Swimming impairment caused by pharmaceuticals in native and invasive fishes: A comparison of fluoxetine effects in two poeciliid species.

Chemical pollution and invasive alien species are currently recognized as two of the major drivers of global biodiversity loss. These two pressures rarely act in isolation, but instead frequently co-occur and interact in human-modified ecosystems. The antidepressant fluoxetine (FLX) is among the neuroactive pharmaceuticals most regularly detected in watersheds worldwide, raising concerns over its potential adverse effects on the behavior of fishes and other aquatic organisms. For native fish species inhabiting FLX-polluted environments, the ability to withstand the ecological pressure from invasive species might be compromised if they prove more vulnerable to chemical pollution than their non-native counterparts. Here, we examined whether FLX impairs swimming performance at lower concentrations in the native ten-spotted livebearer Cnesterodon decemmaculatus than in the invasive mosquitofish Gambusia holbrooki. Fishes were exposed to waterborne FLX for 14 days, under semi-static conditions with daily renewal of test solutions. Following exposure, swimming activity of individual fish was video-recorded and analyzed. We found that fish exposed to 25 μg/L FLX exhibited impaired swimming behaviour, slowing down their movement and remaining for longer time static and close to the surface. Contrarily, exposure to 0.5 μg/L FLX-an environmentally relevant concentration-did not affect swimming performance in either species. Interestingly, exposure to 5 μg/L FLX, which is close to the concentration of combined antidepressants reported in surface waters, had a significant effect on overall locomotion in C. decemmaculatus but not in G. holbrooki. This finding points to a higher susceptibility of the native species as compared to the invasive one, potentially diminishing its capacity to cope with interference competition under aquatic pollution by neuroactive contaminants.