Relationships between the peak hypoxic ventilatory response and critical O tension in larval and adult zebrafish ().

Fish increase ventilation during hypoxia, a reflex termed the hypoxic ventilatory response (HVR). The HVR is an effective mechanism to increase O uptake, but at a high metabolic cost. Therefore, when hypoxia becomes severe enough, ventilation declines, as its benefit is diminished. The water oxygen partial pressure (w ) at which this decline occurs is expected to be near the critical w  (), the w  at which O consumption begins to decline. Our results indicate that in zebrafish (), the relationship between peak HVR and is dependent on developmental stage. Peak ventilation occurred at w  values higher than in larvae, but at a w  significantly lower than in adults. Larval zebrafish use cutaneous respiration to a greater extent than branchial respiration and the cost of sustaining the HVR may outweigh the benefit, whereas adult zebrafish, which rely on branchial respiration, may benefit from using HVR at w  below .