Shoreline armouring has progressively affected the coastal landscapes of countries all over the world, and armouring construction will increase in coming years as a consequence of climate change. Armouring has the potential to affect coastal environments and induce changes in the abundance and diversity of marine communities, and its effects might by increased by wide adoption. Moreover, compared with temperate locations, the effects of armouring have been less studied in tropical and subtropical areas. Okinawa Island, the largest and most populated island of the Ryukyu Archipelago in southern Japan, has been affected by numerous civil and military engineering works. After decades of development, less than 40% of its coastline remains natural, and yet impacts from armouring on local marine communities have been overlooked until recent years. The aim of this research was to evaluate effects of near-shore armouring on the surrounding environment by comparing diversity and abundance of coral rubble mobile cryptofauna benthic communities between armoured and control sites. Across six different geographic locations, coral rubble was sampled in front of subtidal breakwaters and at nearby control sites. Armoured sites were associated with lower cryptofauna abundances and reduced richness and diversity at higher taxonomic levels (phylum and class). Reduction in spatial complexity could be a plausible reason for the observed patterns. Impacts could be mitigated by combining technical innovations, habitat restoration, and use of natural spaces as buffers for coastal protection. Since less than half of Okinawa Island’s coastline remains in a natural state, environmental conservation should be prioritized.

Hypoxia-tolerant bivalve populations could be a key component of ecosystem function because of their predominance in hypoxic environments. In this study, we conducted field surveys at two sites to elucidate the biological traits of the hypoxia-tolerant bivalve Veremolpa micra. We detected new settlements at both sites during summer. More highly reduced conditions in the surface sediment were recorded at Site-A than at Site-B in August 2013. Following this hypoxic event, shell growth of V. micra at Site-A decelerated, whereas that at Site-B increased. The population density after recovery from the hypoxic event decreased more at Site-B than at Site-A. Body size exhibited very little growth at both sites during the fall and winter, but there was rapid growth during the subsequent spring and summer, with sexual maturity being reached in early summer. Our findings demonstrated that the growth rates and survival patterns of the hypoxia-tolerant bivalve populations differed between sites with different degrees of sediment reduction. To understand the biological traits of hypoxia-tolerant bivalve species, it is necessary to consider and evaluate spatial differences in hypoxic stress at the local site level.

Juvenile Japanese Neptune whelk Neptunea arthritica preys on small gastropods in the field. Although the most common prey is Homalopoma sangarense, its density is lower than those of other prey species (e.g., Barleeia angustata, Reticunassa fratercula, and Cantharidus japonicus) in the field. To clarify whether N. arthritica juveniles show an “actual” or “apparent” preference for H. sangarense, we conducted a single-prey feeding experiment. Although N. arthritica attacked H. sangarense earlier and more frequently than B. angustata and C. japonicus, there was no difference in attack frequency and latency between H. sangarense and R. fratercula. Attacks on H. sangarense failed more frequently than on the other species, and the handling time for H. sangarense was significantly longer compared to the other three species. The large difference in handling time (over 10-fold difference) could be caused by the presence of a thick calcareous operculum on H. sangarense. Eventually, N. arthritica consumed more individuals of R. fratercula than of H. sangarense in the experiment. These results indicate that N. arthritica juveniles exhibit both “actual” and “apparent” preferences for H. sangarense. The “actual preference” of N. arthritica juveniles may reflect the past species compositions of small gastropods, because H. sangarense has been previously reported as the dominant species at the study site.

In the Ariake Sea, Japan, pen shells Atrina spp. are an important fishery resource, but now abundance of the stock has decreased. Methods to increase the abundance include transplanting juvenile Atrina spp. in the sea and maintaining the survival rate high. We conducted transplant experiments to compare the survival rates of one genetic lineage of Atrina between 1) transplant sites and 2) preventive measures against predation. We conducted five runs of transplant of Atrina artificial seeds (about 10 cm in shell length) in the inner and outer sea areas, using cages with different mesh opening sizes. We found that 1) Atrina sp. survived better at stations in the outer sea area than in the inner sea area. This trend in the last run was partly explained by a heavy deposition of mud at a station in the inner sea area. 2) Atrina sp. survived better in cages with mesh opening size <20 mm than those with the size >20 mm. This was attributed to predatory animals, which presumably include the whelk Rapana venosa, octopi Octopodidae spp., and swimming crabs Charybdis spp. recorded by underwater time-lapse camera. In conclusion, transplant to the outer sea area and protection from predators with a mesh <20 mm opening are promising to increase the survival rate of one genetic lineage of Atrina transplanted in the Ariake Sea.

The euryhaline mysid Neomysis awatschensis occurs widely in estuaries and brackish lakes in Japan. Although N. awatschensis may serve as an essential energy channel to higher trophic levels, its feeding ecology has been studied only in sheltered and desalinated Lake Kasumigaura. To determine major food sources for N. awatschensis, monthly sampling was conducted during the day along the microtidal Yura River estuary from March 2014 to July 2015. Additional sampling was conducted in May 2019 to assess diel vertical migration in relation to feeding. Microscopic examinations revealed that benthic diatoms (e.g. Bacillaria paxillifer and Navicula spp.) dominated in the stomach irrespective of developmental stage, diel cycle, or season. Stable carbon and nitrogen isotope ratios in N. awatschensis and its potential food sources indicated great contributions to the diet of N. awatschensis from benthic microalgae (mainly benthic diatoms) in 2014 and from particulate organic matter (mainly phytoplankton) in 2015. Although N. awatschensis always concentrated close to the bottom in 2019, diel vertical migration might have happened in 2015, when fresh water occupied the whole estuary possibly due to river mouth clogging. Under the stagnant and oligohaline conditions, N. awatschensis could have swum up into the water column and fed on phytoplankton at night in 2015. This hypothesis may resolve the apparent contradiction between the microscopy and stable isotope analysis in 2015. In the Yura River estuary, N. awatschensis feeds primarily on benthic diatoms, although it can utilize phytoplankton flexibly in response to prevailing environmental conditions.

In early life history, the clupeoid fishes, Ilisha elongata (Pristigasteridae) and Sardinella zunasi (Clupeidae) have similar spawning seasons and nurseries from spring to summer in the Ariake Sound, Shimabara Bay, Japan. Surveys conducted using larva nets and a beam trawl in July showed that the horizontal egg and larva distributions of I. elongata and S. zunasi both overlapped considerably in the inner estuaries; however, I. elongata settled in the estuaries while S. zunasi retained a pelagic distribution. A comparison of the vertical distributions of the two species in the estuary showed that I. elongata larvae were dispersed from the surface to near-bottom layers during the flood tide, and moved downwards to avoid being swept out of the estuary after the high tide, while S. zunasi larvae remained spread throughout the water column in all tidal phases. The findings suggest the existence of niche segregation during the pelagic larval period in the two species. Thus, by examining differences in larval mouth sizes, which are much larger in I. elongata than in S. zunasi, it appears that prey preference varies between the two species, prey specialization occurs, and there is no niche competition between the two species.

This study was undertaken to clarify the downstream drifting of Macrobrachium larvae in the Shimanto River by 24-hour sampling. Stage I zoeae accounted for most larvae collected in fresh and brackish waters. M. formosense and M. japonicum dominated in brackish and fresh waters, respectively, while M. nipponense occurred only in brackish waters. The amount of yolk droplets of M. formosense and M. japonicum reduced over time in freshwater. However, no larvae that had completely used up their yolks were observed. Even though M. japonicum larvae drifted from farther upstream than those of M. formosense, they had a higher residual amount of yolk droplets. Therefore, long-distance and long-time drifting in M. japonicum may be enabled by delaying the absorption rate of droplets.

The rhizostome jellyfishes, Rhopilema hispidum and Lobonemoides robustus, are two of the most abundant and commercially important species in Southeast Asia. However, information on genetic diversity and continuities among local populations remains totally unknown. We explored the genetic structure and population continuities of R. hispidum and L. robustus using genetic markers (COI & ITS1 regions) at 11 locations in four countries in Southeast Asia where fisheries were conducted. Rhopilema populations showed genetic distances (Φ_ST) among locations correlated positively with geographic distances, suggesting that they are in the isolation-by-distance (IBD). In Lobonemoides, molecular analysis revealed three distinct clades corresponding to sampling locations. Genetic distances among locations in L. robustus suggested that all populations maintain significant isolation. Our study reveals that these two blooming species have different phylogeographic patterns and differ in genetic diversity and continuities. Eustatic sea level changes during the Pleistocene and present ocean current systems, as well as differences in biological characteristics of these two species may explain these phylogeographic differences. Our results also suggest that jellyfish fisheries need to be carefully managed to avoid extinction of local populations and maintain the genetic diversity of these species, especially for L. robustus, which exhibits considerable genetic diversity in each location.