Potential grazing intensity directly determines the extent of grazer-induced colony formation in Scenedesmus obliquus

• All different ages of Daphnia can induce colony formation in Scenedesmus.
• At the same biomass, younger Daphnia cause stronger induced effect than older ones.
• At the same feeding intensity, different ages of Daphnia have similar induced effect.
• Grazing intensity directly determines the extent of induced colony formation.

The genus Scenedesmus can form large colonies to provide protection against predation by herbivores. Scenedesmus has been investigated to elucidate the varying extents of colony formation in response to predation. However, limited information is available regarding the factors directly affecting the extent of colony formation responses. This study investigated the colony formation responses of Scenedesmus obliquus to infochemicals derived from Daphnia of different ages with the same biomass or feeding intensity. Results showed that water containing Daphnia infochemicals can induce the colony formation in S. obliquus regardless of age, biomass, or feeding intensity of Daphnia. The infochemicals derived from 1-day-old Daphnia caused a higher number of cells per particle than those from 5-day-old Daphnia when the grazers were of the same biomass; the infochemicals from 1-day-old Daphnia also maintained the algal colonial morph longer than those from 3-day-old Daphnia. The number of cells per particle did not differ among the algal populations treated with infochemicals from Daphnia of different ages when the feeding intensity of grazers was held constant. Therefore, potential grazing intensity directly determines the extent of the grazer-induced colony formation in S. obliquus.