Optimal Foraging by Zooplankton

We describe experiments with several species of the zooplankton, \textit{Daphnia, }while foraging for food. They move in sequences: \textit{hop-pause-turn-hop }etc. While we have recorded hop lengths, hop times, pause times and turning angles, our focus is on histograms representing the distributions of the turning angles. We find that different species, including adults and juveniles, move with similar turning angle distributions described by exponential functions. Random walk simulations and a theory based on active Brownian particles indicate a maximum in food gathering efficiency at an optimal width of the turning angle distribution. Foraging takes place within a fixed size food patch during a fixed time. We hypothesize that the exponential distributions were selected for survival over evolutionary time scales.