Heart structure in the Amazonian teleost Arapaima gigas (Osteoglossiformes, Arapaimidae)

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.

Ontogenetic changes in the body structure of the Arctic fish Leptoclinus maculatus

Histological studies of the ontogenetic changes in Arctic marine fishes are often fragmented and incomplete. Here we present a comprehensive histological ontogenetic analysis of the daubed shanny (Leptoclinus maculatus) from the Arctic, characterizing its development as it undergoes a series of changes in the organ and tissue organization, especially during the postlarvae transition from the pelagic to benthic lifestyle. The thyroid, heart, digestive tract, liver, gonads, blood, and the lipid sac of the postlarvae at different developmental stages (L1-L5) were studied for the first time. We found that L. maculatus has structural characteristics of marine fish developing in cold, high-oxygen polar waters. We conclude that the presence of the lipid sac and the absence of distinguishable red blood cells in pelagic postlarvae are unique features of the daubed shanny most likely linked to its successful growth and development in the Arctic environment.

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.

Development and evolution of the metazoan heart

The mechanisms of the evolution and development of the heart in metazoans are highlighted, starting with the evolutionary origin of the contractile cell, supposedly the precursor of cardiomyocytes. The last eukaryotic common ancestor is likely a combination of several cellular organisms containing their specific metabolic pathways and genetic signaling networks. During evolution, these tool kits diversified. Shared parts of these conserved tool kits act in the development and functioning of pumping hearts and open or closed circulations in such diverse species as arthropods, mollusks, and chordates. The genetic tool kits became more complex by gene duplications, addition of epigenetic modifications, influence of environmental factors, incorporation of viral genomes, cardiac changes necessitated by air‐breathing, and many others. We evaluate mechanisms involved in mollusks in the formation of three separate hearts and in arthropods in the formation of a tubular heart. A tubular heart is also present in embryonic stages of chordates, providing the septated four‐chambered heart, in birds and mammals passing through stages with first and second heart fields. The four‐chambered heart permits the formation of high‐pressure systemic and low‐pressure pulmonary circulation in birds and mammals, allowing for high metabolic rates and maintenance of body temperature. Crocodiles also have a (nearly) separated circulation, but their resting temperature conforms with the environment. We argue that endothermic ancestors lost the capacity to elevate their body temperature during evolution, resulting in ectothermic modern crocodilians. Finally, a clinically relevant paragraph reviews the occurrence of congenital cardiac malformations in humans as derailments of signaling pathways during embryonic development.

The cardiac regulatory toolkit contains many factors including epigenetic, genetic, viral, hemodynamic, and environmental factors, but also transcriptional activators, repressors, duplicated genes, redundancies and dose‐dependancies.

Numerous toolkits regulate mechanisms including cell‐cell interactions, EMT, mitosis patterns, cell migration and differentiation and left/right sidedness involved in the development of endocardial cushions, looping, septum complexes, pharyngeal arch arteries, chamber and valve formation and conduction system.

Evolutionary development of the yolk sac circulation likely preceded the advent of endothermy in amniotes.

Parallel evolutionary traits regulate the development of contractile pumps in various taxa often in conjunction with the gut, lungs and excretory organs.