Motility of zooplankton: fitness, foraging and predation

The relative fitness of planktonic organisms foraging under the risk of predation is examined in terms of their swimming speed, path geometry and jump frequency. Fitness is quantified in terms of encounter and ingestion of prey, respiration and energy cost associated with swimming and mortality due to encounters with predators. It is shown that a convoluted swimming path in the form of meanders, zigzags or spirals confers greater fitness than swimming along a straight path. Optimal path configuration is such that the length-scale of the path-meanders is commensurate with an organism's detection radius to prey, which in turn scales with the size of the organism. Optimal swimming speed for a cruise-feeding organism decreases with increasing prey concentration and increasing risk due to ambush predators. For ambush feeding on motile prey, a benefit is gained by periodically moving to a new location. The time spent swimming is largely a function of energetic costs, whereas the time spent feeding is strongly controlled by prey concentration and the risk posed, in turn, by ambush predators. These predictions are supported by observations drawn from the literature.