Gill Adaptation of the Hong Kong Catfish ( Clarias fuscus ) to Chronic Heat Stress: Tissue Remodeling, Enhanced Antioxidant Defense and Immune Metabolism Regulation

Rising temperature fluctuations associated with global climate change pose an increasing threat to fish survival. This study investigated the adaptive mechanisms in the gills of Hong Kong catfish (Clarias fuscus) under long-term high temperature stress. C. fuscus were cultured at natural-temperature (NT, 26°C) and high-temperature (HT, 34°C) for 90 days, with histopathological, biochemical, and transcriptomic differences analyzed between the two groups. Histopathological findings revealed that continuous HT treatment reduced the gill surface area and induced apoptosis. Biochemical analyses showed that prolonged HT treatment significantly increased the activities of superoxide dismutase (SOD, p = 0.0281), catalase (CAT, p = 0.0388), and glutathione peroxidase (GPX, p = 0.0394), while reducing malondialdehyde (MDA, p = 0.0312) level. The status of the antioxidant system and the survival time may be associated with the re-establishment of antioxidant balance and an overall enhancement of the fish's antioxidant capacity. Transcriptome analysis indicated that long-term HT treatment induced significant expression changes in 675 genes, of which 407 were significantly upregulated and 208 were significantly downregulated. These differentially expressed genes (DEGs) were primarily enriched in the biological processes concerned with immune response and energy metabolism. The results suggest that C. fuscus enhanced thermal resilience by modulating immune pathways to mitigate excessive apoptosis and upregulating energy metabolism to support tissue repair. The findings demonstrate that although chronic heat stress induces structural damage in the gills of C. fuscus, it simultaneously strengthens the fish's antioxidant defenses, improves stress tolerance, and activates repair mechanisms. This study provides a valuable molecular and physiological foundation for understanding the adaptive evolution of fish in a warming environment and provides guidance for the implementation of effective species protection measures.