Bis(2-ethylhexyl) sebacate (DEHS), a commonly used non-phthalate plasticizer considered relatively safe relative to phthalates, has been reported to disrupt the endocrine system, affect reproduction-related genes, and potentially induce thyroid hormone– disrupting and estrogenic effects on Japanese medaka (Oryzias latipes). However, the long-term effects of DEHS exposure on aquatic organisms remain unclear; further, data on residual DEHS concentrations in rivers are extremely limited. Here, the effects of DEHS on the reproductive performance and gonadal sex differentiation of Japanese medaka were determined. Japanese medaka embryos and larvae were exposed to varying DEHS concentrations that have been reported to induce thyroid hormonedisrupting effects. The residual concentrations of DEHS in the Sumiyoshi River were measured weekly from May to July in 2024. The formation of testis-ova was induced in XY medaka exposed to varying DEHS concentrations. DEHS exposure was shown to significantly reduce the number of eggs laid but did not affect fertilization rates. The DEHS levels in the Sumiyoshi River were either undetected or below the method quantification limit. Although significant changes in reproductive capacity and testis-ova were not observed at environmentally relevant residual concentrations, this study highlights the potentially harmful effects of a chemical that was previously considered environmentally friendly.

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