Physiological, plasma biochemical, and heat-shock responses of starry flounder ( Platichthys stellatus ) to acute and chronic high-temperature exposure

High water temperature is a major environmental stressor affecting the survival and physiological condition of cultured fish. This study evaluated survival, plasma biochemical responses, stress-related indicators, and heat-shock protein expression in starry flounder ( Platichthys stellatus ) exposed to gradual or acute high-temperature conditions. A total of 140 fish were used: 20 were sampled to determine the initial physiological status, and the remaining 120 were assigned to control, gradual-exposure, and acute-exposure groups, with two replicate tanks containing 20 fish each per treatment. Fish in the control group were maintained at 20°C, whereas those in the gradual-exposure group were exposed to 26–27°C over 110 h and those in the acute-exposure group were exposed to 29°C for 6 h. Final survival was 100.0%, 77.5%, and 7.5% in the control, gradual-exposure, and acute-exposure groups, respectively. The hepatosomatic index did not differ significantly among the groups. Plasma glucose concentrations measured using a dry-chemistry analyzer were significantly higher in both high-temperature groups than in the control group. Plasma aspartate aminotransferase activity was significantly higher in the acute-exposure group than in the control group, while lactate dehydrogenase activity was significantly higher in the acute-exposure group than in both the control and gradual-exposure groups. No significant differences were observed in the activities of alanine aminotransferase, alkaline phosphatase, or lipase, or in the concentrations of total cholesterol, triglyceride, total protein, albumin, or total carbon dioxide. ELISA measurements showed no significant differences in plasma catalase or glucose concentrations, whereas plasma cortisol concentrations were significantly elevated in the gradual- and acute-exposure groups compared with the initial group. Plasma IgM concentrations showed a numerical decrease following high-temperature exposure, although this difference was not statistically significant. HSP70 expression was significantly affected by high-temperature exposure in the liver, kidney, and muscle, whereas HSP90 expression was significantly affected in the kidney and muscle but not in the liver. These findings indicate that high-temperature exposure is associated with marked physiological stress, metabolic disturbance, tissue-related biochemical responses, and changes in heat-shock protein expression in starry flounder, with acute exposure to 29°C being associated with particularly severe mortality and physiological disruption.