A methodological evaluation of the determination of critical oxygen threshold in an estuarine teleost

Whether hypoxia tolerance improved after short-term fasting is closely related to phylogeny but not to foraging mode in freshwater fish species

The combined stresses of fasting and hypoxia are common events during the life history of freshwater fish species. Hypoxia tolerance is vital for survival in aquatic environments, which requires organisms to down-regulate their maintenance energetic expenditure while simultaneously preserving physiological features such as oxygen supply capacity under conditions of food deprivation. Generally, infrequent-feeding species who commonly experience food shortages might evolve more adaptive strategies to cope with food deprivation than frequent-feeding species. Thus, the present study aimed to test whether the response of hypoxia tolerance in fish to short-term fasting (2 weeks) varied with different foraging modes. Fasting resulted in similar decreases in maintenance energetic expenditure and similar decreases in Pcrit and Ploe between fishes with different foraging modes, whereas it resulted in decreased oxygen supply capacity only in frequent-feeding fishes. Furthermore, independent of foraging mode, fasting decreased Pcrit and Ploe in all Cypriniformes and Siluriformes species but not in Perciformes species. The mechanism for decreased Pcrit and Ploe in Cypriniformes and Siluriformes species is at least partially due to the downregulated metabolic demand and/or the maintenance of a high oxygen supply capacity while fasting. The present study found that the effect of fasting on hypoxia tolerance depends upon phylogeny in freshwater fish species. The information acquired in the present study is highly valuable in aquaculture industries and can be used for species conservation in the field.

Implications of chronic hypoxia during development in red drum

Respiratory plasticity is a beneficial response to chronic hypoxia in fish. Red drum, a teleost that commonly experiences hypoxia in the Gulf of Mexico, have shown respiratory plasticity following sublethal hypoxia exposure as juveniles, but implications of hypoxia exposure during development are unknown. We exposed red drum embryos to hypoxia (40% air saturation) or normoxia (100% air saturation) for 3 days post fertilization (dpf). This time frame encompasses hatch and exogenous feeding. At 3 dpf, there was no difference in survival or changes in size. After the 3-day hypoxia exposure, all larvae were moved and reared in common normoxic conditions. Fish were reared for ∼3 months and effects of the developmental hypoxia exposure on swim performance and whole-animal aerobic metabolism were measured. We used a cross design wherein fish from normoxia (N=24) were exercised in swim tunnels in both hypoxia (40%, n=12) and normoxia (100%, n=12) conditions, and likewise for hypoxia-exposed fish (n=10 in each group). Oxygen consumption, critical swim speed (Ucrit), critical oxygen threshold (Pcrit) and mitochondrial respiration were measured. Hypoxia-exposed fish had higher aerobic scope, maximum metabolic rate, and higher liver mitochondrial efficiency relative to control fish in normoxia. Interestingly, hypoxia-exposed fish showed increased hypoxia sensitivity (higher Pcrit) and recruited burst swimming at lower swim speeds relative to control fish. These data provide evidence that early hypoxia exposure leads to a complex response in later life.

Spatiotemporal variability of oxygen concentration in coral reefs of Gorgona Island (Eastern Tropical Pacific) and its effect on the coral Pocillopora capitata

Dissolved oxygen concentration (DO) is one of the main factors limiting benthic species distribution. Due to ocean warming and eutrophication, the ocean is deoxygenating. In the Eastern Tropical Pacific (ETP), deep waters with low DO (<1 mg L^-1) may reach coral reefs, because upwelling will likely intensify due to climate change. To understand oxygen variability and its effects on corals, we characterize the Spatio-temporal changes of DO in coral reefs of Gorgona Island and calculate the critical oxygen tension (P _crit) to identify the DO concentration that could represent a hypoxic condition for Pocillopora capitata, one of the main reef-building species in the ETP. The mean (±SD) DO concentration in the coral reefs of Gorgona Island was 4.6 ± 0.89 mg L^-1. Low DO conditions were due to upwelling, but hypoxia (<3.71 mg L^-1, defined as a DO value 1 SD lower than the Mean) down to 3.0 mg O₂ L^-1 sporadically occurred at 10 m depth. The P _crit of P. capitata was 3.7 mg L^-1 and lies close to the hypoxic condition recorded on coral reefs during the upwelling season at 10 m depth. At Gorgona Island oxygen conditions lower than 2.3 mg L^-1 occur at >20 m depth and coincide with the deepest bathymetric distribution of scattered colonies of Pocillopora. Because DO concentrations in coral reefs of Gorgona Island were comparably low to other coral reefs in the Eastern Tropical Pacific, and the hypoxic threshold of P. capitata was close to the minimum DO record on reefs, hypoxic events could represent a threat if conditions that promote eutrophication (and consequently hypoxia) increase.

Aerobic scope falls to nil at Pcrit and anaerobic ATP production increases below Pcrit in the tidepool sculpin, Oligocottus maculosus

The critical oxygen tension of whole-animal oxygen uptake rate, or P_crit, has historically been defined as the oxygen partial pressure (P_O2) at which aerobic scope falls to zero and further declines in P_O2 require substrate-level phosphorylation to meet shortfalls in aerobic ATP production, thereby time-limiting survival. Despite the inclusion of aerobic scope and anaerobic ATP production in the definition, little effort has been made to verify that P_crit measurements, the vast majority of which are obtained using respirometry in resting animals, actually reflect the predictions of zero aerobic scope and a transition to increasing reliance on anaerobic ATP production. To test these predictions, we compared aerobic scope and levels of whole-body lactate at oxygen partial pressures (P_O2s) bracketing P_crit obtained in resting fish during progressive hypoxia in the tidepool sculpin, Oligocottus maculosus. We found that aerobic scope falls to zero at P_crit and, in resting fish exposed to P_O2s < P_crit, whole-body lactate accumulated pointing to an increased reliance on anaerobic ATP production. These results support the interpretation of P_crit as a key oxygen threshold at which aerobic scope falls to nil and, below P_crit, survival is time-limited based on anaerobic metabolic capacity.

The role of carbonic anhydrase-mediated tissue oxygen extraction in a marine teleost acclimated to hypoxia

With the growing prevalence of hypoxia (O2 levels ≤2 mg l−1) in aquatic and marine ecosystems, there is increasing interest in the adaptive mechanisms fish may employ to better their performance in stressful environments. Here, we investigated the contribution of a proposed strategy for enhancing tissue O2 extraction – plasma-accessible carbonic anhydrase (CA-IV) – under hypoxia in a species of estuarine fish (red drum, Sciaenops ocellatus) that thrives in fluctuating habitats. We predicted that hypoxia-acclimated fish would increase the prevalence of CA-IV in aerobically demanding tissues to confer more efficient tissue O2 extraction. Furthermore, we predicted the phenotypic changes to tissue O2 extraction that occur with hypoxia acclimation may improve respiratory and swim performance under 100% O2 conditions (i.e. normoxia) when compared with performance in fish that have not been acclimated to hypoxia. Interestingly, there were no significant differences in relative CA-IV mRNA expression, protein abundance or enzyme activity between the two treatments, suggesting CA-IV function is maintained under hypoxia. Likewise, respiratory performance of hypoxia-acclimated fish was similar to that of control fish when tested in normoxia. Critical swim speed (Ucrit) was significantly higher in hypoxia-acclimated fish but translated to marginal ecological benefits with an increase of ∼0.3 body lengths per second. Instead, hypoxia-acclimated fish may have relied more heavily on anaerobic metabolism during their swim trials, utilizing burst swimming 1.5 times longer than control fish. While the maintenance of CA-IV may still be an important contributor for hypoxia tolerance, our evidence suggests hypoxia-acclimated red drum are using other mechanisms to cope in an O2-depleted environment.

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.

Oxygen supply capacity breathes new life into critical oxygen partial pressure (Pcrit)

The critical oxygen partial pressure (Pcrit), typically defined as the PO2 below which an animal's metabolic rate (MR) is unsustainable, is widely interpreted as a measure of hypoxia tolerance. Here, Pcrit is defined as the PO2 at which physiological oxygen supply (α0) reaches its maximum capacity (α; µmol O2 g-1 h-1 kPa-1). α is a species- and temperature-specific constant describing the oxygen dependency of the maximum metabolic rate (MMR=PO2×α) or, equivalently, the MR dependence of Pcrit (Pcrit=MR/α). We describe the α-method, in which the MR is monitored as oxygen declines and, for each measurement period, is divided by the corresponding PO2 to provide the concurrent oxygen supply (α0=MR/PO2). The highest α0 value (or, more conservatively, the mean of the three highest values) is designated as α. The same value of α is reached at Pcrit for any MR regardless of previous or subsequent metabolic activity. The MR need not be constant (regulated), standardized or exhibit a clear breakpoint at Pcrit for accurate determination of α. The α-method has several advantages over Pcrit determination and non-linear analyses, including: (1) less ambiguity and greater accuracy, (2) fewer constraints in respirometry methodology and analysis, and (3) greater predictive power and ecological and physiological insight. Across the species evaluated here, α values are correlated with MR, but not Pcrit. Rather than an index of hypoxia tolerance, Pcrit is a reflection of α, which evolves to support maximum energy demands and aerobic scope at the prevailing temperature and oxygen level.

Coronary blood flow influences tolerance to environmental extremes in fish

Approximately half of all fishes have, in addition to the luminal venous O2 supply, a coronary circulation supplying the heart with fully oxygenated blood. Yet, it is not fully understood how coronary O2 delivery affects tolerance to environmental extremes such as warming and hypoxia. Hypoxia reduces arterial oxygenation, while warming increases overall tissue O2 demand. Thus, as both stressors are associated with reduced venous O2 supply to the heart, we hypothesised that coronary flow benefits hypoxia and warming tolerance. To test this hypothesis, we blocked coronary blood flow (via surgical coronary ligation) in rainbow trout (Oncorhynchus mykiss) and assessed how in vivo cardiorespiratory performance and whole-animal tolerance to acute hypoxia and warming was affected. While coronary ligation reduced routine stroke volume relative to trout with intact coronaries, cardiac output was maintained by an increase in heart rate. However, in hypoxia, coronary-ligated trout were unable to increase stroke volume to maintain cardiac output when bradycardia developed, which was associated with a slightly reduced hypoxia tolerance. Moreover, during acute warming, coronary ligation caused cardiac function to collapse at lower temperatures and reduced overall heat tolerance relative to trout with intact coronary arteries. We also found a positive relationship between individual hypoxia and heat tolerance across treatment groups, and tolerance to both environmental stressors was positively correlated with cardiac performance. Collectively, our findings show that coronary perfusion improves cardiac O2 supply and therefore cardiovascular function at environmental extremes, which benefits tolerance to natural and anthropogenically induced environmental perturbations.

Reduced Oxygen as an Environmental Pressure in the Evolution of the Blind Mexican Cavefish

Extreme environmental features can drive the evolution of extreme phenotypes. Over the course of evolution, certain environmental changes may be so drastic that they lead to extinction. Conversely, if an organism adapts to harsh environmental changes, the adaptations may permit expansion of a novel niche. The interaction between environmental stressors and adaptive changes is well-illustrated by the blind Mexican cavefish, Astyanaxmexicanus, which has recurrently adapted to the stark subterranean environment. The transition from terrestrial rivers and streams (occupied by extant surface morphs of the same species) to the cave has been accompanied by the resorption of eyes, diminished pigmentation and reduced metabolism in cave-dwelling morphs. The principal features of caves most often associated with evolution of these common cave features are the absence of light and limited nutrition. However, a putatively essential cave feature that has received less attention is the frequently low concentration of oxygen within natural karst environments. Here, we review the potential role of limited oxygen as a critical environmental feature of caves in the Sierra de El Abra. Additionally, we review evidence that Astyanax cavefish may have evolved adaptive features enabling them to thrive in lower oxygen compared to their surface-dwelling counterparts.

Personality and physiological traits predict contest interactions in Kryptolebias marmoratus

Personality and physiological traits often have close relationships with dominance status, but the significance and/or direction of the relationships vary between studies. This study examines whether two personality traits (aggressiveness and boldness) and three physiological traits (testosterone and cortisol levels and oxygen consumption rates) are associated with contest decisions/performance using a mangrove killifish Kryptolebias marmoratus. The results show that individuals that attacked their own mirror images (an aggressiveness index) at higher rates or had higher levels of testosterone were more likely to attack their opponent and win non-escalated contests, while individuals that had higher levels of cortisol were more likely to lose. After the contests, (1) individuals that had attacked their opponents or won had higher post-contest oxygen consumption rates, and (2) individuals that had attacked their opponents also had higher post-contest levels of cortisol. Although no significant correlations were detected among pre-contest physiological traits, post-contest levels of cortisol were positively correlated with oxygen consumption rates. Overall, personality and physiological traits provide useful predictors for the fish's contest decisions/performance. Contest interactions subsequently modified post-contest physiological traits and potentially also promoted associations between them. Nevertheless, the fish's physiological traits remained rather consistent over the entire study period.