Herbicide pollution is persistent, which not only has a negative impact on individual organisms, but also may endanger the interspecific relationship between predators and prey. Cladocerans, i.e. zooplankton that plays an important role in the energy flow and material circulation in freshwater ecosystem, usually develop induced defense in response to predation risk. We used atrazine, one of the most used herbicides in the world, and Daphnia pulex, a representative cladocerans, to test the possible interference effect of herbicides on the induced defensive traits of cladocerans in response to predator fish (Rhodeus ocellatus) kairomone, including morphological defense, life history strategies, and the expression of defense-related genes. Atrazine reduced the body size, spine size, growth rate, total offspring, and the relative reproductive output of D. pulex, which further affected the response strength of the morphological and life history defenses, i.e., atrazine significantly reduced the spine size, relative spine size, and fecundity of D. pulex in response to R. ocellatus kairomone. Exposure to atrazine affected the expression of defense-related genes, and we speculated that atrazine affected the signaling process in the induced anti-predation defense of cladocerans. Specially, fish kairomone attenuated the negative effects of high concentrations of atrazine on the life history traits of D. pulex. Our results will help to accurately assess the potential risk of artificial compounds in freshwater ecosystems from the perspective of interspecific relationships, and help to understand the impact of environmental changes on the inducible anti-predator defense of prey in aquatic ecosystems.

When shrimps select a habitat, the presence of elements like predators, shelter and contamination might determine if an area is preferred or avoided. We hypothesised that when shrimps are exposed to a situation in which they have to select whether to avoid contamination, seek shelter or protect themselves against predators, they will avoid the situation that supposes a higher cost for their survival (cost-benefits balance). The current study aimed to assess the plasticity of the selection behaviour of the freshwater shrimp Atyaephyra desmarestii between moving to a clean and unprotected area (no shelter and with a risk of predation), thus avoiding exposure to contamination, or moving to a contaminated and protected area (with shelters), thereby avoiding potential predators. Shrimps were experimentally exposed in a free-choice system simulating a heterogeneous environment with a contaminant (copper), shelter and a predator signal (kairomones of Salmo trutta). The shrimps avoided the copper by moving towards a less contaminated area, both in the absence or presence of shelter. When confronted with a choice between a cleaner zone with no shelter and a contaminated zone with shelter, the shrimps preferred being in the cleanest area. However, when the uncontaminated area contained a predator signal, the shrimps balanced the risk of predation and exposure to contamination by selecting a moderately contaminated area relatively further away from the predator signals. In summary, contamination might favour a plasticity of the habitat selection behaviour of shrimps, modifying the cost-benefits balance of such a selection.

Secondary salinization represents a global threat to freshwater ecosystems. Salts, such as NaCl, can be toxic to freshwater organisms and may also modify the outcome of species interactions (e.g. host-parasite interactions). In nature, hosts and their parasites are embedded in complex communities where they face anthropogenic and biotic (i.e. predators) stressors that influence host-parasite interactions. As human populations grow, considering how anthropogenic and natural stressors interact to shape host-parasite interactions will become increasingly important. We conducted two experiments investigating: (1) the effects of NaCl on tadpole susceptibility to trematodes and (2) whether density- and trait-mediated effects of a parasite-predator (i.e. damselfly) and a host-predator (i.e. dragonfly), respectively, modify the effects of NaCl on susceptibility to trematode infection. In the first experiment, we exposed tadpoles to three concentrations of NaCl and measured parasite infection in tadpoles. In the second experiment, we conducted a 2 (tadpoles exposed to 0 g L−1 NaCl vs. 1 g L−1 NaCl) x 4 (no predator, free-ranging parasite-predator (damselfly), non-lethal host-predator (dragonfly kairomone), and free-ranging parasite-predator + dragonfly kairomone) factorial experiment. In the absence of predators, exposure to NaCl increased parasite infection. Of the predator treatments, NaCl only caused an increase in parasite infection in the presence of the parasite-predator. However, direct consumption of trematodes caused a reduction in overall infection in the parasite-predator treatment. In the dragonfly kairomone treatment, a reduction in tadpole movement (i.e. trematode avoidance behavior) led to an increase in overall infection. In the parasite-predator + dragonfly kairomone treatment, antagonistic effects of the parasite-predator (reduction in trematode abundance) and dragonfly kairomone (reduction in parasite avoidance behavior) resulted in intermediate parasite infection. Collectively, these findings demonstrate that NaCl can increase amphibian susceptibility to parasites, and underscores the importance of considering predator-mediated interactions in understanding how contaminants influence host-parasite interactions.

Antidepressants discharged into natural waters are likely to become a new type of endocrine pollutant, which may impact the interspecific relationship in aquatic ecosystem. Induced defense of cladocerans plays an important role in maintaining the balance of interspecific relationships between cladocerans and higher trophic levels. Here we studied the effects of antidepressant sertraline, a selective serotonin reuptake inhibitor, on the induced defensive traits of Ceriodaphnia cornuta in response to invertebrate predator Chaoborus larvae kairomone, including morphological defense and life history traits. We also conducted the predation experiments to check the selection rate of Chaoborus larvae during directly ingesting C. cornuta that were exposed to Chaoborus larvae kairomone at high concentration of sertraline. Results showed sertraline had an interference effect on the induced morphological defense of C. cornuta in response to Chaoborus larvae kairomone, i.e. the high concentration of sertraline (20 and 100 μg L⁻¹) significantly reduced the horns induction. However, the different concentrations of sertraline generally did not affect the life history traits of C. cornuta, regardless of presence or absence of Chaoborus larvae kairomone. The predation experiment demonstrated that the inhibition of sertraline on the induced morphological defense of C. cornuta can promote the feeding selective efficiency of Chaoborus larvae, and thus cause C. cornuta easily to be predated by Chaoborus larvae. Our results suggested that sertraline at the concentrations that are not direct harmful to life history traits of C. cornuta can seriously affect the predator-prey relationship, indicating that effects of pollutants on interspecific relationships should be considered comprehensively to avoid underestimating the potential risk of pollutants to ecosystems.