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

The common green alga Scenedesmus obliquus may respond morphologicallyto the presence of natural enemies. Exposure to water-bornecues from the herbivorous zooplankton Ceriodaphnia reticulata,Daphnia galeata x hyalina, Daphnia magna and Daphnia pulicariastimulated the formation of protective colonies in S. obliquus.This response seemed strongly related to the amount of algaegrazed upon, because a highly significant correlation betweenthe amount of S. obliquus consumed and the induced colony formationwas found. However, when exposed to medium that had been inhabitedby the carnivorous zooplankton Bythotrephes longimanus and Leptodorakindtii, no colony formation occurred. A similar result wasobtained in two different experiments when S. obliquus was exposedto filtrate from cultures of the freshwater fish ide (Leuciscusidus) and perch (Perca fluviatilis). These results support thehypothesis that S. obliquus responds to a herbivorous zooplanktonchemical cue, rather than to a more general animal excretoryproduct. No support was obtained for the hypothesis that cuesfrom the enemy of their enemy could serve as signals to thealgae: despite the presence of filtrate from fish culture, filtratefrom Daphnia cultures induced the formation of colonies in S.obliquus. The biological activity seems to be linked to thealga–grazer interaction, which ensures a reliable cuethat evokes the morphological response of S. obliquus only whennecessary. The grazer-induced colony formation can be viewedas an adaptive reaction in habitats with variable grazing pressurefrom an assemblage of many different herbivores to pare downmortality through grazing.