Zebrafish larva acquire hypoxia tolerance after temporal oxygen deprivation  in the embryonic period.
Environmental stimuli during embryonic development bring significant effects on physiological traits in animals. Oxygen is one of the most important molecules for survival. This study investigated how transient hypoxia during the embryonic stage alters hypoxic tolerance in later life. Zebrafish embryos were exposed to temporal hypoxia for 8 hours and then returned to ambient conditions. After this, stage/age-matched larvae were exposed to chronic hypoxia to test their hypoxia tolerance. We found that larvae subjected to hypoxic preconditioning (HP) survived significantly longer than control larvae (Ctrl). RNA-seq analysis revealed that embryonic hemoglobin family was more abundant in the HP group than in the Ctrl during chronic hypoxia. Based on the RNA-seq data, we investigated whether the HP group had increased hemoglobin levels using o-dianisidine staining. As a result, the HP group exhibited a significant increase in hemoglobin levels. We then asked about the importance of hemoglobin using artesunate (AS), a potent chemical that induce hemolysis, and dimethyloxalylglycine (DMOG), which leads to erythropoiesis through hypoxia-inducible factor (Hif) stabilization, to test if the changes in hemoglobin were a potential cause of increased hypoxia tolerance in the HP larvae. Results demonstrated that AS-treated HP larvae exhibited the same survival time as Ctrl larvae under chronic hypoxia. DMOG-treated Ctrl larvae exhibited an extended survival time under prolonged hypoxia. These results indicate that hypoxia tolerance in zebrafish was enhanced by experiencing transient oxygen deprivation during the embryonic period via enhanced erythropoiesis or erythrocyte function. These findings may contribute to the development of stress-tolerant fish for larval rearing.