High blood oxygen affinity in the air-breathing swamp eel Monopterus albus

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.

Evolution of hemoglobin function in tropical air-breathing catfishes

The evolution of hemoglobin function in the transition from water- to air-breathing has been highly debated but remains unresolved. Here, we characterized the hemoglobin function in five closely related water- and air-breathing catfishes. We identify distinct directions of hemoglobin evolution in the clades that evolved air-breathing, and we show strong selection on hemoglobin function within the catfishes. These findings show that the lack of a general direction in hemoglobin function in the transition from water- to air-breathing may have resulted from divergent selection on hemoglobin function in independent clades of air-breathing fishes.

A novel acidification mechanism for greatly enhanced oxygen supply to the fish retina

Previously, we showed that the evolution of high acuity vision in fishes was directly associated with their unique pH-sensitive hemoglobins that allow O₂ to be delivered to the retina at PO₂s more than ten-fold that of arterial blood (Damsgaard et al., 2019). Here, we show strong evidence that vacuolar-type H⁺-ATPase and plasma-accessible carbonic anhydrase in the vascular structure supplying the retina act together to acidify the red blood cell leading to O₂ secretion. In vivo data indicate that this pathway primarily affects the oxygenation of the inner retina involved in signal processing and transduction, and that the evolution of this pathway was tightly associated with the morphological expansion of the inner retina. We conclude that this mechanism for retinal oxygenation played a vital role in the adaptive evolution of vision in teleost fishes.

Evolutionary and cardio-respiratory physiology of air-breathing and amphibious fishes

Air-breathing and amphibious fishes are essential study organisms to shed insight into the required physiological shifts that supported the full transition from aquatic water-breathing fishes to terrestrial air-breathing tetrapods. While the origin of air-breathing in the evolutionary history of the tetrapods has received considerable focus, much less is known about the evolutionary physiology of air-breathing among fishes. This review summarizes recent advances within the field with specific emphasis on the cardiorespiratory regulation associated with air-breathing and terrestrial excursions, and how respiratory physiology of these living transitional forms are affected by development and personality. Finally, we provide a detailed and re-evaluated model of the evolution of air-breathing among fishes that serves as a framework for addressing new questions on the cardiorespiratory changes associated with it. This review highlights the importance of combining detailed studies on piscine air-breathing model species with comparative multi-species studies, to add an additional dimension to our understanding of the evolutionary physiology of air-breathing in vertebrates.

Oxygenation properties of hemoglobin and the evolutionary origins of isoform multiplicity in an amphibious air-breathing fish, the blue-spotted mudskipper (Boleophthalmus pectinirostris)

Among the numerous lineages of teleost fish that have independently transitioned from obligate water breathing to facultative air breathing, evolved properties of hemoglobin (Hb)-O₂ transport may have been shaped by the prevalence and severity of aquatic hypoxia (which influences the extent to which fish are compelled to switch to aerial respiration) as well as the anatomical design of air-breathing structures and the cardiovascular system. Here, we examined the structure and function of Hbs in an amphibious, facultative air-breathing fish, the blue-spotted mudskipper (Boleophthalmus pectinirostris). We also characterized the genomic organization of the globin gene clusters of the species and we integrated phylogenetic and comparative genomic analyses to unravel the duplicative history of the genes that encode the subunits of structurally distinct mudskipper Hb isoforms (isoHbs). The B. pectinirostris isoHbs exhibit high intrinsic O₂ affinities, similar to those of hypoxia-tolerant, water-breathing teleosts, and remarkably large Bohr effects. Genomic analysis of conserved synteny revealed that the genes that encode the α-type subunits of the two main adult isoHbs are members of paralogous gene clusters that represent products of the teleost-specific whole-genome duplication. Experiments revealed no appreciable difference in the oxygenation properties of co-expressed isoHbs in spite of extensive amino acid divergence between the alternative α-chain subunit isoforms. It therefore appears that the ability to switch between aquatic and aerial respiration does not necessarily require a division of labor between functionally distinct isoHbs with specialized oxygenation properties.

Multiplicity and Polymorphism of Fish Hemoglobins

The diversity of fish hemoglobins and the association with oxygen availability and physiological requirements during the life cycle has attracted scientists since the first report on multiple hemoglobin in fishes (Buhler and Shanks 1959). The functional heterogeneity of the fish hemoglobins enables many species to tolerate hypoxic conditions and exhausting swimming, but also to maintain the gas pressure in the swim bladder at large depths. The hemoglobin repertoire has further increased in various species displaying polymorphic hemoglobin variants differing in oxygen binding properties. The multiplicity of fish hemoglobins as particularly found in the tetraploid salmonids strongly contrasts with the complete loss of hemoglobins in Antarctic icefishes and illustrates the adaptive radiation in the oxygen transport of this successful vertebrate group.

Systematic evaluation of the gut microbiome of swamp eel (Monopterus albus) by 16S rRNA gene sequencing

Background

The swamp eel (Monopterus albus) is a commercially important farmed species in China. The dysbiosis and homeostasis of gut microbiota has been suggested to be associated with the swamp eel’s disease pathogenesis and food digestion. Although the contributions of gut microbiome in fish growth and health has been increasingly recognized, little is known about the microbial community in the intestine of the swamp eel (Monopterus albus).

Methods

The intestinal microbiomes of the five distinct gut sections (midgut content and mucosa, hindgut content and mucosa, and stools) of swamp eel were compared using Illumina MiSeq sequencing of the bacterial 16S rRNA gene sequence and statistical analysis.

Results

The results showed that the number of observed OTUs in the intestine decreased proximally to distally. Principal coordinate analysis revealed significant separations among samples from different gut sections. There were 54 core OTUs shared by all gut sections and 36 of these core OTUs varied significantly in their abundances. Additionally, we discovered 66 section-specific enriched KEGG pathways. These section-specific enriched microbial taxa (e.g., Bacillus, Lactobacillus) and potential function capacities (e.g., amino acid metabolism, carbohydrate metabolism) might play vital roles in nutrient metabolism, immune modulation and host-microbe interactions of the swamp eel.

Conclusions

Our results showed that microbial diversity, composition and function capacity varied substantially across different gut sections. The gut section-specific enriched core microbial taxa and function capacities may perform important roles in swamp eel’s nutrient metabolism, immune modulation, and host-microbe interactions. This study should provide insights into the gut microbiome of the swamp eel.

Learning to Air-Breathe: The First Steps

Air-breathing in vertebrates has evolved many times among the bony fish while in water. Its appearance has had a fundamental impact on the regulation of ventilation and acid-base status. We review the physico-chemical constraints imposed by water and air, place the extant air-breathing fish into this framework, and show how that the advantages of combining control of ventilation and acid-base status are only available to the most obligate of air-breathing fish, thus highlighting promising avenues for research.

Very high blood oxygen affinity and large Bohr shift differentiates the air-breathing siamese fighting fish (Betta splendens) from the closely related anabantoid the blue gourami (Trichopodus trichopterus)

The Siamese fighting fish, Betta splendens, and the blue gourami, Trichopodus trichopterus, are two closely related air-breathing anabantoid fishes. B. splendens is a sedentary facultative air breather frequenting often hypoxic waters, while T. trichopterus is a more active obligatory air-breather inhabiting better oxygenated waters. Despite their close taxonomic relationship, previous studies have shown inter-specific differences in both physiological and morphological plasticity. Consequently, we hypothesized that B. splendens would have the higher blood oxygen affinity characteristics typical of more hypoxia-tolerant fishes. Whole blood oxygen equilibrium curves were determined at 27 °C and pHs of 7.62, 7.44 and 7.25. At a pH of 7.62, the blood O₂ affinity (P₅₀) of B. splendens was just 2.9 mmHg, while that of T. trichopterus was ~5 times higher at 14.7 mmHg. There were no significant differences in P₅₀ between males and females in either species. The Bohr coefficient in B. splendens and T. trichopterus was -1.79 and - 0.83, respectively. B. splendens, unlike T. trichopterus, showed a large Root effect. Hills cooperatively coefficient, n, was ~2 in both species, indicating a significant binding cooperative between oxygen and hemoglobin. Collectively, these differences in blood O₂ transport characteristics in these two closely related species are likely correlated with the differing habitats in which they breed and inhabit as adults, as well as different activity levels. Finally, the very high blood O₂ affinity of B. splendens is not extraordinary among air-breathing fish, as revealed by a review of the literature of blood oxygen affinity in air-breathing fishes.

Acid-base regulation in the air-breathing swamp eel (Monopterus albus) at different temperatures

Vertebrates reduce arterial blood pH (pHa) when body temperature increases. In water-breathers this response occurs primarily by reducing plasma HCO3− levels with small changes in the partial pressure of CO2 (PCO2). In contrast, air-breathers mediate the decrease in pHa by increasing arterial PCO2 (PaCO2) at constant plasma HCO3− by reducing lung ventilation relative to metabolic CO2 production. Much less is known in bimodal breathers that utilize both water and air. Here, we characterize the influence of temperature on arterial acid-base balance and intracellular pH (pHi) in the bimodal breathing swamp eel, Monopterus albus. This teleost uses the buccopharyngeal cavity for gas exchange and has very reduced gills. When exposed to ecologically relevant temperatures (20, 25, 30 and 35°C) for 24 and 48h, pHa decreased by -0.025 pH units/°C (U/°C) in association with an increased PaCO2, but without changes in plasma [HCO3−]. Intracellular pH (pHi) was also reduced with increased temperature. The slope of pHi of liver and muscle was -0.014 and -0.019 U/°C, while the heart muscle showed a smaller reduction (-0.008U/°C). When exposed to hypercapnia (7 or 14 mmHg) at either 25 or 35°C, Monopterus albus elevated plasma [HCO3−] and therefore seemed to defend the new pHa set-point, demonstrating an adjusted control of acid-base balance with temperature. Overall, the effects of temperature on acid-base balance in Monopterus albus resemble air-breathing amniotes, and we discuss the possibility that this pattern of acid-base balance results from a progressive transition in CO2 excretion from water to air as temperature rises.

Increased temperature tolerance of the air-breathing Asian swamp eel Monopterus albus after high-temperature acclimation is not explained by improved cardiorespiratory performance

This study investigated the hypothesis that in the Asian swamp eel Monopterus albus, an air-breathing fish from south-east Asia that uses the buccopharyngeal cavity for oxygen uptake, the upper critical temperature (TU) is increased by acclimation to higher temperature, and that the increased TU is associated with improved cardiovascular and respiratory function. Monopterus albus were therefore acclimated to 27° C (current average) and 32° C (current maximum temperature as well as projected average within 100-200 years), and both the effect of acclimation and acute temperature increments on cardiovascular and respiratory functions were investigated. Two weeks of heat acclimation increased upper tolerated temperature (TU ) by 2° C from 36·9 ± 0·1° C to 38·9 ± 0·1° C (mean ± s.e.). Oxygen uptake (M˙O2) increased with acclimation temperature, accommodated by increases in both aerial and aquatic respiration. Overall, M˙O2 from air (M˙O2a ) was predominant, representing 85% in 27° C acclimated fish and 80% in 32° C acclimated fish. M˙O2 increased with acute increments in temperature and this increase was entirely accommodated by an increase in air-breathing frequency and M˙O2a . Monopterus albus failed to upregulate stroke volume; rather, cardiac output was maintained through increased heart rate with rising temperature. Overall, acclimation of M. albus to 32° C did not improve its cardiovascular and respiratory performance at higher temperatures, and cardiovascular adaptations, therefore, do not appear to contribute to the observed increase in TU.

High affinity and temperature sensitivity of blood oxygen binding in Pangasianodon hypophthalmus due to lack of chloride-hemoglobin allosteric interaction

Air-breathing fishes represent interesting organisms in terms of understanding the physiological changes associated with the terrestrialization of vertebrates, and, further, are of great socio-economic importance for aquaculture in Southeast Asia. To understand how environmental factors, such as high temperature, affect O2 transport in air-breathing fishes, this study assessed the effects of temperature on O2 binding of blood and Hb in the economically important air-breathing fish Pangasianodon hypophthalmus. To determine blood O2 binding properties, blood was drawn from resting cannulated fishes and O2 binding curves made at 25°C and 35°C. To determine the allosteric regulation and thermodynamics of Hb O2 binding, Hb was purified, and O2 equilibria were recorded at five temperatures in the absence and presence of ATP and Cl(-). Whole blood had a high O2 affinity (O2 tension at half saturation P50 = 4.6 mmHg at extracellular pH 7.6 and 25°C), a high temperature sensitivity of O2 binding (apparent heat of oxygenation ΔH(app) = -28.3 kcal/mol), and lacked a Root effect. Further, the data on Hb revealed weak ATP binding and a complete lack of Cl(-) binding to Hb, which, in part, explains the high O2 affinity and high temperature sensitivity of blood O2 binding. This study demonstrates how a potent mechanism for increasing O2 affinity is linked to increased temperature sensitivity of O2 transport and provides a basic framework for a better understanding of how hypoxia-adapted species will react to increasing temperatures.

Air breathing in the Arctic: influence of temperature, hypoxia, activity and restricted air access on respiratory physiology of the Alaska blackfish Dallia pectoralis

The Alaska blackfish (Dallia pectoralis) is an air-breathing fish native to Alaska and the Bering Sea islands, where it inhabits lakes that are ice-covered in the winter, but enters warm and hypoxic waters in the summer to forage and reproduce. To understand the respiratory physiology of this species under these conditions and the selective pressures that maintain the ability to breathe air, we acclimated fish to 5°C and 15°C and used respirometry to measure: standard oxygen uptake (Ṁ(O₂)) in normoxia (19.8 kPa P(O₂)) and hypoxia (2.5 kPa), with and without access to air; partitioning of standard Ṁ(O₂) in normoxia and hypoxia; maximum Ṁ(O₂) and partitioning after exercise; and critical oxygen tension (P(crit)). Additionally, the effects of temperature acclimation on haematocrit, haemoglobin oxygen affinity and gill morphology were assessed. Standard Ṁ(O₂) was higher, but air breathing was not increased, at 15°C or after exercise at both temperatures. Fish acclimated to 5°C or 15°C increased air breathing to compensate and fully maintain standard Ṁ(O₂) in hypoxia. Fish were able to maintain Ṁ(O₂) through aquatic respiration when air was denied in normoxia, but when air was denied in hypoxia, standard Ṁ(O₂) was reduced by ∼30-50%. P(crit) was relatively high (5 kPa) and there were no differences in P(crit), gill morphology, haematocrit or haemoglobin oxygen affinity at the two temperatures. Therefore, Alaska blackfish depends on air breathing in hypoxia and additional mechanisms must thus be utilised to survive hypoxic submergence during the winter, such as hypoxia-induced enhancement in the capacities for carrying and binding blood oxygen, behavioural avoidance of hypoxia and suppression of metabolic rate.