days. In conclusion, there was a significant difference among microplastic-size treatments, but not among feeding types or in the interaction between microplastic size and feeding type. The size of microplastics, rather than feeding type of nematodes, impacted ingestion rates. It is possible that microplastics in the sediment are ingested by nematodes living in marine benthic ecosystems.

Hydrozoan jellyfish of the genus Blackfordia currently consist of three described species: B. manhattensis, B. virginica and B. polytentaculata. These species are distinguished by combinations of the position and shape of the gonads, the number of marginal tentacles, and the presence or absence of black pigments on the bell rim. We discovered a species of the genus Blackfordia in Vietnam for the first time. Morphological and molecular analyses revealed that this Vietnamese Blackfordia had an intermediate number of tentacles compared with its congeners, and a similar number of tentacles had been reported in Blackfordia only in Brazil, Mexico and India. Mitochondrial DNA analysis indicated that Vietnamese Blackfordia formed a sub-clade with “B. virginica” from Brazil, and differed from B. virginica from other locations, having a K2P distance of >0.13. These results suggest that Blackfordia sp. found in Vietnam (and Brazil) may be a different species from B. virginica found elsewhere, though more information is needed to establish this new species.

A new hydromedusa belonging to the order Anthoathecata is reported from Sagami Bay, eastern Japan. Tiaricodon orientalis sp. nov. can be distinguished from other Tiaricodon species by the umbrella size of the medusa, manubrium length, interradial peaks in the subumbrella, and a red band on the upper part of the manubrium. A comparative table of the primary diagnostic characters of the genus is provided. Our morphological and molecular phylogenetic analyses suggest that Tiaricodon from China is not Tiaricodon coeruleus but Tiaricodon orientalis.

The deep-water bathypelagic nemertean Nectonemertes mirabilis, previously found in the Atlantic and Pacific oceans, was confirmed for the first time in the Gulf of Mexico. This new record is represented by a juvenile male collected by a stratified plankton sampling at 800–1,000 m depth in the southern gulf (20.5°N, 94.5°W), during summer 2014. This is the first record of a pelagic nemertean in the gulf; extending our knowledge about the species’ global distribution, the number of nemertean species present in Mexican Atlantic waters, and the need to explore the mesopelagic zone of the oceans.

Commercially pumped deep-seawater is an easily accessible source of deep-sea animals. In this study, we report the bioluminescence of a cirratulid polychaete, Tharyx sp., obtained from pipeline seawater pumped up from a depth of 384 m in Toyama Bay. Two living worms collected from the nylon mesh attached to the pump outlet produced greenish light when disturbed. They were identified as Tharyx sp. (Cirratulidae) by morphological examination and DNA sequencing. Bioluminescence of cirratulid worms is not widely reported and, to our knowledge, this is the first report of bioluminescence in the genus Tharyx.

Human activities, such as coastline development, water pollution, and the introduction of invasive species, threaten mudflat macrofauna. Many spoon worm species are endangered, possibly because of anthropogenic disturbance of mudflat environments. Understanding worm ecology is necessary for conservation. However, information regarding these species is limited, presumably because of difficulties in sample collection. In Tanabe Bay, Wakayama Prefecture, Japan, we attempted to collect and identify planktonic larvae instead of adults, using DNA barcoding to determine rare species. A larva of the rare spoon worm, Ikedosoma elegans, endemic to Japan, was collected. Thus, the existence of an unknown population of I. elegans likely exists. Furthermore, the use of larval monitoring for detecting rare marine invertebrates is demonstrated.