Serotonin is involved in multiple diverse and complex biological processes. Selective serotonin reuptake inhibitors (SSRIs) are often used to test this, and fluoxetine, a commonly detected SSRI in aquatic ecosystems from pharmaceutical pollution, has been found to disrupt important behaviours and functions in organisms exposed to the antidepressant. Increased serotonin generally has an inhibitory effect on sexual behaviour and decreased serotonin has a facilitatory effect, however in the few existing studies on fish, either no effect or the opposite effect have been reported in females, with most studies focusing on male behaviour. The present research aims to examine the role of the serotonin pathway in female sexual receptivity in the cichlid fish Nyassachromis cf. microcephalus . The sexual dimorphism and lekking system of this species, where male-male competition is aggressive and female choice is important, make them an important and interesting evolutionary system. Serotonin levels were modulated using fluoxetine and after a three-week exposure to their respective treatments (control - 0μg/L, low ecologically relevant dose - 0.54μg/L, high dose - 5.4μg/L), female sexual receptivity to male courtship was measured in a simulated lek mating arena where multiple males were presented simultaneously. Sexual receptivity was inhibited in females exposed to a high dose of fluoxetine. This behaviour change due to a disruption of the serotonin pathway could affect sexual selection in the long term, and in the short term highlights the potential for pollution to affect aquatic life by directly altering mating behaviour. ### Competing Interest Statement The authors have declared no competing interest.

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