Long-term continuous mismatch between grazing cues and real grazing losses causes attenuation of induced morphological defense in Scenedesmus

For the freshwater phytoplankton Scenedesmus , morphological defense induced by infochemicals released from grazers (such as Daphnia ) is considered to be a defensive strategy. In this study, we investigated the response of Scenedesmus obliquus to long-term repeated grazing infochemicals exposure, with two stimulation intervals (short period of 4 days and long period of 7 days per round). Results demonstrated that the defensive response of S. obliquus gradually degenerated with periodically repeated exposure to Daphnia infochemicals and the degeneration rate was positively correlated with stimulation frequency, that is, shorter intervals caused a faster decrease in average algal colony size. According to nonlinear regression fitting, a harmonic oscillator function with exponential attenuation could be applied to illustrate S. obliquus morph changes, with which the growth rate was positively correlated. Our study suggests that the optimal defensive colony size in S. obliquus might depend on trade-offs between its anti-grazing defense and other functions. This insight contributes to the understanding of algal inducible defense mechanism, serving as a complement to the optimal defense theory. Additionally, long-term periodic grazing threats without causing concrete damage may disrupt feedback mechanisms and real biomass loss is assumed to be an important precondition for sustaining enduring defensive phenotypes.