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 damsel fishes and two species of cardinal fishes were held for 14d 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, Chromisatripectoralis, is already living above its thermal optimum of 29°C. The other five species appeared to be living close to their thermal optima (approximately 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.

SUMMARYMetabolic rates of aquatic organisms are estimated from measurements of oxygen consumption rates (Ṁ O2 ) through swimming and resting respirometry. These distinct approaches are increasingly used in ecophysiology and conservation physiology studies; however, few studies have tested whether they yield comparable results. We examined whether two fundamental Ṁ O2 measures, standard metabolic rate (SMR) and maximum metabolic rate (MMR), vary based on the method employed. Ten bridled monocle bream (Scolopsis bilineata) were exercised using (1) a critical swimming speed (U crit ) protocol, (2) a 15min exhaustive chase protocol and (3) a 3min exhaustive chase protocol followed by brief (1min) air exposure. Protocol 1 was performed in a swimming respirometer whereas protocols 2 and 3 were followed by resting respirometry. SMR estimates in swimming respirometry were similar to those in resting respirometry when a three-parameter exponential or power function was used to extrapolate the swimming speed-Ṁ O2 relationship to zero swimming speed. In contrast, MMR using the U crit protocol was 36% higher than MMR derived from the 15min chase protocol and 23% higher than MMR using the 3min chase/1min air exposure protocol. For strong steady (endurance) swimmers, such as S. bilineata, swimming respirometry can produce more accurate MMR estimates than exhaustive chase protocols because oxygen consumption is measured during exertion. However, when swimming respirometry is impractical, exhaustive chase protocols should be supplemented with brief air exposure to improve measurement accuracy. Caution is warranted when comparing MMR estimates obtained with different respirometry methods unless they are cross-validated on a species-specific basis.

The commercial and recreational harvests of red snapper Lutjanus campechanus in the Gulf of Mexico have declined over the past five decades, prompting strict regulations. Release mortality associated with catastrophic decompression (CD) is a possible cause for the continuing decline, although to date no physiological data exist to support this assumption. Using a flow‐through high‐pressure chamber, subadult red snapper were acclimated to 101.2, 405.3, 608.0, and 1,215.9 kPa, simulating depths typical of their distribution (as deep as 200 m), and then decompressed at a rate of 10.1 kPa/s. Lateral and dorsal X‐ray imaging in combination with necropsy showed that swim bladders expanded in a predictable manner. Ventral expansion into the caudal body cavity space occurred at lower pressures, whereas expansion into the cranial portion of the body cavity occurred at the highest pressure. Expansion patterns resulted in over 70 different overexpansion injuries, the most severe being to vital organs. Our results suggest a specific suite of clearly identifiable injuries associated with CD that increase in number and severity as retrieval depth increases. A more thorough understanding of catastrophic decompression syndrome will provide insight into the declining fishery and aid in developing effective physiology‐based management strategies.

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The impacts of ocean acidification will depend on the ability of marine organisms to tolerate, acclimate and eventually adapt to changes in ocean chemistry. Here, we use a unique transgenerational experiment to determine the molecular response of a coral reef fish to short-term, developmental and transgenerational exposure to elevated CO, and to test how these responses are influenced by variations in tolerance to elevated CO exhibited by the parents. Within-generation responses in gene expression to end-of-century predicted CO levels indicate that a self-amplifying cycle in GABAergic neurotransmission is triggered, explaining previously reported neurological and behavioural impairments. Furthermore, epigenetic regulator genes exhibited a within-generation specific response, but with some divergence due to parental phenotype. Importantly, we find that altered gene expression for the majority of within-generation responses returns to baseline levels following parental exposure to elevated CO conditions. Our results show that both parental variation in tolerance and cross-generation exposure to elevated CO are crucial factors in determining the response of reef fish to changing ocean chemistry.

Holding Your Breath Hemoglobin and myoglobin are widely responsible for oxygen transport and storage (see the Perspective by Rezende ). The ability of diving mammals to obtain enough oxygen to support extended dives and foraging is largely dependent on muscle myoglobin (Mb) content. Mirceta et al. (p. 1303 ) found that in mammalian lineages with an aquatic or semiaquatic lifestyle, Mb net charge increases, which may represent an adaptation to inhibit self-association of Mb at high intracellular concentrations. Epistasis results from nonadditive genetic interactions and can affect phenotypic evolution. Natarajan et al. (p. 1324 ) found that epistatic interactions were able to explain the increased hemoglobin oxygen-binding affinity observed in deer mice populations at high altitude. In mammals, the offloading of oxygen from hemoglobin is facilitated by a reduction in the blood's pH, driven by metabolically produced CO 2 . However, in fish, a reduction in blood pH reduces oxygen carrying capacity of hemoglobin. Rummer et al. (p. 1327 ) implanted fiber optic oxygen sensors within the muscles of rainbow trout and found that elevated CO 2 levels in the water led to acidosis and elevated oxygen tensions.

Summary 1.A range of physiological traits are linked with aggression and dominance within social hierarchies, but the role of individual aerobic capacity in facilitating aggression has seldom been studied. Further, links previously observed between an individual's metabolic rate and aggression level may be context dependent and modulated by factors such as social stress and fcompetitor familiarity. 2. We examined these issues in juvenile Ambon damselfish, Pomacentrus amboinensis, which display intraspecific competition for territories during settlement on coral reefs. 3. Individuals were measured for routine metabolic rate, aerobic scope (AS) and anaerobic capacity using intermittent-flow respirometry before dyadic dominance contests. Post-contest, fish were measured for metabolic rate in isolation and while interacting with their previous competitor or a stranger in adjacent transparent respirometers. 4. In arena contests, AS was correlated with aggression and dominance, while routine metabolic rate and anaerobic capacity were not related to dominance. Post-contest, subordinates showed a rise in metabolic rate and decrease in available AS, presumably due to social stress. Dominants increased metabolic rate in the presence of a previous competitor, possibly due to the stresses of hierarchy maintenance. 5. Metabolic rate during aggressive interactions did not approach that measured during exhaustive exercise, suggesting individuals do not fully utilise their AS during aggression. A greater AS may, however, allow faster post-contest recovery.6. These results demonstrate a link between AS and dominance during intraspecific competition for territory. Selection on AS could therefore follow, either indirectly through correlations with other traits influencing resource-holding potential, or directly if AS carries benefits important for territory acquisition or holding, such as an enhanced capacity to cope with socially induced stress.

The oceans are absorbing excess atmospheric CO2, and this is causing ocean acidification. Surprisingly, one coral reef damselfish exhibits enhanced aerobic performance after living at projected future ocean CO2 levels for 17 days. Identifying both the winners and losers under climate change scenarios is vital to conserving marine biodiversity.