A unique mode of tissue oxygenation and the adaptive radiation of teleost fishes

The physiological significance of plasma-accessible carbonic anhydrase in the respiratory systems of fishes

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work.

Coping with environmental degradation: Physiological and morphological adjustments of wild mangrove fish to decades of aquaculture-induced nutrient enrichment

The impact of eutrophication on wild fish individuals is rarely reported. We compared physiological and morphological traits of Siganus lineatus chronically exposed to aquaculture-induced eutrophication in the wild with individuals living at a control site. Eutrophication at the impacted site was confirmed by elevated organic matter (up to 150 % higher), phytoplankton (up to 7 times higher), and reduced oxygen (up to 60 % lower). Physiological and morphological traits of S. lineatus differed significantly between the two sites. Fish from the impacted site exhibited elevated hypoxia tolerance, increased gill surface area, shorter oxygen diffusion distances, and altered blood oxygen-carrying capacity. Elevated blood lactate and scope for anaerobic ATP production were observed, suggesting enhanced survival below critical oxygen levels. A significant 8.5 % increase in metabolic costs and altered allometric scaling, related to environmental degradation, were recorded. Our study underscores eutrophication's profound impact at the organism-level and the importance to mitigate it.

Developing rainbow trout (Oncorhynchus mykiss) lose branchial plasma accessible carbonic anhydrase expression with hatch and the transition to pH-sensitive, adult hemoglobin polymorphs

Salmonids possess a unique respiratory system comprised of three major components: highly pH-sensitive hemoglobins, red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of plasma accessible carbonic anhydrase (paCA), specifically with absence of paCA at the gills. These characteristics are thought to have evolved to enhance oxygen unloading to the tissues while protecting uptake at the gills. Our knowledge of this system is detailed in adults, but little is known about it through development. Developing rainbow trout (Oncorhynchus mykiss) express embryonic RBCs containing hemoglobins that are relatively insensitive to pH; however, availability of gill paCA and RBC pHi protection is unknown. We show that pre-hatch rainbow trout express gill paCA, which is lost in correlation with the emergence of highly pH-sensitive adult hemoglobins and RBC pHi protection. Rainbow trout therefore exhibit a switch in respiratory strategy with hatch. We conclude that gill paCA likely represents an embryonic trait in rainbow trout and is constrained in adults due to their highly pH-sensitive hemoglobins.

Warming-induced "plastic floors" improve hypoxia vulnerability, not aerobic scope, in red drum (Sciaenops ocellatus)

Ocean warming is a prevailing threat to marine ectotherms. Recently the "plastic floors, concrete ceilings" hypothesis was proposed, which suggests that a warmed fish will acclimate to higher temperatures by reducing standard metabolic rate (SMR) while keeping maximum metabolic rate (MMR) stable, therefore improving aerobic scope (AS). Here we evaluated this hypothesis on red drum (Sciaenops ocellatus) while incorporating measures of hypoxia vulnerability (critical oxygen threshold; Pcrit) and mitochondrial performance. Fish were subjected to a 12-week acclimation to 20 °C or 28 °C. Respirometry was performed every 4 weeks to obtain metabolic rate and Pcrit; mitochondrial respirometry was performed on liver and heart samples at the end of the acclimation. 28 °C fish had a significantly higher SMR, MMR, and Pcrit than 20 °C controls at time 0, but SMR declined by 36.2 % over the 12-week acclimation. No change in SMR was observed in the control treatment. Contrary to expectations, SMR suppression did not improve AS relative to time 0 owing to a progressive decline in MMR over acclimation time. Pcrit decreased by 27.2 % in the warm-acclimated fishes, which resulted in temperature treatments having statistically similar values by 12-weeks. No differences in mitochondrial traits were observed in the heart - despite a Δ8 °C assay temperature - while liver respiratory and coupling control ratios were significantly improved, suggesting that mitochondrial plasticity may contribute to the reduced SMR with warming. Overall, this work suggests that warming induced metabolic suppression offsets the deleterious consequences of high oxygen demand on hypoxia vulnerability, and in so doing greatly expands the theoretical range of metabolically available habitats for red drum.

An investigation of gill and blood carbonic anhydrase characteristics in three basal actinopterygian species: alligator gar (Atractosteus spatula), white sturgeon (Acipenser transmontanus) and Senegal bichir (Polypterus senegalus)

Many teleosts possess a unique set of respiratory characteristics allowing enhanced oxygen unloading to the tissues during stress. This system comprises three major components: highly pH sensitive haemoglobins (large Bohr and Root effects), rapid red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of membrane-bound plasma-accessible carbonic anhydrase (paCA; absence in the gills). The first two components have received considerable research effort; however, the evolutionary loss of branchial paCA has received little attention. In the current study, we investigated the availability of branchial membrane-bound CA, along with several other CA-related characteristics in species belonging to three basal actinopterygian groups: the Lepisosteiformes, Acipenseriformes and Polypteriformes to assess the earlier hypothesis that Root effect haemoglobins constrain branchial paCA availability. We present the first evidence suggesting branchial membrane-bound CA presence in a basal actinopterygian species: the Senegal bichir (Polypterus senegalus) and show that like the teleosts, white sturgeon (Acipenser transmontanus) and alligator gar (Atractosteus spatula) do not possess branchial membrane-bound CA. We discuss the varying respiratory strategies for these species and propose that branchial paCA may have been lost much earlier than previously thought, likely in relation to the changes in haemoglobin buffer capacity associated with the increasing magnitude of the Bohr effect. The findings described here represent an important advancement in our understanding of the evolution of the unique system of enhanced oxygen unloading thought to be present in most teleosts, a group that encompasses half of all vertebrates.

A novel perspective on the evolutionary loss of plasma-accessible carbonic anhydrase at the teleost gill

The gills of most teleost fishes lack plasma-accessible carbonic anhydrase (paCA) that could participate in CO2 excretion. We tested the prevailing hypothesis that paCA would interfere with red blood cell (RBC) intracellular pH regulation by β-adrenergic sodium-proton exchangers (β-NHE) that protect pH-sensitive haemoglobin-oxygen (Hb-O2) binding during an acidosis. In an open system that mimics the gills, β-NHE activity increased Hb-O2 saturation during a respiratory acidosis in the presence or absence of paCA, whereas the effect was abolished by NHE inhibition. However, in a closed system that mimics the tissue capillaries, paCA disrupted the protective effects of β-NHE activity on Hb-O2 binding. The gills are an open system, where CO2 generated by paCA can diffuse out and is not available to acidifying the RBCs. Therefore, branchial paCA in teleosts may not interfere with RBC pH regulation by β-NHEs, and other explanations for the evolutionary loss of the enzyme must be considered.

Getting to the heart of anatomical diversity and phenotypic plasticity: fish hearts are an optimal organ model in need of greater mechanistic study

Natural selection has produced many vertebrate 'solutions' for the cardiac life-support system, especially among the approximately 30,000 species of fishes. For example, across species, fish have the greatest range for central arterial blood pressure and relative ventricular mass of any vertebrate group. This enormous cardiac diversity is excellent ground material for mechanistic explorations. Added to this species diversity is the emerging field of population-specific diversity, which is revealing that cardiac design and function can be tailored to a fish population's local environmental conditions. Such information is important to conservation biologists and ecologists, as well as physiologists. Furthermore, the cardiac structure and function of an individual adult fish are extremely pliable (through phenotypic plasticity), which is typically beneficial to the heart's function when environmental conditions are variable. Consequently, exploring factors that trigger cardiac remodelling with acclimation to new environments represents a marvellous opportunity for performing mechanistic studies that minimize the genetic differences that accompany cross-species comparisons. What makes the heart an especially good system for the investigation of phenotypic plasticity and species diversity is that its function can be readily evaluated at the organ level using established methodologies, unlike most other organ systems. Although the fish heart has many merits as an organ-level model to provide a mechanistic understanding of phenotypic plasticity and species diversity, bringing this potential to fruition will require productive research collaborations among physiologists, geneticists, developmental biologists and ecologists.

An atlas of plasma-accessible carbonic anhydrase availability in the model teleost, the rainbow trout

The unique teleost oxygenation system that permits enhanced oxygen unloading during stress comprises three main characteristics: pH-sensitive haemoglobin, red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of plasma-accessible carbonic anhydrase (paCA). A heterogeneous distribution of paCA is essential; its presence permits enhanced oxygen unloading during stress, while its absence at the gills maintains conditions for oxygen uptake by pH-sensitive haemoglobins. We hypothesised that paCA would be absent in all four gill arches, as has been previously indicated for arch two, and that paCA would be present in all other tissues. Through a suite of biochemical and molecular methods, we confirmed the absence of paCA from all four arches. We also found evidence for paCA in nine other tissues and a lack of paCA availability in the stomach. Expression was highly variable between tissues and suggests these differences may be associated with their respective metabolic activities. Additionally, we analysed the specific CA-IV isoform expressed within each tissue and showed almost complete separation of expression between tissues; CA-IVa was detected in the heart, brain, anterior intestine, and liver, whereas CA-IVb was detected in all intestine sections, pyloric caeca, kidney, and white muscle. This adds to a growing collection of work suggesting CA-IVa and b play divergent roles in gas exchange and ion/acid-base balance, respectively. The current study represents the first comprehensive atlas of paCA availability within the circulatory system of the model teleost, rainbow trout, and fills important gaps in our knowledge of this unique oxygenation system.

Functional divergence of teleost carbonic anhydrase 4

The functional role of membrane-bound carbonic anhydrases (CAs) has been of keen interest in the past decade, and in particular, studies have linked CA in red muscle, heart, and eye to enhanced tissue oxygen extraction in bony fishes (teleosts). However, the number of purported membrane-bound CA isoforms in teleosts, combined with the imperfect system of CA isoform nomenclature, present roadblocks for ascribing physiological functions to particular CA isoforms across different teleost lineages. Here we developed an organizational framework for membrane-bound CAs in teleosts, providing the latest phylogenetic analysis of extant CA4 and CA4-like isoforms. Our data confirm that there are three distinct isoforms of CA4 (a, b, and c) that are conserved across major teleost lineages, with the exception of CA4c gene being lost in salmonids. Tissue distribution analyses suggest CA4a functions in oxygen delivery across teleost lineages, while CA4b may be specialized for renal acid-base balance and ion regulation. This work provides an important foundation for researchers to elucidate the functional significance of CA4 isoforms in fishes.

The onset of active gill respiration in post-embryonic zebrafish (Danio rerio) larvae triggers an increased sensitivity to neurotoxic compounds

Originally designed as a general alternative to acute fish toxicity testing (AFT), the fish embryo toxicity test (FET) has become subject to concerns with respect to neurotoxic substances. Whereas oxygen uptake in the fish embryo primarily occurs via diffusion across the skin, juvenile and adult fish rely on active ventilation of the gills. As a consequence, substances including, e.g., neurotoxicants which prevent appropriate ventilation of gills ("respiratory failure syndrome") might lead to suffocation in juvenile and adult fish, but not in skin-breathing embryos. To investigate if this respiratory failure syndrome might play a role for the higher sensitivity of juvenile and adult fish to neurotoxicants, a modified acute toxicity test using post-embryonic, early gill-breathing life-stages of zebrafish was developed with chlorpyrifos, permethrin, lindane, aldicarb, ziram and aniline as test substances. Additionally, a comparative study into bioaccumulation of lipophilic substances with log_Kow > 3.5 and swimbladder deflation as potential side effects of the respiratory failure syndrome was performed with 4 d old skin-breathing and 12 d old gill-breathing zebrafish. With respect to acute toxicity, post-embryonic 12 d larvae proved to be more sensitive than both embryos (FET) and adult zebrafish (AFT) to all test substances except for permethrin. Accumulation of chlorpyrifos, lindane and permethrin was 1.3- to 5-fold higher in 4 d old than in 12 d old zebrafish, suggesting that (intermediate) storage of substances in the yolk might reduce bioavailability and prevent metabolization, which could be a further reason for lower toxicity in 4 d than in 12 d old zebrafish. Whereas ziram and aniline showed no significant effect on the swimbladder, zebrafish exposed to chlorpyrifos, lindane and permethrin showed significantly deflated swimbladders in 12 d old larvae; in the case of aldicarb, there was a significant hyperinflation in 4 d old larvae. Swimbladder deflation in post-embryonic 12 d zebrafish larvae might be hypothesized as a reason for a lack of internal oxygen supplies during the respiratory failure syndrome, whereas in 4 d old embryos cholinergic hyperinflation of the swimbladder dominates over other effects. Regarding acute lethality, the study provides further evidence that the switch from transcutaneous to branchial respiration in post-embryonic zebrafish life-stages might be the reason for the higher sensitivity of juvenile and adult fish to neurotoxic substances.

Local Adaptation and the Evolution of Genome Architecture in Threespine Stickleback

Theory predicts that local adaptation should favor the evolution of a concentrated genetic architecture, where the alleles driving adaptive divergence are tightly clustered on chromosomes. Adaptation to marine versus freshwater environments in threespine stickleback has resulted in an architecture that seems consistent with this prediction: divergence among populations is mainly driven by a few genomic regions harboring multiple quantitative trait loci for environmentally adapted traits, as well as candidate genes with well-established phenotypic effects. One theory for the evolution of these "genomic islands" is that rearrangements remodel the genome to bring causal loci into tight proximity, but this has not been studied explicitly. We tested this theory using synteny analysis to identify micro- and macro-rearrangements in the stickleback genome and assess their potential involvement in the evolution of genomic islands. To identify rearrangements, we conducted a de novo assembly of the closely related tubesnout (Aulorhyncus flavidus) genome and compared this to the genomes of threespine stickleback and two other closely related species. We found that small rearrangements, within-chromosome duplications, and lineage-specific genes (LSGs) were enriched around genomic islands, and that all three chromosomes harboring large genomic islands have experienced macro-rearrangements. We also found that duplicates and micro-rearrangements are 9.9× and 2.9× more likely to involve genes differentially expressed between marine and freshwater genotypes. While not conclusive, these results are consistent with the explanation that strong divergent selection on candidate genes drove the recruitment of rearrangements to yield clusters of locally adaptive loci.

Adrenergically induced translocation of red blood cell β-adrenergic sodium-proton exchangers has ecological relevance for hypoxic and hypercapnic white seabass

White seabass (Atractoscion nobilis) increasingly experience periods of low oxygen (O₂; hypoxia) and high carbon dioxide (CO₂, hypercapnia) due to climate change and eutrophication of the coastal waters of California. Hemoglobin (Hb) is the principal O₂ carrier in the blood and in many teleost fishes Hb-O₂ binding is compromised at low pH; however, the red blood cells (RBC) of some species regulate intracellular pH with adrenergically stimulated sodium-proton-exchangers (β-NHEs). We hypothesized that RBC β-NHEs in white seabass are an important mechanism that can protect the blood O₂-carrying capacity during hypoxia and hypercapnia. We determined the O₂-binding characteristics of white seabass blood, the cellular and subcellular response of RBCs to adrenergic stimulation, and quantified the protective effect of β-NHE activity on Hb-O₂ saturation. White seabass had typical teleost Hb characteristics, with a moderate O₂ affinity (Po₂ at half-saturation; P₅₀ 2.9 kPa) that was highly pH-sensitive (Bohr coefficient -0.92; Root effect 52%). Novel findings from super-resolution microscopy revealed β-NHE protein in vesicle-like structures and its translocation into the membrane after adrenergic stimulation. Microscopy data were corroborated by molecular and phylogenetic results and a functional characterization of β-NHE activity. The activation of RBC β-NHEs increased Hb-O₂ saturation by ∼8% in normoxic hypercapnia and by up to ∼20% in hypoxic normocapnia. Our results provide novel insight into the cellular mechanism of adrenergic RBC stimulation within an ecologically relevant context. β-NHE activity in white seabass has great potential to protect arterial O₂ transport during hypoxia and hypercapnia but is less effective during combinations of these stressors.

Enhanced oxygen unloading in two marine percomorph teleosts

Teleost fishes are diverse and successful, comprising almost half of all extant vertebrate species. It has been suggested that their success as a group is related, in part, to their unique O₂ transport system, which includes pH-sensitive hemoglobin, a red blood cell β-adrenergic Na⁺/H⁺ exchanger (RBC β-NHE) that protects red blood cell pH, and plasma accessible carbonic anhydrase which is absent at the gills but present in some tissues, that short-circuits the β-NHE to enhance O₂ unloading during periods of stress. However, direct support for this has only been examined in a few species of salmonids. Here, we expand the knowledge of this system to two warm-water, highly active marine percomorph fish, cobia (Rachycentron canadum) and mahi-mahi (Coryphaena hippurus). We show evidence for RBC β-NHE activity in both species, and characterize the Hb-O₂ transport system in one of those species, cobia. We found significant RBC swelling following β-adrenergic stimulation in both species, providing evidence for the presence of a rapid, active RBC β-NHE in both cobia and mahi-mahi, with a time-course similar to that of salmonids. We generated oxygen equilibrium curves (OECs) for cobia blood and determined the P₅₀, Hill, and Bohr coefficients, and used these data to model the potential for enhanced O₂ unloading. We determined that there was potential for up to a 61% increase in O₂ unloading associated with RBC β-NHE short-circuiting, assuming a - 0.2 ∆pH_a-v in the blood. Thus, despite phylogenetic and life history differences between cobia and the salmonids, we found few differences between their Hb-O₂ transport systems, suggesting conservation of this physiological trait across diverse teleost taxa.

The effects of warming on red blood cell carbonic anhydrase activity and respiratory performance in a marine fish

Measures of fitness are valuable tools to predict species' responses to environmental changes, like increased water temperature. Aerobic scope (AS) is a measure of an individual's capacity for aerobic processes, and frequently used as a proxy for fitness. However, AS is complicated by individual variation found not only within a species, but within similar body sizes as well. Maximum metabolic rate (MMR), one of the factors determining AS, is constrained by an individual's ability to deliver and extract oxygen (O₂) at the tissues. Recently, data has shown that red blood cell carbonic anhydrase (RBC CA) is rate-limiting for O₂ delivery in red drum (Sciaenops ocellatus). We hypothesized increased temperature impacts MMR and RBC CA activity in a similar manner, and that an individual's RBC CA activity drives individual variation in AS. Red drum were acutely exposed to increased temperature (+6 °C; 22 °C to 28 °C) for 24 h prior to exhaustive exercise and intermittent-flow respirometry at 28 °C. RBC CA activity was measured before temperature exposure and after aerobic performance. Due to enzymatic thermal sensitivity, acute warming increased individual RBC CA activity by 36%, while there was no significant change in the control (22 °C) treatment. Interestingly, average MMR of the acute warming treatment was 36% greater than that of control drum. However, we found no relationships between individual RBC CA activity and their respective MMR and AS at either temperature. While warming similarly affects RBC CA activity and MMR, RBC CA activity is not a predictor of individual MMR.

Hypoxia Performance Curve: Assess a Whole-Organism Metabolic Shift from a Maximum Aerobic Capacity towards a Glycolytic Capacity in Fish

The utility of measuring whole-animal performance to frame the metabolic response to environmental hypoxia is well established. Progressively reducing ambient oxygen (O₂) will initially limit maximum metabolic rate as a result of a hypoxemic state and ultimately lead to a time-limited, tolerance state supported by substrate-level phosphorylation when the O₂ supply can no longer meet basic needs (standard metabolic rate, SMR). The metabolic consequences of declining ambient O₂ were conceptually framed for fishes initially by Fry's hypoxic performance curve, which characterizes the hypoxemic state and its consequences to absolute aerobic scope (AAS), and Hochachka's concept of scope for hypoxic survival, which characterizes time-limited life when SMR cannot be supported by O₂ supply. Yet, despite these two conceptual frameworks, the toolbox to assess whole-animal metabolic performance remains rather limited. Here, we briefly review the ongoing debate concerning the need to standardize the most commonly used assessments of respiratory performance in hypoxic fishes, namely critical O₂ (the ambient O₂ level below which maintenance metabolism cannot be sustained) and the incipient lethal O₂ (the ambient O₂ level at which a fish loses the ability to maintain upright equilibrium), and then we advance the idea that the most useful addition to the toolbox will be the limiting-O₂ concentration (LOC) performance curve. Using Fry & Hart's (1948) hypoxia performance curve concept, an LOC curve was subsequently developed as an eco-physiological framework by Neil et al. and derived for a group of fish during a progressive hypoxia trial by Claireaux and Lagardère (1999). In the present review, we show how only minor modifications to available respirometry tools and techniques are needed to generate an LOC curve for individual fish. This individual approach to the LOC curve determination then increases its statistical robustness and importantly opens up the possibility of examining individual variability. Moreover, if peak aerobic performance at a given ambient O₂ level of each individual is expressed as a percentage of its AAS, the water dissolved O₂ that supports 50% of the individual's AAS (DO_AAS-50) can be interpolated much like the P₅₀ for an O₂ hemoglobin dissociation curve (when hemoglobin is 50% saturated with O₂). Thus, critical O₂, incipient lethal O₂, DO_AAS-50 and P₅₀ and can be directly compared within and across species. While an LOC curve for individual fish represents a start to an ongoing need to seamlessly integrate aerobic to anaerobic capacity assessments in a single, multiplexed respirometry trial, we close with a comparative exploration of some of the known whole-organism anaerobic and aerobic capacity traits to examine for correlations among them and guide the next steps.

Is hypoxia vulnerability in fishes a by-product of maximum metabolic rate?

The metabolic index concept combines metabolic data and known thermal sensitivities to estimate the factorial aerobic scope of animals in different habitats, which is valuable for understanding the metabolic demands that constrain species' geographical distributions. An important assumption of this concept is that the O2 supply capacity (which is equivalent to the rate of oxygen consumption divided by the environmental partial pressure of oxygen: ) is constant at O2 tensions above the critical O2 threshold (i.e. the where O2 uptake can no longer meet metabolic demand). This has led to the notion that hypoxia vulnerability is not a selected trait, but a by-product of selection on maximum metabolic rate. In this Commentary, we explore whether this fundamental assumption is supported among fishes. We provide evidence that O2 supply capacity is not constant in all fishes, with some species exhibiting an elevated O2 supply capacity in hypoxic environments. We further discuss the divergent selective pressures on hypoxia- and exercise-based cardiorespiratory adaptations in fishes, while also considering the implications of a hypoxia-optimized O2 supply capacity for the metabolic index concept.

A lack of red blood cell swelling in five elasmobranch fishes following air exposure and exhaustive exercise

In teleost fishes, catecholamine-induced increases in the activity of cation exchangers compensate for decreases in hemoglobin oxygen affinity and maximum blood oxygen carrying capacity caused by decreases in plasma pH (i.e., metabolic acidosis). The resultant red blood cell (RBC) swelling has been documented in sandbar (Carcharhinus plumbeus) and epaulette (Hemiscyllium ocellatum) sharks following capture by rod-and-reel or after a 1.5 h exposure to anoxia (respectively), although the underlying mechanisms remain unknown. To determine if RBC swelling could be documented in other elasmobranch fishes, we collected blood samples from clearnose skate (Rostroraja eglanteria), blacktip reef shark (Carcharhinus melanopterus), and sicklefin lemon shark (Negaprion acutidens) subjected to exhaustive exercise or air exposure (or both) and measured hematocrit, hemoglobin concentration, RBC count, RBC volume, and mean corpuscular hemoglobin content. We did likewise with sandbar and epaulette sharks to further explore the mechanisms driving swelling when present. We could not document RBC swelling in any species; although hematocrit increased in all species (presumably due to RBC ejection from the spleen or fluid shifts out of the vascular compartment) except epaulette shark. Our results indicate RBC swelling and associated ion shifts in elasmobranch fishes is not inducible by exercise or hypoxia, thus implying this response maybe of lesser importance for maintaining oxygen delivery during acute acidosis than in teleost fishes.

Red blood cell carbonic anhydrase mediates oxygen delivery via the Root effect in red drum

Oxygen (O₂) and carbon dioxide (CO₂) transport are tightly coupled in many fishes as a result of the presence of Root effect hemoglobins (Hb), whereby reduced pH reduces O₂ binding even at high O₂ tensions. Red blood cell carbonic anhydrase (RBC CA) activity limits the rate of intracellular acidification, yet its role in O₂ delivery has been downplayed. We developed an in vitro assay to manipulate RBC CA activity while measuring Hb-O₂ offloading following a physiologically relevant CO₂-induced acidification. RBC CA activity in red drum (Sciaenops ocellatus) was inhibited with ethoxzolamide by 53.7±0.5%, which prompted a significant reduction in O₂ offloading rate by 54.3±5.4% (P=0.0206, two-tailed paired t-test; n=7). Conversely, a 2.03-fold increase in RBC CA activity prompted a 2.14-fold increase in O₂ offloading rate (P<0.001, two-tailed paired t-test; n=8). This approximately 1:1 relationship between RBC CA activity and Hb-O₂ offloading rate coincided with a similar allometric scaling exponent for RBC CA activity and maximum metabolic rate. Together, our data suggest that RBC CA is rate limiting for O₂ delivery in red drum.

Recent genome duplications facilitate the phenotypic diversity of Hb repertoire in the Cyprinidae

Whole-genome duplications (WGDs) are an important contributor to phenotypic innovations in evolutionary history. The diversity of blood oxygen transport traits is the perfect reflection of physiological versatility for evolutionary success among vertebrates. In this study, the evolutionary changes of hemoglobin (Hb) repertoire driven by the recent genome duplications were detected in representative Cyprinidae fish, including eight diploid and four tetraploid species. Comparative genomic analysis revealed a substantial variation in both membership composition and intragenomic organization of Hb genes in these species. Phylogenetic reconstruction analyses were conducted to characterize the evolutionary history of these genes. Data were integrated with the expression profiles of the genes during ontogeny. Our results indicated that genome duplications facilitated the phenotypic diversity of the Hb gene family; each was associated with species-specific changes in gene content via gene loss and fusion after genome duplications. This led to repeated evolutionary transitions in the ontogenic regulation of Hb gene expression. Our results revealed that genome duplications helped to generate phenotypic changes in Cyprinidae Hb systems.

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.

Thermal tolerance and hypoxia tolerance are associated in blacktip reef shark (Carcharhinus melanopterus) neonates

Thermal dependence of growth and metabolism can influence thermal preference and tolerance in marine ectotherms, including threatened and data-deficient species. Here, we quantified the thermal dependence of physiological performance in neonates of a tropical shark species (blacktip reef shark, Carcharhinus melanopterus) from shallow, nearshore habitats. We measured minimum and maximum oxygen uptake rates (ṀO2), calculated aerobic scope, excess post-exercise oxygen consumption and recovery from exercise, and measured critical thermal maxima (CTmax), thermal safety margins, hypoxia tolerance, specific growth rates, body condition and food conversion efficiencies at two ecologically relevant acclimation temperatures (28 and 31°C). Owing to high post-exercise mortality, a third acclimation temperature (33°C) was not investigated further. Acclimation temperature did not affect ṀO2 or growth, but CTmax and hypoxia tolerance were greatest at 31°C and positively associated. We also quantified in vitro temperature (25, 30 and 35°C) and pH effects on haemoglobin–oxygen (Hb–O2) affinity of wild-caught, non-acclimated sharks. As expected, Hb–O2 affinity decreased with increasing temperatures, but pH effects observed at 30°C were absent at 25 and 35°C. Finally, we logged body temperatures of free-ranging sharks and determined that C. melanopterus neonates avoided 31°C in situ. We conclude that C. melanopterus neonates demonstrate minimal thermal dependence of whole-organism physiological performance across a seasonal temperature range and may use behaviour to avoid unfavourable environmental temperatures. The association between thermal tolerance and hypoxia tolerance suggests a common mechanism warranting further investigation. Future research should explore the consequences of ocean warming, especially in nearshore, tropical species.

Evolutionary links between intra- and extracellular acid-base regulation in fish and other aquatic animals

The acid-base relevant molecules carbon dioxide (CO₂ ), protons (H⁺ ), and bicarbonate (HCO₃ ^- ) are substrates and end products of some of the most essential physiological functions including aerobic and anaerobic respiration, ATP hydrolysis, photosynthesis, and calcification. The structure and function of many enzymes and other macromolecules are highly sensitive to changes in pH, and thus maintaining acid-base homeostasis in the face of metabolic and environmental disturbances is essential for proper cellular function. On the other hand, CO₂ , H⁺ , and HCO₃ ^- have regulatory effects on various proteins and processes, both directly through allosteric modulation and indirectly through signal transduction pathways. Life in aquatic environments presents organisms with distinct acid-base challenges that are not found in terrestrial environments. These include a relatively high CO₂ relative to O₂ solubility that prevents internal CO₂ /HCO₃ ^- accumulation to buffer pH, a lower O₂ content that may favor anaerobic metabolism, and variable environmental CO₂ , pH and O₂ levels that require dynamic adjustments in acid-base homeostatic mechanisms. Additionally, some aquatic animals purposely create acidic or alkaline microenvironments that drive specialized physiological functions. For example, acidifying mechanisms can enhance O₂ delivery by red blood cells, lead to ammonia trapping for excretion or buoyancy purposes, or lead to CO₂ accumulation to promote photosynthesis by endosymbiotic algae. On the other hand, alkalinizing mechanisms can serve to promote calcium carbonate skeletal formation. This nonexhaustive review summarizes some of the distinct acid-base homeostatic mechanisms that have evolved in aquatic organisms to meet the particular challenges of this environment.

The importance of a single amino acid substitution in reduced red blood cell carbonic anhydrase function of early-diverging fish

In most vertebrates, red blood cell carbonic anhydrase (RBC CA) plays a critical role in carbon dioxide (CO₂) transport and excretion across epithelial tissues. Many early-diverging fishes (e.g., hagfish and chondrichthyans) are unique in possessing plasma-accessible membrane-bound CA-IV in the gills, allowing some CO₂ excretion to occur without involvement from the RBCs. However, implications of this on RBC CA function are unclear. Through homology cloning techniques, we identified the putative protein sequences for RBC CA from nine early-diverging species. In all cases, these sequences contained a modification of the proton shuttle residue His-64, and activity measurements from three early-diverging fish demonstrated significantly reduced CA activity. Site-directed mutagenesis was used to restore the His-64 proton shuttle, which significantly increased RBC CA activity, clearly illustrating the functional significance of His-64 in fish red blood cell CA activity. Bayesian analyses of 55 vertebrate cytoplasmic CA isozymes suggested that independent evolutionary events led to the modification of His-64 and thus reduced CA activity in hagfish and chondrichthyans. Additionally, in early-diverging fish that possess branchial CA-IV, there is an absence of His-64 in RBC CAs and the absence of the Root effect [where a reduction in pH reduces hemoglobin's capacity to bind with oxygen (O₂)]. Taken together, these data indicate that low-activity RBC CA may be present in all fish with branchial CA-IV, and that the high-activity RBC CA seen in most teleosts may have evolved in conjunction with enhanced hemoglobin pH sensitivity.

Blood and Gill Carbonic Anhydrase in the Context of a Chondrichthyan Model of CO2 Excretion

Pacific spiny dogfish (Squalus suckleyi) have been widely used as a representative species for chondrichthyan CO₂ excretion. Pacific spiny dogfish have a slower red blood cell (RBC) carbonic anhydrase (CA) isoform than teleost fishes, extracellular CA activity, no endogenous plasma CA inhibitor, and plasma-accessible CA IV at the gills. Thus, both the RBC and plasma compartments contribute to bicarbonate ion (HCO3-) dehydration at the gills for CO₂ excretion in contrast to teleost fishes, in which HCO3- dehydration is restricted to RBCs. We compared CA activity levels, subcellular localization, and presence of plasma CA inhibitors in the blood and gills of 13 chondrichthyans to examine the hypothesis that the dogfish model of CO₂ excretion applies broadly to chondrichthyans. In general, blood samples from the 12 other chondrichthyans examined had lower RBC CA activity than teleosts, some extracellular CA activity, and no endogenous plasma CA inhibitor. While type IV-like membrane-associated CA was found in the gills in all four of the chondrichthyans examined, S. suckleyi had three times more CA activity (183±13.2 μmol CO₂ min^-1 mg protein^-1) in the microsomal (membrane) fraction of gills than the other three. In addition, unexpected variation in CA characteristics was observed between chondrichthyan species. Thus, in general, it appears that the pattern of CA distribution in fishes can be generally categorized as either chondrichthyan or teleost models. However, further studies should examine the functional significance of the within-chondrichthyan differences we observed and investigate whether CO₂ excretion patterns exist along a continuum or in discrete groups.

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.

Regulation of erythrocyte function: Multiple evolutionary solutions for respiratory gas transport and its regulation in fish

Gas transport concepts in vertebrates have naturally been formulated based on human blood. However, the first vertebrates were aquatic, and fish and tetrapods diverged hundreds of millions years ago. Water-breathing vertebrates live in an environment with low and variable O₂ levels, making environmental O₂ an important evolutionary selection pressure in fishes, and various features of their gas transport differ from humans. Erythrocyte function in fish is of current interest, because current environmental changes affect gas transport, and because especially zebrafish is used as a model in biomedical studies, making it important to understand the differences in gas transport between fish and mammals to be able to carry out meaningful studies. Of the close to thirty thousand fish species, teleosts are the most species-numerous group. However, two additional radiations are discussed: agnathans and elasmobranchs. The gas transport by elasmobranchs may be closest to the ancestors of tetrapods. The major difference in their haemoglobin (Hb) function to humans is their high urea tolerance. Agnathans differ from other vertebrates by having Hbs, where cooperativity is achieved by monomer-oligomer equilibria. Their erythrocytes also lack the anion exchange pathway with profound effects on CO₂ transport. Teleosts are characterized by highly pH sensitive Hbs, which can fail to become fully O₂ -saturated at low pH. An adrenergically stimulated Na⁺ /H⁺ exchanger has evolved in their erythrocyte membrane, and plasma-accessible carbonic anhydrase can be differentially distributed among their tissues. Together, and differing from other vertebrates, these features can maximize O₂ unloading in muscle while ensuring O₂ loading in gills.

Evidence for embryonic haemoglobins from Sparus aurata under normal and hypoxic conditions

Teleost haemoglobins vary in polymorphisms and primary structure, although display similar functional properties. Key amino acids for Root effect (a reduction in oxygen-carrying capacity and loss of cooperativity with declining pH) are conserved throughout fish evolution. For the first time, we cloned and characterised Sparus aurata L. embryonic globin chains (eα1, eα2, eβ). We also studied haemoglobins (eHbI, eHbII) behaviour in normal and low-oxygen conditions. Several amino acids in fry globins are different in chemical type (e.g. polar → non-polar and vice versa), compared to adult globins. His55α1, crucial for Root effect, is substituted by Ala in fry, presumably enhancing oxygen capture, transport and reducing the dependence of Root effect from pH. Phylogenetic trees demonstrate that eα1 globin diversified more recently than eα2; moreover, eα1, eα2 and eβ globins evolved earlier than adult α and β globins. In low-oxygen conditions, fry haemoglobins display the same behaviour of the adult haemoglobins (probably, embryonic and adult-type I Hbs display a higher oxygen affinity than type II Hbs, operating through a rapid cycle of heme-Fe auto-oxidation/reduction). Therefore, based on our results and on the comparison with adult haemoglobins, we hypothesise that embryonic haemoglobins have evolved to better adapt fry to variable habitats. We studied Sparus aurata for its economical relevance in Mediterranean aquaculture. The information we provide can help understand Sparus aurata behaviour in the wild and in rearing conditions. Further studies with functional assays will deepen the knowledge on the molecular mechanisms of fry haemoglobin physiology.

Functional support for a novel mechanism that enhances tissue oxygen extraction in a teleost fish

A successful spawning migration in salmon depends on their athletic ability, and thus on efficient cardiovascular oxygen (O₂) transport. Most teleost fishes have highly pH-sensitive haemoglobins (Hb) that can release large amounts of O₂ when the blood is acidified at the tissues. We hypothesized that plasma-accessible carbonic anhydrase (paCA; the enzyme that catalyses proton production from CO₂) is required to acidify the blood at the tissues and promote tissue O₂ extraction. Previous studies have reported an elevated tissue O₂ extraction in hypoxia-acclimated teleosts that may also be facilitated by paCA. Thus, to create experimental contrasts in tissue O₂ extraction, Atlantic salmon were acclimated to normoxia or hypoxia (40% air saturation for more than six weeks), and the role of paCA in enhancing tissue O₂ extraction was tested by inhibiting paCA at rest and during submaximal exercise. Our results show that: (i) in both acclimation groups, the inhibition of paCA increased cardiac output by one-third, indicating a role of paCA in promoting tissue O₂ extraction during exercise, recovery and at rest; (ii) the recruitment of paCA was plastic and increased following hypoxic acclimation; and (iii) maximal exercise performance in salmon, and thus a successful spawning migration, may not be possible without paCA.

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.

Effect of dendritic organ ligation on striped eel catfish Plotosus lineatus osmoregulation

Unique amongst the teleost, Plotosidae catfish possess a dendritic organ (DO) as a purported salt secreting organ, whereas other marine teleosts rely on their gill ionocytes for active NaCl excretion. To address the role of the DO in ionregulation, ligation experiments were conducted in brackish water (BW) 3‰ and seawater (SW) 34‰ acclimated Plotosus lineatus and compared to sham operated fish. Ligation in SW resulted in an osmoregulatory impairment in blood (elevated ions and hematocrit) and muscle (dehydration). However, SW ligation did not elicit compensatory changes in gill or kidney Na⁺/K⁺-ATPase (NKA) activity and/or protein expression while a decrease in anterior intestine and increased in posterior intestine were observed but this was not reflected at the protein level. Following ligation in SW, protein levels of carbonic anhydrase (CA) and V-ATPase B subunit (VHAB) were higher in kidney but either lower (CA) or unchanged (VHAB) in other tissues. Taken together, the osmotic disturbance in ligated SW fish indicates the central role of the DO in salt secretion and the absence of a compensatory response from the gill.

Control of air-breathing in fishes: Central and peripheral receptors

This review considers the environmental and systemic factors that can stimulate air-breathing responses in fishes with bimodal respiration, and how these may be controlled by peripheral and central chemoreceptors. The systemic factors that stimulate air-breathing in fishes are usually related to conditions that increase the O₂ demand of these animals (e.g. physical exercise, digestion and increased temperature), while the environmental factors are usually related to conditions that impair their capacity to meet this demand (e.g. aquatic/aerial hypoxia, aquatic/aerial hypercarbia, reduced aquatic hidrogenionic potential and environmental pollution). It is now well-established that peripheral chemoreceptors, innervated by cranial nerves, drive increased air-breathing in response to environmental hypoxia and/or hypercarbia. These receptors are, in general, sensitive to O₂ and/or CO₂/H⁺ levels in the blood and/or the environment. Increased air-breathing in response to elevated O₂ demand may also be driven by the peripheral chemoreceptors that monitor O₂ levels in the blood. Very little is known about central chemoreception in air-breathing fishes, the data suggest that central chemosensitivity to CO₂/H⁺ is more prominent in sarcopterygians than in actinopterygians. A great deal remains to be understood about control of air-breathing in fishes, in particular to what extent control systems may show commonalities (or not) among species or groups that have evolved air-breathing independently, and how information from the multiple peripheral (and possibly central) chemoreceptors is integrated to control the balance of aerial and aquatic respiration in these animals.

Splitting of circulating red blood cells as an in vivo mechanism of erythrocyte maturation in developing zebrafish, chick and mouse embryos

Nucleated circulating red blood cells (RBCs) of developing zebrafish, chick and mouse embryos can actively proliferate. While marrow- or organ-mediated erythropoiesis has been widely studied, transforming in vivo processes of circulating RBCs are under little scrutiny. We employed confocal, stereo- and electron microscopy to document the maturation of intravascular RBCs. In zebrafish embryos (32-72 h post-fertilization), RBC splitting in the caudal vein plexus follows a four-step program: (i) nuclear division with continued cytoplasmic connection between somata; (ii) dumbbell-shaped RBCs tangle at transluminal vascular pillars; (iii) elongation; and (iv) disruption of soma-to-soma connection. Dividing RBCs of chick embryos, however, retain the nucleus in one of their somata. Here, RBC splitting acts to pinch off portions of cytoplasm, organelles and ribosomes. Dumbbell-shaped primitive RBCs re-appeared as circulation constituents in mouse embryos. The splitting of circulating RBCs thus represents a biologically relevant mechanism of RBC division and maturation during early vertebrate ontogeny.

Osmoregulation in the Plotosidae Catfish: Role of the Salt Secreting Dendritic Organ

Unlike other marine teleosts, the Plotosidae catfishes reportedly have an extra-branchial salt secreting dendritic organ (DO). Salinity acclimation [brackishwater (BW) 3aaa, seawater (SWcontrol) 34aaa, and hypersaline water (HSW) 60aaa] for 14 days was used to investigate the osmoregulatory abilities of Plotosus lineatus through measurements of blood chemistry, muscle water content (MWC), Na⁺/K⁺-ATPase (NKA) specific activity and ion transporter expression in gills, DO, kidney and intestine. Ion transporter expression was determined using immunoblotting, immunohistochemistry (IHC) and quantitative polymerase chain reaction (qPCR). HSW elevated mortality, plasma osmolality and ions, and hematocrit, and decreased MWC indicating an osmoregulatory challenge. NKA specific activity and protein levels were significantly higher in DO compared to gill, kidney and intestine at all salinities. NKA specific activity increased in kidney and posterior intestine with HSW but only kidney showed correspondingly higher NKA α-subunit protein levels. Since DO mass was greater in HSW, the total amount of DO NKA activity expressed per gram fish was greater indicating higher overall capacity. Gill NKA and V-ATPase protein levels were greater with HSW acclimation but this was not reflected in NKA activity, mRNA or ionocyte abundance. BW acclimation resulted in lower NKA activity in gill, kidney and DO. Cl^- levels were better regulated and the resulting strong ion ratio in BW suggests a metabolic acidosis. Elevated DO heat shock protein 70 levels in HSW fish indicate a cellular stress. Strong NKA and NKCC1 (Na⁺:K⁺:2Cl^- cotransporter1) co-localization was observed in DO parenchymal cells, which was rare in gill ionocytes. NKCC1 immunoblot expression was only detected in DO, which was highest at HSW. Cystic fibrosis transmembrane regulator Cl^- channel (CFTR) localize apically to DO NKA immunoreactive cells. Taken together, the demonstration of high NKA activity in DO coexpressed with NKCC1 and CFTR indicates the presence of the conserved secondary active Cl^- secretion mechanism found in other ion transporting epithelia suggesting a convergent evolution with other vertebrate salt secreting organs. However, the significant osmoregulatory challenge of HSW indicates that the DO may be of limited use under more extreme salinity conditions in contrast to the gill based ionoregulatory strategy of marine teleosts.

Time course of red blood cell intracellular pH recovery following short-circuiting in relation to venous transit times in rainbow trout, Oncorhynchus mykiss

Accumulating evidence is highlighting the importance of a system of enhanced hemoglobin-oxygen (Hb-O₂) unloading for cardiovascular O₂ transport in teleosts. Adrenergically stimulated sodium-proton exchangers (β-NHE) create H⁺ gradients across the red blood cell (RBC) membrane that are short-circuited in the presence of plasma-accessible carbonic anhydrase (paCA) at the tissues; the result is a large arterial-venous pH shift that greatly enhances O₂ unloading from pH-sensitive Hb. However, RBC intracellular pH (pH_i) must recover during venous transit (31-90 s) to enable O₂ loading at the gills. The halftimes ( t_1/2) and magnitudes of RBC β-adrenergic stimulation, short-circuiting with paCA and recovery of RBC pH_i, were assessed in vitro, on rainbow trout whole blood, and using changes in closed-system partial pressure of O₂ as a sensitive indicator for changes in RBC pH_i. In addition, the recovery rate of RBC pH_i was assessed in a continuous-flow apparatus that more closely mimics RBC transit through the circulation. Results indicate that: 1) the t_1/2 of β-NHE short-circuiting is likely within the residence time of blood in the capillaries, 2) the t_1/2 of RBC pH_i recovery is 17 s and within the time of RBC venous transit, and 3) after short-circuiting, RBCs reestablish the initial H⁺ gradient across the membrane and can potentially undergo repeated cycles of short-circuiting and recovery. Thus, teleosts have evolved a system that greatly enhances O₂ unloading from pH-sensitive Hb at the tissues, while protecting O₂ loading at the gills; the resulting increase in O₂ transport per unit of blood flow may enable the tremendous athletic ability of salmonids.

Enhanced hemoglobin-oxygen unloading in migratory salmonids

Recent findings indicate that some teleost fishes may be able to greatly enhance hemoglobin-oxygen (Hb-O₂) unloading at the tissues under conditions that result in catecholamine release. The putative mechanism relies on the high pH sensitivity of teleost hemoglobin (Hb), intracellular red blood cell (RBC) pH regulation via β-adrenergic Na⁺/H⁺ exchanger (β-NHE) activity, and plasma-accessible carbonic anhydrase at the tissues that short-circuits RBC pH regulation. Previous studies have shown that in rainbow trout, this system may double Hb-O₂ unloading to red muscle compared to a situation without short-circuiting. The present study determined that: (1) in rainbow trout this system may be functional even at low concentrations of circulating catecholamines, as shown by conducting a dose-response analysis; (2) Atlantic and coho salmon also possess β-NHE activity, as shown by changes in hematocrit in adrenergically stimulated cells; and (3) with β-NHE short-circuiting, Atlantic and coho salmon may be able to increase Hb-O₂ unloading by up to 74 and 159%, respectively, as determined by modeling based on O₂ equilibrium curves. Together, these results indicate that a system to enhance Hb-O₂ unloading may be common among salmonids and may be operational even under routine conditions. In view of the life histories of Atlantic and coho salmon, a system to enhance Hb-O₂ unloading during exercise may help determine a successful spawning migration and thus reproductive success.

Effects of water ionic composition on acid-base regulation in rainbow trout, during hypercarbia at rest and during sustained exercise

Aquatic hypercarbia (elevated environmental CO₂) results in a blood acidosis in fish, which is compensated by the exchange of Na⁺ and/or Cl^- for its acid/base counterpart (H⁺, HCO₃^-) across the gill epithelium. To date, no studies exist on how a single species, capable of inhabiting both fresh and saltwater, responds to hypercarbia, at rest or during sustained exercise. Rainbow trout was acclimated to soft water (in mmol l^- 1: Na⁺, 0.08; Cl^-, 0.05; pH 6.7-6.8), hard water (in mmol l^- 1: Na⁺, 2.4; Cl^-, 0.2; pH 7.9-8.0), or 85% saltwater (28 ppt) (in mmol l^- 1: Na⁺, 410; Cl^-, 476; pH 7.8-8.0). Acid-base relevant blood parameters were measured during a 1 kPa CO₂ hypercarbia exposure, both at rest and during sustained exercise (~ 60% U _crit). After 48 h of hypercarbia, resting hard-, and saltwater trout fully restored blood pH, whereas soft-water-acclimated trout was only 60.6 ± 10.5% recovered. In all fish, recovery was associated with an increase in plasma [HCO₃^-] and an equimolar reduction in plasma [Cl^-]. Following 8 h of hypercarbia during sustained exercise, saltwater fish fully restored blood pH, while soft- and hard water fish were 42 ± 18.1 and 64 ± 6.8% recovered, respectively. Results provide intra-specific support demonstrating that saltwater acclimated fish acid-base compensate faster than freshwater fish during hypercarbia. Furthermore, data indicate that recovery during hypercarbia in trout is more rapid during exercise than rest. This not only demonstrates an important link between ambient water ion levels and ability to recover from acid-base disturbances, but also it presents novel data, suggesting that exercise may enhance acid-base regulation.

Beyond just hemoglobin: Red blood cell potentiation of hemoglobin-oxygen unloading in fish

Teleosts comprise 95% of fish species, almost one-half of all vertebrate species, and represent one of the most successful adaptive radiation events among vertebrates. This is thought to be in part because of their unique oxygen (O2) transport system. In salmonids, recent in vitro and in vivo studies indicate that hemoglobin-oxygen (Hb-O2) unloading to tissues may be doubled or even tripled under some conditions without changes in perfusion. This is accomplished through the short circuiting of red blood cell (RBC) pH regulation, resulting in a large arterial-venous pH difference within the RBC and induced reduction in Hb-O2 affinity. This system has three prerequisites: 1) highly pH-sensitive hemoglobin, 2) rapid RBC pH regulation, and 3) a heterogeneous distribution of plasma-accessible CA in the cardiovascular system (presence in the tissues and absence at the gills). Although data are limited, these attributes may be general characteristics of teleosts. Although this system is not likely operational to the same degree in other vertebrates, some of these prerequisites do exist, and the generation and elimination of pH disequilibrium states at the RBC will likely enhance Hb-O2 unloading to some degree. In human disease states, there are conditions that may partly satisfy those for enhanced Hb-O2 unloading, tentatively an avenue for future work that may improve treatment efficacy.

CRF and urocortin 3 protect the heart from hypoxia/reoxygenation-induced apoptosis in zebrafish

Fish routinely experience environmental hypoxia and have evolved various strategies to tolerate this challenge. Given the key role of the CRF system in coordinating the response to stressors and its cardioprotective actions against ischemia in mammals, we sought to characterize the cardiac CRF system in zebrafish and its role in hypoxia tolerance. We established that all genes of the CRF system, the ligands CRFa, CRFb, urotensin 1 (UTS1), and urocortin 3 (UCN3); the two receptor subtypes (CRFR1 and CRFR2); and the binding protein (CRFBP) are expressed in the heart of zebrafish: crfr1 > crfr2 = crfbp > crfa > ucn3 > crfb > uts1 In vivo, exposure to 5% O2 saturation for 15 min and 90 min of recovery resulted in four- to five-fold increases in whole heart crfb and ucn3 mRNA levels but did not affect the gene expression of other CRF system components. In vitro, as assessed by monitoring caspase 3 activity and the number of terminal deoxynucleotidyl transferase dUTP nick-end labeling-positive cells, pretreatment of excised whole hearts with CRF or UCN3 for 30 min prevented the increase in apoptosis associated with exposure to 1% O2 saturation for 30 min with a 24-h recovery. Lastly, the addition of the nonselective CRF receptor antagonist αh-CRF(9-41) prevented the cytoprotective effects of CRF. We show that the CRF system is expressed in fish heart, is upregulated by hypoxia, and is cytoprotective. These findings identify a novel role for the CRF system in fish and a new strategy to tolerate hypoxia.

Hypoxia tolerance and responses to hypoxic stress during heart and skeletal muscle inflammation in Atlantic salmon (Salmo salar)

Heart and skeletal muscle inflammation (HSMI) is associated with Piscine orthoreovirus (PRV) infection and is an important disease in Atlantic salmon (Salmo salar) aquaculture. Since PRV infects erythrocytes and farmed salmon frequently experience environmental hypoxia, the current study examined mutual effects of PRV infection and hypoxia on pathogenesis and fish performance. Furthermore, effects of HSMI on hypoxia tolerance, cardiorespiratory performance and blood oxygen transport were studied. A cohabitation trial including PRV-infected post-smolts exposed to periodic hypoxic stress (4 h of 40% O₂; PRV-H) at 4, 7 and 10 weeks post-infection (WPI) and infected fish reared under normoxic conditions (PRV) was conducted. Periodic hypoxic stress did not influence infection levels or histopathological changes in the heart. Individual incipient lethal oxygen saturation (ILOS) was examined using a standardized hypoxia challenge test (HCT). At 7 WPI, i.e. peak level of infection, both PRV and PRV-H groups exhibited reduced hypoxia tolerance compared to non-infected fish. Three weeks later (10 WPI), during peak levels of pathological changes, reduced hypoxia tolerance was still observed for the PRV group while PRV-H performed equal to non-infected fish, implying a positive effect of the repeated exposure to hypoxic stress. This was in line with maximum heart rate (f_Hmax) measurements, showing equal performance of PRV-H and non-infected groups, but lower f_Hmax above 19°C as well as lower temperature optimum (T_opt) for aerobic scope for PRV, suggesting reduced cardiac performance and thermal tolerance. In contrast, the PRV-H group had reduced hemoglobin-oxygen affinity compared to non-infected fish. In conclusion, Atlantic salmon suffering from HSMI have reduced hypoxia tolerance and cardiac performance, which can be improved by preconditioning fish to transient hypoxic stress episodes.

Long-term hypoxia exposure alters the cardiorespiratory physiology of steelhead trout (Oncorhynchus mykiss), but does not affect their upper thermal tolerance

It has been suggested that exposure to high temperature or hypoxia may confer tolerance to the other oxygen-limited stressor (i.e., 'cross-tolerance'). Thus, we investigated if chronic hypoxia-acclimation (>3 months at 40% air saturation) improved the steelhead trout's critical thermal maximum (CT_Max), or affected key physiological variables that could impact upper thermal tolerance. Neither CT_Max (24.7 vs. 25.3°C) itself, nor oxygen consumption ( [Formula: see text] ), haematocrit, blood haemoglobin concentration, or heart rate differed between hypoxia- and normoxia-acclimated trout when acutely warmed. However, the cardiac output (Q̇) of hypoxia-acclimated fish plateaued earlier compared to normoxia-acclimated fish due to an inability to maintain stroke volume (S_V), and this resulted in a ~50% lower maximum Q̇. Despite this reduced maximum cardiac function, hypoxia-acclimated trout were able to consume more O₂ per volume of blood pumped as evidenced by the equivalent [Formula: see text] . These results provide additional evidence that long-term hypoxia improves tissue oxygen utilization, and that this compensates for diminished cardiac pumping capacity. The limited S_V in hypoxia-acclimated trout in vivo was not associated with changes in cardiac morphology or in vitro maximum S_V, but the affinity and density of myocardial ß-adrenoreceptors were lower and higher, respectively, than in normoxia-acclimated fish. These data suggest that alterations in ventricular filling dynamics or myocardial contractility constrain cardiac function in hypoxia-acclimated fish at high temperatures. Our results do not support (1) 'cross-tolerance' between high temperature and hypoxia when hypoxia is chronic, or (2) that cardiac function is always the determinant of temperature-induced changes in fish [Formula: see text] , and thus thermal tolerance, as suggested by the oxygen- and capacity-limited thermal tolerance (OCLTT) theory.

Evidence for a plasma-accessible carbonic anhydrase in the lumen of salmon heart that may enhance oxygen delivery to the myocardium

Oxygen supply to the heart of most teleosts, including salmonids, relies in part or in whole on oxygen-depleted venous blood. Given that plasma-accessible carbonic anhydrase (CA) in red muscle of rainbow trout has recently been shown to facilitate oxygen unloading from arterial blood under certain physiological conditions, we tested the hypothesis that plasma-accessible CA is present in the lumen of coho salmon (Oncorhynchus kisutch) hearts, and may therefore assist in the luminal oxygen supply to the spongy myocardium, which has no coronary circulation. We demonstrate a widespread distribution of CA throughout the heart chambers, including lumen-facing cells in the atrium, and confirm that the membrane-bound isoform ca4 is expressed in the atrium and ventricle of the heart. Further, we confirm that CA catalytic activity is available to blood in the atrial lumen using a modified electrometric ΔpH assay in intact atria in combination with either a membrane-impermeable CA inhibitor or specific cleavage of the Ca4 membrane anchor. Combined, these results support our hypothesis of the presence of an enhanced oxygen delivery system in the lumen of a salmonid heart, which could help support oxygen delivery when the oxygen content of venous blood becomes greatly reduced, such as after burst exercise and during environmental hypoxia.

Root Effect Haemoglobins in Fish May Greatly Enhance General Oxygen Delivery Relative to Other Vertebrates

The teleost fishes represent over half of all extant vertebrates; they occupy nearly every body of water and in doing so, occupy a diverse array of environmental conditions. We propose that their success is related to a unique oxygen (O₂) transport system involving their extremely pH-sensitive haemoglobin (Hb). A reduction in pH reduces both Hb-O₂ affinity (Bohr effect) and carrying capacity (Root effect). This, combined with a large arterial-venous pH change (ΔpH_a-v) relative to other vertebrates, may greatly enhance tissue oxygen delivery in teleosts (e.g., rainbow trout) during stress, beyond that in mammals (e.g., human). We generated oxygen equilibrium curves (OECs) at five different CO₂ tensions for rainbow trout and determined that, when Hb-O₂ saturation is 50% or greater, the change in oxygen partial pressure (ΔPO₂) associated with ΔpH_a-v can exceed that of the mammalian Bohr effect by at least 3-fold, but as much as 21-fold. Using known ΔpH_a-v and assuming a constant arterial-venous PO₂ difference (P_a-vO₂), Root effect Hbs can enhance O₂ release to the tissues by 73.5% in trout; whereas, the Bohr effect alone is responsible for enhancing O₂ release by only 1.3% in humans. Disequilibrium states are likely operational in teleosts in vivo, and therefore the ΔpH_a-v, and thus enhancement of O₂ delivery, could be even larger. Modeling with known P_a-vO₂ in fish during exercise and hypoxia indicates that O₂ release from the Hb and therefore potentially tissue O₂ delivery may double during exercise and triple during some levels of hypoxia. These characteristics may be central to performance of athletic fish species such as salmonids, but may indicate that general tissue oxygen delivery may have been the incipient function of Root effect Hbs in fish, a trait strongly associated with the adaptive radiation of teleosts.

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.

Rapid regulation of blood parameters under acute hypoxia in the Amazonian fish Prochilodus nigricans

Prochilodus nigricans, locally known as curimatã, is an Amazonian commercial fish that endures adverse environmental conditions, in particular low dissolved oxygen, during its migration. Poorer environmental conditions are expected in the near future. Prochilodus nigricans overcomes current seasonal and diurnal changes in dissolved oxygen by adjusting erythrocytic levels of ATP and GTP, modulators of Hb-O2 affinity. Will this fish species be endangered under more extreme environmental conditions as hypoxia and acidification tend to occur in a shorter period of time? As P. nigricans does not exhibit any apparent morphological alterations to exploit the air-water interface, it must rely on fast adjustments of blood properties. To investigate this aspect, basic hematology indices, pHe, pHi, plasma lactate, erythrocytic levels of ATP and GTP and functional properties of the hemolysate of P. nigricans were analyzed over a period of 6h in hypoxia and subsequent recovery in normoxia. The levels of erythrocytic GTP were four times higher than ATP and were reduced to ¼ of the original level after 3h under hypoxia. Erythrocytic levels of ATP were unaffected over the experimental period. All other analyzed blood parameters exhibited a time-course change in animals under hypoxia and returned to normoxic levels. Considering the hemolysate functional properties and the ability to regulate the above mentioned blood characteristics, P. nigricans is able to endure short-term changes in dissolved oxygen.

Prolonged SDA and reduced digestive efficiency under elevated CO2 may explain reduced growth in Atlantic cod (Gadus morhua)

Land-based aquaculture systems expose fish to elevated dissolved CO2 levels, a factor that is correlated with reduced growth, feed conversion efficiency and body condition index. The physiological basis underlying the pathological effects of environmental hypercapnia is poorly understood, in particular for marine fish species. We investigated whether changes in energy expenditure and the specific dynamic action (SDA) of digestion and assimilation could account for the lower growth of adult Atlantic cod (Gadus morhua) under environmental hypercapnia. Fish acclimated to a partial pressure of 800 μatm CO2 (0.6 mmHg, 1.5 mg/L) and 9200 μatm CO2 (7 mmHg, 18.7 mg/L) exhibited no difference in maintenance metabolic rates, which concurs with previous research for this species and other fish species. At 9200 μatm CO2 Atlantic cod had a significantly diminished (14%) maximum aerobic capacity. While hypercapnia did not affect the amount of oxygen required for the SDA process, it did prolong the SDA duration by 23%. The longer SDA process time may offer an explanation for the observation of lower feed intake, growth and condition factor in long-term hypercapnia studies. Comparison of aerobic scope and cardiac performance during digestion suggested that reduced oxygen delivery capacity under hypercapnia could be one mechanism by which CO2 prolongs SDA, although our results could not definitively demonstrate this effect.