Imidacloprid pesticide alters anti-predator responses in a water flea (Daphnia sinensis)

Organisms develop inducible defense strategies to combat predators, altering behaviour, morphology, and life history to minimize predation risk. However, environmental stressors, such as contaminants, can affect these defense-associated traits. Neonicotinoids such as imidacloprid threaten freshwater organisms, but their effects on predator-prey interactions and predator-induced defensive responses remain unclear. Here we used a single clonal lineage of Daphnia sinensis, a keystone species in aquatic ecosystems, to examine the impact of imidacloprid on its anti-predator responses to fish ( Carassius auratus ) kairomones. Our results showed that fish kairomones exposure triggered significant increases in body and tail spine length, indicative of adaptive anti-predator defenses. Conversely, imidacloprid exposure alone significantly reduced eye diameter and impaired the eye diameter in offspring when combined with fish kairomones. Additionally, exposure to fish kairomones decreased the age at maturity and the reproduction interval, and increased the net reproduction, while imidacloprid did not affect the age at maturity or net reproduction but decreased the reproduction interval. Notably, the combination of imidacloprid and fish kairomones exposure diminished the anti-predator response, including the reproduction interval and offspring eye diameter, which suggests maladaptive effects. Significant morphological changes were observed in offspring exposed to fish kairomones, including elongated tail spines and larger eye diameters, which were absent in imidacloprid-exposed treatments. These findings suggest that imidacloprid disrupts inducible anti-predator defenses in D. sinensis , potentially reducing its survival and fitness, and highlight the necessity for further research on the ecological impacts of neonicotinoid contamination to ensure the long-term viability of zooplankton populations.