Journal of Oceanography, Vol. 61 (No. 5), pp. 973-978, 2005Short Contribution
Ryo Ishii1, Hideo Sekiguchi1* and Yasunari Jinnai2
1Faculty of Bioresources, Mie University, 1515 Kamihama-cho, Tsu, Mie 514-8507, Japan
2Kumamoto Prefectural Fishery Research Center, Oyano-machi, Amakusa, Kumamoto 869-3603, Japan
(Received 18 August 2003; in revised form 20 December 2004; accepted 21 December 2004)
Abstract: We have examined the vertical distributions of planktonic bivalve larvae, particularly the clam Ruditapes philippinarum and the Mussel Musculista senhousia which are common and abundant on tidal flats of eastern Ariake Bay, southern Japan. Submersible pumping gear was used to take samples at 2 stations every 2 hours during the spring tide over a whole tidal cycle and/or through daytime and nighttime. Water samples were pumped up from 3 to 5 depths from the surface to sea bottom. Regardless of tidal cycles, D-shaped larvae were concentrated near the surface, while umbo larvae were found at the surface to intermediate depths. On average, these larval densities were significantly higher at the station close to the shore than the offshore station, with no significant difference between daytime and nighttime and between flood and ebb tides at each station. The velocity and direction of water movement at both stations revealed remarkable difference between the surface and bottom waters. The larvae at the surface may quickly disperse and be transported elsewhere, while those in water close to the sea bottom may tend to be retained.