Effects of bivalve siphonal currents on the settlement of inert particles and larvae
Abstract:Dye studies in a flume revealed that the strong exhalent siphonal current of an individual cockle, Clinocardium nuttallii, behaved much like a cylinder held in the flow, shedding vortices downstream. The inhalent flow was much slower and more diffuse, its effects being limited to the lowermost few centimeters above the bottom. Flume experiments with inert particles having settling velocities similar to those of polychaete larvae revealed that the vortex shedding from the excurrent jet led to variability in deposition of the particles a few centimeters downstream of the jet, and that neither the jet nor the incurrent flow substantially changed the mean number of particles depositing per unit area of bed. Field observations within a few days after settlement of Hobsonia florida, an ampharetid polychaete, using ecologically similar but nonplanktonically recruiting oligochaetes as an internal control, showed similarly enhanced variability in recruitment within a few centimeters of the siphon of resident Mya arenaria (soft-shelled clam). We could find no evidence that isolated clams impede settlement in their immediate surroundings and found, instead, some indication of local settlement being enhanced by the flow convergence toward the incurrent siphon. We thus suggest that any negative influence of suspension-feeding bivalves upon settlement is a larger-scale phenomenon caused by depletion of recruits through the integrated filtering activities of individuals upstream of the settlement site. Hence manipulation of bivalve density in small plots may not be very informative regarding influences upon larval settlement.
Document Type: Research Article
Publication date: November 1, 1988
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