Distinct physiological strategies are used to cope with constant hypoxia and intermittent hypoxia in killifish (Fundulus heteroclitus)

Interactive effects of elevated temperature and venlafaxine on mitochondrial respiration and enzymatic capacity in Nile tilapia (Oreochromis niloticus)

Warming events are becoming more frequent and extreme in aquatic environments worldwide. Concurrently, many environments are polluted with biologically active compounds such as pharmaceuticals. Understanding how these challenges interact is critical for understanding the climate crisis, as contaminants may modulate how ectotherms respond to heat stress or vice versa. One potential site for these heat × contaminant interactions is the mitochondrion, which is central to metabolism, implicated in thermal tolerance, and evolutionarily conserved. Using high-resolution respirometry, we investigated how acute warming (to 35 °C, 40 °C, or 45 °C from 25 °C) impacted the respiration, coupling, and metabolic capacity of liver mitochondria isolated from Nile tilapia, and how exposure to environmentally relevant levels of the ubiquitous antidepressant venlafaxine modulated those effects. Mitochondria exposed to hotter temperatures had higher respiration rates and decreased respiratory control ratio compared to mitochondria exposed to cooler temperatures. The depressive effects of venlafaxine on respiration rates through complex I and II or complex II only (State 3 and State 4), as well as complex IV-linked respiration, were mild except in mitochondria exposed to high temperatures, suggesting an interactive effect of warming and contaminant exposure. Finally, we found that the maximal enzyme activity of intact mitochondria (represented by mitochondrial respiration) showed a different pattern of response to warming and venlafaxine compared to its underlying components (as reflected by the activity of succinate dehydrogenase [complex II] and cytochrome c oxidase [complex IV]), demonstrating the value of incorporating both interactive and reductive approaches in understanding how mitochondria cope with anthropogenic changes in the environment.

A microcontroller-based system for flexible oxygen control in laboratory experiments

Environmental control systems are important tools for experimental researchers studying animal-environment interactions. Commercial systems for the measurement and regulation of environmental oxygen conditions are relatively expensive and cannot always be adapted to varying experimental applications. Here, I present a low-cost and highly flexible oxygen control system using Arduino microcontrollers in combination with a commercial optical oxygen sensor. Hardware and software examples are provided for three different applications: single-setpoint, sequential and long-term dissolved oxygen (DO) control. All tested control systems created the desired DO conditions with high accuracy and repeatability across trials. The resources provided shown here can be adapted and modified to be used in a variety of experimental contexts.

Gene expression and DNA methylation changes in response to hypoxia in toxicant-adapted Atlantic killifish (Fundulus heteroclitus)

Coastal fish populations are threatened by multiple anthropogenic impacts, including the accumulation of industrial contaminants and the increasing frequency of hypoxia. Some populations of the Atlantic killifish (Fundulus heteroclitus), like those in New Bedford Harbor (NBH), Massachusetts, have evolved a resistance to dioxin-like polychlorinated biphenyls (PCBs) that may influence their ability to cope with secondary stressors. To address this question, we compared hepatic gene expression and DNA methylation patterns in response to mild or severe hypoxia in killifish from NBH and Scorton Creek (SC), a reference population from a relatively pristine environment. We hypothesized that NBH fish would show altered responses to hypoxia due to trade-offs linked to toxicant resistance. Our results revealed substantial differences between populations. SC fish demonstrated a dose-dependent changes in gene expression in response to hypoxia, while NBH fish exhibited a muted transcriptional response to severe hypoxia. Interestingly, NBH fish showed significant DNA methylation changes in response to hypoxia, while SC fish did not exhibit notable epigenetic alterations. These findings suggest that toxicant-adapted killifish may face trade-offs in their molecular response to environmental stress, potentially impacting their ability to survive severe hypoxia in coastal habitats. Further research is needed to elucidate the functional implications of these epigenetic modifications and their role in adaptive stress responses.

Time and Tissue-Specific Responses of Mitochondrial Metabolism to Hypoxia in Fish

AbstractPeriods of hypoxia are extremely common in aquatic systems and are predicted to have enduring impacts on aquatic life. Mitochondrial metabolic responses are important for animal performance during hypoxia, but tissue-specific responses and time needed for mitochondria to adjust remain unclear. Here, we investigate how mitochondrial metabolism responds to hypoxia (50% air saturation) over a prolonged period (15-21 wk) in sea bass (Dicentrarchus labrax). We used a longitudinal assessment of mitochondria from three repeated, but nonlethal, samplings of red blood cells (RBCs) at 3-wk intervals (15, 18, and 21 wk of hypoxia) alongside a terminal sampling of two other tissues (liver and heart). We found that hypoxic fish increased their RBC oxidative phosphorylation between weeks 15 and 18 but did not change it between weeks 18 and 21. We also show that mitochondrial respiratory capacities were depressed in the heart but not in the liver or RBCs of sea bass held for 21 wk in hypoxia compared with those of sea bass maintained in normoxia. The time and tissue-specific responses to hypoxia likely have consequences for how organisms adjust their different organ functions under the constraints of oxygen availability. As the occurrence of hypoxia is expected to increase in marine ecosystems, our data also indicate that understanding temporal changes in mitochondrial metabolism is crucial to predict organismal responses in the face of ongoing environmental change.

Gulf toadfish (Opsanus beta) gill neuroepithelial cells in response to hypoxia exposure

Neuroepithelial cells (NECs) within the fish gill contain the monoamine neurochemical serotonin (5-HT), sense changes in the partial pressure of oxygen (PO2) in the surrounding water and blood, and initiate the cardiovascular and ventilatory responses to hypoxia. The distribution of neuroepithelial cells (NECs) within the gill is known for some fish species but not for the Gulf toadfish, Opsanus beta, a fish that has always been considered hypoxia tolerant. Furthermore, whether NEC size, number, or distribution changes after chronic exposure to hypoxia, has never been tested. We hypothesize that toadfish NECs will respond to hypoxia with an increase in NEC size, number, and a change in distribution. Juvenile toadfish (N = 24) were exposed to either normoxia (21.4 ± 0.0 kPa), mild hypoxia (10.2 ± 0.3 kPa), or severe hypoxia (3.1 ± 0.2 kPa) for 7 days and NEC size, number, and distribution for each O2 regime were measured. Under normoxic conditions, juvenile toadfish have similar NEC size, number, and distribution as other fish species with NECs along their filaments but not throughout the lamellae. The distribution of NECs did not change with hypoxia exposure. Mild hypoxia exposure had no effect on NEC size or number, but fish exposed to severe hypoxia had a higher NEC density (# per mm filament) compared to mild hypoxia-exposed fish. Fish exposed to severe hypoxia also had longer gill filament lengths that could not be explained by body weight. These results point to signs of phenotypic plasticity in these juvenile, lab-bred fish with no previous exposure to hypoxia and a strategy to deal with hypoxia exposure that differs in toadfish compared to other fish.

Repeated hypoxic episodes allow hematological and physiological habituation in rainbow trout

Introduction: Under climate change, the increase in temperature in aquatic environments may induce oxygen depletion. In extreme cases, low oxygen may become a limiting factor for fish, thus generating stress. In addition, consecutive hypoxic episodes may complicate the recovery of individuals and hinder their ability to modulate physiological and biochemical responses to maintain homeostasis. Thus, the aim of this study was to determine the hematological and physiological responses of rainbow trout under a condition of repeated hypoxic and manipulation stresses at three different time points. Methods: Every hypoxic episode consisted of exposing the fish to low dissolved oxygen concentrations (2 mgO2/L for 1 h). Following the exposure, the fish were allowed to recover for 1 h, after which they were sampled to investigate hematological and physiological parameters. Results and discussion: The results showed a pattern of habituation reflected by values of hematocrit, hemoglobin, and mean corpuscular volume, indicating a certain ability of rainbow trout to resist this type of repeated hypoxic events, provided that the fish can have some recovery time between the exposures.

Hypoxia-inducible factor 1 alpha protein increases without changes in mRNA during acute hypoxic exposure of the Gulf killifish, Fundulus grandis

The hypoxia inducible factor 1 (HIF1) is a central regulator of the molecular responses of animals to low oxygen. While the hypoxia-responsiveness of HIF1 is generally attributed to the stabilization of the alpha protein subunit (HIF1α) at low oxygen, several studies on fish report increased tissue levels of HIF1A mRNA during hypoxia, suggesting transcriptional regulation. In the current study, HIF1α protein and HIF1A mRNA were determined in parallel in tissues of Gulf killifish, Fundulus grandis, exposed to short-term hypoxia (24 h at 1 mg O2 l-1). HIF1α protein was higher in brain, ovary, and skeletal muscle from fish exposed to hypoxia compared with normoxic controls by 6 h, and it remained elevated in brain and ovary at 24 h. In contrast, HIF1A mRNA levels were unaffected by hypoxia in any tissue. Moreover, HIF1α protein and HIF1A mRNA levels in the same tissues were not correlated with one another during either normoxia or hypoxia. Hence, an increase in HIF1α protein does not depend upon an increase in HIF1A mRNA during acute exposure to low oxygen in this species. The results support the widely accepted mechanism of post-translational protein stabilization, rather than new transcription, during the initial response of fish to hypoxia.

Science communication in experimental biology: experiences and recommendations

During the century of Journal of Experimental Biology's existence, science communication has established itself as an interdisciplinary field of theory and practice. Guided by my experiences as a scientist and science writer, I argue that science communication skills are distinct from scientific communication skills and that engaging in science communication is particularly beneficial to early-career researchers; although taking on these dual roles is not without its difficulties, as I discuss in this Perspective. In the hope of encouraging more scientists to become science communicators, I provide: (i) general considerations for scientists looking to engage in science communication (knowing their audience, storytelling, avoiding jargon) and (ii) specific recommendations for crafting effective contributions on social media (content, packaging, engagement), an emerging, accessible and potentially impactful mode of science communication. Effective science communication can boost the work of experimental biologists: it can impact public opinion by incisively describing the consequences of the climate crisis and can raise social acceptance of fundamental research and experiments on animals.

Intergenerational plasticity to cycling high temperature and hypoxia affects offspring stress responsiveness and tolerance in zebrafish

Predicted climate change-induced increases in heat waves and hypoxic events will have profound effects on fishes, yet the capacity of parents to alter offspring phenotype via non-genetic inheritance and buffer against these combined stressors is not clear. This study tested how prolonged adult zebrafish exposure to combined diel cycles of thermal stress and hypoxia affect offspring early survival and development, parental investment of cortisol and heat shock proteins (HSPs), larval offspring stress responses, and both parental and offspring heat and hypoxia tolerance. Parental exposure to the combined stressor did not affect fecundity, but increased mortality, produced smaller embryos and delayed hatching. The combined treatment also reduced maternal deposition of cortisol and increased embryo hsf1, hsp70a, HSP70, hsp90aa and HSP90 levels. In larvae, basal cortisol levels did not differ between treatments, but acute exposure to combined heat stress and hypoxia increased cortisol levels in control larvae with no effect on larvae from exposed parents. In contrast, whereas larval basal hsf1, hsp70a and hsp90aa levels differed between parental treatments, the combined acute stressor elicited similar transcriptional responses across treatments. Moreover, the combined acute stressor only induced a marked increase in HSP47 levels in the larvae derived from exposed parents. Finally, combined hypoxia and elevated temperatures increased both thermal and hypoxia tolerance in adults and conferred an increase in offspring thermal but not hypoxia tolerance. These results demonstrate that intergenerational acclimation to combined thermal stress and hypoxia elicit complex carryover effects on stress responsiveness and offspring tolerance with potential consequences for resilience.

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.

Physiological and behavioural strategies of aquatic animals living in fluctuating environments

Shallow or near-shore environments, such as ponds, estuaries and intertidal zones, are among the most physiologically challenging of all aquatic settings. Animals inhabiting these environments experience conditions that fluctuate markedly over relatively short temporal and spatial scales. Living in these habitats requires the ability to tolerate the physiological disturbances incurred by these environmental fluctuations. This tolerance is achieved through a suite of physiological and behavioural responses that allow animals to maintain homeostasis, including the ability to dynamically modulate their physiology through reversible phenotypic plasticity. However, maintaining the plasticity to adjust to some stresses in a dynamic environment may trade off with the capacity to deal with other stressors. This paper will explore studies on select fishes and invertebrates exposed to fluctuations in dissolved oxygen, salinity and pH. We assess the physiological mechanisms these species employ to achieve homeostasis, with a focus on the plasticity of their responses, and consider the resulting physiological trade-offs in function. Finally, we discuss additional factors that may influence organismal responses to fluctuating environments, such as the presence of multiple stressors, including parasites. We echo recent calls from experimental biologists to consider physiological responses to life in naturally fluctuating environments, not only because they are interesting in their own right but also because they can reveal mechanisms that may be crucial for living with increasing environmental instability as a consequence of climate change.

Effects of Biophysical Processes on Diel-Cycling Hypoxia in a Subtropical Estuary

In shallow estuaries, fluctuations in bottom dissolved oxygen (DO) at diel (24 h) timescales are commonly attributed to cycles of net production and respiration. However, bottom DO can also be modulated by physical processes, such as tides and wind, that vary at or near diel timescales. Here, we examine processes affecting spatiotemporal variations in diel-cycling DO in Escambia Bay, a shallow estuary along the Gulf of Mexico. We collected continuous water quality measurements in the upper and middle reaches of the Bay following relatively high (> 850 m³ s^-1) and low (< 175 m³ s^-1) springtime freshwater discharge. Variations in diel-cycling amplitude over time were estimated using the continuous wavelet transform, and correlations between DO and biophysical processes at diel timescales were examined using wavelet coherence. Our results reveal that freshwater discharge modulated inter-annual variations in the spatial extent and duration of summertime hypoxia through its effect on vertical density stratification. In the absence of strong stratification (> 15 kg m^-3), vertical mixing by tropic tides and sea breeze enhanced diel fluctuations in deeper areas near the channel, while in shallower areas the largest fluctuations were associated with irradiance. Our findings suggest that processes affecting diel-cycling DO in the bottom layer can vary over a relatively short spatial extent less than 2 km and with relatively small changes in bottom elevation of 1 m or less. Implications for water quality monitoring were illustrated by subsampling DO timeseries, which demonstrates how low-frequency measurements may misrepresent water quality in estuaries where diel-cycling DO is common. In these systems, adequate assessment of hypoxia and its aquatic life impacts requires continuous measurements that capture the variation in DO at diel timescales.

Different patterns of chronic hypoxia lead to hierarchical adaptative mechanisms in goldfish metabolism

Some hypoxia-tolerant species, such as goldfish, experience intermittent and severe hypoxia in their natural habitat causing them to develop multiple physiological adaptations. However, in fish, the metabolic impact of regular hypoxic exposure on swimming performance in normoxia is less well understood. Therefore, we experimentally tested whether chronic exposure to constant (30 days at 10% air saturation) or intermittent hypoxia (3hrs in normoxia and 21hrs in hypoxia, 5 days a week) would result in similar metabolic and swimming performance benefits after reoxygenation. Moreover, half of the normoxic and intermittent hypoxic fish were put on a 20-day normoxic training regime. After these treatments, metabolic rate (standard and maximum metabolic rates: SMR and MMR) and swimming performance (critical swimming speed [Ucrit] and cost of transport [COT]) were assessed. In addition, enzyme activities (citrate synthase CS, cytochrome c oxidase COX and lactate dehydrogenase LDH) and mitochondrial respiration were examined in red muscle fibres. We found that acclimation to constant hypoxia resulted in (1) metabolic suppression (-45% SMR, and -27% MMR), (2) increased anaerobic capacity (+117% LDH), (3) improved swimming performance (+80% Ucrit, -71% COT) and (4) no changes at the mitochondrial level. Conversely, the enhancement of swimming performance was reduced following acclimation to intermittent hypoxia (+45% Ucrit, -41% COT), with a 55% decrease in aerobic scope, despite a significant increase in oxidative metabolism (+201% COX, +49% CS). This study demonstrates that constant hypoxia leads to the greatest benefit in swimming performance and that mitochondrial metabolic adjustments only provide minor help in coping with hypoxia.

Lophiosilurus alexandri, a sedentary bottom fish, adjusts its physiological parameters to survive in hypoxia condition

We investigated blood gas, hematological and biochemical parameters, and gill morphology of Lophiosilurus alexandri juveniles submitted to hypoxia for 48 h, followed by recovery for 48 h. A total of 48 juveniles (360.0 ± 141.6 g) were distributed among eight tanks (120 L) and subjected to hypoxia condition (water with dissolved oxygen at 2.12 ± 0.90 mg L^-1) or normoxia (at 5.60 ± 0.31 mg L^-1). Blood gas values (pH, PvCO₂, PvO₂, sO₂, HCO₃^-, stHCO₃^-, and base excess) in hypoxia were significantly different from normoxia, while for lactate and the electrolytes (K⁺, Na⁺, Cl^-, and Ca²⁺) there was no significant change among treatments. The erythrocytes differed significantly between normoxia and hypoxia at 72 h (24 h of recovery), while for hemoglobin and hematocrit there were no significant differences. There was a significant difference in glucose, triglycerides, and cholesterol for both normoxia and hypoxia, while plasma protein remained unchanged. All gill components (epithelial cells, erythrocytes, pillar cells, mucous cells, ionocytes, undifferentiated cells, and blood capillary lumen) differed significantly between normoxia and hypoxia. A reduction in the length of the primary lamella was observed in the hypoxia and recovery treatments, when compared to normoxia. The secondary branchial lamella showed no significant difference for both treatments. Juveniles of Lophiosilurus alexandri adapted well to hypoxia for 48 h, as they were able to adjust most of their physiological variables to survive this stress condition. After 48 h of hypoxia recovery, fish showed parameters similar to animals in normoxia. Thus, the present study shows that the tolerance to hypoxia conditions of L. alexandri, together with other important beneficial characteristics of the species, such as the high meat quality and high commercial value, demonstrates its great potential for production among regional species.

Seeing in the swamp: hydrogen sulfide inhibits eye metabolism and visual acuity in a sulfide-tolerant fish

In fish, vision may be impaired when eye tissue is in direct contact with environmental conditions that limit aerobic ATP production. We hypothesized that the visual acuity of fishes exposed to hydrogen sulfide (H₂S)-rich water would be altered owing to changes in cytochrome c oxidase (COX) activity. Using the H₂S-tolerant mangrove rivulus (Kryptolebias marmoratus), we showed that a 10 min exposure to greater than or equal to 200 µM of H₂S impaired visual acuity and COX activity in the eye. Visual acuity and COX activity were restored in fish allowed to recover in H₂S-free water for up to 1 h. Since K. marmoratus are found in mangrove pools with H₂S concentrations exceeding 1000 µM, visual impairment may impact predator avoidance, navigation and foraging behaviour in the wild.

Effects of polycyclic aromatic hydrocarbons and abiotic stressors on Fundulus grandis cardiac transcriptomics

Following the 2010 Deepwater Horizon oil spill, extensive research has been conducted on the toxicity of oil and polycyclic aromatic hydrocarbons (PAHs) in the aquatic environment. Many studies have identified the toxicological effects of PAHs in estuarine and marine fishes, however, only recently has work begun to identify the combinatorial effect of PAHs and abiotic environmental factors such as hypoxia, salinity, and temperature. This study aims to characterize the combined effects of abiotic stressors and PAH exposure on the cardiac transcriptomes of developing Fundulus grandis larvae. In this study, F. grandis larvae were exposed to varying environmental conditions (dissolved oxygen (DO) 2, 6 ppm; temperature 20, 30 °C; and salinity 3, 30 ppt) as well as to a single concentration of high energy water accommodated fraction (HEWAF) (∑PAHs 15 ppb). Whole larvae were sampled for RNA and transcriptional changes were quantified using RNA-Seq followed by qPCR for a set of target genes. Analysis revealed that exposure to oil and abiotic stressors impacts signaling pathways associated with cardiovascular function. Specifically, combined exposures appear to reduce development of the systemic vasculature as well as strongly impact the cardiac musculature through cardiomyocyte proliferation resulting in inhibited cardiac function and modulated blood pressure maintenance. Results of this study provide a holistic view of impacts of PAHs and common environmental stressors on the cardiac system in early life stage estuarine species. To our knowledge, this study is one of the first to simultaneously manipulate oil exposure with abiotic factors (DO, salinity, temperature) and the first to analyze cardiac transcriptional responses under these co-exposures.

Parental Males of the Plainfin Midshipman Are Physiologically Resilient to the Challenges of the Intertidal Zone

The decision of where to rear young is influenced by both the needs of offspring and the costs parents incur in certain rearing environments. Plainfin midshipman fish (Porichthys notatus) provide extended paternal care in rocky intertidal zones, where they experience regular bouts of aquatic hypoxia and air exposure during low-tide events. We investigated the physiological responses of plainfin midshipman males to three conditions for 6 h that simulate what these fish naturally experience during tidal cycles while nesting: normoxia, progressive hypoxia, or air exposure. Hypoxia- and air-exposed fish exhibited shifts in energy metabolites, driven largely by elevated lactate and glucose content and reduced glycogen content in several tissues (muscle, heart, liver, and brain), but the magnitude of these changes was relatively modest. Hematocrit increased most in air-exposed fish relative to normoxia-exposed fish, contributing to an increase in whole-blood hemoglobin concentration. Air exposure reduced swim bladder oxygen content, suggesting that internal O₂ stores are drawn on during air exposure. In a second experiment, we found that aquatic surface respiration and gill ventilation frequency increased in hypoxia-exposed fish relative to normoxia-exposed fish. Overall, our results suggest that plainfin midshipman overcome the challenges of the intertidal environment through a variety of physiological strategies and exhibit little physiological disturbance in response to the fluctuating and extreme conditions created by regular low tides.

Identification of Hypoxia-Specific Biomarkers in Salmonids Using RNA-Sequencing and Validation Using High-Throughput qPCR

Identifying early gene expression responses to hypoxia (i.e., low dissolved oxygen) as a tool to assess the degree of exposure to this stressor is crucial for salmonids, because they are increasingly exposed to hypoxic stress due to anthropogenic habitat change, e.g., global warming, excessive nutrient loading, and persistent algal blooms. Our goal was to discover and validate gill gene expression biomarkers specific to the hypoxia response in salmonids across multi-stressor conditions. Gill tissue was collected from 24 freshwater juvenile Chinook salmon (Oncorhynchus tshawytscha), held in normoxia [dissolved oxygen (DO) > 8 mg L^-1] and hypoxia (DO = 4‒5 mg L^-1) in 10 and 18° temperatures for up to six days. RNA-sequencing (RNA-seq) was then used to discover 240 differentially expressed genes between hypoxic and normoxic conditions, but not affected by temperature. The most significantly differentially expressed genes had functional roles in the cell cycle and suppression of cell proliferation associated with hypoxic conditions. The most significant genes (n = 30) were selected for real-time qPCR assay development. These assays demonstrated a strong correlation (r = 0.88; P < 0.001) between the expression values from RNA-seq and the fold changes from qPCR. Further, qPCR of the 30 candidate hypoxia biomarkers was applied to an additional 322 Chinook salmon exposed to hypoxic and normoxic conditions to reveal the top biomarkers to define hypoxic stress. Multivariate analyses revealed that smolt stage, water salinity, and morbidity status were relevant factors to consider with the expression of these genes in relation to hypoxic stress. These hypoxia candidate genes will be put into application screening Chinook salmon to determine the identity of stressors impacting the fish.

Fluctuating environments during early development can limit adult phenotypic flexibility: insights from an amphibious fish

The interaction between developmental plasticity and the capacity for reversible acclimation (phenotypic flexibility) is poorly understood, particularly in organisms exposed to fluctuating environments. We used an amphibious killifish (Kryptolebias marmoratus) to test the hypotheses that organisms reared in fluctuating environments (i) will make no developmental changes to suit any one environment because fixing traits to suit one environment could be maladaptive for another, and (ii) will be highly phenotypically flexible as adults because their early life experiences predict high environmental variability in the future. We reared fish under constant (water) or fluctuating (water-air) environments until adulthood and assessed a suite of traits along the oxygen cascade (e.g. neuroepithelial cell density and size, cutaneous capillarity, gill morphology, ventricle size, red muscle morphometrics, terrestrial locomotor performance). To evaluate the capacity for phenotypic flexibility, a subset of adult fish from each rearing condition was then air-exposed for 14 days before the same traits were measured. In support of the developmental plasticity hypothesis, traits involved with O2 sensing and uptake were largely unaffected by water-air fluctuations during early life, but we found marked developmental changes in traits related to O2 transport, utilization and locomotor performance. In contrast, we found no evidence supporting the phenotypic flexibility hypothesis. Adult fish from both rearing conditions exhibited the same degree of phenotypic flexibility in various O2 sensing- and uptake-related traits. In other cases, water-air fluctuations attenuated adult phenotypic flexibility despite the fact that phenotypic flexibility is hypothesized to be favoured when environments fluctuate. Overall, we conclude that exposure to environmental fluctuations during development in K. marmoratus can dramatically alter the constitutive adult phenotype, as well as diminish the scope for phenotypic flexibility in later life.

Plasticity, repeatability and phenotypic correlations of aerobic metabolic traits in a small estuarine fish

Standard metabolic rate (SMR), maximum metabolic rate (MMR), absolute aerobic scope (AAS) and critical oxygen tension (P_crit) were determined for the Gulf killifish, Fundulus grandis, an ecologically dominant estuarine fish, acclimated to lowered salinity, elevated temperature and lowered oxygen concentration. Acclimation to low salinity resulted in a small, but significant, elevation of P_crit (suggesting lower tolerance of hypoxia); acclimation to elevated temperature increased SMR, MMR, AAS and P_crit; acclimation to low oxygen led to a small increase in SMR, but substantial decreases in MMR, AAS and P_crit Variation in these metabolic traits among individuals was consistent and repeatable when measured during multiple control exposures over 7 months. Trait repeatability was unaffected by acclimation condition, suggesting that repeatability of these traits is not context dependent. There were significant phenotypic correlations between specific metabolic traits: SMR was positively correlated with MMR and P_crit; MMR was positively correlated with AAS; and AAS was negatively correlated with P_crit In general, within-individual variation contributed more than among-individual variation to these phenotypic correlations. The effects of acclimation on these traits demonstrate that aerobic metabolism is plastic and influenced by the conditions experienced by these fish in the dynamic habitats in which they occur; however, the repeatability of these traits and the correlations among them suggest that these traits change in ways that maintain the rank order of performance among individuals across a range of environmental variation.

Plasticity, repeatability and phenotypic correlations of aerobic metabolic traits in a small estuarine fish

Standard metabolic rate (SMR), maximum metabolic rate (MMR), absolute aerobic scope (AAS) and critical oxygen tension (Pcrit) were determined for the Gulf killifish, Fundulus grandis, an ecologically dominant estuarine fish, acclimated to lowered salinity, elevated temperature and lowered oxygen concentration. Acclimation to low salinity resulted in a small, but significant, elevation of Pcrit (suggesting lower tolerance of hypoxia); acclimation to elevated temperature increased SMR, MMR, AAS and Pcrit; acclimation to low oxygen led to a small increase in SMR, but substantial decreases in MMR, AAS and Pcrit Variation in these metabolic traits among individuals was consistent and repeatable when measured during multiple control exposures over 7 months. Trait repeatability was unaffected by acclimation condition, suggesting that repeatability of these traits is not context dependent. There were significant phenotypic correlations between specific metabolic traits: SMR was positively correlated with MMR and Pcrit; MMR was positively correlated with AAS; and AAS was negatively correlated with Pcrit In general, within-individual variation contributed more than among-individual variation to these phenotypic correlations. The effects of acclimation on these traits demonstrate that aerobic metabolism is plastic and influenced by the conditions experienced by these fish in the dynamic habitats in which they occur; however, the repeatability of these traits and the correlations among them suggest that these traits change in ways that maintain the rank order of performance among individuals across a range of environmental variation.

Hypoxia acclimation alters reactive oxygen species homeostasis and oxidative status in estuarine killifish (Fundulus heteroclitus)

Hypoxia is common in aquatic environments, and exposure to hypoxia followed by re-oxygenation is often believed to induce oxidative stress. However, there have been relatively few studies of reactive oxygen species (ROS) homeostasis and oxidative status in fish that experience natural hypoxia-re-oxygenation cycles. We examined how exposure to acute hypoxia (2 kPa O2) and subsequent re-oxygenation (to 20 kPa O2) affects redox status, oxidative damage and anti-oxidant defenses in estuarine killifish (Fundulus heteroclitus), and whether these effects were ameliorated or potentiated by prolonged (28 days) acclimation to either constant hypoxia or intermittent cycles of nocturnal hypoxia (12 h:12 h normoxia:hypoxia). Acute hypoxia and re-oxygenation led to some modest and transient changes in redox status, increases in oxidized glutathione, depletion of scavenging capacity and oxidative damage to lipids in skeletal muscle. The liver had greater scavenging capacity, total glutathione concentrations and activities of anti-oxidant enzymes (catalase, glutathione peroxidase) than muscle, and generally experienced less variation in glutathiones and lipid peroxidation. Unexpectedly, acclimation to constant hypoxia or intermittent hypoxia led to a more oxidizing redox status (muscle and liver) and it increased oxidized glutathione (muscle). However, hypoxia-acclimated fish exhibited little to no oxidative damage (as reflected by lipid peroxidation and aconitase activity), in association with improvements in scavenging capacity and catalase activity in muscle. We conclude that hypoxia acclimation leads to adjustments in ROS homeostasis and oxidative status that do not reflect oxidative stress, but may instead be part of the suite of responses that killifish use to cope with chronic hypoxia.

Elevated pCO2 and Hypoxia Alter the Acid-Base Homeostasis of Developing Sheepshead Minnows, Cyprinodon variegatus

Low dissolved oxygen and increased acidification are two environmental variables that concomitantly change in an estuarine environment, both of which are exacerbated by nutrient pollution and subsequent eutrophication. To better understand how estuarine residents compensate for daily fluctuations in these environmental variables, the interactive effects of acidification and hypoxia were assessed in developing sheepshead minnows (Cyprinodon variegatus) using a 2 by 2 factorial design over a 42-day exposure. Embryos were exposed to either acidic (partial pressure of CO₂, pCO₂, ~2000 μatm), hypoxic (reduced dissolved oxygen, ~2 mg l^-1), or combined acidic and hypoxic conditions and monitored for development, hatch rate, and survival. Changes in oxygen consumption, anaerobic metabolism, oxidative stress, and acid-base balance were evaluated at three life stages (embryo, larval, and juvenile fish) to discern if and how fish compensate for these stressors during development. The combination of acidification and hypoxia delayed hatching in embryos and significantly decreased oxygen consumption (p<0.001) in all three life-stages. Neither acidification, hypoxia, nor the combination of the stressors impacted the anaerobic metabolism or oxidative stress of juvenile fish, but acid-base equilibrium was disrupted by all three treatments in larval fish. Elevated carbonic anhydrase activity was observed in the multi-stress treatment in embryos and larval fish, but not in juvenile fish. These results show that developing sheepshead minnows can re-establish cellular homeostasis in compensating to acidified and hypoxic waters.

Interspecific variation in hypoxia tolerance and hypoxia acclimation responses in killifish from the family Fundulidae

Hypoxia is a pervasive stressor in aquatic environments, and both phenotypic plasticity and evolutionary adaptation could shape the ability to cope with hypoxia. We investigated evolved variation in hypoxia tolerance and the hypoxia acclimation response across fundulid killifishes that naturally experience different patterns of hypoxia exposure. We compared resting O2 consumption rate (ṀO2 ), and various indices of hypoxia tolerance [critical O2 tension (Pcrit), regulation index (RI), O2 tension (PO2 ) at loss of equilibrium (PLOE) and time to LOE (tLOE) at 0.6 kPa O2] in Fundulus confluentus, Fundulus diaphanus, Fundulus heteroclitus, Fundulus rathbuni, Lucania goodei and Lucania parva We examined the effects of chronic (28 days) exposure to constant hypoxia (2 kPa) or nocturnal intermittent hypoxia (12 h normoxia:12 h hypoxia) in a subset of species. Some species exhibited a two-breakpoint model in ṀO2  caused by early, modest declines in ṀO2  in moderate hypoxia. We found that hypoxia tolerance varied appreciably across species: F. confluentus was the most tolerant (lowest PLOE and Pcrit, longest tLOE), whereas F. rathbuni and F. diaphanus were the least tolerant. However, there was not a consistent pattern of interspecific variation for different indices of hypoxia tolerance, with or without taking phylogenetic relatedness into account, probably because these different indices are underlain by partially distinct mechanisms. Hypoxia acclimation generally improved hypoxia tolerance, but the magnitude of plasticity and responsiveness to different hypoxia patterns varied interspecifically. Our results therefore suggest that hypoxia tolerance is a complex trait that is best appreciated by considering multiple indices of tolerance.

Oxygen-mediated plasticity confers hypoxia tolerance in a corallivorous polychaete

There is mounting evidence that the deoxygenation of coastal marine ecosystems has been underestimated, particularly in the tropics. These physical conditions appear to have far-reaching consequences for marine communities and have been associated with mass mortalities. Yet little is known about hypoxia in tropical habitats or about the effects it has on reef-associated benthic organisms. We explored patterns of dissolved oxygen (DO) throughout Almirante Bay, Panama and found a hypoxic gradient, with areas closest to the mainland having the largest diel variation in DO, as well as more frequent persistent hypoxia. We then designed a laboratory experiment replicating the most extreme in situ DO regime found on shallow patch reefs (3 m) to assess the response of the corallivorous fireworm, Hermodice carnaculata to hypoxia. Worms were exposed to hypoxic conditions (8 hr ~ 1 mg/L or 3.2 kPa) 16 times over an 8-week period, and at 4 and 8 weeks, their oxygen consumption (respiration rates) was measured upon reoxygenation, along with regrowth of severed gills. Exposure to low DO resulted in worms regenerating significantly larger gills compared to worms under normoxia. This response to low DO was coupled with an ability to maintain elevated oxygen consumption/respiration rates after low DO exposure. In contrast, worms from the normoxic treatment had significantly depressed respiration rates after being exposed to low DO (week 8). This indicates that oxygen-mediated plasticity in both gill morphology and physiology may confer tolerance to increasingly frequent and severe hypoxia in one important coral predator associated with reef decline.

Influence of 96h sub-lethal copper exposure on aerobic scope and recovery from exhaustive exercise in killifish (Fundulus heteroclitus)

Production of industrial effluents have led to increased copper (Cu) pollution of aquatic ecosystems, impacting the physiology of aquatic vertebrates. Past work has shown that Cu exerts its toxicity by disruption ion regulation and/ or increasing oxidative stress. However, it remains unclear how Cu may influence aerobic metabolism and hypoxia tolerance, two possible targets of its toxicity. To address this issue, we exposed freshwater acclimated killifish (F. heteroclitus) to a 96 h Cu exposure at a target concentration of 100 μg L^-1. We determined resting oxygen consumption (ṀO₂), ṀO_2max after exhaustive exercise, and followed ṀO₂ for 3 h in post-exercise recovery in water with either no Cu or 100 μg L^-1 Cu. We assessed hypoxia tolerance by determining the critical oxygen tension (P_crit). It was found that killifish exposed to combined 96 h Cu exposure and Cu present during metabolic measurements, showed a significant decrease in ṀO_2max and in aerobic scope (ṀO_2max - ṀO_2rest), compared to control fish. However, changes in blood and muscle lactate and muscle glycogen were not consistent with an upregulation of anaerobic metabolism as compensation for reduced aerobic performance in Cu exposed fish. Hypoxia tolerance was not influenced by the 96 h Cu exposure or by presence or absence of Cu during the P_crit test. This study suggests that Cu differentially influences responses to changes in oxygen demand and oxygen availability.

Cortisol modulates metabolism and energy mobilization in wild-caught pumpkinseed (Lepomis gibbosus)

Acute elevation of cortisol via activation of the hypothalamic-pituitary-interrenal (HPI) axis aids the fish in dealing with a stressor. However, chronic elevation of cortisol has detrimental effects and has been studied extensively in lab settings. However, data pertaining to wild teleosts are lacking. Here, we characterized the metabolic consequences of prolonged cortisol elevation (96 h) in wild-caught pumpkinseed (Lepomis gibbosus). Pumpkinseed were implanted with cocoa butter alone (sham) or containing cortisol (25 mg kg^-1 body weight), and at 24, 48, 72, and 96 h, tissue samples were collected, whole-body ammonia excretion was determined, and whole-organism metabolism was assessed using intermittent flow respirometry. Cortisol-treated pumpkinseed exhibited the highest plasma cortisol concentration at 24 h post-implantation, with levels decreasing over the subsequent time points although remaining higher than in sham-treated fish. Cortisol-treated fish exhibited higher standard and maximal metabolic rates than sham-treated fish, but the effect of cortisol treatment on aerobic scope was negligible. Indices of energy synthesis/mobilization, including blood glucose concentrations, hepatosomatic index, hepatic glycogen concentrations, and ammonia excretion rates, were higher in cortisol-treated fish compared with controls. Our work suggests that although aerobic scope was not diminished by prolonged elevation of cortisol levels, higher metabolic expenditures may be of detriment to the animal's performance in the longer term.

Hypoxia during incubation does not affect aerobic performance or haematology of Atlantic salmon (Salmo salar) when re-exposed in later life

Hypoxia in aquatic ecosystems is becoming increasingly prevalent, potentially reducing fish performance and survival by limiting the oxygen available for aerobic activities. Hypoxia is a challenge for conserving and managing fish populations and demands a better understanding of the short- and long-term impacts of hypoxic environments on fish performance. Fish acclimate to hypoxia via a variety of short- and long-term physiological modifications in an attempt to maintain aerobic performance. In particular, hypoxia exposure during early development may result in enduring cardio-respiratory modifications that affect future hypoxia acclimation capacity, yet this possibility remains poorly investigated. We incubated Atlantic salmon (Salmo salar) in normoxia (~100% dissolved oxygen [DO, as percent air saturation]), moderate hypoxia (~63% DO) or cyclical hypoxia (100-25% DO daily) from fertilization until 113 days post-fertilization prior to rearing all groups in normoxia for a further 8 months. At ~11 months of age, subsets of each group were acclimated to hypoxia (50% DO) for up to 44 days prior to haematology, aerobic metabolic rate and hypoxia tolerance measurements. Hypoxia exposure during incubation (fertilization to 113 days post-fertilization) did not affect the haematology, aerobic performance or hypoxia tolerance of juvenile salmon in later life. Juveniles acclimated to hypoxia increased maximum aerobic metabolic rate and aerobic scope by ~23 and ~52%, respectively, when measured at 50% DO but not at 100% DO. Hypoxia-incubated juveniles also increased haematocrit and haemoglobin concentration but did not affect acute hypoxia tolerance (critical oxygen level and DO at LOE). Thus, while Atlantic salmon possess a considerable capacity to physiologically acclimate to hypoxia by improving aerobic performance in low oxygen conditions, we found no evidence that this capacity is influenced by early-life hypoxia exposure.

Cardiac mitochondrial function, nitric oxide sensitivity and lipid composition following hypoxia acclimation in sablefish

In fishes, the effect of O₂ limitation on cardiac mitochondrial function remains largely unexplored. The sablefish (Anoplopoma fimbria) encounters considerable variations in environmental oxygen availability, and is an interesting model for studying the effects of hypoxia on fish cardiorespiratory function. We investigated how in vivo hypoxia acclimation (6 months at 40% then 3 weeks at 20% air saturation) and in vitro anoxia-reoxygenation affected sablefish cardiac mitochondrial respiration and reactive oxygen species (ROS) release rates using high-resolution fluorespirometry. Further, we investigated how hypoxia acclimation affected the sensitivity of mitochondrial respiration to nitric oxide (NO), and compared mitochondrial lipid and fatty acid (FA) composition between groups. Hypoxia acclimation did not alter mitochondrial coupled or uncoupled respiration, or respiratory control ratio, ROS release rates, P ₅₀ or superoxide dismutase activity. However, it increased citrate synthase activity (by ∼20%), increased the sensitivity of mitochondrial respiration to NO inhibition (i.e., the NO IC₅₀ was 25% lower), and enhanced the recovery of respiration (by 21%) and reduced ROS release rates (by 25-30%) post-anoxia. In addition, hypoxia acclimation altered mitochondrial FA composition [increasing arachidonic acid (20:4ω6) and eicosapentaenoic acid (20:5ω3) proportions by 11 and 14%, respectively], and SIMPER analysis revealed that the phospholipid:sterol ratio was the largest contributor (24%) to the dissimilarity between treatments. Overall, these results suggest that hypoxia acclimation may protect sablefish cardiac bioenergetic function during or after periods of O₂ limitation, and that this may be related to alterations in mitochondrial sensitivity to NO and to adaptive changes in membrane composition (fluidity).

The utility and determination of P crit in fishes

The critical O₂ tension (P _crit) is the lowest P _O2  at which an animal can maintain some benchmark rate of O₂ uptake (Ṁ _O2 ). This P _O2  has long served as a comparator of hypoxia tolerance in fishes and aquatic invertebrates, but its usefulness in this role, particularly when applied to fishes, has recently been questioned. We believe that P _crit remains a useful comparator of hypoxia tolerance provided it is determined using the proper methods and hypoxia tolerance is clearly defined. Here, we review the available methods for each of the three steps of P _crit determination: (1) measuring the most appropriate benchmark Ṁ _O2  state for P _crit determination (Ṁ _O2,std, the Ṁ _O2  required to support standard metabolic rate); (2) reducing water P _O2 ; and (3) calculating P _crit from the Ṁ _O2  versus P _O2  curve. We make suggestions on best practices for each step and for how to report P _crit results to maximize their comparative value. We also discuss the concept of hypoxia tolerance and how P _crit relates to a fish's overall hypoxia tolerance. When appropriate methods are used, P _crit provides useful comparative physiological and ecological information about the aerobic contributions to a fish's hypoxic survival. When paired with other hypoxia-related physiological measurements (e.g. lactate accumulation, calorimetry-based measurements of metabolic depression, loss-of-equilibrium experiments), P _crit contributes to a comprehensive understanding of how a fish combines aerobic metabolism, anaerobic metabolism and metabolic depression in an overall strategy for hypoxia tolerance.

The effect of normoxia exposure on hypoxia tolerance and sensory sampling in a swamp-dwelling mormyrid fish

Effects of energetic limitations on the performance of sensory systems are generally difficult to quantify. Weakly electric fishes provide an ideal model system to quantify the effects of metabolic stressors on sensory information acquisition, because they use an active-sensing strategy that permits easy measurement of the sensing effort. These fishes discharge an electric signal and sense perturbations of the resulting electric field. We used the mormyrid Petrocephalus degeni to quantify the relationship between routine metabolic rate and the rate of sensory sampling (rate of electric organ discharge, EOD) while under progressive hypoxia by quantifying the critical oxygen tension (P_C-MR) and the critical electric organ discharge threshold (P_C-EOD). P_C-MR was significantly higher in fish acclimated to normoxia for over 40 days compared to animals tested within 1-5 days of capture from a hypoxic swamp, which suggests high costs of maintaining hypoxia tolerance; however, there was no acclimation effect on P_C-EOD. All P. degeni reached their P_C-EOD prior to their P_C-MR. However, below the respective critical tension value, EOD rate decreased more gradually than the metabolic rate suggesting that the fish were increasing the proportion of their energy budget allocated to acquiring sensory information as dissolved-oxygen levels dropped. Trade-offs between sensory sampling and other physiological functions are also suggested by the increase in routine EOD rate with long-term normoxia acclimation, in contrast to metabolic rate, which showed no significant changes. These results highlight the relationship between sensory sampling and metabolic rate in response to progressive hypoxia and the plasticity of hypoxia tolerance.

Amphibious fish 'get a jump' on terrestrial locomotor performance after exercise training on land

Many amphibious fishes rely on terrestrial locomotion to accomplish essential daily tasks, but it is unknown whether terrestrial exercise improves the locomotor performance of fishes on land. Thus, we tested the hypothesis that terrestrial exercise improves locomotion in amphibious fishes out of water as a result of skeletal muscle remodeling. We compared the jumping performance of Kryptolebias marmoratus before and after an exercise training regimen, and assessed the muscle phenotype of control and exercise-trained fish. We found that exercise-trained fish jumped 41% farther and 48% more times before reaching exhaustion. Furthermore, exercise training resulted in the hypertrophy of red muscle fibers, and an increase in red muscle capillarity and aerobic capacity. Lactate accumulation after jumping indicates that white muscle is also important in powering terrestrial jumps. Overall, skeletal muscle in K. marmoratus is highly responsive to terrestrial exercise, and muscle plasticity may assist in the effective exploitation of terrestrial habitats by amphibious fishes.

The osmorespiratory compromise: physiological responses and tolerance to hypoxia are affected by salinity acclimation in the euryhaline Atlantic killifish (Fundulus heteroclitus)

The characteristics of the fish gill that maximize gas exchange are the same that promote diffusion of ions and water to and from the environment; therefore, physiological trade-offs are likely to occur. Here, we investigated how salinity acclimation affects whole-animal respiratory gas exchange during hypoxia using Fundulus heteroclitus, a fish that inhabits salt marshes where salinity and oxygen levels vary greatly. Salinity had marked effects on hypoxia tolerance, with fish acclimated to 11 and 35 ppt showing much longer time to loss of equilibrium (LOE) in hypoxia than 0 ppt-acclimated fish. Fish acclimated to 11 ppt (isosmotic salinity) exhibited the greatest capacity to regulate oxygen consumption rate (Ṁ _O2 ) under hypoxia, as measured through the regulation index (RI) and P _crit At 35 ppt, fish had a higher routine metabolic rate (RMR) but a lower RI than fish at 11 ppt, but there were no differences in gill morphology, ventilation or blood O₂ transport properties between these groups. In contrast, 0 ppt-acclimated fish had the highest ventilation and lowest O₂ extraction efficiency in normoxia and hypoxia, indicating a higher ventilatory workload in order to maintain similar levels of Ṁ _O2 These differences were related to alterations in gill morphology, where 0 ppt-acclimated fish had the smallest lamellar surface area with the greatest epithelial cell coverage (i.e. thicker lamellae, longer diffusion distance) and a larger interlamellar cell mass, contrasting with 11 ppt-acclimated fish, which had overall the highest respiratory surface area. The alteration of an array of physiological parameters provides evidence for a compromise between salinity and hypoxia tolerance in killifish acclimated to freshwater.

Metabolic implications of exposure to wastewater effluent in bluegill sunfish

Effluent from wastewater treatment plants (WWTP) contains a complex mixture of contaminants and is a major worldwide source of aquatic pollution. We examined the effects of exposure to treated effluent from a municipal WWTP on the metabolic physiology of bluegill sunfish (Lepomis macrochirus). We studied fish that were wild-caught or experimentally caged (28 d) downstream of the WWTP, and compared them to fish that were caught or caged at clean reference sites. Survival was reduced in fish caged at the effluent-contaminated site compared to those caged at the reference site. Resting rates of O₂ consumption (MO₂) were higher in fish from the contaminated site, reflecting a metabolic cost of wastewater exposure. The increases in routine MO₂ did not reduce aerobic scope (difference or quotient of maximal MO₂ and resting MO₂), suggesting that physiological compensations accompanied the metabolic costs of wastewater exposure. Fish exposed to wastewater also had larger hearts and livers. The activity of mitochondrial enzymes (cytochrome c oxidase, citrate synthase) per liver mass was unaltered across treatments, so the increased mass of this organ increased its cumulative oxidative capacity in the fish. Wastewater exposure also reduced glycogen content per liver mass. The effects of caging itself, based on comparisons between fish that were wild-caught or caged at clean sites, were generally subtle and not statistically significant. We conclude that exposure to wastewater effluent invokes a metabolic cost that leads to compensatory physiological adjustments that partially offset the detrimental metabolic impacts of exposure.

Ageing impacts phenotypic flexibility in an air-acclimated amphibious fish

The ability to tolerate environmental change may decline as fishes age. We tested the hypothesis that ageing influences the scope for phenotypic flexibility in the mangrove rivulus (Kryptolebias marmoratus), an amphibious fish that transitions between two vastly different environments, water and land. We found that older fish (4-6 years old) exhibited marked signs of ageing; older fish were reproductively senescent, had reduced fin regenerative capacity and body condition, and exhibited atrophy of both oxidative and glycolytic muscle fibers relative to younger adult fish (1-2 years old). However, age did not affect routine O₂ consumption. We then acclimated adult fish (1-6 years) to water (control) or air for 10 days to assess the scope for phenotypic flexibility in response to terrestrial exposure. In support of our hypothesis, we found that older air-acclimated fish had a diminished scope for gill remodeling relative to younger fish. We also found that older fish exhibited poorer terrestrial locomotor performance relative to younger adult fish, particularly when acclimated to air. Our results indicate that ageing diminishes skeletal muscle integrity and locomotor performance of amphibious fishes, and may, therefore, impair terrestrial foraging ability, predator avoidance, or dispersal across the terrestrial environment. Remarkably, older fish voluntarily left water to a similar degree as younger fish despite the age-related deterioration of traits important for terrestrial life.

Metabolic rate and hypoxia tolerance in Girardinichthys multiradiatus (Pisces: Goodeidae), an endemic fish at high altitude in tropical Mexico

The darkedged splitfin (Amarillo fish), Girardinichthys multiradiatus is a vulnerable endemic fish species inhabiting central Mexico's high altitude Upper Lerma Basin, where aquatic hypoxia is exacerbated by low barometric pressures (lower P_O2s), large aquatic oxygen changes, poor aquatic systems management and urban, agricultural and industrial pollution. The respiratory physiology of G. multiradiatus under such challenging conditions is unknown - therefore the main goal of the present study was to determine metabolic rates and hypoxia tolerance to elucidate possible physiological adaptations allowing this fish to survive high altitude and increasingly eutrophic conditions. Fish came from two artificial reservoirs - San Elías and Ex Hacienda - considered refuges for this species. Both reservoirs showed high dial P_O2 variation, with hypoxic conditions before midday and after 20:00 h, ~4 h of normoxia (15 kPa) from 16:00-20:00, and ~4 h of hyperoxia (16-33 kPa) from 12:00-16:00. Standard metabolic rate at 20 ± 0.5 °C of larvae from Ex Hacienda was significantly higher than those from San Elías, but these differences disappeared in juveniles and adults. Metabolic rate at 20 ± 0.5 °C for adults was 9.8 ± 0.1 SEM μmol O₂/g/h. The metabolic scaling exponent for adults was 0.58 for San Elías fish and 0.83 for Ex Hacienda fish, indicating possible ecological effects on this variable. Post-larval fish in Ex Hacienda and all stages in San Elias site showed considerable hypoxia tolerance, with P_Crit mean values ranging from 1.9-3.1 kPa, lower than those of many tropical fish at comparable temperatures. Collectively, these data indicate that G. multiradiatus is well adapted for the hypoxia associated with their high-altitude habitat.

Hydrogen sulphide toxicity and the importance of amphibious behaviour in a mangrove fish inhabiting sulphide-rich habitats

We investigated amphibious behaviour, hydrogen sulphide (H₂S) tolerance, and the mechanism of H₂S toxicity in the amphibious mangrove rivulus (Kryptolebias marmoratus). We found that fish emersed (left water) in response to acutely elevated [H₂S] (~ 130-200 µmol l^-1). The emersion response to H₂S may be influenced by prior acclimation history due to acclimation-induced alterations in gill morphology and/or the density and size of neuroepithelial cells (NECs) on the gills and skin. Thus, we acclimated fish to water (control), H₂S-rich water, or air and tested the hypotheses that acclimation history influences H₂S sensitivity due to acclimation-induced changes in (i) gill surface area and/or (ii) NEC density and/or size. Air-acclimated fish emersed at significantly lower [H₂S] relative to fish acclimated to control or H₂S-rich water, but exhibited no change in gill surface area or in NEC density or size in the gills or skin. Despite possessing exceptional H₂S tolerance, all fish lost equilibrium when unable to emerse from environments containing extremely elevated [H₂S] (2272 ± 46 µmol l^-1). Consequently, we tested the hypothesis that impaired blood oxygen transport (i.e., sulphemoglobin formation) causes H₂S toxicity in amphibious fishes. In vitro exposure of red blood cells to physiologically relevant [H₂S] did not cause a substantial increase in sulphemoglobin formation. We found evidence, however, for an alternative hypothesis that H₂S toxicity is caused by impaired oxidative phosphorylation (i.e., cytochrome c oxidase inhibition). Collectively, our results show that amphibious behaviour is critical for the survival of K. marmoratus in H₂S-rich environments as fish experience impaired oxidative phosphorylation when unable to emerse.

Phenotypic flexibility in respiratory traits is associated with improved aerial respiration in an amphibious fish out of water

Amphibious fishes have evolved multiple adaptive strategies for respiring out of water, but there has been less focus on reversible plasticity. We tested the hypothesis that when amphibious fishes leave water, enhanced respiratory performance on land is the result of rapid functional phenotypic flexibility of respiratory traits. We acclimated four isogenic strains of Kryptolebias marmoratus to air for 0, 1, 3 or 7 days. We compared respiratory performance out of water with traits linked to the O₂ cascade. Aerial O₂ consumption rate was measured over a step-wise decrease in O₂ levels. There were significant differences between strains, but time out of water had the largest impact on measured parameters. K ryptolebias marmoratus had improved respiratory performance [lower aerial critical oxygen tension (P _crit), higher regulation index (RI)] after only 1 day of air exposure, and these changes were strongly associated with the change in hematocrit and dorsal cutaneous angiogenesis. Additionally, we found that 1 h of air exposure induced the expression of four angiogenesis-associated genes - vegfa, angpt2, pecam-1 and efna1 - in the skin. After 7 days in air, respiratory traits were not significantly linked to the variation in either aerial P _crit or RI. Overall, our data indicate that there are two phases involved in the enhancement of aerial respiration: an initial rapid response (1 day) and a delayed response (7 days). We found evidence for the hypothesis that respiratory performance on land in amphibious fishes is the result of rapid flexibility in both O₂ uptake and O₂ carrying capacity.

Diel cycling hypoxia enhances hypoxia-tolerance in rainbow trout (Oncorhynchus mykiss): evidence of physiological and metabolic plasticity

Many fish naturally encounter a daily cycle of hypoxia but it is unclear whether this exposure hardens hypoxia-intolerant fish to future hypoxia or leads to accumulated stress and death. Rainbow trout (Oncorhynchus mykiss) is a putatively hypoxia-sensitive species found in rivers and estuaries that may routinely experience hypoxic events. Trout were exposed to 1 of 4 135h treatments in a swim-tunnel respirometer: 1) air-saturated control (20.7 kPa PO2); 2) diel cycling O2 (20.7-4.2 kPa over 24h); 3) acute hypoxia (130h at 20.7 kPa PO2 followed by 5h at 4.2 kPa PO2); 4) the mean oxygen tension (12.4 kPa PO2) experienced by the diel cycled fish. Some responses were similar in diel O2 cycled and mean PO2-treated fish but overall exposure to ecologically-representative diel hypoxia cycles improved hypoxia tolerance. Diel hypoxia-induced protective responses included increased inducible HSP70 concentration and mean corpuscular hemoglobin concentration, as well as reduced plasma cortisol. Acclimation to diel hypoxia allowed metabolic rates to decline during hypoxia, reduced oxygen debt following subsequent exposures, and allowed fish to return to an anabolic phenotype. The data demonstrate that acute diel cycling hypoxia improves hypoxia tolerance in previously intolerant fish through the activation of cellular protective mechanisms and a reduction in metabolic O2 requirements.

Combined effects of warming and hypoxia on early life stage Chinook salmon physiology and development

Early life stages of salmonids are particularly vulnerable to warming and hypoxia, which are common stressors in hyporheic, gravel bed, rearing habitat (i.e. a 'redd'). With the progression of global climate change, high temperatures and hypoxia may co-occur more frequently within redds, particularly for salmonid species at their southern range limit. Warming and hypoxia have competing effects on energy supply and demand, which can be detrimental to energy-limited early life stages. We examined how elevated temperature and hypoxia as individual and combined stressors affected the survival, physiological performance, growth, and development of Chinook salmon (Oncorhynchus tshawytscha). We reared late fall-run Chinook salmon from fertilization to the fry stage in a fully factorial design of two temperatures [10°C (ambient) and 14°C (warm)] and two oxygen levels [normoxia (100% air saturation, 10 mg O₂/l) and hypoxia (50% saturation, 5.5 mg O₂/l)]. Rearing in hypoxia significantly reduced hatching success, especially in combination with warming. Both warming and hypoxia improved acute thermal tolerance. While rearing in hypoxia improved tolerance to acute hypoxia stress, warming reduced hypoxia tolerance. Hypoxia-reared fish were smaller at hatch, but were able to reach similar sizes to the normoxia-reared fish by the fry stage. High temperature and normoxia resulted in the fastest rate of development while low temperature and hypoxia resulted in the slowest rate of development. Despite improved physiological tolerance to acute heat and hypoxia stress, hypoxia-reared embryos had reduced survival and growth, which could have larger population-level effects. These results suggest that both warming and hypoxia are important factors to address in conservation strategies for Chinook salmon.

Distinct metabolic adjustments arise from acclimation to constant hypoxia and intermittent hypoxia in estuarine killifish (Fundulus heteroclitus)

Many fish experience daily cycles of hypoxia in the wild, but the physiological strategies for coping with intermittent hypoxia are poorly understood. We examined how killifish adjust O2 supply and demand during acute hypoxia, and how these responses are altered after prolonged acclimation to constant or intermittent patterns of hypoxia exposure. We acclimated killifish to normoxia (∼20 kPa O2), constant hypoxia (2 kPa) or intermittent cycles of nocturnal hypoxia (12 h:12 h normoxia:hypoxia) for 28 days, and then compared whole-animal O2 consumption rates (ṀO2) and tissue metabolites during exposure to 12 h of hypoxia followed by reoxygenation in normoxia. Normoxia-acclimated fish experienced a pronounced 27% drop in ṀO2 during acute hypoxia, and modestly increased ṀO2 upon reoxygenation. They strongly recruited anaerobic metabolism during acute hypoxia, indicated by lactate accumulation in plasma, muscle, liver, brain, heart and digestive tract, as well as a transient drop in intracellular pH, and they increased hypoxia inducible factor (HIF)-1α protein abundance in muscle. Glycogen, glucose and glucose-6-phosphate levels suggested that glycogen supported brain metabolism in hypoxia, while the muscle used circulating glucose. Acclimation to constant hypoxia caused a stable ∼50% decrease in ṀO2 that persisted after reoxygenation, with minimal recruitment of anaerobic metabolism, suggestive of metabolic depression. By contrast, fish acclimated to intermittent hypoxia maintained sufficient O2 transport to support normoxic ṀO2, modestly recruited lactate metabolism and increased ṀO2 dramatically upon reoxygenation. Both groups of hypoxia-acclimated fish had similar glycogen, ATP, intracellular pH and HIF-1α levels as normoxic controls. We conclude that different patterns of hypoxia exposure favour distinct strategies for matching O2 supply and O2 demand.

The fallacy of the P crit - are there more useful alternatives?

P _crit - generally defined as the P _O2  below which the animal can no longer maintain a stable rate of O₂ consumption (Ṁ _O2 ), such that Ṁ _O2  becomes dependent upon P _O2  - provides a single number into which a vast amount of experimental effort has been invested. Here, with specific reference to water-breathers, I argue that this focus on the P _crit is not useful for six reasons: (1) calculation of P _crit usually involves selective data editing; (2) the value of P _crit depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) P _crit is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) P _crit is not a reliable index of hypoxia tolerance; and (6) P _crit carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full Ṁ _O2  versus P _O2  profiles accompanied by measurements of ventilation, lactate appearance and metabolic rate by calorimetry. If the goal is to assess the ability of the animal to regulate Ṁ _O2  from this profile in a mathematical fashion, promising, more informative alternatives to P _crit are the regulation index and Michaelis-Menten or sigmoidal allosteric analyses.

Can variation among hypoxic environments explain why different fish species use different hypoxic survival strategies?

In aquatic environments, hypoxia is a multi-dimensional stressor that can vary in O₂ level (partial pressure of O₂ in water, Pw_O2 ), rate of induction and duration. Natural hypoxic environments can therefore be very different from one another. For the many fish species that have evolved to cope with these different hypoxic environments, survival requires adjusting energy supply and demand pathways to maintain energy balance. The literature describes innumerable ways that fishes combine aerobic metabolism, anaerobic metabolism and metabolic rate depression (MRD) to accomplish this, but it is unknown whether the evolutionary paths leading to these different strategies are determined primarily by species' phylogenetic histories, genetic constraint or their native hypoxic environments. We explored this idea by devising a four-quadrant matrix that bins different aquatic hypoxic environments according to their duration and Pw_O2  characteristics. We then systematically mined the literature for well-studied species native to environments within each quadrant, and, for each of 10 case studies, described the species' total hypoxic response (THR), defined as its hypoxia-induced combination of sustained aerobic metabolism, enhanced anaerobic metabolism and MRD, encompassing also the mechanisms underlying these metabolic modes. Our analysis revealed that fishes use a wide range of THRs, but that distantly related species from environments within the same matrix quadrant have converged on similar THRs. For example, environments of moderately hypoxic Pw_O2  favoured predominantly aerobic THRs, whereas environments of severely hypoxic Pw_O2  favoured MRD. Capacity for aerial emergence as well as predation pressure (aquatic and aerial) also contributed to these responses, in addition to other biotic and abiotic factors. Generally, it appears that the particular type of hypoxia experienced by a fish plays a major role in shaping its particular THR.

Daily rhythms of swimming activity, synchronization to different feeding times and effects on anesthesia practice in an Amazon fish species (Colossoma macropomum)

This study aimed to investigate the existence of day-night differences in the time for anesthesia and recovery in tambaqui exposed to the anesthetic eugenol and the influence of feeding time. Thus, we evaluated: (1) swimming activity; (2) food anticipatory activity (FAA) as a synchronizer of swimming activity and change to susceptibility to anesthetic; and (3) the effects of diurnal/nocturnal anesthesia exposure of fish feeding in the mid-light phase: 12:00 h (ML) and fish feeding in the mid-dark phase: 00:00 h (MD). Our findings revealed strictly nocturnal activity for tambaqui (94.2%), known as diurnal fish to date. Moreover, FAA was observed in tambaqui fed at MD, which showed a sustained increase in activity that began 2 h before feeding time and lasted until feeding. In contrast, no FAA was observed in fish fed at ML. Regarding anesthesia by day or night, the tambaqui treated with eugenol exhibited no difference in induction time. However, differences were observed in recovery times, with fish anesthetized at day recovering in 1-2 min and fish anesthetized at night recovering in 5-7 min. In short, our findings revealed for the first time the nocturnal behavior of tambaqui. These results indicated that recovery by day/night by eugenol in tambaqui has a strong dependence of behavioral patterns and the time of day.

Oxygen drives skeletal muscle remodeling in an amphibious fish out of water

Skeletal muscle remodeling in response to terrestrial acclimation improves the locomotor performance of some amphibious fishes on land, but the cue for this remodeling is unknown. We tested the hypothesis that muscle remodeling in the amphibious Kryptolebias marmoratus on land is driven by higher O2 availability in atmospheric air, and the alternative hypothesis that remodeling is induced by a different environmental or physiological condition fish experience on land. Fish were acclimated to 28 days of air, aquatic hyperoxia, hypercapnia, hypoxia, elevated temperature, or fasting conditions. Air, fasting, and hyperoxic conditions increased (&gt;25%) the size of oxidative fibers in K. marmoratus while hypoxia had the reverse effect (23% decrease). Surprisingly, hyperoxia-acclimation also resulted in a transformation of the musculature to include large bands of oxidative-like muscle. Our results show that K. marmoratus is highly responsive to environmental O2 levels and capitalize on O2-rich opportunities to enhance O2 utilization by skeletal muscle.

Intraspecific variation and plasticity in mitochondrial oxygen binding affinity as a response to environmental temperature

Mitochondrial function has been suggested to underlie constraints on whole-organism aerobic performance and associated hypoxia and thermal tolerance limits, but most studies have focused on measures of maximum mitochondrial capacity. Here we investigated whether variation in mitochondrial oxygen kinetics could contribute to local adaptation and plasticity in response to temperature using two subspecies of the Atlantic killifish (Fundulus heteroclitus) acclimated to a range of temperatures (5, 15, and 33 °C). The southern subspecies of F. heteroclitus, which has superior thermal and hypoxia tolerances compared to the northern subspecies, exhibited lower mitochondrial O₂ P50 (higher O₂ affinity). Acclimation to thermal extremes (5 or 33 °C) altered mitochondrial O₂ P50 in both subspecies consistent with the effects of thermal acclimation on whole-organism thermal tolerance limits. We also examined differences between subspecies and thermal acclimation effects on whole-blood Hb O₂-P50 to assess whether variation in oxygen delivery is involved in these responses. In contrast to the clear differences between subspecies in mitochondrial O₂-P50 there were no differences in whole-blood Hb-O₂ P50 between subspecies. Taken together these findings support a general role for mitochondrial oxygen kinetics in differentiating whole-organism aerobic performance and thus in influencing species responses to environmental change.

Using comparative biology to understand how aging affects mitochondrial metabolism

Lifespan varies considerably among even closely related species, as exemplified by rodents and primates. Despite these disparities in lifespan, most studies have focused on intra-specific aging pathologies, primarily within a select few systems. While mice have provided much insight into aging biology, it is unclear if such a short-lived species lack defences against senescence that may have evolved in related longevous species. Many age-related diseases have been linked to mitochondrial dysfunction that are measured by decreased energy generation, structural damage to cellular components, and even cell death. Post translational modifications (PTMs) orchestrate many of the pathways associated with cellular metabolism, and are thought to be a key regulator in biological senescence. We propose hyperacylation as one such modification that may be implicated in numerous mitochondrial impairments affecting energy metabolism.