GRAZER‐INDUCED DEFENSE IN SCENEDESMUS OBLIQUUS (CHLOROPHYCAEA)

The non‐spiny Scenedesmus obliquus may express considerable phenotypic plasticity in response to changes in their environment. When exposed to chemicals released from the grazer Ceriodaphnia , unicellular S. obliquus populations were rapidly transformed into colonial ones. The morphological response appeared strongly correlated with the amount of algae grazed upon by the Ceriodaphnia. The induced S. obliquus colonies were protected against grazing, as were large spined Scenedesmus , whereas unicellular S. obliquus and spined Scenedesmus remained vulnerable to grazing by Ceriodaphnia. Growth rates of unicellular and induced‐colonial S. obliquus populations were identical. However, sinking rates were significantly higher for the induced‐colonial S. obliquus. Hence, the phenomenon of grazer‐induced colony formation in S. obliquus can be interpreted as an inducible defense at the expense of higher sinking losses.