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caused by a fire at a neighboring oil refinery following the Great East Japan Earthquake in March 2011 as well as unpublished data on PAH concentrations in the sediment through 2022.

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.

The objective of this study was to determine the characteristics of the composition and spatiotemporal distribution of operational taxonomic units (OTUs) of nematodes in two tidal flats, namely, Nakatsu in Oita and Umino-Koen in Kanagawa, which differed in the density of Manila clams (Ruditapes philippinarum) and the other macrobenthos. There was no difference in the median particle size of the sediment between the two tidal flats, but the sorting coefficient was smaller in Umino-Koen, and the amount of organic matter was larger in the Nakatsu tidal flat. The density of nematodes was higher, and the number of OTUs was lower in Umino-Koen, which has a higher clam production than the Nakatsu tidal flat. A total of 219 OTUs were confirmed in both tidal flats, but 16 common OTUs were confirmed even by one individual on both tidal flats. In Nakatsu tidal flats, the OTU presumed to be Daptonema sp., which is common during all seasons at the two stations, was a dominant species that always inhabited extensively. It has been reported that the genus Daptonema is a non-selective deposit feeder (1B feeding type) nematode. In the Umino-Koen, where macrobenthos density is high, the presumed OTU Oncholaimus sp. was the dominant species that stably inhabited every year. Oncholaimus nematodes may thrive in tidal flats with a large amount of benthos because they are omnivore predators (2B feeding type). The characteristics of the composition and spatiotemporal distribution of nematode OTUs in the two tidal flats were different.

The macrozoobenthic community structure was examined at 21 sites along the Pacific coast of Japan, from Chiba to Aomori Prefectures after the 2011 tsunamis, using government survey datasets. The taxonomic diversity was low (20–42 taxa) in river-dominated sites on the Sanriku Coast, including Unosumaigawa and Orikasagawa in Iwate Prefecture. Endangered species were scarce at six sites along the Sanriku Coast (2–5 taxa) compared with the other 15 sites, including Aomori Prefecture (7–21 taxa). Cluster analyses based on the qualitative survey data classified the macrozoobenthic community types into four groups: (1) Low saline estuaries, (2) Sanriku Coast, (3) High saline inlets, and (4) Moderate saline estuaries. Qualitative analyses revealed that the community structure in Iwate Prefecture was typically characterized by marine andepifaunal organisms, such as limpets, gastropods, and barnacles, due to the steep, marine-affected, and substrate-rich river mouth habitats. Contrastingly, the quantitative survey data showed that the community structure at three sites in Iwate Prefecture was characterized by the highest diversity and density. Based on this survey, we listed 31 taxa that were absent from Iwate Prefecture, including 18 endangered species. This clearly showed that the macrozoobenthic community structure in Iwate Prefecture differed from those at other sites on the northeastern coast of Japan. Such discontinuity may partly be caused by the effects of cold Oyashio Currents in the Sanriku Region.

Corals are known to synchronously undergo gonadal development during a period of 1 year and release gametes simultaneously during the spawning season. Synchronization of reproduction may be caused by changes in the external environment; particularly, the effects of light on reproduction have been extensively reported. In this study, we specifically focused on the type of wavelength, with the intention to elucidate whether there are wavelengths that promote reproduction in two Acropora. We exposed coral colonies to four types of light-emitting diode conditions with wavelengths suitable for keeping coral: “Coral,” “Reef,” “Fresh,” and “Sunset,” and monitored gamete maturation every 3 months, as well as the number of released gametes during the spawning season. In A. intermedia, “Sunset” conditions promoted gamete maturation; however, there was no spawning observed. In contrast, in A. muricata, the same conditions promoted a series of processes ranging from gamete maturation to gamete release. In “Fresh” and “Coral” conditions, gamete maturation was also promoted. “Sunset” condition is characterized by longer wavelengths within the red-light visible spectrum, which is relatively predominant in the shallow waters where the two target species live. Therefore, this indicates that these longer wavelengths represent a light stimulus that promotes reproduction. By contrast, gamete maturation was promoted by even shorter wavelengths in A. muricata; therefore, further experiments considering different light sensitivities among species and growth promotion through the photosynthesis of zooxanthellae are recommended. It is also suggested that, in addition to wavelength type, spawning may be affected by water temperature increase after winter season.

Spartina alterniflora, a halophytic plant native to the east coast of North America and the Gulf of Mexico, grows in river estuaries and tidal flats, forming large colonies. Given that the invasion of this species often has substantial detrimental effects on local ecosystems, including the survival of benthic animals and native plants, it needs to be appropriately managed. Since 2009, S. alterniflora has been spreading into the Ohno River, which flows into in the inner region of the Yatsushiro Sea. As the Ohno River environment has obstructed access to dredging machines and thus restricted the removal of S. alterniflora, colonies of this plant have rapidly proliferated. To characterize its spreading patterns in this river, we quantified chronological changes in the area covered by S. alterniflora colonies based on a comparison of aerial photographs. The colonies were found to be distributed primarily in the intertidal zone of the Ohno River. Although the number and area of S. alterniflora colonies have increased markedly along the river estuary up to 2 km upstream, these colonies are rarely found in the upper reaches of the tidal zone, which could be attributable to the fact that large stands of Phragmites australis reed cover extensive parts of the habitat in the upper part of the intertidal zone. Nevertheless, there remain areas in the intertidal zone that are potentially suitable for further growth of S. alterniflora (i.e., bare fields), indicating that it will continue to expand its distribution along the Ohno River in the future. Continuous monitoring of the Ohno River and neighboring rivers will facilitate the early detection of this spread and thereby minimize the costs of controlling S. alterniflora growth.