In spite of the fact that the area of Sagami Bay occupies only 0.05% of the total area of Japanese waters, the total species richness is 16.5% of the total Japanese species richness. This suggests high species richness due to the array of habitats present in Sagami Bay. The high species diversity of Sagami Bay has been recognized previously, and one of the proposed reasons for this degree of diversity is the influence of cold-water masses derived from the Oyashio Current, as well as tropical or subtropical water masses derived from the Kuroshio Current. Sagami Bay’s high reported diversity probably contains bias, because marine biologists have historically paid more attention to Sagami Bay.