Effects of Copepod Foraging Behavior on Predation Risk: An Experimental Study of the Predatory Copepod Pareuchaeta norvegica Feeding on Acartia clausi and A. tonsa (Copepoda)

The effect of foraging behavior on predation risk was studied by exposing the two small calanoid copepods Acartia clausi and Acartia tonsa to 0 or 1 ppm$(\ticksim 1,500 cells ml^-1)$of the diatom Thalassiosira weissflogii and to presence of the predator Pareuchaeta norvegica. In filtered water, predation rate was the same on the two species. In algal suspension, predation rate on A. clausi was half that in filtered water and half that on A. tonsa. Video observations revealed distinct differences in motility of Acartia depending on algal concentration. Both species performed frequent short feeding bouts in algal suspension; nonfeeding copepods in filtered water alternately sank or adjusted their vertical distribution by stronger jumps. Jump frequency nearly doubled for A. clausi in filtered water, but no significant difference was observed for A. tonsa. To explain the predation, assuming that P. norvegica is a rheotactic predator, we developed a model of potential hydrodynamic disturbance associated with each foraging behavior. Increased encounter rate with P. norvegica caused by frequent strong jumps by A. clausi in the absence of algae could explain >40% of the observed increase in predation rate. For A. tonsa, jump frequencies and predation rates were similar in both food treatments, which is in accordance with the model.