Low O2 acclimation shifts the hypoxia avoidance behaviour of snapper (Pagrus auratus) with only subtle changes in aerobic and anaerobic function

Abstract

Summary It was hypothesized that chronic hypoxia acclimation (preconditioning) would alter the behavioural low O2 avoidance strategy of fish as a result of both aerobic and anaerobic physiological adaptations. Avoidance and physiological responses of juvenile snapper (Pagrus auratus) were therefore investigated following a 6 week period of moderate hypoxia exposure (10.2-12.1 kPa PO2, 21 ± 1°C) and compared to those of normoxic controls (PO2= 20-21 kPa, 21 ± 1°C). The critical oxygen pressure (i.e. Pcrit) limit of both groups was unchanged at ~7 kPa, as were standard, routine and maximum metabolic rates. However, hypoxia acclimated fish showed increased tolerances to hypoxia in behavioral choice chambers by avoiding lower PO2 levels (3.3 ± 0.7 vs 5.3 ± 1.1 kPa) without displaying greater perturbations of lactate or glucose. This behavioural change was associated with unexpected physiological adjustments. For example, a decrease in blood O2 carrying capacity was observed after hypoxia-acclimation. Also unexpected was an increase in whole blood P50 following acclimation to low O2, perhaps facilitating Hb-O2 off-loading to tissues. In addition, cardiac mitochondria measured in situ using permeabilised fibres showed improved O2 uptake efficiencies. The proportion of the anaerobic enzyme lactate dehydrogenase (LDH), at least relative to the aerobic marker enzyme citrate synthase (CS), also increased in heart and skeletal red muscle indicating enhanced anaerobic potential, or in situ lactate metabolism, in these tissues. Overall these data suggest that a prioritization of O2 delivery and O2 utilization over O2 uptake during long-term hypoxia may convey a significant survival benefit to snapper in terms of behavioural low O2 tolerance.