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

Abstract

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.

Do air-breathing fish suffer branchial oxygen loss in hypoxic water?

“…The aquatic CO 2 excretion fraction, however, only changes from 99% in normoxia to 91% during severe hypoxia. This continuous aquatic CO 2 excretion in severe aquatic hypoxia has also been reported in A. gigas [20] (aquatic PO 2 ≈ 1.1 mmHg), L. oculatus [16] (PO 2 = 12 mmHg), Anabas testudineus [41] (PO 2 = 2.7-7.5 mmHg) and in Ancistrus chagresi [42] (PO 2 : 5-20 mmHg) as well as a host of other species where only CO 2 excretion to the air-phase was measured (table 5.7 in [2]). Since CO 2 is continuously excreted into the water in the current study, blood must come into near contact with the water, most notably during branchial passage, and the anatomical diffusion factor (ADF) of the gills of this species under these conditions can be expected to be very large and comparable to active water breathers such as rainbow trout [30].…”

“…The respirometer and methods have previously been described in detail in [20]. The respirometer used was a water-filled cylinder (30.1 l) with a small air-filled chamber (≈800 ml) at the top of one end.…”

“…Arapaima gigas is an obligate airbreather with strongly reduced gills, where the secondary lamellae are almost eliminated in juvenile and adult stages [21]. However, the separation of gas exchange exhibited by A. gigas must require other adaptations that likely include shunting of blood past the gills but also some degree of flow separation of blood streams, through the undivided heart [20]. A reduction of the gills is normally seen as a key adaptation to life in hypoxic water and expected to reduce oxygen loss [2,15,[21][22][23].…”

“…The main purpose of this study was therefore to determine the extent of oxygen loss in aquatic hypoxia of P. hypophthalmus and to further understand this oxygen loss by characterizing the morphological and functional characteristics of the cardiovascular system. We quantified the partitioning of oxygen uptake and CO 2 excretion in both air-and water-phases allowing measurement of actual branchial O 2 loss in deep aquatic hypoxia using a newly developed two-phase respirometer [20]. Further, we charted the circulatory bauplan of P. hypophthalmus with special attention to the branchial basket and modifications for air-breathing using Mercox casts, CT scans with Microfil or barium sulfate and dissections.…”

“…This difficulty was recently overcome in a study of the air-breathing Arapaima gigas . Here, it was shown that even at a water PO 2 of only 1 mmHg where 100% of oxygen uptake was sourced from air, 77% of the produced CO 2 was still excreted into the water and that further, there was no evidence of branchial oxygen loss [20]. Arapaima gigas is an obligate air-breather with strongly reduced gills, where the secondary lamellae are almost eliminated in juvenile and adult stages [21].…”