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Martin Horstmann, Linda C. Weiss, Ralph Tollrian

• Inducible defenses are a wide-spread defensive mechanism in \(\it Daphnia\). For example, \(\textit {D. cucullata}\) is known to form different adaptive morphologies under changing environmental conditions. In this species, predator presence elicits defensive helmets. Defended animals perform better and survive more frequently in predation attempts. Another peculiarity of \(\textit {D. cucullata}\) is that they develop these defenses also in response to turbulence. The mechanisms that underly this defensive effect are so far unclear. Therefore, we investigated the three-dimensional morphology of the typical, the turbulence- and \(\it Chaoborus\)-exposed morph and analyzed their surface and volume alterations. We furthermore measured the swimming velocities of these morphs. With the determined shapes and the measured velocities, we simulated the drag force under different angles of attack and determined the energy necessary to move through the water. In the light of previous findings and hypotheses we here discuss biomechanical mechanisms that improve \(\textit {D. cucullata}\)'s fitness in the investigated environments.