How Daphnia magna Defends Itself against Predators: Mechanisms and Adaptations in a Freshwater Microcosm

The freshwater water flea (Daphnia magna Straus, 1820) is prey for numerous predators. Yet it possesses a wide range of strategies to defend itself against predation. The aim of this work is to investigate the defensive mechanisms employed by D. magna to reduce predation by the coelenterate Hydra viridissima, and two planarians, Polycelis felina and Dugesia gonocephala. To do this, we used a freshwater microcosm. An additional aim is to investigate interactions with the presence of the isolated endosymbiotic algae from green hydra, thus combining and observing the interaction of the zooplankton and microalgal component. Each experiment included five replicates (13.5 degrees C, 25 degrees C), in crystallizing glass containers (60 mL volume, 60 mm diameter, 35 mm height), including satiated (fed with larvae of Artemia salina) and starved predators, respectively (one or five individuals of a particular predator species in one microcosm). As the isolated microalgae are unique, we tracked the following three mechanisms of Daphnia defense for the first time including precisely this microalgal component: (i) grouping (visual magnification), i.e., two or more Daphnia holding together; (ii) the phenomenon of overproduction, i.e., any number of Daphnia in one container above the 10 initially added individuals; and (iii) accelerated movement ("bullet movement"), i.e., high-speed movements in particular microcosms. The results provide new information for a better understanding of the interspecific relationships in systems that include both zooplankton and microalgal components.