Embryonic developmental toxicity and potential endocrine disruption in an estuarine benthic fish under binary and ternary exposures to organic ultraviolet absorbents.

Estuarine ecosystems are increasingly threatened by organic ultraviolet absorbers (OUVs). However, their acute and combined toxicity to estuarine aquatic organisms at environmentally relevant concentrations remains poorly understood. This study evaluated the acute lethality and sublethal developmental toxicity of ethylhexyl salicylate (EHS), homosalate (HMS), and UV-329 on Mugilogobius chulae embryos. It also investigated the developmental effects and thyroid-related transcriptional responses of binary and ternary mixtures at environmentally relevant concentrations, and assessed their combined effects at both phenotypic and transcriptional levels. While individual exposure exhibited low acute lethality, co-exposure to binary and ternary mixtures increased embryonic mortality, although these changes were not statistically significant. Co-exposure significantly decreased embryonic heart rate (p < 0.05). Notably, at the tested environmentally relevant concentration, the heart rate response to the HMS + UV-329 mixture showed an interaction pattern consistent with potential synergism based on the CI and IA models, whereas other mixtures were classified as antagonistic. In addition, co-exposure significantly upregulated HPT axis-related genes (p < 0.05), suggesting potential perturbation of thyroid-related molecular pathways. Co-exposure to EHS + HMS and EHS + UV-329 showed synergistic effects on tg, but additive effects on tpo and nis, whereas HMS + UV-329 and the ternary mixture produced synergistic effects on all three genes. The ternary mixture significantly upregulated dio3, trhrα, tshr, and thrα (p < 0.05), and these responses were classified as synergistic at the tested environmentally relevant concentration. Overall, co-exposure to environmentally relevant concentrations of OUVs was associated with greater developmental and molecular-level responses in benthic fish embryos than did exposure to individual OUVs. These findings provide useful information for assessing the mixture toxicity of OUVs in estuarine environments, particularly with respect to early developmental effects in fish.