Bull. Jpn. Soc. Fish. Oceanogr. 69(2), Page 73-82, 2005   Changes in Behavioral Characteristics, in Relation to Salinity Selection and Vertical Movement at Different Growth Stages of the Planktonic Larvae of the Japanese Littleneck Clam, Ruditapes philippinarum Motoo Ishida†1, Momoko Ogasawara2, Chisato Murakami2, Mikio Momoi3, Tetsuya Ichikawa3 and Teruaki Suzuki1 1Aichi Fisheries Research Institute, 97 Wakamiya, Miyatyo, Gamagori, 443-0021, Japan 2Marine Biological Research Institute of Japan Co., Ltd., 4-6 Yutakamati, Sinagawa-ku, Tokyo 142-0042, Japan 3ChudenCTI Co., Ltd., 1-27-2 Meiekiminami, Nakamura-ku, Nagoya 450-0003, Japan †e-mail: motoo_ishida@pref.aichi.lg.jp For the restoration of tides [do they mean “tidal zones” or “tidal areas” ?] there is a need for techniques which can reveal features of planktonic larvae of filter feeding macrobenthos, such as bivalves. For understanding and predicting the sources of planctonic larvae, the vertical distribution of larvae is the most important factor, because it affects the drift route. In this study, the optimum range of salinities chosen by the larvae and the vertical movement velocity at each growth stage, from fertilized egg to full-grown stage, was measured using a density-gradient water column apparatus. The results show that trochophore larvae select low salinity water (18~24 psu). D-shaped larvae move toward more saline water (29 psu) in the early stages, and then gradually select lower salinity layers (21~23 psu) as they mature. Results are similar in both dark and light conditions. We derive an empirical equation describing changes in vertical movement velocity at different stages. The temporal changes in the vertical movement of larvae can be simulated by this equation using the values for salinity and vertical movement velocity. Key words: Japanese littleneck clam, planktonic larvae, salinity selection, vertical movement