Behavioral and physiological toxicity of dietary methylmercury exposure to larval dragonflies

While ecotoxicology has traditionally focused on impacts to reproduction and mortality, sublethal effects on behavior can also have important consequences that affect toxicity risk. Mercury is a potent neurotoxicant with well-documented effects on vertebrate behavior, although behavioral toxicity to invertebrates is understudied. Over a series of experiments, we assessed how a gradient of dietary methylmercury exposure influenced behavioral and physiological endpoints in dragonfly larvae, a widely used mercury biosentinel. Mercury exposure induced suboptimal behaviors in an artificially ideal trophic environment, suggesting potentially important impairments to prey detection and predator avoidance in larval dragonflies. For each doubling of dragonfly total mercury concentration, we estimated that dragonflies took 32 to 508 s longer to initiate foraging behavior and were 3% to 18% more active when exposed to a predation cue (95% confidence intervals). Other behavioral endpoints were unaffected by mercury exposure, including prey capture rates, foraging efficiency, and refuge use under predatory threat. Physiological endpoints, including growth, body condition, molting, wingpad symmetry, and immune response, were similarly insensitive to mercury toxicity. These results help support the utility of dragonfly larvae to monitor mercury in freshwater food webs and confirm that quantifying behavioral toxicity is important to fully characterize and mitigate exposure risk.