Vertical Distribution of Ark Shell Arcidae Species Larvae along Axial Transects in Sendai Bay, Northeastern Japan

Vertical distribution of planktonic larvae affects their passive horizontal transport. We investigated vertical distributions of ark shell Arcidae species larvae at successive developmental stages monthly from August to October 2009 along axial transects in Sendai Bay, an open embayment along the Pacific coast of northeastern Japan, where the ark shell Anadara broughtonii (Schrenck 1867) is the dominant arcid species. Younger larvae (early and mid-umbo stages) were typically distributed in the surface near the shore with salinity <33 psu. Older larvae (late umbo and competent stages) were distributed in the bottom far from the shore with salinity >33 psu. Surface axial distributions of younger larvae were marginally affected by the river discharge volume, tidal height and southerly wind strength. The younger and older larvae were distributed closer inshore and further offshore than the main habitat of the adults, respectively. Therefore, younger larvae were first transported inshore and moved upward to the surface layer, and then transported offshore, increasing their distribution depth. These results indicate that ark shell larvae ontogenetically change their swimming behaviour to increase their distribution depth and their axial distribution patterns may be explained by the different flow patterns among the layers of the water column in Sendai Bay.