Splenic tissue injury and physiological response mechanisms in juvenile yellowfin tuna (Thunnus albacares) under acute cold stress.

Abnormal seawater temperatures driven by global climate change are profoundly disrupting the physiological homeostasis and immune regulation of marine fish. As a warm-blooded pelagic species, yellowfin tuna (Thunnus albacares) possesses partial thermoregulatory capability but still experiences significant physiological stress under abrupt cold exposure. The spleen, a key immune and metabolic organ, is highly sensitive to temperature fluctuations and serves as a critical indicator of cold stress effects. In this study, juvenile yellowfin tuna were subjected to cold stress at 24 °C (LT group) and 18 °C (ULT group), with 30 °C as the control (CG group). Sampling was conducted at 0, 12, 24, and 36 h. By evaluating splenic antioxidant and metabolic enzyme activities, histopathological changes, and immune gene expression profiles, we systematically assessed tissue injury and physiological responses under different cold intensities. Results showed that acute cold stress induced notable splenic damage, including nuclear deformation, vacuolization, and melano-macrophage aggregation, with the most severe lesions observed in the ULT group. Antioxidant responses revealed significantly elevated CAT activity at 36 h and increased MDA levels at 0 h and 36 h in both cold-stressed groups (p < 0.05). Metabolic enzymes such as ALT, AST, LDH, and ACP exhibited dynamic fluctuations, with ACP activity significantly increased at 36 h in the ULT group. Immune-related genes (hspa8b, irf3, b2m, blmh) displayed time- and temperature-dependent expression, with upregulation at 24 h and partial downregulation at 36 h, indicating immune activation followed by potential suppression. These findings highlight the vulnerability of the splenic immune-metabolic axis in yellowfin tuna to cold stress and offer important implications for understanding temperature-induced physiological dysfunction in regionally endothermic marine fish.