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We analyzed total mercury content (THg) and carbon (δ13C) and nitrogen (δ15N) stable isotope ratios in fish, subtidal macrobenthos, and particulate organic matter (POM) as a proxy for pelagic phytoplankton and attached microalgae as a proxy for microphytobenthos to investigate the mercury exposure pathway in fish. For four seasons, samples of the above-mentioned organisms were collected on five occasions (July and October 2018 and January, April, and July 2019) in Minamata Bay. Isotope analysis showed that Minamata Bay food web structures were almost entirely fueled by microphytobenthos. The THg values of the fish and macrobenthos species were positively correlated with their δ13C. This indicates that their diets, which were highly fueled by microphytobenthos, led to high THg bioaccumulation in both macrobenthos and fish. The feeding habits of fishes differ depending on the species, and they prey on organisms of many taxa, including fish (mainly Japanese anchovy), crabs, shrimp, copepods, annelids, and algae. Fish species that preyed on benthic crustaceans had high THg. These results suggest that the main pathway of Hg bioaccumulation in fish from Minamata Bay is the benthic food chain, which is primarily linked to benthic crustaceans fueled by microphytobenthos.

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The fecal pellets of Marphysa sp. E sensu Abe et al. (2019) (Annelida, Eunicidae) living in the Yoro tidal flat (Ichihara, Chiba, Japan) contain high levels of polycyclic aromatic hydrocarbons (PAHs), and the concentrations rapidly decrease over time. To investigate the origin of the high-concentration PAHs in the fecal pellets and food sources of the worms, the PAH concentrations, carbon and nitrogen stable isotope ratios (δ13C and δ15N), total organic carbon, and total nitrogen for two types of sediment (sands and reduced muds), fecal pellets, and the body of the worms were determined. The PAH concentrations and chemical properties of the fecal pellets were similar to those of the reduced muds (20-30 cm sediment depth). The δ13C, δ15N, and C/N values of reduced muds were the same as the typical values of terrestrial C3 plants, suggesting that reduced muds were derived from terrestrial plants. These data indicated that the worms selectively take up reduced muds containing high levels of PAHs. The δ13C and δ15N values of the worm bodies indicated that the worms did not use the organic carbon derived from terrestrial C3 plants as primary nutrition. Taking into consideration their selective uptake of reduced muds, excretion, and subsequent rapid decrease of PAHs in the fecal pellets, the worms could contribute to the remediation of chemical pollutants in the tidal flat sediments.

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Choanostoma Yamaguti, 1934 was established for Choanostoma plectorhynchi Yamaguti, 1934 (Trematoda: Opecoelidae). The generic name Choanostoma, however, is a junior homonym of Choanostoma Wang, 1930 (Ciliophora: Haptorida: Didiniidae), and we propose a new replacement name, Choanotrema nom. nov.

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Hartman (1965) described a nereidid species, Namalycastis profundus[sic], based on a deep-sea specimen collected from off Bermuda (1000 m deep) in the NW Atlantic. Although Hartman (1965) did not mention its etymology, the specific name is believed to be derived from the Latin adjective "profund-" meaning "deep." Because Namalycastis is feminine in gender (Glasby et al. 2016), the original specific name as masculine was incorrect and must be corrected to N. profunda for mandatory gender agreement under Articles 31.2 and 34.2 of the International Code of Zoological Nomenclature (hereafter, "the Code") (International Commission of Zoological Nomenclature 1999).

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After a long-standing taxonomic confusion, the echiurid genus Ikedosoma Bock, 1942 , endemic to Japan and surroundings, is redefined on the basis of morphological and molecular analyses of many new Japanese materials and some museum specimens. The re-examination of a syntype of I. elegans ( Ikeda, 1904 ), the type species of the genus, first revealed that its oblique muscle layer is continuous throughout and never fasciculate between longitudinal muscle bands, unlike those described in the definitions that have prevailed for ca. 70 years, making this genus indistinguishable from Listriolobus Spengel, 1912 . Two Japanese species of Ikedosoma, I. elegans and I. gogoshimense (Ikeda, 1904), which were thus redefined, had also been poorly defined in the past to the point of being nearly indistinguishable from each other, largely due to incomplete descriptions and poor collections. Molecular phylogenetic analyses using 18S and 28S ribosomal RNA, histone H3, and cytochrome c oxidase subunit I (COI) genes clearly confirmed the distinction between these two species, their monophyletic origin, and their distinction from L. sorbillans (Lampert, 1883). The genus Ikedosoma thus validated is morphologically distinguishable from Listriolobus by the absence of a rectal caecum. Ikedosoma elegans and I. gogoshimense also differ in the disposition of gonoduct pairs. The third known species, I. qingdaoense Li, Wang and Zhou, 1994 , from Qingdao, North China, lacks information on oblique muscle layers, which makes even its generic affiliation uncertain.

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Large numbers of swimming and stranding Urechis unicinctus were observed at night during low tide in Sasuhama, Miyagi Prefecture, northeastern Japan, during the periods from January to February in 2012 and 2013. Worms did not drift passively but swam actively, therefore hinting at a certain purpose for such behavior. As trochophore larvae of U. unicinctus were observed to occur simultaneously in the plankton, we infer the possibility that this is an event of reproductive swarming. Anatomical observations of both swimming and stranding U. unicinctus showed that none of the specimens had gametes, which may suggest that these were completely spent after spawning. Urechis unicinctus seemed to begin swimming after dusk and the observed swimming behavior occurred during the evening ebb tide throughout the night low tide during winter time. Stranding U. unicinctus have long been known in Japan and have been attributed to sea storms. The present study shows for the first time the possibility that U. unicinctus swims in order to reproduce at night and that this swimming behavior is closely linked to the stranding of U. unicinctus individuals.

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A new echiuran, Arhynchite hayaoi sp. n., is described from newly collected specimens from sandy flats of the Seto Inland Sea, Japan, together with many museum specimens, including those once identified as Thalassema owstoni Ikeda, 1904 or Arhynchite arhynchite (Ikeda, 1924). The new species is clearly distinguishable from its congeners by the smooth margin of gonostomal lips and lack of rectal caecum. Brief references are also made to the morphological distinction between the new species and Thalassema owstoni, originally described from the deep bottom on the Japanese Pacific coast.