Impacts of Acute Hypothermia Stress on Gastrointestinal Digestive Enzyme Function and Pathological Changes in Yellowfin Tuna ( <i>Thunnus albacares</i> )

This study investigated the effects of acute cold stress on digestive function and gastrointestinal pathology in juvenile yellowfin tuna (Thunnus albacares). The experimental setup encompassed a control group (CG, 30°C), a low-temperature group (HT, 24°C), and an ultra-low-temperature group (LT, 18°C), with sampling conducted at 0, 12, 24, and 36 h, respectively, over a total experimental duration of 36 h. We analyzed digestive enzyme activities in the gastric tissue and intestine, expression of key digestive genes, and gastrointestinal histopathology. Results showed that low-temperature stress significantly altered digestive enzyme activities. During the early stage of acute cold exposure (12-24 h), intestinal amylase and trypsin showed enzyme-specific changes, indicating an energy-saving response. With continued exposure over the 36-h experimental period, lipase activity increased substantially in the LT group, suggesting a metabolic shift toward lipid mobilization for energy and membrane fluidity maintenance. Pepsin activity was highest in the HT group and fluctuated in the LT group, reflecting complex gastric microenvironment disruption. Gene expression analysis revealed transcriptional-physiological decoupling. Histopathological findings showed cellular edema, necrosis, and mucous cell hyperplasia in both HT and LT groups, worsening with colder temperatures and longer exposure. By 36 h, the LT group exhibited severe mucosal detachment and glandular structure disruption. In summary, juvenile yellowfin tuna responded to acute cold stress through a biphasic metabolic strategy-initial inhibition followed by compensation-and dynamic gene-enzyme decoupling. While short-term stress activated metabolic plasticity, severe acute cold exposure caused significant gastric tissue damage and digestive dysfunction. These findings provide new insights into thermal adaptation mechanisms and support improved temperature management in aquaculture.