Behavior as additional endpoint for effect assessment of diazinon in Daphnia magna

This study examined the effects of acute and chronic exposure to diazinon on the locomotor behaviour and reproduction of Daphnia magna. Acute exposure led to significant alterations in swimming parameters, likely associated with acetylcholinesterase (AChE) inhibition, resulting in immobility and mortality at high concentrations. In contrast, after 21 days of chronic exposure to environmentally relevant concentrations (0.1-300 ng/L), no significant effect on locomotion or AChE activity was observed, despite a significant reduction in reproductive output from the lowest tested concentration (0.1 ng/L). This dissociation between behavioral and reproductive responses suggests that locomotor activity and AChE activity may be maintained under chronic low-level exposure, while other physiological processes such as reproduction are more sensitive to energetic constraints. These results are consistent with the DEBtox model assumptions, which propose that organisms under chemical stress may adjust their energy allocation to prioritize essential functions. Overall, this study underscores the relevance of locomotor activity as a biomarker of acute neurotoxicity, while highlighting the need to consider sublethal reproductive impacts under chronic exposure conditions.