Inducible defense is an effective way for prey to improve its fitness under fluctuating predation risks. While the formation and reversibility of diverse inducible defensive traits are still not fully understood. This study focused on the inducible morphological and behavioral traits of Daphnia magna in the process of predator come and go, which were simulated by adding and removing fish kairomone, respectively. Results showed that D. magna exposed to fish kairomone inhabited deeper water and owned smaller individual size. The adult D. magna exposed to fish kairomone could develop the same inducible defensive traits as the individuals exposed to fish kairomone from neonate. In terms of the complete formation time, behavior (6 days) was faster than morphology (17 days). Besides, by removing kairomone after maturity, the inducible defensive traits of D. magna could reverse to the traits of the individuals never exposed to fish kairomone. In terms of the reversal time, behavior (18 days) was similar to morphology (21 days). Furthermore, the reversal degrees of morphological and behavioral defenses were equal at any specific time point. Such results suggest that inducible behavioral defensive traits are not always more reversible than inducible morphological defensive traits.

In aquatic ecosystems, cladocerans, an important part of zooplankton, are often exposed to new pollutant nanoparticles and poor quality food simultaneously. To evaluate the combined effects of poor quality food and nanoparticles on the development and early reproductive performance of cladocerans, we exposed Daphnia magna, a representative cladoceran, to different concentrations of ZnO nanoparticles under the food conditions containing different proportions of non-toxic Microcystis, recorded some key indicators of the early life history, and analyzed the possible differential dose effects. The results showed that non-toxic Microcystis and ZnO nanoparticles significantly delayed the times to maturation and reproduction and also significantly reduced the survival time, the body length at maturation, the number of offspring in the first brood, and the total offspring per female of D. magna. When the food contained non-toxic Microcystis, D. magna did not reach sexual maturity at 0.25 mg L−1 ZnO nanoparticles and did not develop eggs and reproduce offspring at 0.20 mg L−1 ZnO nanoparticles, especially non-toxic Microcystis and ZnO nanoparticles had a significant or nearly significant synergistic effect on the time to maturation, time to first brood, and the total offspring per female of D. magna, whereas for other life history indicators, non-toxic Microcystis and ZnO nanoparticles showed an additive effect, based on comparing the slopes of linear fitting of the relevant indicators with nanoparticle concentration under different food quality conditions. Such findings are helpful to realistically evaluate the comprehensive stress effect on zooplankton under the complex conditions of poor food quality and coexistence of pollutants.