Integrated ecotoxicological assessment of the UV filter 4-MBC in zebrafish larvae: developmental, neurobehavioral, and redox–metabolic responses

Organic ultraviolet (UV) filters are increasingly recognized as emerging aquatic contaminants, yet the mechanistic basis of their early life ecotoxicity remains insufficiently resolved. Here, we investigated the developmental, neurobehavioral, biochemical, and transcriptomic responses of zebrafish ( Danio rerio ) embryos and larvae exposed to 4-methylbenzylidene camphor (4-MBC), a widely used UV-filter compound. Embryonic exposure to 4-MBC caused concentration-dependent developmental toxicity, including increased mortality, delayed hatching, reduced body length, and morphological abnormalities, with a 96-h LC₅₀ of 929.5 μg/L. Sublethal exposure further impaired larval locomotor performance and attenuated light–dark adaptive responses, indicating functional neurobehavioral disruption with potential ecological relevance. Biochemical analyses indicated redox perturbation, characterized by increased ROS fluorescence, increased SOD activity at 500 μg/L, and a significant increase in MDA only at 500 μg/L. The different concentration-response patterns of these biomarkers indicate that elevated ROS did not uniformly translate into detectable lipid peroxidation. Transcriptomic profiling revealed extensive gene expression remodeling, with enriched pathways associated with oxidative-reduction processes, mitochondrial function, amino acid and carbohydrate metabolism, ubiquinone biosynthesis, cell-cycle regulation, and apoptosis. Integrated phenotypic, biochemical, and transcriptomic observations were consistent with a proposed redox–metabolic response framework linking 4-MBC exposure with developmental and behavioral impairment. The higher exposure concentrations were used primarily for acute hazard characterization, whereas lower sublethal concentrations were used to identify sensitive biochemical and transcriptional responses. These findings provide hazard information and candidate biomarkers that may support future ecological risk assessment of organic UV filters, although environmentally realistic exposure scenarios require further investigation using analytically verified concentrations and internal-dose measurements.