The transition from water-breathing to air-breathing is associated with a shift in ion uptake from gills to gut: a study of two closely related erythrinid teleosts, Hoplerythrinus unitaeniatus and Hoplias malabaricus

Gill structure and respiratory ability of Euchiloglanis kishinouyei (Osteichthyes: Siluriformes: Sisoridae)

Glyptosternoid fishes are distributed in the torrent environment of alpine canyons, where they often leave the water to climb rocky cliffs. As one of the most primitive species of glyptosternoid fishes, Euchiloglanis kishinouyei was examined in the current study to analyse its gill microstructure and respiratory ability. We first found that the oxygen consumption rate was relatively high and negatively correlated with body mass and that the average oxygen consumption at night was higher than during the day. The asphyxiation point of E. kishinouyei (5.05 ± 0.22 g) was c. 1.93 mg/L. Subsequently, the surface morphology, gross gill tissue structure, and ultra-microstructure of gill lamellae were investigated using optical microscopy and SEM. The gills showed an overall trend of regression, with five pairs of gill arches in each gill cavity. The adjacent gill filaments had large gaps, and the gill lamellae were thick. The gill filaments were closely arranged on the gill arches, their folded respiratory surface was highly vascularized with no tiny crest, and there were obvious tiny crests, grooves, pits, and pores on the nonrespiratory surface. The gill lamellae were closely embedded on both sides of gill filaments, which were composed of flat epithelial cells, basement membrane, pillar cells, and mucous cells. The gill total respiratory area correlated positively with body mass and length, whereas the gill relative respiratory area correlated negatively with body mass. We comprehensively analysed the gill microstructure and respiratory capacity of E. kishinouyei to provide fundamental data for the adaptive evolution of the gill structures of bimodally respiring fishes and offer insights into further study on the accessory air-breathing function of skin.

The physiological consequences of a very large natural meal in a voracious marine fish, the staghorn sculpin (Leptocottus armatus)

Little information exists on physiological consequences when wild fish eat natural food. Staghorn sculpins at 10-13°C voluntarily consumed 15.8% of their body mass in anchovies. Gastric clearance was slow with >60% of the meal retained in the stomach at 48 h, and was not complete until 84 h. At 14-24 h post-feeding, pH was depressed by 3 units and Cl- concentration was elevated 2-fold in gastric chyme, reflecting HCl secretion, while in all sections of the intestine, pH declined by 1 pH unit but Cl- concentration remained unchanged. PCO2 and total ammonia concentration were greatly elevated throughout the tract, whereas PNH3 and HCO3- concentration were depressed. Intestinal HCO3- secretion rates, measured in gut sacs in vitro, were also lower in fed fish. Whole-animal O2 consumption rate was elevated approximately 2-fold for 72 h post-feeding, reflecting 'specific dynamic action', whereas ammonia and urea-N excretion rates were elevated about 5-fold. Arterial blood exhibited a modest 'alkaline tide' for about 48 h, but there was negligible excretion of metabolic base to the external seawater. PaCO2 and PaO2 remained unchanged. Plasma total amino acid concentration and total lipid concentration were elevated about 1.5-fold for at least 48 h, whereas small increases in plasma total ammonia concentration, PNH3 and urea-N concentration were quickly attenuated. Plasma glucose concentration remained unchanged. We conclude that despite the very large meal, slow processing with high efficiency minimizes internal physiological disturbances. This differs greatly from the picture provided by previous studies on aquacultured species using synthetic diets and/or force-feeding. Questions remain about the role of the gastro-intestinal microbiome in nitrogen and acid-base metabolism.

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.

Endogenic upregulations of HIF/VEGF signaling pathway genes promote air breathing organ angiogenesis in bimodal respiration fish

Air-breathing has evolved independently serval times with a variety of air-breathing organs (ABOs) in fish. The physiology of the air-breathing in bimodal respiration fish has been well understood, while studies on molecular mechanisms of the character are very limited. In the present study, we first determined the gill indexes of 110 fish species including 25 and 85 kinds of bimodal respiration fishes and non-air-breathing fishes, respectively. Then combined with histological observations of gills and ABOs/non-ABOs in three bimodal respiration fishes and two non-air breathing fishes, we found that the bimodal respiration fish was always of a degeneration gill and a well-vascularized ABO. Meanwhile, a comparative transcriptome analysis of posterior intestines, namely a well vascularized ABO in Misgurnus anguillicaudatus and a non-ABO in Leptobotia elongata, was performed to expound molecular variations of the air-breathing character. A total of 5,003 orthologous genes were identified. Among them, 1,189 orthologous genes were differentially expressed, which were enriched in 14 KEGG pathways. More specially, the expressions of hemoglobin genes and various HIF/VEGF signaling pathway genes were obviously upregulated in the ABO of M. anguillicaudatus. Moreover, we found that HIF-1α, VEGFAa, and MAP2K1 were co-expressed dramatically higher in ABOs of bimodal respiration fishes than those of non-ABOs of non-air-breathing fishes. These results indicated that the HIF/VEGF pathway played an important role in ABO angiogenesis/formation to promote fish to do aerial respiration. This study will contribute to our understanding of molecular mechanisms of air-breathing in fish.

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 effects of dissolved organic carbon on the reflex ventilatory responses of the neotropical teleost (Colossoma macropomum) to hypoxia or hypercapnia

The tambaqui (Colossoma macropomum), migrates annually between whitewater and blackwater rivers of the Amazon. Unlike the whitewater, blackwater is characterized by higher levels of dissolved organic carbon (DOC), including humic acids (HA). Because humic substances impair sensory processes, the current study tested the hypothesis that O₂ and/or CO₂ chemoreception is impeded in blackwater owing to the presence of HA. Thus, the ventilatory responses of tambaqui to hypoxia or hypercapnia were assessed in well water transported from Manaus, local blackwater, and in well water containing HA either extracted from Rio Negro water or obtained commercially (Sigma Aldrich; SA). In well water, tambaqui exhibited typical hyperventilatory responses to hypoxia or hypercapnia. These responses were prevented by simultaneously exposing fish to SA HA (20 mg l^-1). The negative effects of SA HA on ventilation were prevented when natural DOC (30 mg l^-1; extracted from Rio Negro water after first removing the endogenous HA fraction) was added concurrently, indicating a protective effect of this non-humic acid DOC fraction. The hyperventilatory responses were unaffected during acute exposure or after acclimation of fish to Rio Negro water. HA extracted from Rio Negro water did not impair the hyperventilatory responses to hypoxia or hypercapnia. This study, while demonstrating a negative effect of SA HA derived from peat (coal) on the control of breathing in tambaqui, failed to reveal any detrimental consequences of HA (derived from the decomposition of a variety of lignin-rich plants) naturally occurring in the blackwaters of the Rio Negro.

Effect of hypoxia and air-breathing restricted on respiratory physiology of air-breathing loach (Paramisgurnus dabryanus)

This aim of this study was to determine the respiratory physiology response in the gill and gut of Paramisgurnus dabryanus under different breathing treatment patterns. The experimental design included the following three conditions: a control group without any stress treatments, an inhibited group with intestinal respiration inhibited, and an air-exposed group with gill respiration inhibited. The results indicated that the total static metabolic rate in the air-exposed group (188.92 ± 13.67 mg h^-1 kg^-1) was much higher than that of the other group after 7 days, decreased significantly after the first day of recovery (81.64 ± 7.85 mg h^-1 kg^-1). The air metabolic rate in the air-exposed group increased significantly after 7 days (P < 0.05). There was no significant difference among the groups. Histological observation on the gill and hindgut of P. dabryanus showed that the gill filament area of inhibited group became larger, while the gill structure of air exposed group showed some damage. The number of capillariesin the hindgut mucosal epithelial in air-exposed group showed a rapidly increase (P < 0.05). Likewise, the gas diffusion distance (1.24 ± 0.36 μm) became significantly shorter (P < 0.05). Lactate dehydrogenase activity of gill in the air-exposed group (846.68 ± 88.78 U mg^-1 protein) significantly increased after 7 days whereas succinate dehydrogenase (1.02 ± 0.21 U mg^-1 protein) and Na⁺/K⁺ ATPase (0.57 ± 0.20 U mg^-1 protein) activity decreased significantly (P < 0.05). However, there was no significant change in the hindgut. After recovery, there was no significant difference in lactate dehydrogenase, succinate dehydrogenase, and Na⁺/K⁺ ATPase activity in the gill or hindgut in groups. P. dabryanus had a high viability in air-exposed condition. When recovery occurred under normoxic conditions, the physical levels of respiration returned back to the normal level quickly.

Gills versus kidney for ionoregulation in the obligate air-breathing Arapaima gigas, a fish with a kidney in its air-breathing organ

In Arapaima gigas, an obligate air-breather endemic to ion-poor Amazonian waters, a large complex kidney runs through the air-breathing organ (ABO). Previous indirect evidence suggested that the kidney, relative to the small gills, may be exceptionally important in ionoregulation and nitrogen (N) waste excretion, with support of kidney function by direct O2 supply from the airspace. We tested these ideas by continuous urine collection and gill flux measurements in ∼700 g fish. ATPase activities were many-fold greater in kidney than gills. In normoxia, gill Na+ influx and efflux were in balance, with net losses of Cl- and K+ Urine flow rate (UFR, ∼11 ml kg-1 h-1) and urinary ions (< 0.2 mmol l-1) were exceptional, with [urine]:[plasma] ratios of 0.02-0.002 for K+, Na+, and Cl-, indicating strong reabsorption with negligible urinary ion losses. Urinary [ammonia] was very high (10 mmol l-1, [urine]:[plasma] ∼17) indicating strong secretion. The kidney accounted for 21-24% of N excretion, with ammonia dominating (95%) over urea-N through both routes. High urinary [ammonia] was coupled to high urinary [HCO3-]. Aerial hypoxia (15.3 kPa) and aerial hyperoxia (>40.9 kPa) had no effects on UFR, but both inhibited branchial Na+ influx, revealing novel aspects of the osmorespiratory compromise. Aquatic hypoxia (4.1 kPa), but not aquatic hyperoxia (>40.9 kPa), inhibited gill Na+ influx, UFR and branchial and urinary ammonia excretion. We conclude that the kidney is more important than gills in ionoregulation, and is significant in N excretion. Although not definitive, our results do not indicate direct O2 supply from the ABO for kidney function.

Cellular oxygen consumption, ROS production and ROS defense in two different size-classes of an Amazonian obligate air-breathing fish (Arapaima gigas)

In air-breathing fish a reduction of gill surface area reduces the danger of losing oxygen taken up in the air-breathing organ (ABO) to hypoxic water, but it also reduces the surface area available for ion exchange, so that ion regulation may at least in part be transferred to other organs, like the kidney or the gut. In the air-breathing Arapaima gigas, gill lamellae regress as development proceeds, and starting as a water-breathing embryo Arapaima turns into an obligate air-breathing fish with proceeding development, suggesting that ion regulation is shifted away from the gills as the fish grows. In Arapaima the kidney projects medially into the ABO and thus, probably a unique situation among fishes, is in close contact to the gas of the ABO. We therefore hypothesized that the kidney would be predestined to adopt an increased importance for ion homeostasis, because the elevated ATP turnover connected to ion transport can easily be met by aerobic metabolism based on the excellent oxygen supply directly from the ABO. We also hypothesized that in gill tissue the reduced ion regulatory activity should result in a reduced metabolic activity. High metabolic activity and exposure to high oxygen tensions are connected to the production of reactive oxygen species (ROS), therefore the tissues exposed to these conditions should have a high ROS defense capacity. Using in vitro studies, we assessed metabolic activity and ROS production of gill, kidney and ABO tissue, and determined the activity of ROS degrading enzymes in small (~ 5g, 2–3 weeks old) and larger (~ 670 g, 3–4 months old) A. gigas. Comparing the three tissues revealed that kidney tissue oxygen uptake by far exceeded the uptake measured in gill tissue or ABO. ROS production was particularly high in gill tissue, and all three tissues had a high capacity to degrade ROS. Gill tissue was characterized by high activities of enzymes involved in the glutathione pathway to degrade ROS. By contrast, the tissues of the ABO and in particular the kidney were characterized by high catalase activities, revealing different, tissue-specific strategies in ROS defense in this species. Overall the differences in the activity of cells taken from small and larger fish were not as pronounced as expected, while at the tissue level the metabolic activity of kidney cells by far exceeded the activity of ABO and gill cells.

Gills and air-breathing organ in O2 uptake, CO2 excretion, N-waste excretion, and ionoregulation in small and large pirarucu (Arapaima gigas)

In the pirarucu (Arapaima gigas), gill surface area and thus gas exchange capacity of the gills are reduced with proceeding development. It, therefore, is expected that A. gigas, starting as a water breather, progressively turns into an obligate air-breathing fish using an air-breathing organ (ABO) for gas exchange. We assessed the air-breathing activity, O₂ and CO₂ exchange into air and water, ammonia-N and urea-N excretion, ion flux rates, and activities of ion transport ATPases in large versus small pirarucu. We found that even very young A. gigas (4-6 g, 2-3 weeks post-hatch) with extensive gills are air-breathers (18.1 breaths*h^-1) and cover most (63%) of their O₂ requirements from the air whereas 600-700-g animals (about 3-4 months post-hatch), with reduced gills, obtain 75% of their O₂ from the air (10.8 breaths*h^-1). Accordingly, the reduction in gill surface area hardly affected O₂ uptake, but development had a significant effect on aerial CO₂ excretion, which was very low (3%) in small fish and increased to 12% in larger fish, yielding a hyper-allometric scaling coefficient (1.12) in contrast to 0.82-0.84 for aquatic and total CO₂ excretion. Mass-specific ammonia excretion decreased in approximate proportion to mass-specific O₂ consumption as the fish grew, but urea-N excretion dropped from 18% (at 4-6 g) to 8% (at 600-700 g) of total N-excretion; scaling coefficients for all these parameters were 0.70-0.80. Mass-specific sodium influx and efflux rates, as well as potassium net loss rates, departed from this pattern, being greater in larger fish; hyper-allometric scaling coefficients were > 1.0. Gill V-type H⁺ ATPase activities were greater than Na⁺, K⁺-ATPase activities, but levels were generally low and comparable in large and small fish, and similar activities were detected in the ABO. A. gigas is a carnivorous fish throughout its lifecycle, and, despite fasting, protein oxidation accounted for the major portion (61-82%) of aerobic metabolism in both large and small animals. ABO PO₂ and PCO₂ (measured in 600-700-g fish) were quite variable, and aerial hypoxia resulted in lower ABO PO₂ values. Under normoxic conditions, a positive correlation between breath volume and ABP PO₂ was detected, and on average with a single breath more than 50% of the ABO volume was exchanged. ABO PCO₂ values were in the range of 1.95-3.89 kPa, close to previously recorded blood PCO₂ levels. Aerial hypoxia (PO₂ down to 12.65 kPa) did not increase either air-breathing frequency or breath volume.

Renal acid excretion contributes to acid-base regulation during hypercapnia in air-exposed swamp eel (Monopterus albus)

The swamp eel (Monopterus albus) uses its buccal cavity to air breathe, while the gills are strongly reduced. It burrows into mud during the dry season, is highly tolerant of air exposure, and experiences severe hypoxia both in its natural habitat and in aquaculture. To study the ability of M. albus to compensate for respiratory acidosis, we implanted catheters to sample both arterial blood and urine during hypercapnia (4% CO₂) in either water or air, or during whole-animal air exposure. These hypercapnic challenges caused an immediate reduction in arterial pH, followed by progressive compensation through a marked elevation of plasma HCO₃ ^- over the course of 72 h. There was no appreciable rise in urinary acid excretion in fish exposed to hypercapnia in water, although urine pH was reduced and ammonia excretion did increase. In the air-exposed fish, however, hypercapnia was attended by a large elevation of ammonia in the urine and a large rise in titratable acid excretion. The time course of the increased renal acid excretion overlapped with the time period required to elevate plasma HCO₃ ^-, and we estimate that the renal compensation contributed significantly to whole-body acid-base compensation.

Ionoregulatory and oxidative stress issues associated with the evolution of air-breathing

Aquatic areas frequently face hypoxic conditions. In order to get sufficient oxygen to support aerobic metabolism, a number of freshwater fish resort to aerial respiration to supplement gill respiration especially in situations with reduced oxygen availability in the water. In many species a concomitant reduction in gill surface area or in gill perfusion reduces possible loss of aerially acquired oxygen to the water at the gills, but it also compromises the ion regulatory capacity of gill tissue. In consequence, the reduced gill contact area with water requires appropriate compensation to maintain ion and acid-base homeostasis, often with important ramifications for other organs. Associated modifications in the structure and function of the gills themselves, the skin, the gut, the kidney, and the physiology of water exchange and ion-linked acid-base regulation are discussed. In air-breathing fish, the gut may gain particular importance for the uptake of ions. In addition, tissues frequently exposed to environmental air encounter much higher oxygen partial pressures than typically observed in fish tissues. Physostomous fish using the swimbladder for aerial respiration, for example, will encounter aerial oxygen partial pressure at the swimbladder epithelium when frequently gulping air in hypoxic water. Hyperoxic conditions or rapid changes in oxygen partial pressures result in an increase in the production of reactive oxygen species (ROS). Accordingly, in air-breathing fish, strategies of ionoregulation may be greatly modified, and the ROS defense capacity of air-exposed tissues is improved.

Air-breathing behavior, oxygen concentrations, and ROS defense in the swimbladders of two erythrinid fish, the facultative air-breathing jeju, and the non-air-breathing traira during normoxia, hypoxia and hyperoxia

The jeju Hoplerythrinus unitaeniatus and the traira Hopliasmalabaricus are two neighboring genera from the family of erythrinid fish, both possessing a two-chambered physostomous swimbladder. In the jeju the anterior section of the posterior bladder is highly vascularized, and the swimbladder is used for aerial respiration; the traira, in turn, is a water-breather that uses the swimbladder as a buoyancy organ and not for aerial oxygen uptake. Measurement of swimbladder oxygen partial pressure (PO₂) of fish kept at 26 °C in normoxic, hyperoxic (28–32 mg O₂ L^− 1) or hypoxic (1–1.5 mg O₂ L^− 1) water revealed constant values in traira swimbladder. Under normoxic conditions in the jeju swimbladder PO₂ was higher than in traira, and the PO₂ significantly increased under hyperoxic conditions, even in the absence of air breathing. In jeju, air-breathing activity increased significantly under hypoxic conditions. Hypoxic air-breathing activity was negatively correlated to swimbladder PO₂, indicating that the swimbladder was intensely used for gas exchange under these conditions. In traira, the capacity of the ROS defense system, as assessed by measurement of activities of enzymes involved in ROS degradation and total glutathione (GSH + GSSG) concentration, was elevated after 4 h of hyperoxic and/or hypoxic exposure, although swimbladder PO₂ was not affected. In jeju, experiencing a higher variability in swimbladder PO₂ due to the air-breathing activity, only a reduced responsiveness of the ROS defense system to changing environmental PO₂ was detected.