Physiological and Tissue-Specific Transcriptomic Responses to Cold Stress in Hybrid Grouper (Epinephelus fuscoguttatus × E. lanceolatus)

The hybrid grouper (Epinephelus fuscoguttatus × E. lanceolatus) is a high-value aquaculture species with limited cold tolerance in temperate regions such as South Korea. Two complementary temperature experiments were conducted. In Experiment I, smaller juveniles (33.9 ± 10.2 g) were subjected to water temperatures (20, 18, 16, 14, and 12 °C) lower than 27 °C for 30 days to assess survival. In Experiment II, larger juveniles (281.1 ± 2.9 g) were reared for 90 days at 27, 24, 21, 18, and 15 °C to evaluate chronic physiological responses; brain and liver RNA-Seq analyses compared the 15 °C low-temperature condition with the 27 °C control condition. Fish maintained at 15 °C exhibited progressive mortality, whereas fish reared at 18–27 °C showed 100% survival during the 90-day trial. Exposure to 15 °C was associated with significant decreases in hemoglobin and hematocrit, a >5-fold increase in plasma cortisol, and elevated glucose levels. SOD and CAT activities were also elevated at low temperatures, indicating an enhanced antioxidant response; however, these enzyme measurements do not directly quantify reactive oxygen species. Transcriptomic profiling revealed temperature-associated expression differences, with more DEGs in the liver (1365) than in the brain (221). Brain DEGs were associated with calcium-signaling- and neural-related categories, whereas liver DEGs were associated with metabolic, antioxidant response, and extracellular-matrix-related categories. Functional enrichment findings that did not meet FDR < 0.05 were treated as suggestive or exploratory rather than statistically significant. These findings help define the lower thermal tolerance of hybrid grouper and provide information relevant to aquaculture management under low-temperature conditions.