1Tokyo Bay Fisheries Laboratory, Chiba Prefectural Fisheries Research Center, Futtsu, Chiba 293-0042, 2Department of Marine Bioscience, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan
Larvae of Manila clam were sampled at eight sites along an on-offshore transect line in Banzu, Tokyo Bay. Clam larvae were sampled at vertical points at each site during a tidal cycle (06:00 to 18:00) at 90-min intervals in a spring tide (August 7, 2001) and a neap tide (August 13, 2001). Small-sized larvae (SSL, shell length <143 μm) were abundant in the surface and middle points throughout the tidal cycle in both the spring and neap tides. Medium-sized (MSL, 144-175 μm) and large-sized (LSL, >176 μm) larvae were abundant in the middle and bottom points in the spring tide and in the surface and middle points in the neap tide. Densities of the SSL were not different between onshore and offshore divisions (foreset slope, subtidal flat) of the sampled sites. In the spring tide, high densities of MSL and LSL were observed in the bottom of the foreset slope during an ebbing tide and in the tidal flat in a flooding tide. SSL appeared to be abundant in surface and middle points in relation to higher water temperature and lower salinity. However, the aggregation of MSL and LSL in the bottom of the foreset slope and in the tidal flat may not have been related to water temperature and salinity.