Shoreline topography is believed to affect the physical environment of tidal flats and composition of benthic animals. Different degrees of topographic openness on tidal flats are expected to alter the feeding modes and life forms of the benthic organisms that inhabit them. We investigated the distribution of tidal flat benthos, blow-off distance(Fetch)as topographical openness, grain size, and the elevation on tidal flats on islands in the Seto Inland Sea. In total, 135 macroscopic benthic animals were identified. Fetch was weakly correlated with the silt-clay content and elevation. Nonmetric multi-dimensional scaling(NMDS)ordering of species compositions with a frequency of occurrence of 5% enabled the construction of a two-dimensional plot of the NMDS related to with the fetch and the elevation. The median grain size, silt clay contents, and ignition loss also showed relationships with the plot, although the effective directions on the plot were different from those of the fetch and elevation. Deposit & Filter-feeding and epifaunal species were positively and negatively correlated, respectively, with the first axis. Filter-feeding and infaunal species were also positively and negatively correlated, respectively, with the first axis and were also correlated with the second axis. These relationships suggested that the Deposit & Filter-feeding and epifaunal species are more likely to occur on closed tidal flats characterized by finer grain sizes and higher organic contents. In addition, filter-feeding and infaunal species became more abundant as the grain size increased. Therefore, the topographic openness of tidal flats allowed us to predict the environments of tidal flats and the functional taxonomy type of dominant macrozoobenthos in their habitats.

This study examined the possibility of sexual reproduction of Acropora pruinosa at Uchiura Bay, Shizuoka. Observations of spawning, early development, settlement, and post-settlement survival were conducted as follows. Mature A. pruinosa were transferred to flow-through tanks just before the expected spawning period in August 2021. Spawning was observed from 4–15 August, and egg–sperm bundles were collected and fertilized. Early development to the planula larval stage was observed every 12 hours. Numbers of settled individuals on two types of settlement plates, made of Amakusa pottery stone and of flexible board, were counted. Survival rates of settled individuals were also observed for four months in the sea. More than 50% of fertilized eggs developed into planula larvae after 61 hours. In total, 6,378 planulae were acquired, 41% of which settled on settlement plates. Settlement did not differ between the two types of plates. After transferring the plates to the sea, the number of polyps decreased drastically for the first 14 days. After 112 days, survival rates were 21.3% and 7.71% on Amakusa pottery stones and flexible boards, respectively. These results demonstrate the possibility of artificial fertilization and of seeding larvae of A. pruinosa in the field of its northern distributional limit. However, survival rates of acquired individuals as planulae were very low; therefore, further study is required to enhance survival rates by improving the settlement plates and/or by adding the symbiotic algae to planulae.

Stingrays, including longheaded eagle ray(Aetobatus flagellum),have been the target of eradication activities as they are considered to prey on short-neck clams(Ruditapes philippinarum).However, stingrays play a critical role as a keystone species in tidal flat communities. To date, the assessment of stingray predation on short-neck clams has only been limited to laboratory experiments and gut content analyses. Therefore, to conserve both stingrays and short-neck clams, it is necessary to quantitatively evaluate the stingray predation on short-neck clams. In this study, we focused on the foraging pits made by stingrays on the Midorikawa River tidal flat in Ariake Bay, Kyushu, Japan. Moreover, we estimated the rate of stingray predation on short-neck clams and examined the magnitude of the effects of predation on the decline of short-neck clams using unmanned aerial vehicle and quantitative surveys. We found that the maximum rate of stingray predation on short-neck clams was 0.29% per day, which accounted for approximately 17.7% of the overall mortality rate of short-neck clams during the same month. Therefore, our findings suggest that stingray predation exerts only a limited effect on the decline of short-neck clam stocks in the study area.

Catch of kuruma prawn Penaeus japonicus in Japan has declined since the mid-1990s. It has been reported that artificial seeds released for stock enhancement are heavily predated by fish in tidal flats, but this has not been reported for natural postlarvae. Therefore, to clarify the predation pressure on natural postlarvae, the postlarvae and surrounding fish were simultaneously collected using a rake sledge net on tidal flats at three sites along the Pacific coast of central Japan during the daytime from July to December 2021. The highest density of natural postlarvae in this study(5.7 individuals/m²)was less than that recorded immediately after the release of artificial seeds, but comparable to that recorded before the mid-1990s when the catch was higher. A total of 888 individuals of 21 fish species were collected with the prawn, with a high abundance of the goby Favonigobius gymnauchen. This goby has been reported as a predator on artificial seeds and occurred during the periods when natural postlarvae were abundant. However, few postlarvae were observed in the stomach contents of these fish, except for a small specimen of crescent sweetlips Plectorhinchus cinctus. These results suggest that natural postlarvae are less vulnerable to predators than artificial seeds potentially due to behavioral difference in addition to the lower density. It is probable that the sinking behavior in the sand during daytime in abundant artificial seeds attracts predators, although natural postlarvae do not perform such behavior because of the lower developmental stage.

The density and community composition of meiobenthos were investigated on the leaves of Zostera marina from spring to autumn in 2010 and 2011 at two stations in Kiritappu Cove, Hokkaido, Japan. The operational taxonomic unit(OTU)composition of nematodes on Z. marina leaves was analyzed using 18S ribosomal RNA during the summer and autumn 2011 and 2012. The estimated density of nematodes on the leaves was low in spring, and increased and dominated to the other meiobenthic animals from summer to autumn, when the community of Z. marina declined. The nematode OTUs living in epiphytic biofilms structures on the leaves of Z. marina were few, mostly dominated by one OTU(Chromadorina sp.: Chromadoridae),which has been reported as the dominant nematodes in epiphytic biofilms on submerged macrophytes and artificial structures in freshwater and brackish waters. The data suggest that the activity of these nematodes may contribute to biofilm development on leaves of Z. marina in the sea.

The ecological characteristics of Ena Bay, southern Miura Peninsula, Japan, were reviewed as a preface for the feature article “Macrozoobenthos in Ena Bay, Miura Peninsula (Japan): a hotspot for benthic biodiversity retained at the mouth of highly urbanized Tokyo Bay” in the Japanese Journal of Benthology. We first demonstrate the ecological importance of ecotones retained in Ena Bay at the southern portion of the Miura Peninsula. The intertidal zones consist of reed marshes, sandy to muddy tidal flats, boulder zones, rocky shores, coastal forests, and seagrass beds, which support the high diversity of macroinvertebrates (356 taxa) in Ena Bay, which had been found in a citizen-based survey conducted from 2013 to 2022. Secondary, the ecological importance of the reed marsh is also discussed based on a citizen census of semi-terrestrial crabs, including the endangered taxa Chasmagnathus convexus, Orisarma intermedium, and Parasesarma tripectinis. Thirdly, the history of scientific research and conservation activities were also reviewed based on the literature and our activities. Finally, the changes in topography and habitat structure, mainly due to massive soil deposition from the catchment and erosion during storms, are discussed. We concluded that the biodiversity in Ena Bay is vulnerable to natural and anthropogenic disturbances, and attention is needed to assure the conservation of ecotones in southern Miura Peninsula.

Survey by the benthos researchers and citizen-based monitoring of intertidal macrozoobenthos was conducted in Ena Bay, southern Miura Peninsula, Kanagawa Prefecture, Japan, from 2013 to 2022. The species richness, including endangered species in Ena Bay, was compared with those for 9 tidal flats from Tokyo Bay to the Kii Peninsula (<500 km spatial scale). Species richness was highest in Ena-Bay (356 taxa), which was characterized by high numbers of mollusks, annelids, and arthropods inhabiting various habitats(reed marshes, seagrass, rocky and boulder zones, and sandy to muddy bottoms). The number of endangered taxa was highest in Tanabe (59 taxa), followed by Ago Bay (50 taxa), Ena Bay (44 taxa), Banzu (40 taxa). Tanabe had 14 indigenous endangered taxa, followed by Ago Bay with 8 taxa, Ena Bay with 6 taxa. Therefore, these tidal flats were considered to precious habitat for conserving Red List macrozoobenthos species. The high macrozoobenthic diversity in Ena Bay was partly due to the high diversities of habitats and environments within a relatively small area at a small spatial scale (<1 km). The macrozoobenthos observed in the southern Miura Peninsula (i.e., Ena, Bishamon, and Koajiro Bays)included many taxa that were nearly extinct in the adjacent Tokyo Bay, a highly urbanized and anthropogenically disturbed bay. The macrozoobenthic communities at the tidal flats in southern Miura Peninsula are a potential source of endangered macrozoobenthos in the metapopulation network in the region.

The macrozoobenthos community structure was investigated at 27 tidal flats in Japan using citizen-based monitoring data. First, we compared the results of free surveys completed by citizens versus scientists conducted repeatedly at Ena, Bishamon, and Koajiro Bays, in the southern Miura Peninsula, and found that species numbers were comparable, although scientists found more deep-dwellers, symbionts, and habitat-specific taxa (e.g., muddy sediments, boulder zones, and reed marshes). Species richness showed no apparent latitudinal trends, whereas community structure had a geographical pattern from west to east, and could be grouped in clusters: A, tidal flats with reed marshes in the Yatsushiro Sea; B, Kii Peninsula; C, Koajiro Bay and open shores of the Yatsushiro Sea; D, Ena and Bishamon Bays; E, inner Tokyo and Ise Bays; and F and G, Tohoku region. The reed marshes in the Yatsushiro Sea hosted marsh-specific taxa, including endangered species, leading to specific community compositions at these sites. Species richness was greatest in Ena Bay (239 taxa in the citizen survey), mainly due to the high heterogeneity of habitat structure within a small spatial scale. Tidal flats in southern Miura Peninsula are valuable habitat for macrozoobenthos, including endangered taxa that are nearly extinct in the adjacent Tokyo Bay. Long-term monitoring of tidal flats is necessary for conserving faunal diversity and detecting the possible impacts of natural and anthropogenic disturbances and future climate changes in Japanese coastal waters.

In recent years, many subtropical brachyuran crab species distributed mainly along the southern Japanese coast have been recorded in the southern portion of the Izu, Miura, and Boso Peninsulas in eastern Japan near the warm Kuroshio Current axis. In this study, we conducted multiple surveys of the crab fauna in the Aono and Ogamo Rivers in the southern Izu Peninsula from 2014 to 2018, and assessed their population stability based on their occurrence frequency. We also compared the composition of the crab fauna using cluster analysis based on the presence-absence data with those at Yukashi Lagoon (Kii Peninsula), Ena Bay (Miura Peninsula), and Banzu tidal flat (Tokyo Bay). The results showed that the subtropical crabs Parasesarma bidens and Ptychognathus ishii were highly abundant in the Aono and Ogamo Rivers, and that the crab fauna in the Izu Peninsula was similar to that of the Kii Peninsula, but differed from those of the Miura Peninsula and Tokyo Bay. Similarity in the community structure between Kii and Izu Peninsulas suggested that the southern portion of the Izu Peninsula functions as a habitat for the subtropical crabs, and acts as “stepping stones” for expanding their distribution toward northeastern regions. Further, the southern Izu Peninsula had various microhabitats, such as gravel and cobble zones, mangroves, reed marshes, and soft bottoms in the estuaries, which could function as stable habitats preferred by these subtropical crabs.