Rapid physiological and transcriptomic changes associated with oxygen delivery in larval anemonefish suggest a role in adaptation to life on hypoxic coral reefs.

Connectivity of coral reef fish populations relies on successful dispersal of a pelagic larval phase. Pelagic larvae must exhibit high swimming abilities to overcome ocean and reef currents, but once settling onto the reef, larvae transition to endure habitats that become hypoxic at night. Therefore, coral reef fish larvae must rapidly and dramatically shift their physiology over a short period of time. Taking an integrative, physiological approach, using swimming respirometry, and examining hypoxia tolerance and transcriptomics, we show that larvae of cinnamon anemonefish (Amphiprion melanopus) rapidly transition between "physiological extremes" at the end of their larval phase. Daily measurements of swimming larval anemonefish over their entire early development show that they initially have very high mass-specific oxygen uptake rates. However, oxygen uptake rates decrease midway through the larval phase. This occurs in conjunction with a switch in haemoglobin gene expression and increased expression of myoglobin, cytoglobin, and neuroglobin, which may all contribute to the observed increase in hypoxia tolerance. Our findings indicate that critical ontogenetic changes in the gene expression of oxygen-binding proteins may underpin the physiological mechanisms needed for successful larval recruitment to reefs.

Expression of prolyl hydroxylase domains, the upstream regulators of HIF, in the brain of the anoxia-tolerant crucian carp during anoxia-reoxygenation.

The hypoxia-inducible factor (HIF) is considered key in the transcriptional response to low oxygen. Yet, the role of HIF in the absence of oxygen (anoxia) and in preparation for reoxygenation remains unclear. Recent studies suggest that mounting a HIF response may be counterproductive for anoxia survival. We here studied one of the champions of anoxia survival, the crucian carp (Carassius carassius), and hypothesized that expression of prolyl hydroxylase domains (PHDs; the upstream regulators of HIF) are upregulated to circumvent an energy-costly activation of HIF in anoxia and to prepare for reoxygenation. We measured whole brain mRNA and protein levels of the three isoforms PHD1, PHD2, and PHD3, coded for by multiple paralogs of the genes egln2, egln1, and egln3, using quantitative PCR and Western blotting in the brain of crucian carps exposed to 5 days normoxia or anoxia, and 5 days anoxia followed by 3 or 24 h of reoxygenation. The mRNA levels of most egln paralogs were increased in anoxia and upon reoxygenation, with egln3 showing the largest increase in mRNA level (up to 17-fold) and highest relative mRNA abundance (up to 75% of expressed egln). The protein level of all PHDs was maintained in anoxia and increased upon reoxygenation. We then explored PHD distribution in different brain regions and found PHD immunoreactivity to be associated with axonal branches and showing region-specific changes during anoxia-reoxygenation. Our results support an overall upregulation of egln under prolonged anoxia and PHDs upon reoxygenation in crucian carp, likely aimed at suppressing HIF responses, although regional differences are apparent in such a complex organ as the brain.NEW & NOTEWORTHY We report a profound upregulation of most egln paralog mRNA levels in anoxia and upon reoxygenation, with egln3ii showing the largest, a 17-fold increase, and highest relative mRNA abundance. The relative abundance of prolyl hydroxylase domain (PHD) proteins was maintained during anoxia and increased at reoxygenation. PHD immunoreactivity was localized to axonal branches with region-specific changes during anoxia-reoxygenation. These dynamic and regional changes in crucian carp, champion of anoxia tolerance, are most likely adaptive and call for further mechanistic studies.

Maintained mitochondrial integrity without oxygen in the anoxia-tolerant crucian carp.

Very few vertebrates survive without oxygen (anoxia) for more than a few minutes. Crucian carp (Carassius carassius) are one example, surviving months of anoxia at low temperatures, and we hypothesised that they maintain mitochondrial membrane potential and function. Isolated crucian carp cardiomyocytes indeed maintained mitochondrial membrane potential after blocking complex IV of the electron transport system with cyanide, while those of anoxia-intolerant trout depolarised. When complexes I-III were inhibited, crucian carp mitochondria depolarised, indicating that these complexes need to function during anoxia. Mitochondrial membrane potential depended on reversal of ATP synthase in chemical anoxia, as blocking with cyanide combined with oligomycin to inhibit ATP synthase led to depolarisation. ATP synthase activity was reduced in the heart after 1 week of anoxia in crucian carp, together with a downregulation of ATP synthase subunit gene expression. However, the morphology of cardiac mitochondria was not affected by 1 week of anoxia, even with a large increase in mitofusin 2 mRNA expression. Cardiac citrate synthase activity was not affected by anoxia, while cytochrome c oxidase activity was increased. We show how mitochondria respond to anoxia. A mechanistic understanding of how mitochondrial function can be maintained in anoxia may provide new perspectives to reduce mitochondrial damage in anoxia-sensitive organisms.

Two decades of research on anoxia tolerance - mitochondria, -omics and physiological diversity.

Just over two decades ago, Bob Boutilier published a much-cited Review in this journal on the mechanisms of cell survival in hypoxia and hypothermia. Here, we celebrate this important Review by describing how our knowledge of the mechanisms behind anoxia tolerance have progressed since 2001, including new key roles of mitochondria, something Boutilier had started exploring. Evidence now suggests that, in anoxia-tolerant brains, mitochondria initiate responses aimed at suppressing electrical activity and energy use. These responses are largely dependent on gamma-aminobutyric acid (GABA) release. Animals that survive anoxia must also tolerate reoxygenation - a major challenge that could cause a massive production of damaging reactive oxygen species (ROS). Here, the handling of succinate, which builds up during anoxia, is critical. Interestingly, there are clear species differences in succinate handling among anoxia-tolerant vertebrates (Trachemys and Chrysemys turtles and crucian carp, Carassius carassius). Trachemys turtles suppress succinate build-up during anoxia, presumably to limit ROS production during reoxygenation. By contrast, in crucian carp, reduction of fumarate to succinate during anoxia appears to be essential for keeping their mitochondria charged and viable. Consequently, during anoxia, crucian carp accumulate much more succinate than Trachemys turtles. Moreover, during anoxia, succinate is apparently transported from crucian carp brain and heart to the liver, which handles succinate upon reoxygenation. This is one example of the striking physiological diversity among vertebrates that survive long-term anoxia. More examples are given, and we argue that -omics approaches are, and will be, helpful in providing new insight and moving the field forward.

Effects of lactate ions on the cardiorespiratory system in rainbow trout (Oncorhynchus mykiss).

Lactate ions are involved in several physiological processes, including a direct stimulation of the carotid body, causing increased ventilation in mammals. A similar mechanism eliciting ventilatory stimulation in other vertebrate classes has been demonstrated, but it remains to be thoroughly investigated. Here, we investigated the effects of lactate ions on the cardiorespiratory system in swimming rainbow trout by manipulating the blood lactate concentration. Lactate elicited a vigorous, dose-dependent elevation of ventilation and bradycardia at physiologically relevant concentrations at constant pH. After this initial confirmation, we examined the chiral specificity of the response and found that only l-lactate induced these effects. By removal of the afferent inputs from the first gill arch, the response was greatly attenuated, and a comparison of the responses to injections up- and downstream of the gills collectively demonstrated that the lactate response was initiated by branchial cells. Injection of specific receptor antagonists revealed that a blockade of serotonergic receptors, which are involved in the hypoxic ventilatory response, significantly reduced the lactate response. Finally, we identified two putative lactate receptors based on sequence homology and found that both were expressed at substantially higher levels in the gills. We propose that lactate ions modulate ventilation by stimulating branchial oxygen-sensing cells, thus eliciting a cardiorespiratory response through receptors likely to have originated early in vertebrate evolution.

In modelling effects of global warming, invalid assumptions lead to unrealistic projections.

In their recent Opinion, Pauly and Cheung () provide new projections of future maximum fish weight (W∞ ). Based on criticism by Lefevre et al. (2017) they changed the scaling exponent for anabolism, dG . Here we find that changing both dG and the scaling exponent for catabolism, b, leads to the projection that fish may even become 98% smaller with a 1°C increase in temperature. This unrealistic outcome indicates that the current W∞ is unlikely to be explained by the Gill-Oxygen Limitation Theory (GOLT) and, therefore, GOLT cannot be used as a mechanistic basis for model projections about fish size in a warmer world.

Experimental strategies to assess the biological ramifications of multiple drivers of global ocean change-A review.

Marine life is controlled by multiple physical and chemical drivers and by diverse ecological processes. Many of these oceanic properties are being altered by climate change and other anthropogenic pressures. Hence, identifying the influences of multifaceted ocean change, from local to global scales, is a complex task. To guide policy-making and make projections of the future of the marine biosphere, it is essential to understand biological responses at physiological, evolutionary and ecological levels. Here, we contrast and compare different approaches to multiple driver experiments that aim to elucidate biological responses to a complex matrix of ocean global change. We present the benefits and the challenges of each approach with a focus on marine research, and guidelines to navigate through these different categories to help identify strategies that might best address research questions in fundamental physiology, experimental evolutionary biology and community ecology. Our review reveals that the field of multiple driver research is being pulled in complementary directions: the need for reductionist approaches to obtain process-oriented, mechanistic understanding and a requirement to quantify responses to projected future scenarios of ocean change. We conclude the review with recommendations on how best to align different experimental approaches to contribute fundamental information needed for science-based policy formulation.

Physiological Challenges to Fishes in a Warmer and Acidified Future.

With the projected levels of global warming and ocean acidification, fishes have to face warmer waters with CO2 levels that are the highest in over 30 million years. The resultant rise in body temperature means that metabolic rates of fish will increase, and some may become energetically compromised. No less worrying, and maybe more surprising, is that rising CO2 concentrations appear to trigger pH regulatory mechanisms that disrupts neural ion gradients, leading to altered neurotransmitter function and maladaptive behavioral changes. We point out the many outstanding questions, including the ultimate one: Will fish be able to adapt to these challenges?

Models projecting the fate of fish populations under climate change need to be based on valid physiological mechanisms.

Some recent modelling papers projecting smaller fish sizes and catches in a warmer future are based on erroneous assumptions regarding (i) the scaling of gills with body mass and (ii) the energetic cost of 'maintenance'. Assumption (i) posits that insurmountable geometric constraints prevent respiratory surface areas from growing as fast as body volume. It is argued that these constraints explain allometric scaling of energy metabolism, whereby larger fishes have relatively lower mass-specific metabolic rates. Assumption (ii) concludes that when fishes reach a certain size, basal oxygen demands will not be met, because of assumption (i). We here demonstrate unequivocally, by applying accepted physiological principles with reference to the existing literature, that these assumptions are not valid. Gills are folded surfaces, where the scaling of surface area to volume is not constrained by spherical geometry. The gill surface area can, in fact, increase linearly in proportion to gill volume and body mass. We cite the large body of evidence demonstrating that respiratory surface areas in fishes reflect metabolic needs, not vice versa, which explains the large interspecific variation in scaling of gill surface areas. Finally, we point out that future studies basing their predictions on models should incorporate factors for scaling of metabolic rate and for temperature effects on metabolism, which agree with measured values, and should account for interspecific variation in scaling and temperature effects. It is possible that some fishes will become smaller in the future, but to make reliable predictions the underlying mechanisms need to be identified and sought elsewhere than in geometric constraints on gill surface area. Furthermore, to ensure that useful information is conveyed to the public and policymakers about the possible effects of climate change, it is necessary to improve communication and congruity between fish physiologists and fisheries scientists.

Bigger is not better: cortisol-induced cardiac growth and dysfunction in salmonids.

Stress and elevated cortisol levels are associated with pathological heart growth and cardiovascular disease in humans and other mammals. We recently established a link between heritable variation in post-stress cortisol production and cardiac growth in salmonid fish too. A conserved stimulatory effect of the otherwise catabolic steroid hormone cortisol is probably implied, but has to date not been established experimentally. Furthermore, whereas cardiac growth is associated with failure of the mammalian heart, pathological cardiac hypertrophy has not previously been described in fish. Here, we show that rainbow trout (Oncorhynchus mykiss) treated with cortisol in the diet for 45 days have enlarged hearts with lower maximum stroke volume and cardiac output. In accordance with impaired cardiac performance, overall circulatory oxygen-transporting capacity was diminished as indicated by reduced aerobic swimming performance. In contrast to the well-known adaptive/physiological heart growth observed in fish, cortisol-induced growth is maladaptive. Furthermore, the observed heart growth was associated with up-regulated signature genes of mammalian cardiac pathology, suggesting that signalling pathways mediating cortisol-induced cardiac remodelling in fish are conserved from fish to mammals. Altogether, we show that excessive cortisol can induce pathological cardiac remodelling. This is the first study to report and integrate the etiology, physiology and molecular biology of cortisol-induced pathological remodelling in fish.

Re-oxygenation after anoxia induces brain cell death and memory loss in the anoxia-tolerant crucian carp.

Crucian carp (Carassius carassius) survive without oxygen for several months, but it is unknown whether they are able to protect themselves from cell death normally caused by the absence, and particularly return, of oxygen. Here, we quantified cell death in brain tissue from crucian carp exposed to anoxia and re-oxygenation using the terminal deoxy-nucleotidyl transferase dUTP nick-end labelling (TUNEL) assay, and cell proliferation by immunohistochemical staining for proliferating cell nuclear antigen (PCNA) as well as PCNA mRNA expression. We also measured mRNA and protein expression of the apoptosis executer protease caspase 3, in laboratory fish exposed to anoxia and re-oxygenation and fish exposed to seasonal anoxia and re-oxygenation in their natural habitat over the year. Finally, a behavioural experiment was used to assess the ability to learn and remember how to navigate in a maze to find food, before and after exposure to anoxia and re-oxygenation. The number of TUNEL-positive cells in the telencephalon increased after 1 day of re-oxygenation following 7 days of anoxia, indicating increased cell death. However, there were no consistent changes in whole-brain expression of caspase 3 in either laboratory-exposed or naturally exposed fish, indicating that cell death might occur via caspase-independent pathways or necrosis. Re-oxygenated crucian carp appeared to have lost the memory of how to navigate in a maze (learnt prior to anoxia exposure), while the ability to learn remained intact. PCNA mRNA was elevated after re-oxygenation, indicating increased neurogenesis. We conclude that anoxia tolerance involves not only protection from damage but also repair after re-oxygenation.

Responses of neurogenesis and neuroplasticity related genes to elevated CO2 levels in the brain of three teleost species.

The continuous increase of anthropogenic CO2 in the atmosphere resulting in ocean acidification has been reported to affect brain function in some fishes. During adulthood, cell proliferation is fundamental for fish brain growth and for it to adapt in response to external stimuli, such as environmental changes. Here we report the first expression study of genes regulating neurogenesis and neuroplasticity in brains of three-spined stickleback (Gasterosteus aculeatus), cinnamon anemonefish (Amphiprion melanopus) and spiny damselfish (Acanthochromis polyacanthus) exposed to elevated CO2 The mRNA expression levels of the neurogenic differentiation factor (NeuroD) and doublecortin (DCX) were upregulated in three-spined stickleback exposed to high-CO2 compared with controls, while no changes were detected in the other species. The mRNA expression levels of the proliferating cell nuclear antigen (PCNA) and the brain-derived neurotrophic factor (BDNF) remained unaffected in the high-CO2 exposed groups compared to the control in all three species. These results indicate a species-specific regulation of genes involved in neurogenesis in response to elevated ambient CO2 levels. The higher expression of NeuroD and DCX mRNA transcripts in the brain of high-CO2-exposed three-spined stickleback, together with the lack of effects on mRNA levels in cinnamon anemonefish and spiny damselfish, indicate differences in coping mechanisms among fish in response to the predicted-future CO2 level.

Ambient CO2, fish behaviour and altered GABAergic neurotransmission: exploring the mechanism of CO2-altered behaviour by taking a hypercapnia dweller down to low CO2 levels.

Recent studies suggest that projected rises of aquatic CO2 levels cause acid-base regulatory responses in fishes that lead to altered GABAergic neurotransmission and disrupted behaviour, threatening fitness and population survival. It is thought that changes in Cl(-) and HCO3 (-) gradients across neural membranes interfere with the function of GABA-gated anion channels (GABAA receptors). So far, such alterations have been revealed experimentally by exposing species living in low-CO2 environments, like many oceanic habitats, to high levels of CO2 (hypercapnia). To examine the generality of this phenomenon, we set out to study the opposite situation, hypothesizing that fishes living in typically hypercapnic environments also display behavioural alterations if exposed to low CO2 levels. This would indicate that ion regulation in the fish brain is fine-tuned to the prevailing CO2 conditions. We quantified pH regulatory variables and behavioural responses of Pangasianodon hypophthalmus, a fish native to the hypercapnic Mekong River, acclimated to high-CO2 (3.1 kPa) or low-CO2 (0.04 kPa) water. We found that brain and blood pH was actively regulated and that the low-CO2 fish displayed significantly higher activity levels, which were reduced after treatment with gabazine, a GABAA receptor blocker. This indicates an involvement of the GABAA receptor and altered Cl(-) and HCO3 (-) ion gradients. Indeed, Goldman calculations suggest that low levels of environmental CO2 may cause significant changes in neural ion gradients in P. hypophthalmus. Taken together, the results suggest that brain ion regulation in fishes is fine-tuned to the prevailing ambient CO2 conditions and is prone to disruption if these conditions change.

Oxygen- and temperature-dependent expression of survival protein kinases in crucian carp (Carassius carassius) heart and brain.

Living without oxygen is limited to very few vertebrates, one species being the fresh water fish crucian carp (Carassius carassius), which can survive months of anoxia at low temperatures. Mammalian heart and brain are particularly intolerant to oxygen deprivation, yet these organs can be conditioned to display increased resistance, possibly due to activation of several protein kinases. We hypothesized increased phosphorylation status of these kinases in hypoxic and anoxic crucian carp heart and brain. Moreover, we wanted to investigate whether the kinases showing the strongest phosphorylation during hypoxia/anoxia, ERK 1/2, p38-MAPK, JNK, PKCε, and PKCδ, also had increased expression and phosphorylation at cold temperatures, to better cope with the anoxic periods during winter. We found small differences in the phosphorylation status of ERK 1/2, p38-MAPK, JNK, PKCε, and PKCδ during 10 days of severe hypoxia in both heart and brain (0.3 mg O2/l) and varying responses to reoxygenation. In contrast, 7 days of anoxia (<0.01 mg O2/l) markedly increased phosphorylation of ERK 1/2, p38-MAPK, JNK in the heart, and p38-MAPK and PKCε in the brain. Similarly, varying acclimation temperature between 4, 10 and 20°C induced large changes in phosphorylation status. Total protein expression in heart and brain neither changed during different oxygen regimes nor with different acclimation temperatures, except for ERK 1/2, which slightly decreased in the heart at 4°C compared with 20°C. A phylogenetic analysis confirmed that these protein kinases are evolutionarily conserved across a wide range of vertebrate species. Our findings indicate important roles of several protein kinases during oxygen deprivation.

Will jumping snails prevail? Influence of near-future CO2, temperature and hypoxia on respiratory performance in the tropical conch Gibberulus gibberulus gibbosus.

Tropical coral reef organisms are predicted to be especially sensitive to ocean warming because many already live close to their upper thermal limit, and the expected rise in ocean CO2 is proposed to further reduce thermal tolerance. Little, however, is known about the thermal sensitivity of a diverse and abundant group of reef animals, the gastropods. The humpbacked conch (Gibberulus gibberulus gibbosus), inhabiting subtidal zones of the Great Barrier Reef, was chosen as a model because vigorous jumping, causing increased oxygen uptake (MO2 ), can be induced by exposure to odour from a predatory cone snail (Conus marmoreus). We investigated the effect of present-day ambient (417-454 µatm) and projected-future (955-987 µatm) PCO2 on resting (MO2 , rest) and maximum (MO2 , max) MO2 , as well as MO2 during hypoxia and critical oxygen tension (PO2 , crit), in snails kept at present-day ambient (28°C) or projected-future temperature (33°C). MO2 , rest and MO2 , max were measured both at the acclimation temperature and during an acute 5°C increase. Jumping caused a 4- to 6-fold increase in MO2 , and MO2 , max increased with temperature so that absolute aerobic scope was maintained even at 38°C, although factorial scope was reduced. The humpbacked conch has a high hypoxia tolerance with a PO2 , crit of 2.5 kPa at 28°C and 3.5 kPa at 33°C. There was no effect of elevated CO2 on respiratory performance at any temperature. Long-term temperature records and our field measurements suggest that habitat temperature rarely exceeds 32.6°C during the summer, indicating that these snails have aerobic capacity in excess of current and future needs.

Reversible brain swelling in crucian carp (Carassius carassius) and goldfish (Carassius auratus) in response to high external ammonia and anoxia.

Increased internal ammonia (hyperammonemia) and ischemic/anoxic insults are known to result in a cascade of deleterious events that can culminate in potentially fatal brain swelling in mammals. It is less clear, however, if the brains of fishes respond to ammonia in a similar manner. The present study demonstrated that the crucian carp (Carassius carassius) was not only able to endure high environmental ammonia exposure (HEA; 2 to 22 mmol L(-1)) but that they experienced 30% increases in brain water content at the highest ammonia concentrations. This swelling was accompanied by 4-fold increases in plasma total ammonia (TAmm) concentration, but both plasma TAmm and brain water content were restored to pre-exposure levels following depuration in ammonia-free water. The closely related, ammonia-tolerant goldfish (Carassius auratus) responded similarly to HEA (up to 3.6 mmol L(-1)), which was accompanied by 4-fold increases in brain glutamine. Subsequent administration of the glutamine synthetase inhibitor, methionine sulfoximine (MSO), reduced brain glutamine accumulation by 80% during HEA. However, MSO failed to prevent ammonia-induced increases in brain water content suggesting that glutamine may not be directly involved in initiating ammonia-induced brain swelling in fishes. Although the mechanisms of brain swelling are likely different, exposure to anoxia for 96 h caused similar, but lesser (10%) increases in brain water content in crucian carp. We conclude that brain swelling in some fishes may be a common response to increased internal ammonia or lower oxygen but further research is needed to deduce the underlying mechanisms behind such responses.

Marine mollusc predator-escape behaviour altered by near-future carbon dioxide levels.

Ocean acidification poses a range of threats to marine invertebrates; however, the potential effects of rising carbon dioxide (CO2) on marine invertebrate behaviour are largely unknown. Marine gastropod conch snails have a modified foot and operculum allowing them to leap backwards rapidly when faced with a predator, such as a venomous cone shell. Here, we show that projected near-future seawater CO2 levels (961 µatm) impair this escape behaviour during a predator-prey interaction. Elevated-CO2 halved the number of snails that jumped from the predator, increased their latency to jump and altered their escape trajectory. Physical ability to jump was not affected by elevated-CO2 indicating instead that decision-making was impaired. Antipredator behaviour was fully restored by treatment with gabazine, a GABA antagonist of some invertebrate nervous systems, indicating potential interference of neurotransmitter receptor function by elevated-CO2, as previously observed in marine fishes. Altered behaviour of marine invertebrates at projected future CO2 levels could have potentially far-reaching implications for marine ecosystems.

Aerobic vs. anaerobic scope: sibling species of fish indicate that temperature dependence of hypoxia tolerance can predict future survival.

The temperature dependence of aerobic scope has been suggested to be a major determinant of how marine animals will cope with future rises in environmental temperature. Here, we present data suggesting that in some animals, the temperature dependence of anaerobic scope (i.e., the capacity for surviving severe hypoxia) may determine present-day latitudinal distributions and potential for persistence in a warmer future. As a model for investigating the role of anaerobic scope, we studied two sibling species of coral-dwelling gobies, Gobiodon histrio, and G. erythrospilus, with different latitudinal distributions, but which overlap in equal abundance at Lizard Island (14°40'S) on the Great Barrier Reef. These species did not differ in the temperature dependence of resting oxygen consumption or critical oxygen concentration (the lowest oxygen level where resting oxygen consumption can be maintained). In contrast, the more equatorial species (G. histrio) had a better capacity to endure anaerobic conditions at oxygen levels below the critical oxygen concentration at the high temperatures (32-33 °C) more likely to occur near the equator, or in a warmer future. These results suggest that anaerobic scope, in addition to aerobic scope, could be important in determining the impacts of global warming on some marine animals.

Life on the edge: thermal optima for aerobic scope of equatorial reef fishes are close to current day temperatures.

Equatorial populations of marine species are predicted to be most impacted by global warming because they could be adapted to a narrow range of temperatures in their local environment. We investigated the thermal range at which aerobic metabolic performance is optimum in equatorial populations of coral reef fish in northern Papua New Guinea. Four species of damselfishes and two species of cardinal fishes were held for 14 days at 29, 31, 33, and 34 °C, which incorporated their existing thermal range (29-31 °C) as well as projected increases in ocean surface temperatures of up to 3 °C by the end of this century. Resting and maximum oxygen consumption rates were measured for each species at each temperature and used to calculate the thermal reaction norm of aerobic scope. Our results indicate that one of the six species, Chromis atripectoralis, is already living above its thermal optimum of 29 °C. The other five species appeared to be living close to their thermal optima (ca. 31 °C). Aerobic scope was significantly reduced in all species, and approached zero for two species at 3 °C above current-day temperatures. One species was unable to survive even short-term exposure to 34 °C. Our results indicate that low-latitude reef fish populations are living close to their thermal optima and may be more sensitive to ocean warming than higher-latitude populations. Even relatively small temperature increases (2-3 °C) could result in population declines and potentially redistribution of equatorial species to higher latitudes if adaptation cannot keep pace.