Responses of larval zebrafish to low pH immersion assay. Comment on Lopez-Luna et al.

Lopez-Luna et al. (2017) observed behavioral responses of larval zebrafish (Danio rerio) exposed for 10 min to pH 2.6–3.6 when acetic acid (0.01–0.25%) or citric acid (0.1–5%) was added to the tank water in the presence or absence of aspirin (1–2.5 mg l−1), morphine sulfate (1–48 mg l−1), lidocaine (1–5 mg l−1) and flunixin (8–20 mg l−1). Fish exposed to 0.1–0.25% acetic acid were less active than controls while those exposed to citric acid and 0.01% acetic acid were more active. Administration of high doses of aspirin, morphine and lidocaine for 30 min before exposure prevented the reduction in activity induced by 0.1–0.25% aceticacid. These behavioral responses were interpreted as evidence that acetic acid immersion provided a noxious stimulus (i.e. activated nociceptors) that was reliable for use as a model system for the study of analgesic substances. We identify methodological weaknesses and inconsistencies in the interpretation of results, and emphasize that activation of nociceptors was assumed, not demonstrated. As a result of several processes and interactions that were not accounted for or discussed, we warn their conclusions are unfounded. A critical omission was the failure to report water conductivity, hardness and alkalinity data. These determine the magnitude of acute osmoregulatory effects that occur in fish exposed to highly acidic water (Wood, 1989). Trials by other researchers using water with different conductivity, hardness or alkalinity profiles could, therefore, generate significantly different results. Immersion of fishin low pH water also introduces several unavoidable and uncontrolled interactions that prevent unequivocal interpretation of the behavioral changes observed.