Bathymetric distribution and biodiversity of cold-water octocorals (Coelenterata: Octocorallia) in Sagami Bay and adjacent waters of Japan

in total, 260 octocoral species, including 144 gorgonians, 80 alcyonaceans, and 36 pennatulaceans have been recorded in the literature at Sagami Bay, Japan. Fifteen of the octocoral species were newly recorded at littoral to bathyal depths (13 gorgonians, one alcyonacean, and one pennatulacean). Thirty families of octocorals were recorded between the depth of 100–200 m (deep littoral) and 23% of all recorded species and 37% of newly recorded species were obtained at this depth. A total of 114 species of octocoral is endemic to Japan and adjacent waters, while 74 of these octocorals are only found in or around Sagami Bay. These results reconfirmed the high biodiversity of octocoral fauna in Sagami Bay, and suggest there are two main explanations for these high levels of biodiversity in this bay and in this area. One explanation appears to be that the recorded species represent a faunal boundary which is formed by converging elements of temperate, sub-tropical, and sub-arctic regions. Another explanation may be that the 100–200 m depth range may be a benthic/pelagic ecotone, a transitional zone between two ecosystems, which results in a high degree of biodiversity due to benthicpelagic coupling.