It has been known that some aquatic organisms change their morphology to defend themselves in response to kairomones released from their predators, although very few kairomones are chemically identified. Scenedesmus, unicellular fresh-water phytoplankton, forms colonies in the presence of its grazer, Daphnia. Hessen and van Donk (1993) discovered the involvement of a chemical substance released from the Daphnia in stimulation of colonies. The addition of filtered medium from a Daphnia magna culture to unicellular Scenedesmus subspicatus achieved within a few days morphological change into 2-, 4-, and 8-colonies, while the controls remained unicellular. Their report has aroused the interest of many scientists to attempt to identify the kairomone, but the chemical substance that triggers this behavior has not been identified. Here we report the identification of the Daphnia kairomones as aliphatic sulfates that cause the morphological change in a unicellular green alga Scenedesmus gutwinskii var. heterospina (NIES-802) at 0.1-1000ng/ml concentrations. The kairomones were synthesized to rule out the possibility that the isolated substances are inactive and they might still be contaminated with a minute amount of 'super active compound'.
