Abstract: Ammonia is one of the most common and highly toxic nitrogenous pollutants in water, which not only directly poisons aquatic organisms, but also seriously threatens the interspecific relationships of aquatic organisms without causing direct harm. Induced defense is a typical adaptive response of prey to the predation risk, and it is an important relationship between predation and defense. However, whether ammonia interfere with the responses of prey to their predators is still unclear. In this study, the effects of ammonia with different concentrations on the induced anti-predation (Rhodeus ocellatus as the predator) life-history defense responses of Daphnia pulex, a representative species of zooplankton, were examined and the changes in susceptibility of D. pulex under induced defense to ammonia stress were investigated. Results show that ammonia had an inhibitory effect on the life-history traits of D. pulex. For example, at an ammonia concentration of 0.6 mg·L^-1, there was a delay of 4.2 hours in the first brood time, a 5.72% and 3.74% reduction in maturation body size and spine length, respectively, and a 39.7% decrease in offspring numbers. As to the induced life-history defense of D. pulex, with the increase of ammonia concentrations, the performance of D. pulex in response to predation risk, i.e., body size reduction (-11.13%) and somatic growth rate slowdown (-24.18%), were significantly inhibited. The inhibitory effect accumulated with prolonged ammonia exposure. In addition, the predator-induced defense makes D. pulex more susceptible to ammonia, i.e., predation risk weakens D. pulex's tolerance to ammonia. This study not only further reveals the extensive negative effects of ammonia from the perspective of interspecific relationships, but also provides new insights into the ability changes of organisms in response to environmental stress under predation risk.

Key words: Daphnia pulex, induced defense, ammonia pollution, predation risk, kairomone