Bull. Jpn. Soc. Fish. Oceanogr. 79(1), Page 1-11, 2015   Effects of hypoxic water mass on vertical distributions of planktonic larvae of the short-necked clam (Ruditapes philippinarum) Satoshi Yamada1†, Yasuhiro Iwata1, Toshihiro Horiguchi2 and Teruaki Suzuki3 1 Aichi Fisheries Research Institute, 97 Wakamiya, Miyacyo, Gamagori, Aichi 443-0021, Japan 2 National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan 3 Graduate School of Environmental and Human Sciences, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya, Aichi 468-8502, Japan † E-mail: satoshi_5_yamada@pref.aichi.lg.jp We investigated vertical distributions of the short-necked clam larvae and dissolved oxygen (DO) concentrations in Mikawa Bay, Japan, from spring 2010 to autumn 2012. Larvae of the short-necked clam were usually found in the bottom layer or almost uniformly distributed in the water column. However, in the summer, hypoxic water mass formed at the bottom layer, and the larval distribution was restricted to the surface or middle water layers. The occurrence of D-shaped larvae was rare in layers with DO below 4 mg·l−1, while that of umbo and full grown larvae was limited in layers with DO below 2 mg·l−1. Diurnal vertical distribution of the larvae was observed when no hypoxic water mass formed at the bottom layer: larvae moved from the bottom at daytime to the surface layers at nighttime. Interestingly, they remained at the surface or middle layers for the whole day when the bottom layer of the water mass became hypoxic. These phenomena suggest that short-necked clam larvae avoid hypoxic water masses. Larval occurrence peaks (mainly D-shaped larvae) were observed during May to June (before hypoxic water mass formation) and in October (when the hypoxic water mass disappeared). These occurrence peaks were accompanied with spawning peaks. The survival rates of larvae in spring, however, seemed to be lower than those in autumn, suggesting that hypoxic water mass formation in spring may lead to high larval mortality. Key words: vertical distributions, larvae of the short-necked clam, dissolved oxygen, Mikawa Bay, hypoxic