Effects of water-accommodated fractions of crude oil and chemical dispersant on the antioxidant defense system, tissue damage, and gene expression in Ruditapes philippinarum

Oil spills pose significant ecological risks to marine benthic organisms, particularly bivalves. In this study, Ruditapes philippinarum was exposed to different concentrations of crude oil water accommodated fraction (WAF) and chemical enhanced water accommodated fraction (CEWAF) prepared with the addition of an oil dispersant. Six exposure groups were established: low (5 %), medium (10 %), and high (40 %) concentrations for both WAF and CEWAF treatments. Mortality analysis revealed that, at the same nominal concentrations, cumulative mortality in the M-CEWAF and H-CEWAF groups was significantly higher than in the corresponding M-WAF and H-WAF groups (P < 0.05). Histological examination showed more severe tissue damage in CEWAF groups, including vacuolization, structural disintegration, and cellular loss in the hepatopancreas and gills. Biochemical assays indicated prolonged oxidative stress in CEWAF groups, as evidenced by sustained elevations of CAT, GST, and MDA levels. Transcriptome analysis identified several upregulated stress-responsive genes involved in antioxidant defense (PHGDH), detoxification (CYP2J2, ABCC1), environmental stress (Dio1), and cellular protection (HspB11). The expression levels of PHGDH, Dio1, CYP2J2, and ABCC1 were significantly elevated under both WAF and CEWAF exposures (P < 0.05), showing a general pattern of early activation followed by a gradual decline over time. HspB11 exhibited dose-dependent upregulation, particularly under CEWAF exposure, indicating a prolonged activation of cellular protective mechanisms during the late exposure phase. These results demonstrate that CEWAF exposure induces stronger toxicological and transcriptomic responses than WAF, and that oxidative biomarkers and gene expression profiles are effective tools for evaluating the ecological risks of chemically dispersed oil.