Nonylphenol (NP) and polystyrene microplastics (PS-MPs) commonly coexist in aquatic environments, yet their chronic and combined effects on adult marine fish remain poorly understood. Adult marine medaka (Oryzias melastigma) were exposed for 28 days to Control, NP, PS-MPs, or combined NP and PS-MPs treatments. Female and male fish were analyzed separately using physiological, histopathological, transcriptomic, qPCR, and metabolomic approaches. No mortality or overt abnormalities were observed. NP alone exerted limited effects on most physiological endpoints but reduced hepatocyte nuclear area in both sexes.
PS-MPs produced more pronounced physiological effects, altering body weight and condition factor in both sexes and gonadosomatic index in females. Histopathological alterations included sinusoidal congestion, hepatocellular swelling, cytoplasmic vacuolation, and abnormal nuclear morphology. Combined exposure induced the broadest transcriptional response, with 361 and 238 differentially expressed genes in females and males, respectively. Significant NP × PS-MPs interactions were detected for selected physiological and hepatic endpoints. Transcriptomic and metabolomic analyses suggested sex-related differences in lipid- and energy-related metabolic responses, particularly following PS-MPs and combined exposure. Overall, PS-MPs caused comparatively stronger physiological disturbances, whereas combined exposure elicited the most extensive molecular responses, highlighting sex-related differences in selected toxicological responses.