Evaluating the physiological significance of hypoxic hyperventilation in larval zebrafish (Danio rerio)

In water-breathing fishes, the hypoxic ventilatory response (HVR) represents an increase in water flow over the gills during exposure to lowered ambient O₂ levels. The HVR is a critical defense mechanism that serves to delay the negative consequences of hypoxia on aerobic respiration. However, the physiological significance of the HVR in larval fishes is unclear as they do not have a fully developed gill and rely primarily on cutaneous gas transfer. Using larval zebrafish (4, 7, 10 and 15 days post-fertilization; dpf), we examined HVR under three levels of hypoxia (25, 45 and 60 mmHg). The larvae exhibited widely different HVRs as a function of developmental age and level of the hypoxia. Yet, critical O₂ tensions (P_crit) remained constant (30-34 mmHg) over the same period of development. Micro-optrode O₂ sensors were used to measure a significant decrease in buccal cavity water O₂ tensions in 4 and 7 dpf larvae compared with the water they inspired, demonstrating significant extraction of O₂ from the buccal cavity. To assess the physiological significance of the HVR, ventilatory water flow was prevented in larvae at 4 and 7 dpf by embedding their heads in agar. An increase in P_crit was observed in larvae at 7 dpf but not 4 dpf, suggesting that buccal ventilation is important for O₂ extraction by 7 dpf. Combined, these data indicate that branchial/buccal gas transfer plays a significant role in O₂ uptake during hypoxia, and supports a physiological benefit of the HVR in early life stages of zebrafish.