Relationships between the peak hypoxic ventilatory response and critical O2 tension in larval and adult zebrafish (Danio rerio).

Fish increase ventilation during hypoxia, a reflex termed the hypoxic ventilatory response (HVR). The HVR is an effective mechanism to increase O2 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 (PwO2 ) at which this decline occurs is expected to be near the critical PwO2  (P crit), the PwO2  at which O2 consumption begins to decline. Our results indicate that in zebrafish (Danio rerio), the relationship between peak HVR and P crit is dependent on developmental stage. Peak ventilation occurred at PwO2  values higher than P crit in larvae, but at a PwO2  significantly lower than P crit 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 PwO2  below P crit.