Behavioural uniformity as a response to cues of predation risk

When confronted with a predator, individuals often alter their behaviour to minimize predation risk. To investigate behavioural changes induced by predator presence and light in a zooplankton species, we video-recorded the swimming behaviour of waterfleas, Daphnia pulicaria. Chemical cues from two predators with different hunting techniques (a visual predator: bluegill sunfish, Lepomis macrochirus; and a tactile predator: phantom midge larvae, Chaoborus sp.) were used in two sets of experiments. We filmed Daphnia individually in either control or predator-treated water, in a light or dark environment (four treatments in total) and recorded swimming speed, hop rate, sink rate and net angle of swimming. All swimming parameters in the fish experiment and two in the Chaoborus sp. experiment changed significantly between treatments. The coefficient of variation (CV) of all parameters except sink rate decreased significantly in the presence of light and/or a predator. In the presence of light, CV values declined by at least two-fold in both fish and Chaoborus experiments, while the changes in CV values between treatments varied depending on the type of predator used in the experiment. Waterfleas increased their swimming uniformity while isolated from other individuals. We therefore conclude that this swimming behaviour pattern is based on individual perception of both kairomones and light in the environment rather than on social interactions. Our results suggest that uniformity in swimming behaviour may be an important defence mechanism in zooplankton, aiming to minimize oddity among group members in order to decrease predation risk.