Swimming behaviour of Brachionus plicatilis in relation to food concentration and feeding rates

We examined feeding rates and swimming speed in amictic females of Brachionus plicatilis over algal cell concentrations ranging from 15 x 10³ to 30 x 10⁶ cell ml-¹, to determine to what extent filtration rate is a consequence of a real modulation of swimming speed in response to food availability. Swimming rates were measured using an automated motion analysis system via video recording. The results showed that swimming speed changed as a function of food density. Swimming speed increased from the lowest tested concentration of algae to reach a maximum at 6 x 10⁶ cell ml-¹. Above this density, swimming speed declined slightly and then remained constant at a mean speed of 0.45 mm s-¹. Filtration and ingestion rates changed as cell concentration increased, following patterns consistent with those generally described for suspension feeders. However, the observed swimming pattern did not explain the recorded changes in clearance rate. These results suggest that filtration, and therefore ingestion, is mainly regulated by modifying particle retention efficiency.