Group versus individual exposure: Do methodological decisions in aquatic toxicology alter experimental results?

In aquatic toxicology, methods that are chosen for exposures have profound consequences on experimental outcomes and thus can skew policy initiatives. For example, as compared to single-organism exposures, toxicity test results of group exposures may be impacted by confounding factors such as social interactions between animals or individual variation in accumulation rates. To test for differences in organismal response between group and individual toxicological exposures, we exposed Daphnia magna to copper and subsequently compared the toxicity (median lethal concentration or LC50) between groups and individuals. Results suggested that water chemistry had a larger effect on experimental outcomes than the number of animals exposed in the same tank. Methodological decisions with respect to replication type can affect toxicity tests, and LC50s calculated using different exposure types (such as group and individual exposures) may not be comparable.