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

The missing effect of temperature on branchial O2 loss in an air-breathing catfish

Air-breathing fish risk losing aerially sourced O2 to hypoxic water during branchial passage. Two adaptations thought to mitigate this loss are reduced gill size and increased blood O2 affinity. Both are affected by temperature in the facultative air-breathing catfish Pangasianodon hypophthalmus, where increased temperature results in larger gills and reduced blood O2 affinity. Here, we tested whether branchial O2 loss increases with temperature, by measuring this loss and the aerial and aquatic gas exchange at 25°C and 33°C in near aquatic anoxia. Surprisingly, increasing temperature did not change the absolute O2 loss while metabolic rate increased by 75%. Hence, animals suffered a 10% loss of the aerial O2 uptake at 25°C compared with only a 5% loss at 33°C. Our results indicate an increased hypoxia-induced reduction in gill ventilation at 33°C, negatively affecting aquatic exchange of both CO2 and O2, resulting in unchanged O2 loss and a CO2 partitioning shift towards the air phase.

Comparison of the respiratory vasculature of two species of swamp eels, Monopterus albus and Ophisternon bengalense (Synbranchidae)

We compare the cardiovascular anatomy of two synbranchids (Actinopterygii, Synbranchiformes); Ophisternon bengalense, which only infrequently breathes air when in hypoxic water, and Monopterus albus, which is more strongly dependent on air-breathing. Both species use the buccopharyngeal cavity for aerial respiration. The gill vasculature in O. bengalense comprises four pairs of holobranchs. Filaments are lined with secondary lamellae, the blood space of which is studded with the pillar cells, as in most other teleosts. In comparison, M. albus has only three pairs of gill arches exposed to the surrounding water. Filaments are rudimentary, with the afferent and efferent filamental arteries connected by 8-10 (first arch) or fewer (second and third) parallel vessels. There also are shunt vessels directly connecting the afferent and efferent branchial arteries. The fourth arch artery is a large throughfare vessel embedded in tissue with no branchial ramifications. The aerial respiratory capillaries are distributed with no particular pattern in O. bengalense, whereas the capillaries occur in clusters, each composed of repeatedly turning capillaries in M. albus. The arterial architecture of O. bengalense shows no deviation from the typical teleost pattern. The respiratory capillaries over the buccopharyngeal cavity surface are supplied mainly by the branches of the first efferent branchial artery and drained by the anterior cardinal vein. The efferent branchial arteries are connected by the lateral dorsal aorta. In contrast, the arterial system of M. albus shows notable anomalies. These include complete disruption of the lateral dorsal aorta and the presence of pre-gill arteries to the aerial respiratory capillaries (hyoidean artery, ventral esophageal artery and other smaller ramifications of the first to third branchial arches). We discuss the functional implications of these findings and hypothesize a sequence of evolutionary steps from adoption of air-breathing in fish to the development of double circulation as seen in lungfish and tetrapods.

Is the air-breathing organ a significant route for CO2 excretion during aquatic hypercapnia in the pirarucu, Arapaima gigas?

The pirarucu is one of the very few obligate air-breathing fish, employing a gigantic, highly vascularized air-breathing organ (ABO). Traditionally, the ABO is thought to serve mainly for O2 uptake (ṀO2), with the gills providing the major route for excretion of CO2 (ṀCO2) and N-waste. However, under aquatic hypercapnia, a common occurrence in its natural environment, branchial ṀCO2 to the water may become impaired. Under these conditions, does the ABO become an important route of ṀCO2 excretion to the air? We have answered this question by measuring ṀCO2 and ṀO2 in both air and water phases, as well as the pattern of air-breathing, in pirarucu under aquatic normocapnia and hypercapnia (3% CO2). Indeed, ṀCO2 to the air phase via the ABO increased 2- to 3-fold during exposure to high water PCO2, accounting for 59-71% of the total, with no change in the dominant contribution of the ABO to ṀO2 (71-75% of the total). These adjustments were quickly reversed upon restoration of aquatic normocapnia. During aquatic hypercapnia, ṀCO2 via the ABO became more effective over time, and the pattern of air-breathing changed, exhibiting increased frequency and decreased breath volume. Ammonia-N excretion (86-88% of total) dominated over urea-N excretion and tended to increase during exposure to aquatic hypercapnia. We conclude that the ability of the ABO to take on the dominant role in CO2 excretion when required may have been an important driver in the original evolution of air-breathing, as well as in the functionality of the ABO in modern air-breathing fish.

Modulation of gill surface area does not correlate with oxygen loss in Chitala ornata

Air-breathing fish risk losing aerially sourced oxygen to ambient hypoxic water since oxygenated blood from the air-breathing organ returns through the heart to the branchial basket before distribution. This loss is thought to help drive the evolutionary reduction in gill size with the advent of air-breathing. In many teleost fish, gill size is known to be highly plastic by modulation of their anatomic diffusion factor (ADF) with inter-lamellar cell mass (ILCM). In the anoxia-tolerant crucian carp, ILCM recedes with hypoxia but regrows in anoxia. The air-breathing teleost Chitala ornata has been shown to increase gill ADF from normoxic to mildly hypoxic water by reducing ILCM. Here, we test the hypothesis that ADF is modulated to minimize oxygen loss in severe aquatic hypoxia by measuring ADF, gas-exchange, and by using computed tomography scans to reveal possible trans-branchial shunt vessels. Contrary to our hypothesis, ADF does not modulate to prevent oxygen loss and despite no evident trans-branchial shunting, C. ornata loses only 3% of its aerially sourced O2 while still excreting 79% of its CO2 production to the severely hypoxic water. We propose this is achieved by ventilatory control and by compensating the minor oxygen loss by extra aerial O2 uptake.

Air-breathing behavior in Heterotis niloticus fingerlings: Response to changes in oxygen, temperature, and exercise regimes

Air-breathing in fish is believed to have arisen as an adaptation to aquatic hypoxia. Although air-breathing has been widely studied in numerous fish species, little is known about the obligate air-breathing African bonytongue, Heterotis niloticus. We evaluated if abiotic factors and physical activity affect air-breathing patterns in fingerlings. The air-breathing frequency (fAB ) and behavioral responses of H. niloticus fingerlings were assessed in response to environmental oxygen, temperature, and exhaustion and activity in a series of experiments. The air-breathing behavior of H. niloticus fingerlings under optimum water conditions was characterized by swift excursions lasting less than 1 s to the air-water interface to gulp air. The intervals between air-breaths were highly variable, ranging from 3 to 259 s. Body size only slightly affected fAB , while hypoxia, hyperthermia, and exercise stress significantly increased fAB . Progressive hypoxia from 17.69 to 2.17 kPa caused a ~2.5-fold increase in fAB . Increasing temperatures to 27 and 32°C, from a baseline temperature of 22°C, significantly increased fAB from 0.4 ± 0.2 to 1.3 ± 0.5 and 1.6 ± 0.4 breaths min-1 , respectively. Lastly, following exhaustive exercise, fAB increased up to 3-fold. These observations suggest that H. niloticus fingerlings are very reliant on aerial oxygen, and their air-breathing behavior is sensitive to environmental changes and activity levels.

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

In hypoxia, air-breathing fish obtain O2 from the air but continue to excrete CO2 into the water. Consequently, it is believed that some O2 obtained by air-breathing is lost at the gills in hypoxic water. Pangasionodon hypophthalmus is an air-breathing catfish with very large gills from the Mekong River basin where it is cultured in hypoxic ponds. To understand how P. hypophthalmus can maintain high growth in hypoxia with the presumed O2 loss, we quantified respiratory gas exchange in air and water. In severe hypoxia (PO2: ≈ 1.5 mmHg), it lost a mere 4.9% of its aerial O2 uptake, while maintaining aquatic CO2 excretion at 91% of the total. Further, even small elevations in water PO2 rapidly reduced this minor loss. Charting the cardiovascular bauplan across the branchial basket showed four ventral aortas leaving the bulbus arteriosus, with the first and second gill arches draining into the dorsal aorta while the third and fourth gill arches drain into the coeliacomesenteric artery supplying the gut and the highly trabeculated respiratory swim-bladder. Substantial flow changes across these two arterial systems from normoxic to hypoxic water were not found. We conclude that the proposed branchial oxygen loss in air-breathing fish is likely only a minor inefficiency.

Ontogeny of hemoglobin‑oxygen binding and multiplicity in the obligate air-breathing fish Arapaima gigas

The evolutionary and ontogenetic changes from water- to air-breathing result in major changes in the cardiorespiratory systems. However, the potential changes in hemoglobin's (Hb) oxygen binding properties during ontogenetic transitions to air-breathing remain poorly understood. Here we investigated Hb multiplicity and O₂ binding in hemolysates and Hb components from juveniles and adults of the obligate air-breathing pirarucu (Arapaima gigas) that starts life as water-breathing hatchlings. Contrasting with previous electrophoresis studies that report one or two isoHbs in adults, isoelectric focusing (IEF) resolved the hemolysates from both stages into four major bands, which exhibited identical O₂ binding properties (i.e. O₂ affinities, cooperativity coefficients, and sensitivities to pH and the major organic phosphate effectors), also as compared to the cofactor-free hemolysates. Of note, the multiplicity pattern recurred upon reanalyses of the most-abundant fractions isolated from the juvenile and the adult stages, suggesting possible stabilization of different quaternary states with different isoelectric points during the purification procedure. The study demonstrates unchanged Hb-O₂ binding properties during development, despite the pronounced differences in O₂ availability between the two media, which harmonizes with findings based on a broader spectrum of interspecific comparisons. Taken together, these results disclose that obligate air-breathing in Arapaima is not contingent upon changes in Hb multiplicity and O₂ binding characteristics.

Global change and physiological challenges for fish of the Amazon today and in the near future

Amazonia is home to 15% (>2700, in 18 orders) of all the freshwater fish species of the world, many endemic to the region, has 65 million years of evolutionary history and accounts for 20% of all freshwater discharge to the oceans. These characteristics make Amazonia a unique region in the world. We review the geological history of the environment, its current biogeochemistry and the evolutionary forces that led to the present endemic fish species that are distributed amongst three very different water types: black waters [acidic, ion-poor, rich in dissolved organic carbon (DOC)], white waters (circumneutral, particle-rich) and clear waters (circumneutral, ion-poor, DOC-poor). The annual flood pulse is the major ecological driver for fish, providing feeding, breeding and migration opportunities, and profoundly affecting O2, CO2 and DOC regimes. Owing to climate change and other anthropogenic pressures such as deforestation, pollution and governmental mismanagement, Amazonia is now in crisis. The environment is becoming hotter and drier, and more intense and frequent flood pulses are now occurring, with greater variation between high and low water levels. Current projections are that Amazon waters of the near future will be even hotter, more acidic, darker (i.e. more DOC, more suspended particles), higher in ions, higher in CO2 and lower in O2, with many synergistic effects. We review current physiological information on Amazon fish, focusing on temperature tolerance and ionoregulatory strategies for dealing with acidic and ion-poor environments. We also discuss the influences of DOC and particles on gill function, the effects of high dissolved CO2 and low dissolved O2, with emphasis on water- versus air-breathing mechanisms, and strategies for pH compensation. We conclude that future elevations in water temperature will be the most critical factor, eliminating many species. Climate change will likely favour predominantly water-breathing species with low routine metabolic rates, low temperature sensitivity of routine metabolic rates, high anaerobic capacity, high hypoxia tolerance and high thermal tolerance.