Growth and Morphological Changes in Scenedesmus dimorphus Induced by Substances Released from Grazers, Daphnia magna and Moina macrocopa

Some freshwater phytoplankton species are plastic in their morphology, physiological characteristics, genetic structure, and biochemical composition. These characteristics of planktonic algae vary considerably under different environmental conditions in aquatic ecosystems. Algae such as Staurastrum sometimes lose their spiny form, and large multi-f ilament ‘f lakes’ of Aphanizomenon grows as single filament (Lynch, 1980). Various blue-green algae, including Microcystis aeruginosa Kützing, have potential for both colonial morphology and chemical toxicity. In nutrient-deficient systems, where algal growth rate is slow, these features of algae might be effective defense mechanisms against zooplankton grazing (Van Donk et al., 1997). Defenses such as large size, hardness of cell wall structures, mucus excretion, and the production of toxins must offer considerable advantages that overcome the negative aspects such as increased energy demand and greater sinking losses of cells with these features (Van Donk et al., 1998). Recently, Bronmark and Hansson (2000) summarized the concept of defense mechanisms of prey being induced by chemical factors released by aquatic predators. ‘Info-chemicals’ (Dicke and Sabelis, 1988) convey information between two individuals, evoking a behavioral or physiological response in the receiver. This information transfer by chemicals in aquatic ecosystems has gained attention recently but has mainly focused on the response of zooplankton to the presence of fish and invertebrate predators (Larsson and Dodson, 1993). Relatively little information exists regarding grazer-mediated anti-herbivore responses in phytoplankton. There is some evidence that chemicals released