The feeding performance of zooplankton influences their evolution and can explain their behaviour. A commonly used metric for feeding performance is the volume of fluid that flows through a filtering surface and is scanned for food. Here, we show that such a metric may give incorrect results for organisms that produce recirculatory flows, so that fluid flowing through the filter may have been already filtered of food. In a numerical model, we construct a feeding metric that correctly accounts for recirculation in a sessile model organism inspired by our experimental observations of Vorticella and its flow field. Our metric tracks the history of current-borne particles to determine if they have already been filtered by the filtering surface. Examining the pathlines of food particles reveals that the capture of fresh particles preferentially involves the tips of cilia, which we corroborate in observations of feeding Vorticella. We compare the amount of fresh nutrient particles carried to the organism with other metrics of feeding, and show that metrics that do not take into account the history of particles cannot correctly compute the volume of freshly scanned fluid.
Keywords: Vorticella; feeding current; filter feeding; flow visualization; zooplankton.