Tissue-specific expression responses of antioxidant, immune, and metabolic genes in juvenile yellowfin tuna ( Thunnus albacares ) under acute low-pH stress

Acute reductions in seawater pH may disrupt acid–base regulation and physiological homeostasis in marine fish, but tissue-specific molecular responses in juvenile yellowfin tuna remain poorly understood. Juveniles of yellowfin tuna were exposed for 48 h to four seawater pH levels (8.1, 7.6, 7.1, and 6.6), with three replicate tanks per treatment. The expression of selected antioxidant-, immune-, inflammatory-, and metabolism-related genes in the liver and gill was quantified using qRT-PCR. In the gill, most examined genes were upregulated under low-pH exposure, although the magnitude and pH dependence of the responses varied among genes. In the liver, antioxidant genes and most immune-related genes were downregulated, whereas tnf-α was upregulated at pH 7.1. Hepatic metabolic genes also exhibited treatment-dependent responses: mdh2 was significantly downregulated across all low-pH treatments, whereas pck2 was significantly upregulated only at pH 7.1 and 6.6. In contrast, dgat1b was significantly downregulated exclusively at pH 7.1 and 6.6. These contrasting expression patterns demonstrate marked tissue specificity in the acute low-pH response of juvenile yellowfin tuna, with broader transcriptional activation in the gill and selective suppression and metabolic adjustment in the liver. The findings provide an initial molecular basis for further evaluating the physiological consequences of acute and chronic low-pH exposure in this high-metabolic-rate pelagic species.