Linking predator kairomones and turbulence: synergistic effects and ultimate reasons for phenotypic plasticity in Daphnia cucullata

The seasonal change in helmet size in Daphnia cucullata has been studied for over one century. Recently it has been shown that helmets in D. cucullata, which have been found to reduce predator caused mortality, can be induced by chemical cues released by several predatory invertebrates. However, it also has been shown that tur- bulence induces this trait. The relation and interplay of both inducing cues is not known. Here we present results from lab experiments showing that predator cues and turbulence can act synergistically. Both factors in combination induced significantly larger responses, compared to each factor alone, and helmets reached the maximum sizes found in natural lakes. This result might help to explain the observation of large helmets in this species in nature. The ultimate reason behind the turbulence induction is yet unknown. We link both induction factors to predation, as the ultimate reason, by testing the hypothesis that D. cucullata can respond to turbulence produced by swimming invertebrates. We found that helmet growth increased significantly in direct contact to both the heterospecific Daphnia magna and the predator Cyclops sp. Treat- ments which accounted for chemical cues alone did not increase helmet growth in re- sponse to cues from D. magna ,o rCyclops under these conditions. Together, these re- sults indicate that D. cucullata is able to respond to mechanical cues produced by swimming invertebrates. Thus, our study suggests that both chemical cues and turbu- lence generated by predatory invertebrates might act synergistically to induce helmets as effective protection against invertebrate predation.