Psychoactive Pollutant Impacts Baseline Immune Traits but Not Behavior and Microbiome in a Fish

Pharmaceutical pollution is an increasing global threat to freshwater ecosystems. Psychoactive drugs, such as antidepressants, are of particular concern because they affect evolutionarily conserved neuroendocrine pathways in nontarget organisms. As these pathways regulate interconnected biological systems, understanding the ecological consequences of pharmaceutical exposure requires an integrative approach. Here, we investigated the effects of 28-day exposure to control (freshwater), low (4.2 ± 0.3 ng/L), and high (637.5 ± 173.84 ng/L) fluoxetine concentrations on behaviors, baseline immune traits, and skin mucus microbiome composition in the Murray River rainbowfish (Melanotaenia fluviatilis). Fluoxetine exposure did not impact fish behavior, including activity, freezing behavior, shelter use, or microbiome composition. In contrast, immune traits were significantly altered. Both fluoxetine concentrations reduced total white blood cell (WBC) count, while only fish exposed to the low concentration showed reduced lymphocyte counts and elevated neutrophil counts, consistent with a stress-associated WBC redistribution response. Our findings show that different, but interconnected, biological systems can differ in their sensitivity to pollutants and suggest that immune traits may be an early, sensitive indicator of exposure to psychoactive pharmaceuticals. These findings highlight the importance of integrating multiple biological end points to obtain a more comprehensive understanding of the ecological consequences of pharmaceutical contamination.