GRAZER-INDUCED COLONY FORMATION IN SCENEDESMUS ACUTUS (CHLOROPHYCEAE): ECOMORPH EXPRESSION AT DIFFERENT TEMPERATURES

Scenedesmus acutus Meyen was cultured at four temperatures (9.5°, 16.5°, 24°, and 29° C) in standard medium or in medium with filtered water from a Daphnia culture. Growth was significantly reduced at low temperatures. At 9.5° C it took more than a week before formation of eight-celled coenobia occurred in both the absence and presence of water from a Daphnia culture. At higher temperatures, formation of four- and 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 temperature, 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 with declining temperature. The morphological expression may be interpreted as a cyclomorphosis driven by nutrients, temperature, and chemical cues from grazers.