Grazer density-dependent response of induced colony formation of Scenedesmus obliquus to grazing-associated infochemicals

To investigate the effects of infochemicals from Daphnia magna reared at different densities on the growth and morphological development of Scenedesmus obliquus, we cultured S. obliquus with filtrates from D. magna with a density of 0, 6, 30, 150, 300, and 600 ind L−1, respectively. Results showed no significant differences in S. obliquus growth rates among the treatments with different Daphnia densities. On days 3 and 5, the proportion of eight-celled colonies in the treatments constituted 30% of the S. obliquus populations. Importantly, the number of cells per particle increased significantly with increasing density of Daphnia, indicating a grazer density-dependent response. Specifically, the density-response relationship followed a rectangular hyperbolic model in the first five days of treatment, and then switched to a linear model, which implies that the relationship between the inducible colony size of S. obliquus and Daphnia density may also be dependent upon incubation time.