Reproduction of the Marine Debris Distribution in the Seto Inland Sea Immediately after the July 2018 Heavy Rains in Western Japan Using Multidate Landsat-8 Data

Understanding the spatiotemporal environment of the ocean after a heavy rain disaster is critical for satellite remote sensing research and disaster prevention. We attempted to reproduce changes in marine debris distributions using multidate data of Landsat-8 spectral reflectance acquired immediately after a heavy rain disaster in western Japan in July 2018. Data from cleaning ships were used for screening the marine debris area. As most of the target marine debris consisted of plant fragments, a method based on the corrected floating algae index (cFAI) was applied to Landsat-8 data. Data from cleaning ships clarify that most of the marine debris accumulated in the waters in the northern part of Aki Nada, a part of the Seto Inland Sea. The spectral characteristics of the corresponding marine debris spectral reflectance obtained from the Landsat-8 data were explained by the FAI with band 5 (central wavelength: 865 nm) as the maximum value. Unlike traditional FAI, cFAI eliminated the effect of background water turbidity. The Otsu method was effective for the automatic threshold determination for cFAI. Although Landsat-8 data have limited spatial resolution and observation frequency, these data were useful for understanding marine debris distribution after a heavy rain disaster.

Sediment Transport and Associated Long-term Bathymetric Changes due to Tidal Currents in The Seto Inland Sea, Japan

The topography of the seafloor is essential for determining physical phenomena such as ocean currents, favorable habitats for marine organisms, optimal vessel navigation, and so on. Prevailing currents and waves, as well as associated shear stresses acting on the ocean floor, are responsible for the formation of typical topographic features including sea caldrons and sandbanks through erosion of bedrock and sediments and their deposition processes. In the Seto Inland Sea (SIS), the most extensive semi-enclosed estuary in Japan, tidal currents affect pronouncedly the formation of seafloor topographic features; however, they have not been fully studied, particularly from a hydrodynamic viewpoint. This study aims to understand bathymetric formation under the predominance of tidal currents in the SIS. A 3-D high-resolution SIS circulation model based on the JCOPE2-ROMS system in a triple-nested configuration was utilized to examine the detailed hydrodynamic processes for the topography formations. A high correlation between the bottom shear stress and the scour depth of the erosive areas was observed, demonstrating that local tidal forcing has continuously been exerted on the seafloor to erode. A diagnostic sediment budget analysis was then conducted for sediments typical of the SIS, that is, gravel, sand, and clay, using the modeled circulation field. The horizontal divergence of the residual flows indicates consistency between divergence (convergence) and erosion (deposition). The sediment budget model also shows that these sediments are generally transported from deep to shallow areas in eroded terrains to form deposited terrains fringing the eroded terrains, whereas sedimentation tendency differs largely from location to location.

Morphological observations and molecular confirmation of larvae of Levisquilla inermis (Crustacea: Stomatopoda) from the Seto Inland Sea

Mantis shrimps are commercially important crustaceans in many areas of the world. In contrast to the relatively common studies of adults, limited studies have been attempted of larvae because of a lack of identification keys. The objectives of this study were two-fold: (1) to link wild-caught larval specimens from the Seto Inland Sea to a certain species and (2) to present a detailed morphological description of the last larval stage. The resulting molecular phylogenetic tree based on 16S rRNA gene sequences strongly implies that our larval specimens were linked to Levisquilla inermis, which was barcoded from a newly collected adult specimen with a morphological identification. K2P genetic divergence was found to be 0% among the larval and adult specimens. The congeneric species L. jurichi was the sister group of the L. inermis cluster, but the K2P distance between them was 3.4%, and it was considered a distinct species. Morphological observations provided five major distinguishing characteristics of the last-stage larva of L. inermis: (1) propodus of second maxilliped with three basal spines, (2) all five pleopods possessing an appendix interna with cincinnuli, (3) exopod of uropod longer than the endopod, (4) distal margin of the exopod of the uropod reaching the lateral tooth of telson, and (5) telson with nine intermediate denticles. Overall, our results reveal the importance of combining molecular and morphological analyses for solving stomatopod larval identification. This finding can be used to support and facilitate future research on the taxonomy and biodiversity of stomatopod larvae.

Dynamics and Mass Balance of Polycyclic Aromatic Hydrocarbons in and around the Seto Inland Sea, Japan

Polycyclic aromatic hydrocarbons (PAHs) were analyzed to elucidate the distribution, ecological risk, pathways, and fluxes of these pollutants in and around the Seto Inland Sea, Japan. High molecular weight PAHs (5–6 rings) were primarily found in regions close to the bay estuaries, and their proportions decreased at distances further from the estuaries (offshore areas), where low molecular weight PAHs (2–4 rings) were more ubiquitous. Ecological risk assessments revealed that the PAHs found in the sediments should have no adverse effects on benthic communities, with the exception of fluorene, which was detected in high levels in the sediment from Beppu Bay. A mass balance for PAHs in the Seto Inland Sea, calculated based on data collected in the field and published literature findings, showed the PAH flux into the Seto Inland Sea from atmospheric deposition were ca. 5 times higher than that from riverine inflows. Comparison of the amount of the PAH flux between the Seto Inland Sea and other seas suggested that the Seto Inland Sea is less polluted than the Gulf of Lion and the Bohai Sea, and more polluted than the Yellow Sea.

Caltsacoryne setouchiensis (Hydrozoa, Anthoathecata) a new genus and species of hydrozoan jellyfish from Japan

We report a new genus and species of hydrozoan jellyfish belonging to the order Anthoathecata collected from the Seto Inland Sea, western Japan. Caltsacoryne setouchiensis gen. et sp. n. can be distinguished from other species of Corynidae based on the following combination of morphological characters: number of tentacles, cnidocyst pads, manubrium length, and the shape of the gonad and tentacles. A table comparing the primary diagnostic characters of this new genus of Corynidae is presented.