Bull. Jpn. Soc. Fish. Oceanogr. 77(4), Page 282-289, 2013   Influences of swimming stop and death due to sedimentation by hypoxia on larvae of the Japanese littleneck clam, Ruditapes philippinarum Satoru Kamohara1†, Satoshi Yamada1, Ryota Sone1, Toshihiro Horiguchi2 and Teruaki Suzuki3 1 Aichi Fisheries Research Institute, 97 Wakamiya, Miyatyo, 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: satoru kamohara@pref.aichi.lg.jp In Japan, gross reduction of water quality-related substances has been conducted to achieve the environmental standard levels for chemical oxygen demand, total nitrogen, and total phosphorus in bay areas. However, damage to fisheries, such as catch decline, is still a serious problem. Therefore, establishment of a new environmental standard for dissolved oxygen concentration (DO) in the bottom layers of bay areas is currently being discussed by the government of Japan. Unfortunately, there have been only a few studies on the adverse effects of hypoxic water on fishes, crustaceans, and mollusks, particularly on their fries, larvae, and juveniles; these stages of the organisms are required to determine DO standard values. Therefore, we performed laboratory experiments to investigate potential behavior relation to hypoxic water by using the early life stages of the Japanese littleneck clam, Ruditapes philippinarum, as the representative species in bay areas. The following stages of the Japanese littleneck clam showed the highest sedimentation percentages in hypoxic water: pedi veliger>umbonal veliger>D-shaped veliger. The sedimentation percentages in hypoxic water were increased by decreasing the DO percentage in the same stage. The following stages of the Japanese littleneck clam showed the highest lethality percentages in hypoxic water: D-shaped veliger>umbonal veliger>pedi veliger. The lethality percentages for hypoxia were increased by decreasing the DO percentage in the same stage. On the basis of the results, we formulated their sedimentation and lethality values against low DO for understanding the effects of hypoxic water on the pelagic larvae of the Japanese littleneck clam in marine environments. A DO of 3 mg・l-1 or more is required in the bottom layers of the bay areas to prevent the influence of hypoxia on the clam larvae. In addition, a DO of at least 2.5 mg・l-1 or more is required in the bottom layers in order to preserve about 90% of the clam larvae. Key words: Japanese littleneck clam larvae, effects of hypoxia, density-gradient water column, swimming stop and following lethality