摘要Background Hypoxia is a pervasive challenge in aquaculture that poses a significant threat to aquatic organisms.Since fish cannot synthesize vitamin A endogenously,it must be supplied through diet,and it plays a vital role in sup-porting fish stress resistance.This study aimed to investigate the protective effects of VA on the gills of adult grass carp(Ctenopharyngodon idella)against hypoxia and to elucidate the underlying mechanisms.Methods Six experimental diets with graded VA levels(375,862,1,614,2,099,2,786,and 3,118 IU/kg)were fed to grass carp(initial weight:726±1.2 g)for 60 d.After the trial,24 fish per treatment were selected,divided equally into normoxic and hypoxic groups,fasted for 24 h,and then subjected to a 96-h acute hypoxic challenge.Results The results demonstrated that VA supplementation mitigated hypoxia-induced damage in gill tissue,as evidenced by histological examination.Furthermore,VA alleviated oxidative stress,as indicated by reduced levels of lactate(LD),lactate dehydrogenase(LDH),reactive oxygen species(ROS),protein carbonyl(PC),and malondialde-hyde(MDA).Further investigations indicated that VA alleviated mitochondrial stress,potentially through suppress-ing the canonical UPRmt axis while activating both the UPRmt sirtuin axis and the UPRIMS/Erα axis.VA also modulated mitochondrial mass via multiple mechanisms,including the promotion of mitochondrial biogenesis,maintenance of dynamics by stimulating fusion and reducing fission,and inhibition of mitophagy.The suppression of mitophagy likely involved downregulating both the Pink1/Parkin-dependent pathway and the Hif1a-Bnip3 pathway.Taken together,these adaptations suggested an essential role for VA in preserving mitochondrial homeostasis.Based on the quadratic regression analysis of ROS and MDA levels from the hypoxic group,the estimated VA requirements for adult grass carp were 2,013 and 2,056 IU/kg diet,respectively.Conclusions In summary,this study provided the first evidence that VA conferred protective effects against hypoxia-induced gill damage in grass carp.