Respiratory frequency, antioxidant enzyme activities and histological observation in a backcross progeny between Acanthopagrus schlegelii and Pagrus major subjected to thermal stress

Temperature plays a crucial role in regulating fish growth, development, and feeding behavior. However, the limited progress in establishing a comprehensive thermal tolerance evaluation system for seawater fish can be attributed to insufficient research on heat-resistant breeding. This study investigated the thermal tolerance of Backcross F1 (APA), Pagrus major (PP), and Acanthopagrus schlegelii (AA) through multiple physiological indicators. We examined the effects of elevated temperatures (20℃ as control, and 26℃, 29℃, 32℃, 35℃) on respiratory frequency, gill histopathology, Superoxide dismutase（SOD） and Catalase（CAT), Reactive oxygen species（ROS） and Malondialdehyde（MDA). The results demonstrated distinct thermal responses among the three groups. APA showed higher respiratory frequency than PP but similar patterns to AA. Histopathological analysis revealed PP is greater susceptibility to temperature-induced gill damage, which could impair respiratory function. Antioxidant enzyme activities were significantly influenced by thermal stress, with PP exhibiting markedly higher ROS and MDA levels compared to APA and AA. These findings suggest that PP has lower heat tolerance, while APA and AA demonstrate superior thermal resistance. The thermal tolerance observed in APA may be associated with genetic contributions from its AA lineage, suggesting successful trait transfer through backcrossing. This study provides valuable insights into the physiological responses of sea bream to heat stress and contributes to the development of heat-resistant breeding strategies in aquaculture.