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Estimated Population Densities of Megafauna in Two Chemosynthesisbased Communities: a Cold Seep in Sagami Bay and a Hydrothermal Vent in the Okinawa Trough

This study investigates the population density characteristics of the megafauna at two previously described chemosynthesis-based communities associated with cold seepage at the Off Hatsushima Island site (OHl) in Sagami Bay and hytlrothermalism at the Minami-Ensei Knoll (MEK) in the Okinawa Trough. Quantitative sampling was conducted using a submersible, ROV and Deep Tow Camera array near each community. A total of 26 and 27 megafaunal species were recorded at OHI and MEK, respectively. The dominant organism with respect to population density was the provannid snail Provanna glabra at OHI. This species also occurred at MEK, but it had a lower density than at OHL Instead, Cantrainea jamsteci was the dominant gastropod species at MEK. The community at MEK was dominated with respect to population density by the symbiont-containing mussel Bathymodiolus japonicus, but this species was not so common at OHI. The mean population densities of almost all megafaunal species, except for a few conspicuously predominant species at OHI and MEK, are similar to those for the dominant species in other deep-sea photosynthesis-based communities. Both sites were characterized by large populations of only a few dominant species (Provanna glabra, Calyptogena okutanii, and C. soyoae at OHI and Bathymodiolus japonicus at MEK). This pattern may be due to the extreme environments that occur near chemosynthesis-based communities.