Discovery and Genetic Characterization of a Vestimentiferan, Lamellibrachia satsuma, from the Submarine Volcano Omuro Dashi in the Izu-Ogasawara Arc.

Vestimentiferan tube worms (Annelida: Siboglinidae) were discovered in a hydrothermal field at a depth of 195 m in the crater of the submarine volcano Omuro Dashi in the Izu-Ogasawara Arc. Based on the nucleotide sequences of the mitochondrial cytochrome c oxidase subunit I (COI) gene in individuals sampled in 2022, they were identified as Lamellibrachia satsuma Miura, Tsukahara & Hashimoto, 1997. STRUCTURE analysis and discriminant analysis of principal components (DAPC) based on 14 microsatellite markers showed a large genetic deviation of the population of Omuro Dashi from those of Kagoshima Bay and the north Mariana Arc (the Nikko and Daikoku Seamounts), whereas the population of Omuro Dashi did not show significant genetic deviation from that of the Nikko Seamount based on the COI gene. All analyses showed that individuals of a methane seep area on the Kanasu-No-Se Bank, the Nankai Trough, which were collected only in 1994, were more closely related to those of Omuro Dashi than to those of other habitats. These results suggest that the ancestors of the Omuro Dashi and Nankai Trough populations originated from migrants from the north Mariana Arc and that there might be undiscovered source population(s) of L. satsuma around the Nankai Trough.

Food Web Structures and Mercury Exposure Pathway to Fish in Minamata Bay.

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.