Physiological responses of red-spotted grouper (Epinephelus akaara) to water temperature and dietary supplementation of isolated intestinal bacteria.

Water temperature and intestinal bacteria are key regulators of physiological processes in teleost fish; however, growth performance alone may not accurately reflect physiological stability. This study investigated the effects of dietary supplementation with host-derived intestinal bacteria and rearing water temperature on physiological responses in the red-spotted grouper (Epinephelus akaara), with emphasis on metabolic regulation, stress response, and immune homeostasis. A total of 14 bacterial strains were identified, and three bacteria species with potential antioxidant activity were selected. In a dietary experiment with three intestinal bacteria species, we observed increased growth performance, hepatocyte diameter in the liver and goblet cell counts in the intestine. In experiments on water temperature, enhanced growth performance was observed at 29 °C, followed by 26 °C, 32 °C, and 23 °C. The diameter of hepatocytes increased at 26 °C and 29 °C, and the largest number of goblet cells was observed at 29 °C. Expression levels of crh, and npy increased at 32 °C. In the combined treatment (bacterial diet at 26-27 °C), the experimental group exhibited significantly enhanced growth. No adverse expression of immune response-related genes was observed. Our findings suggest that the integration of specific intestinal bacteria and optimal water temperature activates digestive physiology and promotes growth without inducing thermal stress or immune responses.