Spatial and Temporal Distributions of Planktonic and Benthic Phases of Bivalves in a Tidal Estuary

Abstract

The present study describes the spatial and temporal distributions of the planktonic and benthic phases of bivalves in a tidal estuary of the Kiso River, and gives insights into larval retention within the estuary. Patches of planktonic larvae of Corbicula japonica, absolutely dominant among the sampled larvae in July, were found along two troughs in the estuary, and were thought to move back and forth together with the saline water which intruded into the estuary during flooding and ceased from there during the ebb tide. These larvae were detected mainly in the lower, saline water strata. Shortly after the settling of planktonic larvae onto the bottom sediment, juveniles with shell length of less than 0.5 mm were scattered widely within the whole surveyed area. Juveniles of Nuttalia olivacea were not found in August, but youngs of this species with shell length of 1.0-1.5 mm were found scattered widely within the downstream sector of the surveyed area and those with shell length of more than 2 mm were densely distributed at the stations along both of two troughs.