Combined exposure to hypoxia and 17α-ethinylestradiol alters reproductive traits in marine medaka

Aquatic environments increasingly experience the co-occurrence of endocrine-disrupting contaminants and declining oxygen levels, yet their combined influence on reproductive fitness remains insufficiently understood. Here, we examined the reproductive consequences of developmental exposure to hypoxia and the synthetic estrogen 17α-ethinylestradiol (EE2) in marine medaka ( Oryzias melastigma ). Embryos were exposed to environmentally relevant (71 ng/L) or elevated (668 ng/L) EE2 under normoxic or hypoxic (1.8 mg O₂/L) conditions until hatching and subsequently reared in seawater to adulthood. EE2 induced feminization of sex ratios, while hypoxia altered several physiological responses during estrogenic exposure. Co-exposure resulted in distinct endpoint-specific patterns, including increased hepatosomatic and gonadosomatic indices. Although fecundity increased under combined EE2 high dose and hypoxia exposure, hatching success declined across all the EE2 treatments, indicating a shift between reproductive output and offspring viability. Transcriptomic profiling revealed stronger gene expression perturbations under co-exposure than under either stressor alone, with enriched pathways related to steroid hormone biosynthesis, progesterone-mediated oocyte maturation, extracellular matrix organization, and PI3K–Akt signaling. These molecular responses provide biological context for the observed physiological changes. Collectively, these findings demonstrate that transient developmental hypoxia can modify estrogen driven reproductive outcomes later in life, linking early life oxygen limitation with altered endocrine susceptibility. This study highlights the importance of considering co-occurring chemical and environmental stressors when evaluating reproductive risks in increasingly deoxygenated coastal ecosystems.