Dietary crude oil exposure during sex differentiation skewed adult sex ratio towards males in the zebrafish

Dietary crude oil exposure has detrimental morpho-physiological effects in fishes, including endocrine disruption. However, little is known about how it influences sex differentiation and its potential for skewing sex ratios of popula-tions. Appropriate sex ratio is important for maintaining effective population size and structure. Deviations of these ratios can compromise population growth and maintenance and may induce changes in a species' evolutionary trajec-tory. We assessed the potential of dietary exposure to crude oil (6.5, 11.4, and 17.5 mg/kg food) to alter sex differen-tiation in the zebrafish (Danio rerio) (20-35 days post fertilization (dpf)) and subsequently skew the adult (90 dpf) sex ratio. Multiple health-and fitness-related phenotypic traits (i.e., body mass and length, condition factor, heart rate, ox-ygen consumption, and their capacity to cope with hypoxia) were also assessed to better understand the effects of di-etary crude oil exposure. We showed that dietary exposure to crude oil during the process of sex differentiation skewed sex ratio towards males (up to 0.34:1 female to male ratio in the highest oil concentration). Remarkably, this effect oc-curred independently of affecting physiological variables and female gonad characteristics, thus highlighting just how subtle the effects of dietary crude oil exposure can be. Our results suggest that, although fish were in an apparently healthy state during experimentation, sex ratio was still impacted, potentially compromising the resilience of the pop-ulation. Therefore, considering how complex chemical mixtures affect organisms at several levels (molecular -individ-ual) in experimental designs is warranted to better understand the implications of the exposures and the hazards that populations face in the wild.