Limnol. Oceanogr., 44(8), 1999, 1894–1903

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. The induction of colonies in the alga Scenedesmus by the zooplankter Daphnia has been the subject of many studies. This is mainly due to the original work by Hessen and van Donk (1993), which that showed that when Daphnia magna and Daphnia water were introduced to single-celled cultures of Scenedesmus subspicatus, the algae formed coenobia (the term coenobium is often used for Scenedesmus colonies, although strictly speaking the definition of coenobium is ‘‘a colony of unicellular organisms having a definite form and organisation, which behaves as an individual and reproduces to give daughter coenobia’’ [Lawrence 1997]) and that spine formation was also enhanced. Lampert et al. (1994) followed up this work and showed that coenobia were induced by adding Daphnia water to single-celled cultures of Scenedesmus acutus (now known to be Scenedesmus obliquus, strain SAG 276–3a, Göttingen, Hegewald). Both sets of authors showed that Daphnia had to be fed in order for colony induction to occur. As Daphnia is a grazer on Scenedesmus, it has been postulated that colony formation represents an algal defense mechanism (Hessen and van Donk 1993). However, the extensive work of van Donk et al. (1999) showed that colony induction of Scenedesmus is not specific to Daphnia magna. Indeed most cladocerans, a copepod (Eudiaptomus gracilis), and some rotifers seem to induce colony formation in S. acutus/obliquus. In terms of the postulated algal defense mechanism, this is rather intriguing, as some of these organisms are not known to consume Scenedesmus. Other incongruities have been revealed by Lürling and others (Lürling and van Donk 1996, 1997; Lürling et al. 1997) in studies on the differences between single-celled and multicelled Scenedesmus in terms of food quality for ingesting Daphnia.