SWIMMING BEHAVIOR OF MARINE BIVALVE LARVAE IN A FLUME BOUNDARY-LAYER FLOW - EVIDENCE FOR NEAR-BOTTOM CONFINEMENT

The swimming behaviour of settling larvae of the bivalve Cerastoderma edule (L.) was studied in still water and in a flume boundary-layer flow. In still water and slow flow the larvae swam upward in helices with high directionality, which is interpreted as morphological geotaxis. By reducing velar propulsion larvae regularly descended towards the bottom due to gravity. In moderate and fast flow (5 to 10 cm s-1) the larvae became confined to the viscous sublayer, where they slowly drifted in the streamwise direction at 0.45 to 1.6 mm s-1, less than 1 mm above the sediment surface, periodically making contact with the bottom. At flow velocities exceeding 15 cm s-1 the steady near-bottom drift changed to bed-load transport of tumbling larvae with high probability of resuspension. Based on kinematic and force analysis of swimming behaviour we suggest that boundary-shear induced torque restricts the settling larvae to the viscous sublayer. This near-bottom confinment may explain how habitat selection in benthic bivalves is realized under natural flow regimes.