Grazer-induced colony formation in Scenedesmus acutus (Chlorophyceae): Ecomorph expression at different temperatures

Scenedesmus acutus Meyen was cultured at four temperatures (9.5 degrees, 16 .5 degrees, 24 degrees, and 29 degrees C) in standard medium or in medium w ith filtered water from a Daphnia culture, Growth was significantly reduced at low temperatures. At 9.5 degrees C it took more than a week before form ation of eight-celled coenobia occurred in both the absence and presence of water from a Daphnia culture, At higher temperatures, formation of four- a nd eight-celled coenobia occurred more rapidly and was already observed in the presence of Daphnia water within 2 days, As cultures aged, also in the absence of Daphnia water, four-celled coenobia became dominant. At cold tem perature, cell volume initially was significantly larger but declined after 3-4 weeks. Grazer-induced colony formation had occurred independent of the incubation temperature, but the number of cells per colony was increased w ith declining temperature. The morphological expression may be interpreted as a cyclomorphosis driven by nutrients, temperature, and chemical cues fro m gazers.