Influence of predation-based chemical cues on contaminant sensitivity in fathead minnows (Pimephales promelas) and Daphnia pulex

Scope and Method of Study: Anthropogenic contaminants can have significant effects on aquatic organisms and understanding these effects is critical. Single-species laboratory bioassays are used to evaluate effects of pollutants on organisms, but often fail to evaluate how "natural stressors," such as predation, can influence contaminant effects. Many prey fish such as minnows have specialized epithelial cells that release alarm substance when a fish is injured, alerting other organisms of danger. Invertebrates also have the ability to detect and respond to predation-based chemical cues. Predator animals release chemical cues that prey species detect, and the stress induced on organisms as a result of these various predatory cues may increase sensitivity to pollutants. Bioassays were performed to determine the effects of predatory cues on fathead minnows (Pimephales promelas) and Daphnia pulex exposed to standard chemical stressors. The main hypothesis is that fathead minnows and D. pulex will be more sensitive to the chemical when exposure solutions include pheromones derived predatory fish, and fathead minnows will be more sensitive when alarm substance is added.;Findings and Conclusions: Exposure to predation based chemical cues can cause induction of the physiological stress response in aquatic organisms, as does encountering anthropogenic or natural toxicants. Species specific anti-predator behavior affects the level of exposure to a chemical. Our results show that Daphnia respond to fish predators by reducing their swimming activity, causing a reduction in metabolic rate, and decreased uptake of sodium chloride, decreasing sensitivity. Fathead minnows responded to predatory stressors with increased metabolism, which may have increased the uptake of sodium chloride and therefore sensitivity. While the addition of sodium chloride to exposure solutions containing alarm substance caused a synergistic effect, this may not always be true. Some contaminants may hinder normal behavior that would increase vulnerability to predation, such as reduction of predator detection in fish exposed to cadmium. It is apparent that incorporating multiple abiotic and biotic stressors into future protocols for standardized toxicity testing is imperative.