GRAZER-INDUCED COENOBIAL FORMATION IN CLONAL CULTURES OF SCENENDESMUS OBLIQUUS (CHLOROCOCCALES, CHLOROPHYCEAE)

Five strains of the phenotypically plastic alga Scenedesmus obliquus (Turpin) Kützing were cultured in standard medium or in medium containing filtered water from a Daphnia culture to examine grazer-induced coenobia formation. Strains varied considerably in their response to Daphnia chemicals. A rapid formation of four- and eight-celled coenobia was observed in the presence of Daphnia water in three strains. One strain showed numerous irregular aggregates with more than eight cells per colony, and no colony formation occurred in a fifth strain. In standard medium, all cultures remained unicellular. Growth rates were affected by Daphnia water in four strains and were different among strains. The chl a concentration was significantly different among strains but was not influenced by Daphnia water. The specific absorption coefficient was reduced at larger colony sizes, suggesting a possible cost associated with colony formation. The phenomenon of grazer-induced coenobia formation seems widespread but not universal within the species S. obliquus.