Acclimation and recovery dynamics of behavioral and coloration responses of a common fish (Squalius cephalus) to paracetamol exposure

Freshwater ecosystems are increasingly exposed to pharmaceutical contamination, impacting non-target species. Concentrations can vary over time and location, allowing for potential acclimation or recovery effects. Additionally, parasites might interfere with the absorption and adverse outcomes pathways of pollutants. We examined the combined effects of paracetamol and parasite on the behavior and coloration of the European chub (Squalius cephalus), a ubiquitous fish species, from natural populations. Fish were exposed in mesocosms to acute doses of paracetamol (16 μg g−1 once a day over two days), followed by lower doses during a long-term exposure (1.6 μg g−1 once a week over three weeks), followed by a three-week recovery phase. Acute exposure induced marginal decreases in behavioral activity, and changes in dorsal brightness, hue and UV luminance. Interestingly, the long-term phase alone did not yield notable results on behavior and coloration. However, some effects of the acute exposure persisted during the long-term phase, highlighting that the expression of biological responses may be delayed in relation to past high exposure. Parasitism did not attenuate acute impacts, suggesting parasites may not help mitigate effects of paracetamol on behavior and coloration, but alone increased activity levels slightly. No effects of pollutant exposure, either of the acute or long-term phase, were observed after a recovery phase, indicating ability for recovery dynamics. Overall, our findings emphasize that pollutants effects can be highly transient, with rapid recovery when pollutant exposure ceased. Considering different exposure phases is crucial when assessing the ecological consequences of environmental contaminants. © 2025 Elsevier Ltd