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The common green alga Scenedesmus obliquus may respond morphologically to the presence of natural enemies. Exposure to water-borne cues from the herbivorous zooplankton Ceriodaphnia reticulata, Daphnia galeata × hyalina, Daphnia magna and Daphnia pulicaria stimulated the formation of protective colonies in S. obliquus. This response seemed strongly related to the amount of algae grazed upon, because a highly significant correlation between the amount of S. obliquus consumed and the induced colony formation was found. However, when exposed to medium that had been inhabited by the carnivorous zooplankton Bythotrephes longimanus and Leptodora kindtii, no colony formation occurred. A similar result was obtained in two different experiments when S. obliquus was exposed to filtrate from cultures of the freshwater fish ide (Leuciscus idus) and perch (Perca fluviatilis). These results support the hypothesis that S. obliquus responds to a herbivorous zooplankton chemical cue, rather than to a more general animal excretory product. No support was obtained for the hypothesis that cues from the enemy of their enemy could serve as signals to the algae: despite the presence of filtrate from fish culture, filtrate from Daphnia cultures induced the formation of colonies in S. obliquus. The biological activity seems to be linked to the alga–grazer interaction, which ensures a reliable cue that evokes the morphological response of S. obliquus only when necessary. The grazer-induced colony formation can be viewed as an adaptive reaction in habitats with variable grazing pressure from an assemblage of many different herbivores to pare down mortality through grazing.

The effect of substances from different zooplankton species and fish on the induction of defensive morphology in the green alga Scenedesmus obliquus