The presence of pharmaceutical products in the aquatic environment has been reported in several studies. However, the impact of these drugs on living organisms is still uncharacterized. Here, we investigated the effects of acute exposure to either diazepam or fluoxetine on the stress response in Danio rerio. We showed that diazepam and fluoxetine inhibited the stress axis in zebrafish. Intermediate concentrations of diazepam suppressed the stress response as measured by cortisol levels, whereas fluoxetine inhibited cortisol increase at concentrations similar to those found in the environment. These data suggest that the presence of psychoactive drugs in aquatic ecosystems could cause neuroendocrine dysfunction in fish.

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Chemical communication relating to predation risk is a trait common among fish species. Prey fish under threat of predation can signal risk to conspecific fish, which then exhibit defensive responses. Fish also assess predation risk by visual cues and change their behavior accordingly. Here, we explored whether these behavioral changes act as visual alarm signals to conspecific fish that are not initially under risk. We show that shoals of zebrafish (Danio rerio) visually exposed to a predator display antipredator behaviors. In addition, these defensive maneuvers trigger antipredator reactions in conspecifics and, concomitantly, stimulate the hypothalamus-pituitary-interrenal axis, leading to cortisol increase. Thus, we conclude that zebrafish defensive behaviors act as visual alarm cues that induce antipredator and stress response in conspecific fish.

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In fish, defensive reactions are induced by different chemical cues that emanate from sense-related stresses [physical, chemical, and visual (visual contact with predator)] or food stresses (acute fasting and chronic food restriction). Using a shuttle box with a two-chamber unmixed laminar flow that allowed fish to remain or flee from a chemical cue, we showed that the avoidance response depended on the type of the chemical cue. We show that zebrafish (Danio rerio) retreated from water conditioned with chemical cues released by chemically or physically stressed fish and acutely fasted fish, but not from water with cues from fish experiencing visual contact with predatory fish or fish suffering from chronic food restriction. Our data reinforced the hypothesis that fish use a combination of information and the context of the situation to determine their evasion strategy.