The effects of cover-nets installed to prevent the feeding of fishes on Asari clams Ruditapes philippinarum were evaluated to the environment, the dynamics of macrobenthos including clams and meiobenthos in tidal flat of Yamaguchi Bay, Yamaguchi Prefecture, Japan. Five months after net installation, the mud content and total organic carbon inside the nets have been higher than those outside the nets. The densities of major macrobenthos, with the exception of Arcuatula senhousia, inside the nets tended to be higher than those outside the nets. The densities of nematodes inside the nets also tended to be higher than those outside the nets, but the same fluctuation trend was also observed for the densities outside the nets. The densities of copepods inside and outside the nets fluctuated similarly. The densities of R. philippinarum larger than a shell length of 15 mm inside the nets hardly exceeded 50 individuals/m² until two years after the nets were installed. R. philippinarum larger than a shell length of 15 mm rarely occurred outside the nets, suggesting that juveniles of R. philippinarum did not survive to adult size outside of the nets. On the other hand, the densities of juvenile R. philippinarum smaller than the shell length of 15 mm, which peaks at approximately a shell length of 5 mm in May and June every year, were always higher outside the nets, suggesting that the nets reduced the density of juveniles of R. philippinarum.

Peregrinamor ohshimai is a symbiotic bivalve associated with upogebiid mud shrimp, typically distributed along the Pacific coast of Japan, from Ibaraki Prefecture to Kumamoto Prefecture. During a survey conducted by high school students, a specimen of P. ohshimai was found attached to Upogebia yokoyai in Shizugawa Bay, Miyagi Prefecture. This finding extends the known distribution of P. ohshimai to northern Japan.

A newly established population of the sentinel crab Macrophthalmus japonicus in Otomo-ura, Rikuzentakata, Iwate, exemplifies coastal organism range expansions in Japan. However, it remains unclear whether M. japonicus larvae migrated northward from southern Pacific coast populations or southward from the Mutsu Bay population in Aomori. This study analyzed mitochondrial COI gene sequences from 13 M. japonicus specimens collected from Otomo-ura to determine Otomo-ura population larval sources by comparing genetic diversity with published data on other Japanese populations. The Otomo-ura population exhibited high genetic diversity comparable to southern Pacific coast populations, contrasting with the low genetic diversity of the Asadokoro population in Mutsu Bay. These findings suggest that the Otomo-ura population primarily originated from southern Pacific coast larvae. Despite potential founder effects and genetic drift in new populations, the Otomo-ura population’s high genetic diversity suggests its establishment in recent years and/or by repeated larval influx. Thus, founder effects may not have occurred.

Gamo Lagoon, a shallow brackish lagoon in Sendai, Miyagi Prefecture, northeastern Japan, was struck by a 7.2-m-high tsunami on 11 March 2011. The tsunami washed away the sediment and vegetation in the lagoon, leading to changes in lagoon topography, sediment characteristics, and areas of tidalflats and vegetation. Reed marsh and sand dune vegetation (SDV) mostly disappeared after the tsunami, while the endangered opportunistic halophyte Suaeda maritima became proliferated in the bare upper tidal zone formed by the tsunami. It is expected that reed marsh and SDV are fully recovered respective in 2030 and 2026, while S. maritima will disappear due to the competition with reed. In 2011, lagoon sediment become courser, organic poor, and more oxidized due to the removal of reduced mud and deposition of sea sand, and opportunistic macrozoobenthos including polychaetes and amphipods rapidly recovered their population size within three months after the tsunamis. Sand deposition caused by a typhoon and Fukushima earthquake tsunamis in 2016 also modified lagoon topography and sediment characteristics, resulting in the increase in macrozoobenthic density and diversity in 2017. Contrastingly, water stagnation caused by the topographical changes and restoration works after 2017 negatively affected the density and diversity of macrozoobenthos in Gamo Lagoon. Present results demonstrated the impacts of pulsed disturbances including tsunami, typhoon, and restoration works on the lagoonal ecosystem, and the process of recovery and succession of coastal ecotones for 13 years following the 2011 tsunamis.

We examined structural changes in the molluscan community around Matsushima Bay, Miyagi Prefecture, northeastern Japan, for 10 years (2001–2010) before and 13 years (2011–2023) after the 2011 Great East Japan Earthquake and the subsequent tsunami. Before the earthquake and tsunami, Ruditapes philippinarum, Macoma incongrua, Pillucina pisidium, and Batillaria attramentaria were dominant, whereas the alien species Laguncula pulchella appeared in 2002 and increased in number. After the tsunami, R. philippinarum and M. incongrua populations quickly recovered in 2012; however, the intertidal snail B. attramentaria population did not start to recover until 2020. In contrast, Arcuatula senhousia, Mya japonica, Decorifer matusimanus, Solen strictus, and Cylichnatys yamakawai were not recorded or were found at low densities before the tsunami; however, their abundance rapidly increased after the tsunami. These results suggest that the molluscan community on the Tona Coast was drastically altered by the earthquake and tsunami. Some species mainly inhabiting the intertidal–subtidal zone may have increased in number because of land subsidence and sediment scouring near the seawall caused by the tsunami, which expanded the subtidal habitat. Although the molluscan community on the Tona Coast is approaching its original state, the recovery is slower than that of other tidal flats because of the decrease in intertidal habitat. Without restoration of the intertidal habitat, the molluscan community is likely to enter another state as a mixture of characteristic species, before and after the earthquake and tsunami.

After the Great East Japan Earthquake of 2011, the tidal flats of Matsubara Beach, located in the inner part of Shizugawa Bay, were conserved by setting back the location of a large seawall when that washed-away seawall was rebuilt at the request of a local citizens. Since 2017, members of the natural science club of a local high school have conducted a series of benthos surveys. During the restoration work on this tidal flat, efforts were made to conserve the tidal flat environment, and monitoring, including recording of the subsequent recovery process, is ongoing. Furthermore, restoration work was conducted at Oritate Beach in 2015, which is also located in the inner part of Shizugawa Bay, with the construction of a submerged breakwater on the inland side and the laying of a large quantity of crushed stone along the tidal flats, which significantly deteriorated the habitat quality for benthos. Since 2022, the local fishery association has taken the lead in partially removing the crushed stone from the tidal flats, seeking to restore the tidal flat environment. Local elementary and high school students are monitoring the effects of environmental improvement. These surveys using citizen survey methods that are accessible to non-professional scientists have not only elevated the significance of an educational activity but also reinforced the case for continued assessment of biodiversity recovery through cooperative teamwork between local children and scientists. Cooperative activities have played an important role in the conservation of local areas after the Great East Japan Earthquake.

Polycyclic aromatic hydrocarbons (PAHs) are persistent environmental pollutants generated by the combustion of fossil fuels, oil spills, and other anthropogenic and natural activities. Because some PAHs have carcinogenic and mutagenic properties, it is important to analyze their concentrations and behavior in the environment. In this review, we present our previous research findings on the concentration and degradation behavior of PAHs in the fecal pellets of Marphysa sp. E sensu Abe et al. 2019 (Annelida, Eunicidae), inhabiting the sediment of the Yoro tidal flat, which is located within Tokyo Bay, Ichihara, Chiba, Japan. Based on analyses of PAH concentrations, stable isotope ratios of carbon and nitrogen, total organic carbon, total nitrogen, and the particle size of fecal pellets of the worm and sediments (sand and reduced mud), we found that this worm ingested reduced muds (organics-rich silt and clay in a reductive state) containing high levels of PAHs. Furthermore, the PAH levels rapidly decreased in the fecal pellets after excretion. These results suggest that the feeding and excretion behaviors of the worm may play important roles in the purification of tidal flat environments. Additionally, we review previous studies on changes in PAH concentrations in tidal flat sediments caused by a fire at a neighboring oil refinery following the Great East Japan Earthquake in March 2011 as well as unpublished data on PAH concentrations in the sediment through 2022.

In marine macrozoobenthos, the northward expansion of the species distribution along the Japanese coastline has become a popular topic in recent years. Simultaneously, the paucity of information on their status and distribution is a major impediment in evaluating the rarity of each species. In Ena Bay and Bishamon Bay, on the southern Miura Peninsula, located on the Pacific coast of central Japan, citizen-based monitoring and surveys of intertidal macrozoobenthos by experts were conducted in parallel from 2013 to 2023. In this study, we provide habitat information on marine macrozoobenthos, including endangered and/or poorly known species, on four cnidarias, eight mollusks, four annelids and ten decapods obtained from these surveys. Based on this information, we discuss the characteristics and importance of Ena Bay and Bishamon Bay as habitats for marine macrozoobenthos to contribute to the maintenance of regional populations of these species.