Apseudes nipponicus SHIINO, 1937 is the first described tanaidacean species in Japan. This species is a relatively large-sized member among tanaidaceans, reaching 16 mm in length. Its type specimens were collected from shallow water in Sagami Bay and the Sagami Sea, the best-studied marine areas in Japan. Despite the large body size of this species and great sampling efforts in this area, additional published records of this tanaidacean are limited to an illustrated encyclopedia. In this study, we report A. nipponicus from Sagami Bay and the Sagami Sea for the first time in decades. More than 190 individuals were collected from experimental aquaria at the Shimoda Marine Research Center of the University of Tsukuba, where colonial ascidians or gastropods were reared. In aquaria, tanaidaceans inhabited a blackened muddy bottom, and reproduced there. In addition, three animals were collected from another natural environment, a muddy substrate under fist-sized rocks at a depth of 2 m around the Misaki Marine Biological Station of the University of Tokyo. Considering the environments where SHIINO's and our samples were collected, A. nipponicus likely prefers less-disturbed, sulfide-containing muddy substrates, and this probable specialized habitat may be one reason why so few records exist for this species.
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