Urea excretion by Daphnia:: A colony-inducing factor in Scenedesmus?

It is known that the zooplankter Daphnia induces colonies in the alga Scenedesmus. As Daphnia grazes on Scenedesmus, it has been postulated that colony formation represents an algal defense mechanism. This induction of Scenedesmus coenobia/colonies by Daphnia could be associated with a substance exuded by the animals that acts as a specific infochemical (kairomone) for Scenedesmus. However, the chemical nature of such a substance is still unknown. Because coenobia can be formed in the absence of Daphnia kairomone, as a result of different N concentrations in the algae and media, we checked different nitrogen excretory products of Daphnia as potential candidates. In this paper we test the hypothesis that ammonia and urea excreted by zooplankters (Daphnia) may induce coenobia formation in Scenedesmus. Using experiments with dialysis separation of Daphnia and algae we show that one of the excretory products of zooplankters, urea, can induce colony formation in Scenesdesmus, whereas ammonia has no effect. Although the effect of urea on coenobia formation may simply be a nutrient effect, this does not exclude the possibility that it results in grazing protection of the algae. Hence, urea produced by zooplankton may serve as a colony inducer to algae.