The vasculature of the gills in the aquatic and aestivating lungfish (Protopterus aethiopicus)

A single-cell atlas of West African lungfish respiratory system reveals evolutionary adaptations to terrestrialization

The six species of lungfish possess both lungs and gills and are the closest extant relatives of tetrapods. Here, we report a single-cell transcriptome atlas of the West African lungfish (Protopterus annectens). This species manifests the most extreme form of terrestrialization, a life history strategy to survive dry periods that can last for years, characterized by dormancy and reversible adaptive changes of the gills and lungs. Our atlas highlights the cell type diversity of the West African lungfish, including gene expression consistent with phenotype changes of terrestrialization. Comparison with terrestrial tetrapods and ray-finned fishes reveals broad homology between the swim bladder and lung cell types as well as shared and idiosyncratic changes of the external gills of the West African lungfish and the internal gills of Atlantic salmon. The single-cell atlas presented here provides a valuable resource for further exploration of the respiratory system evolution in vertebrates and the diversity of lungfish terrestrialization.

Coronary artery established through amniote evolution

Coronary arteries are a critical part of the vascular system and provide nourishment to the heart. In humans, even minor defects in coronary arteries can be lethal, emphasizing their importance for survival. However, some teleosts survive without coronary arteries, suggesting that there may have been some evolutionary changes in the morphology and function of coronary arteries in the tetrapod lineage. Here, we propose that the true ventricular coronary arteries were newly established during amniote evolution through remodeling of the ancestral coronary vasculature. In mouse (Mus musculus) and Japanese quail (Coturnix japonica) embryos, the coronary arteries unique to amniotes are established by the reconstitution of transient vascular plexuses: aortic subepicardial vessels (ASVs) in the outflow tract and the primitive coronary plexus on the ventricle. In contrast, amphibians (Hyla japonica, Lithobates catesbeianus, Xenopus laevis, and Cynops pyrrhogaster) retain the ASV-like vasculature as truncal coronary arteries throughout their lives and have no primitive coronary plexus. The anatomy and development of zebrafish (Danio rerio) and chondrichthyans suggest that their hypobranchial arteries are ASV-like structures serving as the root of the coronary vasculature throughout their lives. Thus, the ventricular coronary artery of adult amniotes is a novel structure that has acquired a new remodeling process, while the ASVs, which occur transiently during embryonic development, are remnants of the ancestral coronary vessels. This evolutionary change may be related to the modification of branchial arteries, indicating considerable morphological changes underlying the physiological transition during amniote evolution.

Using the swimbladder as a respiratory organ and/or a buoyancy structure-Benefits and consequences

A swimbladder is a special organ present in several orders of Actinopterygians. As a gas-filled cavity it contributes to a reduction in overall density, but on descend from the water surface its contribution as a buoyancy device is very limited because the swimbladder is compressed by increasing hydrostatic pressure. It serves, however, as a very efficient organ for aerial gas exchange. To avoid the loss of oxygen to hypoxic water at the gills many air-breathing fish show a reduced gill surface area. This, in turn, also reduces surface area available for other functions, so that breathing air is connected to a number of physiological adjustments with respect to ion homeostasis, acid-base regulation and nitrogen excretion. Using the swimbladder as a buoyancy structure resulted in the loss of its function as an air-breathing organ and required the development of a gas secreting mechanism. This was achieved via the Root effect and a countercurrent arrangement of the blood supply to the swimbladder. In addition, a detachable air space with separated blood supply was necessary to allow the resorption of gas from the swimbladder. Gas secretion as well as gas resorption are slow phenomena, so that rapid changes in depth cannot instantaneously be compensated by appropriate volume changes. As gas-filled cavities the respiratory swimbladder and the buoyancy device require surfactant. Due to high oxygen partial pressures inside the bladder air-exposed tissues need an effective reactive oxygen species defense system, which is particularly important for a swimbladder at depth.

The NOS/NO system in an example of extreme adaptation: The African lungfish

African dipnoi (lungfish) are aestivating fish and obligate air breathers that, throughout their complex life cycle, undergo remarkable morpho-functional organ readjustment from biochemical to morphological level. In the present review we summarize the changes of the NOS/NO (Nitric Oxide Synthase/Nitric Oxide) system occurring in lungs, gills, kidney, heart, and myotomal muscle of African lungfish of the genus Protopterus (P. dolloi and P. annectens), in relation to the switch from freshwater to aestivation, and vice-versa. In particular, the expression and localization patterns of NOS, and its protein partners Akt, Hsp-90 and HIF-1α, have been discussed, together with the apoptosis rate, evaluated by TUNEL technique. We hypothesize that all these molecular components are crucial in signalling transduction/integration networks induced by environmental challenges (temperature, dehydration, inactivity)experienced at the beginning, during, and at the end of the dry season.

Acute effects of temperature and hypercarbia on cutaneous and branchial gas exchange in the South American lungfish, Lepidosiren paradoxa

The South American lungfish, Lepidosiren paradoxa inhabits seasonal environments in the Central Amazon and Paraná-Paraguay basins that undergo significant oscillations in temperature throughout the year. They rely on different gas exchange organs, such as gills and skin for aquatic gas exchange while their truly bilateral lungs are responsible for aerial gas exchange; however, there are no data available on the individual contributions of the skin and the gills to total aquatic gas exchange in L. paradoxa. Thus, in the present study we quantify the relative contributions of skin and gills on total aquatic gas exchange during warm (35°C) and cold exposure (20°C) in addition to the effects of aerial and aquatic hypercarbia on aquatic gas exchange and gill ventilation rate (f_G; 25°C), respectively. Elevated temperature (35°C) caused a significant increase in the contribution of cutaneous (from 0.61±0.13 to 1.34±0.26ml. STPD.h^-1kg^-1) and branchial (from 0.54±0.17 to 1.73±0.53ml. STPD.h^-1kg^-1) gas exchange for V̇CO₂ relative to the lower temperature (20°C), while V̇O₂ remained relatively unchanged. L. paradoxa exhibited a greater branchial contribution in relation to total aquatic gas exchange at lower temperatures (20 and 25°C) for oxygen uptake. Aerial hypercarbia decreased branchial V̇O₂ whereas branchial V̇CO₂ was significantly increased. Progressive increases in aquatic hypercarbia did not affect f_G. This response is in contrast to increases in pulmonary ventilation that may offset any increase in arterial partial pressure of CO₂ owing to CO₂ loading through the animals' branchial surface. Thus, despite their reduced contribution to total gas exchange, cutaneous and branchial gas exchange in L. paradoxa can be significantly affected by temperature and aerial hypercarbia.

Gill remodelling and growth rate of striped catfish Pangasianodon hypophthalmus under impacts of hypoxia and temperature

Gill morphometric and gill plasticity of the air-breathing striped catfish (Pangasianodon hypophthalmus) exposed to different temperatures (present day 27°C and future 33°C) and different air saturation levels (92% and 35%) during 6weeks were investigated using vertical sections to estimate the respiratory lamellae surface areas, harmonic mean barrier thicknesses, and gill component volumes. Gill respiratory surface area (SA) and harmonic mean water - blood barrier thicknesses (HM) of the fish were strongly affected by both environmental temperature and oxygen level. Thus initial values for 27°C normoxic fish (12.4±0.8g) were 211.8±21.6mm²g^-1 and 1.67±0.12μm for SA and HM respectively. After 5weeks in same conditions or in the combinations of 33°C and/or PO₂ of 55mmHg, this initial surface area scaled allometrically with size for the 33°C hypoxic group, whereas branchial SA was almost eliminated in the 27°C normoxic group, with other groups intermediate. In addition, elevated temperature had an astounding effect on growth with the 33°C group growing nearly 8-fold faster than the 27°C fish.

Ionoregulatory physiology of two species of African lungfishes Protopterus dolloi and Protopterus annectens

Basic ionoregulatory physiology was characterized in two species of African lungfish, slender African lungfish Protopterus dolloi and West African lungfish Protopterus annectens, largely under aquatic conditions. There were no substantive differences between the two species. Plasma [Na], [Cl] and [Ca] were only 60-80% of those typical of freshwater teleosts, and plasma Ca activity was particularly low. Unidirectional Na and Cl influx rates from water were also very low, only c. 10% of teleost values, whereas unidirectional Ca influx rates were comparable with teleost rates. Protopterus spp. were fed a 3% ration of bloodworms every 48 h. The bloodworm diet provided similar amounts of Na and Ca as uptake from water, but almost no Cl. Efflux rates of Na and Cl through the urine were greater than via the faeces, whereas the opposite was true for Ca. Net ion flux measurements and ionic balance sheet calculations indicated that (1) both water and dietary uptake routes are important for Na and Ca acquisition; (2) the waterborne route predominates for Cl uptake; (3) unidirectional ion effluxes across the body surface (gills and skin) rather than urine and faeces are the major routes of loss for Na, Cl and Ca. Tissues (muscle, liver, lung, kidney, intestine and heart) and plasma ions were also examined in P. dolloi'terrestrialized' in air for up to 5 months, during which plasma ion concentrations (Na, Cl, Ca and Mg) did not change and there were only a few alterations in tissue ions, that is, increased [Na] in intestine, decreased [Cl] in kidney and increased [Ca] in liver and kidney.

The African Lungfish (Protopterus dolloi): Ionoregulation and Osmoregulation in a Fish out of Water

Although urea production and metabolism in lungfish have been thoroughly studied, we have little knowledge of how internal osmotic and electrolyte balance are controlled during estivation or in water. We tested the hypothesis that, compared with the body surface of teleosts, the slender African lungfish (Protopterus dolloi) body surface was relatively impermeable to water, Na(+), and Cl(-) due to its greatly reduced gills. Accordingly, we measured the tritiated water ((3)H-H(2)O) flux in P. dolloi in water and during air exposure. In water, (3)H-H(2)O efflux was comparable with the lowest measurements reported in freshwater teleosts, with a rate constant (K) of 17.6% body water h(-1). Unidirectional ion fluxes, measured using (22)Na(+) and (36)Cl(-), indicated that Na(+) and Cl(-) influx was more than 90% lower than values reported in most freshwater teleosts. During air exposure, a cocoon formed within 1 wk that completely covered the dorsolateral body surface. However, there were no disturbances to blood osmotic or ion (Na(+), Cl(-)) balance, despite seven- to eightfold increases in plasma urea after 20 wk. Up to 13-fold increases in muscle urea (on a dry-weight basis) were the likely explanation for the 56% increase in muscle water content observed after 20 wk of air exposure. The possibility that muscle acted as a "water reservoir" during air exposure was supported by the 20% decline in body mass observed during subsequent reimmersion in water. This decline in body mass was equivalent to 28 mL water in a 100-g animal and was very close to the calculated net water gain (approximately 32 mL) observed during the 20-wk period of air exposure. Tritiated water and unidirectional ion fluxes on air-exposed lungfish revealed that the majority of water and ion exchange was via the ventral body surface at rates that were initially similar to aquatic rates. The (3)H-H(2)O flux declined over time but increased upon reimmersion. We conclude that the slender lungfish body surface, including the gills, has relatively low permeability to water and ions but that the ventral surface is an important site of osmoregulation and ionoregulation. We further propose that an amphibian-like combination of ventral skin water and ion permeability, plus internal urea accumulation during air exposure, allows P. dolloi to extract water from its surroundings and to store water in the muscle when the water supply becomes limited.

Pulmonary Oxygen Diffusing Capacity of the South American LungfishLepidosiren paradoxa: Physiological Values by the Bohr Method

Lungfish (Dipnoi) may represent the sister group to all land vertebrates and are therefore important for reconstructing the conquest of land by tetrapods. We determined venous and arterial blood gases, pulmonary O(2) uptake, and the form of the hemoglobin-O(2) dissociation curves in the South American lungfish Lepidosiren paradoxa. Measurements were performed at 25 degrees and 35 degrees C. Based on this information, we calculated its pulmonary O(2) diffusing capacity (D(L)O(2)), using the Bohr integration procedure. D(L)O(2) increased with temperature to reach about 0.04 mL stpd kg(-1) min(-1) mmHg(-1) at 35 degrees C. This value represents about 40% of the morphometric diffusing capacity and is similar to physiological values in some amphibians and reptiles.

Ammonia tolerance in the slender lungfish (Protopterus dolloi): the importance of environmental acidification

Protopterus dolloi Boulenger, 1900 is an obligate air-breather and exhibits ammoniotely (88% ammonia-N excretion, 12% urea-N excretion) under normal aquatic conditions, but tolerates 7 days of exposure to 30 mmol·L–1NH4Cl, a treatment fatal to most other fish. Internal N accumulation is minimal and the subsequent washout of ammonia-N and urea-N after return to control conditions is negligible, indicating that N excretion continues and (or) that N metabolism is markedly depressed. Exposure to 30 mmol·L–1NH4Cl in a closed system without aeration results in depressed urea-N excretion. The lungfish greatly acidifies the external water, a volume 25-fold greater than its own volume. The extent of this acidification increases with time. After several days, the external pH falls from about 7.0 to below 5.0 over a 24-h period, thereby markedly reducing the concentration of NH3(the form that diffuses across biological membranes). CO2excretion is partially responsible for this acidification, because vigorous water aeration reduces but does not eliminate the acidification, and urea-N excretion increases moderately. However, a substantial excretion of titratable acid (non-CO2acidity) also occurs. One exceptional lungfish was able to maintain its aerated environment at a stable pH of 3.7. Environmental acidification may be a less costly strategy for avoiding toxicity than detoxifying ammonia by increasing urea production.

Branchial innervation

Inspection of the dorsal end of fish gills reveals an impressive set of nerve trunks, connecting the gills to the brain. These trunks are branches of cranial nerves VII (the facial) and especially IX (the glossopharyngeal) and X (the vagus). The nerve trunks carry a variety of nervous pathways to and from the gills. A substantial fraction of the nerves running in the branchial trunks carry afferent (sensory) information from receptors within the gills. There are also efferent (motor) pathways, which control muscles within the gills, blood flow patterns and possibly secretory functions. Undertaking a more careful survey of the gills, it becomes evident that the arrangement of the microanatomy (particularly the blood vessels) and its innervation are strikingly complex. The complexity not only reflects the many functions of the gills but also illustrates that the control of blood flow patterns in the gills is of crucial importance in modifying the efficiency of its chief functions: gas transfer and salt balance. The "respiratory-osmoregulatory compromise" is maintained by minimizing the blood/water exchange (functional surface area of the gills) to a level where excessive water loss (marine teleosts) or gain (freshwater teleosts) is kept low while ensuring sufficient gas exchange. This review describes the arrangement and mechanisms of known nervous pathways, both afferent and efferent, of fish (notably teleosts) gills. Emphasis is placed primarily on the autonomic nervous system and mechanisms of blood flow control, together with an outline of the afferent (sensory) pathways of the gill arches.

Effects of induced aestivation in Protopterus annectens: a histomorphological study

The aim of this research was to induce, at will in the laboratory, aestivation of the Dipnoan Protopterus annectens, in order to compare the structure of organs in lungfish adapted to aquatic or aestivating conditions. The animals were placed in a glass tank containing warm water, and the bottom of the tank was filled with clay and sand. To start aestivation the water was allowed to slowly evaporate; as soon as the fish was in a dry environment, it began to excavate a hole in the mud and to burrow into it. Scanning electron microscopy and histological techniques compared the morphology of skin, gills, and lungs in aestivating and free-swimming animals. In the aestivating animals, the secondary lamellae of the gills became thick and were covered by mucus that pasted the lamellae together. The epidermis of the skin was thin and composed of layers of flattened cells. In contrast, in free-swimming animals, the secondary lamellae of the gills were widely separated and the epidermis of the skin was thick and contained numerous mucus-laden cells. The lungs, thin bloodless threads in the aquatic conditions, were, in the air-breathing animals, rich in blood and showed thick walls with ridges and pillars that protruded into the lung cavity, producing small alveolar protrusions. The features of the skin and lungs were similar to that of amphibians, testifying to the convergence of some tissue morphology in aquatic animals utilizing land as a cohabitat.

Vasculature of the fish gill: anatomical correlates of physiological functions

The fish gill is a multifunctional organ responsible for respiration, osmoregulation, acid base balance, nitrogen excretion, and metabolism of circulating hormones. Two or more microcirculatory systems subserve these activities and form one of the most complex vascular networks found in any vertebrate. In this article the vascular anatomy of the teleost gill and the role of gill vessels in mediating physiological function are examined. Vascular corrosion replication techniques have been instrumental in resolving the spatial organization of gill microcirculation. Variations in the replication procedures provide information on the interrelationships between the vascular pathways, factors that govern flow distribution, and the physical characteristics of the vessels themselves. Anatomically, gill vessels are as diverse as their physiological functions. Pillar cells, unique to the fish gill, form the lining of the respiratory vasculature and may have substantial metabolic effects on circulating hormones. The non-respiratory pathways appear to be lined with both typical and unusual endothelial cells, although the fine structure and function(s) of these vessels are largely unknown. To date most of the information on gill vessels has been derived from descriptive morphological studies. Further evaluation of the anatomical and physiological correlations of these tissues is predicated upon the application of histocytochemical, morphometric, and other quantitative methodologies as well as an examination of gills from fish with various evolutionary and environmental backgrounds.

Angiotensin-converting enzyme in organs of air-breathing fish

Angiotensin-converting enzyme (ACE) was measured in tissue homogenates from the African lungfish and six species of air-breathing teleosts (Heteropneustes fossilis, Clarias batrachus, Channa gachua, Anabas testudineus, Notopterus chitala, and Monopterus cuchia) using a standard spectrophotometric assay. In most species, the highest levels of ACE activity were found in the respiratory organs (gills and/or accessory respiratory organs). ACE was also found in heart and kidney tissues from most species and occasionally in liver. Converting enzyme was not found in skin or skeletal muscle from any species and only in blood from H. fossilis and brain from C. batrachus. Captopril, a potent inhibitor of mammalian ACE, inhibited enzymatic activity from all tissues except C. gachua heart and liver and A. testudineus heart. As fish make the transition from aquatic to aerial respiration the gill microcirculation is usually reduced in size and the accessory respiratory organs become elaborated and occupy a more central position in the vascular tree. The presence of ACE in accessory respiratory organs of air-breathing fish appears to greatly enhance the metabolic efficiency of this enzyme on circulating substrates.