Vertical distribution of Manila clam Ruditapes philippinarum larvae characterized through year-round observations in Tokyo Bay, Japan

The spatial density of the Manila clam Ruditapes philippinarum larvae was monitored during 2001-2003 at intervals of 2-4 weeks in Tokyo Bay. Clam larvae were sampled at 3-6 depth layers at four coastal sites (water depth 6-8 m) and four central sites (10-24 m) of the bay. In the coastal sites, small clam larvae (shell length < 144 mu m) were abundant in the surface and middle layers, whereas large larvae ( >176 mu m) were mainly sampled at the bottom layers. Similar patterns of vertical distribution of larvae were observed commonly in three divisions of water temperature (15.1-20.0, 20.1-25.0, 25.1-30.0 degrees C) and in two divisions of salinity (25.1-30.0, 30.1-35.0). Clam larvae appeared to shift swimming position to deeper depths in accordance with their growth in the coastal sites. The vertical shift of larval abundance was assumed to be influenced by the change of swimming behavior of larvae. Relationships between larval size and sampled depth were less clear at the central sites than the coastal sites. Before clam larvae reach a sufficient growth stage to settle, their swimming depth may change to near the bottom in the coastal areas in Tokyo Bay.