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 O-2 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 (Pw(O2)) at which this decline occurs is expected to be near the critical Pw(O2) (P-crit), the Pw(O2) at which O-2 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 Pw(O2) values higher than P-crit in larvae, but at a Pw(O2) 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 Pw(O2) below P-crit.