Estuaries and coastal areas, including the upper intertidal zone, serve many ecosystem functions and provide habitat and reproductive sites for fish and benthic species. In addition to being widely distributed in marine, brackish, and freshwater areas around the world, the family Apogonidae is characterized by having developed male mouthbrooding. Species in the genus Gymnapogon are seldom seen, and little is known about their ecology. In June and July 2022, we collected Gymnapogon japonicus specimens (n=13, including four mouthbrooders), in sediment-covered chambers connected to sympatric shrimp burrows in the upper intertidal zone of the estuary in Shizugawa Bay, Miyagi Prefecture, Japan. Dissection of the gonads revealed that the mouthbrooding individuals were male. The eggs were oval and connected by bundles of adhesive filaments. The developmental stages of the eggs at the time of collection ranged from the cleavage stage to immediately before hatching. In conclusion, G. japonicus is unique in that it exhibits mouthbrooding in subterranean chambers connected to shrimp burrows in the upper intertidal zone of estuaries. This is the first report of mouthbrooding in the genus Gymnapogon. Moreover, no other species in the Apogonidae have been reported to brood in subterranean chambers, making this a surprising discovery.

We evaluated preliminarily the spatial difference in meiofaunal community structure at higher taxonomic level and their association with sediment grain composition of the seafloor around several aquaculture nets in Nagashima Island, facing a strait connecting the Yatsushiro Sea. No significant difference in meiofauna community structure was detected between aquaculture sites and their adjacent sites. Statistical analyses revealed that the heterogeneities in sediment particles were the primary factors associated with 14% of the observed spatial changes in the meiofauna community structure: with increases in size and variances of sediment grains, the relative abundance of nematodes, which are usually the most abundant among meiofaunal taxa, decreased. On the other hand, the proportions of benthic copepods increased along the environmental gradients. This suggests that variations in interstitial space between sediment particles contribute to a certain degree to regulate meiofauna composition around the investigated aquaculture nets.

Upogebiid shrimp (Decapoda: Upogebiidae) are important ecosystem engineers in shallow marine environments because their filter feeding reduces the abundance of phototrophic microorganisms in the water column. Although many previous studies have examined upogebiid populations in intertidal settings such as tidal flats and estuaries, quantitative observations of offshore populations along wave-dominated open coasts remain sparse. Here, we report the abundance of upogebiid burrows in the nearshore zone of a wave-dominated sandy beach, the Kujukuri coast in central Japan. Upogebiid burrows occurred at water depths of 20.1–30.2 m (corresponding to 5.1–13.1 km offshore). The maximum burrow density was 2591.6 openings m⁻² at a water depth of 26.6 m (corresponding to 10.1 km offshore) observed in December 2021. This roughly corresponds to a population density of 1296 m⁻² because upogebiid burrows typically have two openings. The results suggest that a dense shrimp population comparable to the high value reported in tidal flats exists in the nearshore zone of a wave-dominated sandy beach.

Behavioral modifications of the snail host caused by trematode infections may result in spatial variations in the snail population along the intertidal elevational gradient. In this study, we investigated the impact of trematode infections on the behavior and distribution of the first intermediate host snail, Batillaria multiformis, on the rocky tidal flat of Kan-onji in the inner Seto Inland Sea, Japan. Seventy sampling points were randomly selected using quadrats across the intertidal gradient. A total of 1,322 collected B. multiformis snails were examined for parasitism. A higher number of infected snails, including those infected by Cercariae batillariae and Cercaria hosoumininae, were observed in the lower elevational gradient. The mark and recapture field experiment was conducted to assess the movements of the snails across the intertidal gradient. About a hundred samples were recovered after four months from the release. Most of the infected snails moved to the lower shore, whereas a large number of uninfected snails migrated to the higher shore. These findings support our hypothesis that C. batillariae and C. hosoumininae alter the behavior of Batillaria multiformis, possibly to pursue favorable habitats for parasite transmission to the second intermediate host such as fishes or crustaceans.

The camptandriid crab Deiratonotus cristatus inhabits brackish, muddy-bottomed tidal channels and streams, but has experienced a drastic decline due to habitat deterioration, leading to its designation as an endangered species in Japan. Although historically abundant in Tokyo Bay, sightings of the species had ceased for several decades until 2008 when the species was rediscovered. Since D. cristatus had been undetected in Tokyo Bay for decades, we considered the possibility that the population in Tokyo Bay might have been introduced from other regions or overseas. Therefore, as an initial step in our research, we conducted a 13-month study from October 2011 to November 2012 to elucidate the life history traits of the study population. This revealed D. cristatus longevity of approximately 2 years, with recruitment in late summer and ovigerous females holding eggs in the second overwintering period. The D. cristatus population in Tokyo Bay, where females appear in the cool season, differed from several nearby Pacific Coast populations that exhibit this trait in the warm season. The presence of eggs during the cool season is considered consistent with the traits of genotypes from Kyushu and the Seto Inland Sea, which differ from the Pacific type, where ovigerous females appear during the warm season. Thus, although the possibility of introduction via ballast water or fisheries trade cannot be ruled out, the distinct life history of the Tokyo Bay population could represent the uniqueness of the breeding season in the Pacific coast populations.

Adsorption onto the host cell is an important step in viral infection. In this study we aimed to clarify host-strain-specific adsorption of diatom viruses. The adsorption kinetics of Chaetoceros tenuissimus DNA virus (CtenDNAV-II) and Chaetoceros tenuissimus RNA virus (CtenRNAV-II) were compared using the marine planktonic diatoms Chaetoceros socialis f. radians strain NIES-3713 and Chaetoceros tenuissimus strains NIES-3714 and NIES-3715, which possess different viral susceptibility spectra. CtenDNAV-II and CtenRNAV-II viruses showed significant adsorption towards C. tenuissimus strains NIES-3714 and NIES-3715, respectively, which were the original host strains used for their isolation, and the estimated adsorption coefficients were 1.4±0.9×10⁻⁹ mL min⁻¹ and 2.7±1.1×10⁻⁹ mL min⁻¹, respectively. In this study, adsorption was not detected for incompatible host–virus combinations (that is, absence of host cell lysis after viral inoculation). However, virus adsorption was not detected even for compatible combinations of C. tenuissimus NIES-3715 and CtenDNAV-II and C. socialis NIES-3713 and CtenRNAV-II, which may be attributed to the limit of detection in the present experiment. Nevertheless, the results suggest that host–virus affinities differ widely even among compatible host–virus combinations, resulting in changes in clonal composition and population dynamics in natural environments. Thus, this study provides important information for understanding the ecological relationships between diatoms and viruses.

Kagoshima Bay is a deep and semi-enclosed embayment with submarine fumarolic activities. High mercury contents have occasionally been reported for some fishes and this was hypothesized to be due to a bioaccumulation of discharged mercury from the submarine fumaroles. However, there is still no information about how the discharged mercury is accumulated into fishes through the planktonic food web. Here, we demonstrate a bioaccumulation of discharged mercury through copepod life history adapted to the embayment. Calanus sinicus was a minor contribution to the numerical abundance of zooplankton communities in both surface and deep layers during summer. In the deepest basin in northern Kagoshima Bay, copepodite stage 5 of this copepod species concentrated occurred at high abundances below the thermocline throughout the day in summer. Zooplankton communities residing in the deep waters exhibited much lower protein synthetase activity than those in the surface waters, indicating low physiological activity. Metabarcoding analysis of their gut contents demonstrated that copepods, dinoflagellates and diatoms composed more than half of gut content DNA. Mercury contents were higher for all taxonomic groups of zooplankton in the deep layers compared with those in the surface layers. C. sinicus residing in the deep waters exhibited high mercury contents equivalent to carnivorous groups. These findings suggest that bioaccumulation of discharged mercury from submarine fumaroles is accelerated by aspects of this copepod’s life history such as lipid accumulation for dormancy after actively feeding on copepod fecal pellets and phytoplankton and dormant stock accumulation restricted to the deepest basin in northern Kagoshima Bay.

Biodiversity is predicted to decline due to the expansion of low dissolved oxygen waters following global warming. Because most benthic species with poor mobility must tolerate low-oxygen environments, identifying species capable of surviving under these conditions and understanding their survival duration can provide a basis for assessing changes in community composition and organism conservation. In this study, a simplified experimental system using a time-lapse camera was used to assess the low-oxygen tolerance of 10 bivalve species, including rare and endangered species, inhabiting the inner part of Ariake Bay, where seasonal hypoxia occurs. Survival curves were found to be significantly shorter under low-oxygen conditions without antibiotic compared to normoxic conditions for all species, indicating the feasibility of assessing tolerance to hypoxic treatment. The duration of survival (median lethal time: LT₅₀) varied among the 10 species under low-oxygen conditions at 25°C, offering insights into which endangered species can endure oxygen depletion. This approach is expected to provide a simple and quick assessment of tolerance to low-oxygen conditions for the conservation of marine organisms.

Ghost crabs, Ocypode species of the Ocypodidae, are common in temperate to tropical sandy beaches and are well known for producing sounds using their chelipeds. Ocypode sinensis Dai & Yang in Song & Yang, 1985 and its sister species Ocypode cordimana Latreille, 1818 are distinct in this genus in lacking the specialized stridulatory apparatus on the palm that the other congeneric members use for sound production. In this study, we report the sound production behavior of O. sinensis collected from the sandy beaches in southwestern part of the Kii Peninsula (Shirahama and Susami), Wakayama Prefecture, Japan. Three males of O. sinensis collected in June, September, and October, respectively, produced loud sounds by rapidly vibrating their major cheliped at certain intervals at the burrow entrance or inside the burrows. A similar cheliped vibration behavior has been reported as ‘rapping’ in Ocypode jousseaumei (Nobili, 1905) and some other congeneric species, or ‘quivering’ in Ocypode stimpsoni Ortmann, 1897, although O. sinensis apparently did not knock the sand substrate directly, unlike the other species. This sound production behavior was observed only during the day under laboratory conditions. Additionally, the crab sporadically produced clicking sounds, likely by rubbing the mandibles. According to quantitative observations of a male O. sinensis in Shirahama, a train of cheliped vibration lasted 36–193 s, consisting of 9–35 pulses, each of which lasted about 0.98 s on average. The average interval between the pulses within a train was 4.8 s. Although the functions of the sound production by cheliped vibration remain unknown, it may function to attract mates or warn intruders, as suggested in other Ocypode species. In addition, we also briefly observed a similar cheliped vibration behavior in Ocypode ceratophthalmus (Pallas, 1772) in the burrow during this study. Our findings provide a better understanding of the evolution of sound production behavior in Ocypode and Ocypodidae.