Arapaima gigas maintains gas exchange separation in severe aquatic hypoxia but does not suffer branchial oxygen loss

One of the most air-reliant obligate air-breathing fish is the South American Arapaima gigas, with substantially reduced gills impeding gas diffusion, thought to be a result of recurring aquatic hypoxia in its habitat. In normoxic water, A. gigas is reported to satisfy 70-80% of its O₂ requirement from the air while excreting 60-90% of its CO₂ to the water. If this pattern of gas exchange were to continue in severely hypoxic water, O₂ loss at the gills would be expected. We hypothesized therefore that partitioning of CO₂ would shift to the air phase in severe aquatic hypoxia eliminating the risk of branchial O₂ loss. By adapting a respirometer designed to measure aquatic MO₂/MCO₂ we were able to run intermittent closed respirometry on both water and air phase for both of these gasses as well as sample water for N-waste measurements (ammonia-N, urea-N) so as to calculate metabolic fuel utilization. In contrast to our prediction, we found that partitioning of CO₂ excretion changed little between normoxia and severe hypoxia (83% vs 77% aquatic excretion respectively) and at the same time there was no evidence of branchial O₂ loss in hypoxia. This indicates that A. gigas can utilize distinct transfer pathways for O₂ and CO₂. Routine and standard MO₂, N-waste excretion, and metabolic fuel utilization did not change with water oxygenation. Metabolism was fueled mostly by protein oxidation (53%) while carbohydrates and lipids accounted for 27% and 20% respectively.