Analysis of hepatic and serum responses to acute cold stress in juvenile yellowfin tuna Thunnus albacares (Bonnaterre 1788).

A decline in environmental temperature markedly affects various physiological processes of marine organisms, posing significant challenges to the aquaculture of yellowfin tuna Thunnus albacares (Bonnaterre 1788), particularly under winter cold conditions. Understanding how yellowfin tuna physiologically respond to environmental fluctuations is essential for revealing their adaptive strategies under stress conditions. In this study, two cold stress groups, LT (24°C) and ULT (18°C), were established, with a control group (CG, 30°C), and samples were collected at 0, 12, 24 and 36 h. We systematically evaluated hepatic and serum responses of juvenile T. albacares to acute cold stress using liver transcriptome sequencing, histopathological analysis [haematoxylin and eosin (HE) staining], serum biochemical assays (cortisol, glucose, creatinine, phenoloxidase, total cholesterol, pyruvate kinase) and measurements of hepatic antioxidant [superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA), glutathione peroxidase (GSH-Px)] and metabolic enzyme [lactate dehydrogenase (LDH), alanine aminotransferase (ALT), alkaline phosphatase (AKP), acid phosphatase (ACP)] activities. Under 24°C stress, the fish primarily activated the 'cytokine-cytokine receptor interaction' pathway, moderately adjusted SOD and GSH-Px activities, showed minor fluctuations in cortisol, glucose and cholesterol levels and exhibited only mild vacuolation and oedema in liver tissue, suggesting a certain cold tolerance and maintenance of physiological homoeostasis. In contrast, 18°C stress resulted in significant enrichment of 16 KEGG pathways, including glycolysis, cholesterol metabolism, HIF-1 and PPAR signalling, indicating the mobilization of more complex stress responses. Enzyme activity analysis revealed suppressed antioxidant capacity, marked fluctuations in metabolic and transaminase activities and aggravated histological damage, such as hepatocyte oedema and necrosis. The findings reveal distinct adaptive mechanisms employed by yellowfin tuna when exposed to cold environments of differing severity.