Clearance rate of Mytilus edulis (L.) as a function of current velocity and mussel aggregation

The aim of the current study was to investigate the effect of water current velocities on the clearance rate of Mytilus edulis when different numbers of mussels were used in the experiments. An automatic setup, which controlled and monitored the algal concentration continually, was used to measure the effect of increasing current velocity (0.05-1.4 m/sec) on the M. edulis clearance rate. Clearance rate measurements were performed under constant food concentrations of 3,000 cells/mL of Rhodomonas salina on either 3 mussels or 20 mussels. We found that the clearance rate of 20 mussels was unaffected by current velocities up to 1.4 m/sec, whereas experiments with 3 mussels showed that clearance of the mussels decreased progressively at current velocities greater than 0.2 m/sec, and reached 0 L/h per individual at current velocities greater than 0.6 m/sec. The constant feeding at all current velocities observed in the experiments with 20 mussels was most likely a result of a combination of reduced current velocity within the aggregated mussels, a current velocity-dependent change of siphon orientation, and attachment of the mussels at current velocities less than 1.2 m/sec, whereas in experiments with 3 mussels, mussels were detached at >= 0.4 m/sec and the mussel valves were closed in general. Thus, it is evident that the number of mussels present in the experimental tank affects the results and emphasizes the importance of discriminating between the individual level and the level of mussel aggregations in the study of current velocity effects on clearance, especially when the study is made on a bivalve species that form dense beds in nature, to ensure consistency with field observations.