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Climate change stressors are progressively simplifying biogenic habitats in the terrestrial and marine realms, and consequently altering the structure of associated species communities. Here, we used a volcanic CO 2 seep in Papua New Guinea to test in situ if altered reef architecture due to ocean acidification reshuffles associated fish assemblages. We observed replacement of branching corals by massive corals at the seep, with simplified coral architectural complexity driving abundance declines between 60% and 86% for an assemblage of damselfishes associated with branching corals. An experimental test of habitat preference for a focal species indicated that acidification does not directly affect habitat selection behaviour, with changes in habitat structural complexity consequently appearing to be the stronger driver of assemblage reshuffling. Habitat health affected anti-predator behaviour, with P. moluccensis becoming less bold on dead branching corals relative to live branching corals, irrespective of ocean acidification. We conclude that coral reef fish assemblages are likely to be more sensitive to changes in habitat structure induced by increasing pCO 2 than any direct effects on behaviour, indicating that changes in coral architecture and live cover may act as important mediators of reef fish community structures in a future ocean., (© 2024 The Author(s). Journal of Animal Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society.)

Ocean acidification (OA) is postulated to affect the physiology, behavior, and life-history of marine species, but potential for acclimation or adaptation to elevated pCO 2 in wild populations remains largely untested. We measured brain transcriptomes of six coral reef fish species at a natural volcanic CO 2  seep and an adjacent control reef in Papua New Guinea. We show that elevated pCO 2 induced common molecular responses related to circadian rhythm and immune system but different magnitudes of molecular response across the six species. Notably, elevated transcriptional plasticity was associated with core circadian genes affecting the regulation of intracellular pH and neural activity in Acanthochromis polyacanthus. Gene expression patterns were reversible in this species as evidenced upon reduction of CO 2 following a natural storm-event. Compared with other species, Ac. polyacanthus has a more rapid evolutionary rate and more positively selected genes in key functions under the influence of elevated CO 2 , thus fueling increased transcriptional plasticity. Our study reveals the basis to variable gene expression changes across species, with some species possessing evolved molecular toolkits to cope with future OA., (© 2022 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Elevated CO 2 levels have been shown to affect metabolic performance in some coral reef fishes. However, all studies to date have employed stable elevated CO 2 levels, whereas reef habitats can experience substantial diel fluctuations in pCO 2 ranging from ±50 to 600 μatm around the mean, fluctuations that are predicted to increase in magnitude by the end of the century. Additionally, past studies have often investigated the effect of elevated CO 2 in isolation, despite the fact that ocean temperatures will increase in tandem with CO 2 levels. Here, we tested the effects of stable (1000 μatm) versus diel-cycling (1000 ± 500 μatm) elevated CO 2 conditions and elevated temperature (+2 °C) on metabolic traits of juvenile spiny damselfish, Acanthochromis polyacanthus. Resting oxygen uptake rates (ṀO 2 ) were higher in fish exposed to stable elevated CO 2 conditions when compared to fish from stable control conditions, but were restored to control levels under diel CO 2 fluctuations. However, the benefits of diel CO 2 fluctuations were diminished at elevated temperature. Factorial aerobic scope showed a similar pattern, but neither maximal ṀO 2 nor absolute aerobic scope was affected by CO 2 or temperature. Our results suggest that diel CO 2 cycles can ameliorate the increased metabolic cost associated with elevated CO 2 , but elevated temperature diminishes the benefits of diel CO 2 cycles. Thus, previous studies may have misestimated the effect of ocean acidification on the metabolic performance of reef fishes by not accounting for environmental CO 2 fluctuations. Our findings provide novel insights into the interacting effects of diel CO 2 fluctuations and temperature on the metabolic performance of reef fishes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Aquatic acidification, caused by elevating levels of atmospheric carbon dioxide (CO 2 ), is increasing in both freshwater and marine ecosystems worldwide. However, few studies have examined how acidification will affect oxygen (O 2 ) transport and, therefore, performance in fishes. Although data are generally lacking, the majority of fishes investigated in this meta-analysis exhibited no effect of elevated CO 2 at the level of O 2 uptake, suggesting that they are able to maintain metabolic performance during a period of acidosis. Notably, the mechanisms that fish employ to maintain performance and O 2 uptake have yet to be verified. Here, we summarize current data related to one recently proposed mechanism underpinning the maintenance of O 2 uptake during exposure to aquatic acidification, and reveal knowledge gaps that could be targeted for future research. Most studies have examined O 2 uptake rates while fishes were resting and did not calculate aerobic scope, even though aerobic scope can aid in predicting changes to whole-animal metabolic performance. Furthermore, research is lacking on different age classes, freshwater species and elasmobranchs, all of which might be impacted by future acidification conditions. Finally, this Review further seeks to emphasize the importance of developing collaborative efforts between molecular, physiological and ecological approaches in order to provide more comprehensive predictions as to how future fish populations will be affected by climate change., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

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 2 , and to test how these responses are influenced by variations in tolerance to elevated CO 2 exhibited by the parents. Within-generation responses in gene expression to end-of-century predicted CO 2 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 2 conditions. Our results show that both parental variation in tolerance and cross-generation exposure to elevated CO 2 are crucial factors in determining the response of reef fish to changing ocean chemistry.

Sharks play a key role in the structure of marine food webs, but are facing major threats due to overfishing and habitat degradation. Although sharks are also assumed to be at relatively high risk from climate change due to a low intrinsic rate of population growth and slow rates of evolution, ocean acidification (OA) has not, until recently, been considered a direct threat. New studies have been evaluating the potential effects of end-of-century elevated CO 2 levels on sharks and their relatives' early development, physiology and behaviour. Here, we review those findings and use a meta-analysis approach to quantify the overall direction and magnitude of biological responses to OA in the species of sharks that have been investigated to date. While embryo survival and development time are mostly unaffected by elevated CO 2 , there are clear effects on body condition, growth, aerobic potential and behaviour (e.g. lateralization, hunting and prey detection). Furthermore, studies to date suggest that the effects of OA could be as substantial as those due to warming in some species. A major limitation is that all past studies have involved relatively sedentary, benthic sharks that are capable of buccal ventilation-no studies have investigated pelagic sharks that depend on ram ventilation. Future research should focus on species with different life strategies (e.g. pelagic, ram ventilators), climate zones (e.g. polar regions), habitats (e.g. open ocean), and distinct phases of ontogeny in order to fully predict how OA and climate change will impact higher-order predators and therefore marine ecosystem dynamics., (© 2017 The Author(s).)

The gill is widely accepted to have played a key role in the adaptive radiation of early vertebrates by supplanting the skin as the dominant site of gas exchange. However, in the most basal extant craniates, the hagfishes, gills play only a minor role in gas exchange. In contrast, we found hagfish gills to be associated with a tremendous capacity for acid-base regulation. Indeed, Pacific hagfish exposed acutely to severe sustained hypercarbia tolerated among the most severe blood acidoses ever reported (1.2 pH unit reduction) and subsequently exhibited the greatest degree of acid-base compensation ever observed in an aquatic chordate. This was accomplished through an unprecedented increase in plasma [HCO3(-)] (>75 mM) in exchange for [Cl(-)]. We thus propose that the first physiological function of the ancestral gill was acid-base regulation, and that the gill was later co-opted for its central role in gas exchange in more derived aquatic vertebrates.

Ocean warming and acidification are serious threats to marine life. While each stressor alone has been studied in detail, their combined effects on the outcome of ecological interactions are poorly understood. We measured predation rates and predator selectivity of two closely related species of damselfish exposed to a predatory dottyback. We found temperature and CO2 interacted synergistically on overall predation rate, but antagonistically on predator selectivity. Notably, elevated CO2 or temperature alone reversed predator selectivity, but the interaction between the two stressors cancelled selectivity. Routine metabolic rates of the two prey showed strong species differences in tolerance to CO2 and not temperature, but these differences did not correlate with recorded mortality. This highlights the difficulty of linking species-level physiological tolerance to resulting ecological outcomes. This study is the first to document both synergistic and antagonistic effects of elevated CO2 and temperature on a crucial ecological process like predator-prey dynamics., (© 2014 John Wiley & Sons Ltd.)

Ocean surface CO2 levels are increasing in line with rising atmospheric CO2 and could exceed 900μatm by year 2100, with extremes above 2000μatm in some coastal habitats. The imminent increase in ocean pCO2 is predicted to have negative consequences for marine fishes, including reduced aerobic performance, but variability among species could be expected. Understanding interspecific responses to ocean acidification is important for predicting the consequences of ocean acidification on communities and ecosystems. In the present study, the effects of exposure to near-future seawater CO2 (860μatm) on resting (M˙ O2rest) and maximum (M˙O2max) oxygen consumption rates were determined for three tropical coral reef fish species interlinked through predator-prey relationships: juvenile Pomacentrus moluccensis and Pomacentrus amboinensis, and one of their predators: adult Pseudochromis fuscus. Contrary to predictions, one of the prey species, P. amboinensis, displayed a 28-39% increase in M˙O2max after both an acute and four-day exposure to near-future CO2 seawater, while maintaining M˙O2rest. By contrast, the same treatment had no significant effects on M˙O2rest or M˙O2max of the other two species. However, acute exposure of P. amboinensis to 1400 and 2400μatm CO2 resulted in M˙O2max returning to control values. Overall, the findings suggest that: (1) the metabolic costs of living in a near-future CO2 seawater environment were insignificant for the species examined at rest; (2) the M˙O2max response of tropical reef species to near-future CO2 seawater can be dependent on the severity of external hypercapnia; and (3) near-future ocean pCO2 may not be detrimental to aerobic scope of all fish species and it may even augment aerobic scope of some species. The present results also highlight that close phylogenetic relatedness and living in the same environment, does not necessarily imply similar physiological responses to near-future CO2., (© 2013.)

Increased oceanic uptake of atmospheric carbon dioxide (CO2) is a threat to marine organisms and ecosystems. Among the most dramatic consequences predicted to date are behavioural impairments in marine fish which appear to be caused by the interference of elevated CO2 with a key neurotransmitter receptor in the brain. In this study, we tested the effects of elevated CO2 on the foraging and shelter-seeking behaviours of the reef-dwelling epaulette shark, Hemiscyllium ocellatum. Juvenile sharks were exposed for 30 d to control CO2 (400 µatm) and two elevated CO2 treatments (615 and 910 µatm), consistent with medium- and high-end projections for ocean pCO2 by 2100. Contrary to the effects observed in teleosts and in some other sharks, behaviour of the epaulette shark was unaffected by elevated CO2. A potential explanation is the remarkable adaptation of H. ocellatum to low environmental oxygen conditions (hypoxia) and diel fluctuations in CO2 encountered in their shallow reef habitat. This ability translates into behavioural tolerance of near-future ocean acidification, suggesting that behavioural tolerance and subsequent adaptation to projected future CO2 levels might be possible in some other fish, if adaptation can keep pace with the rate of rising CO2 levels. [ABSTRACT FROM AUTHOR]

Experiments have shown that the behaviour of reef fishes can be seriously affected by projected future carbon dioxide (CO2) concentrations in the ocean. However, whether fish can acclimate to elevated CO2 over the longer term, and the consequences of altered behaviour on the structure of fish communities, are unknown. We used marine CO2 seeps in Papua New Guinea as a natural laboratory to test these questions. Here we show that juvenile reef fishes at CO2 seeps exhibit behavioural abnormalities similar to those seen in laboratory experiments. Fish from CO2 seeps were attracted to predator odour, did not distinguish between odours of different habitats, and exhibited bolder behaviour than fish from control reefs. High CO2 did not, however, have any effect on metabolic rate or aerobic performance. Contrary to expectations, fish diversity and community structure differed little between CO2 seeps and nearby control reefs. Differences in abundances of some fishes could be driven by the different coral community at CO2 seeps rather than by the direct effects of high CO2. Our results suggest that recruitment of juvenile fish from outside the seeps, along with fewer predators within the seeps, is currently sufficient to offset any negative effects of high CO2 within the seeps. However, continuous exposure does not reduce the effect of high CO2 on behaviour in natural reef habitat, and this could be a serious problem for fish communities in the future when ocean acidification becomes widespread as a result of continued uptake of anthropogenic CO2 emissions. [ABSTRACT FROM AUTHOR]

Increased oceanic uptake of atmospheric carbon dioxide (CO2) is a threat to marine organisms and ecosystems. Among the most dramatic consequences predicted to date are behavioural impairments in marine fish which appear to be caused by the interference of elevated CO2 with a key neurotransmitter receptor in the brain. In this study, we tested the effects of elevated CO2 on the foraging and shelter-seeking behaviours of the reef-dwelling epaulette shark, Hemiscyllium ocellatum. Juvenile sharks were exposed for 30 d to control CO2 (400 µatm) and two elevated CO2 treatments (615 and 910 µatm), consistent with medium- and high-end projections for ocean pCO2 by 2100. Contrary to the effects observed in teleosts and in some other sharks, behaviour of the epaulette shark was unaffected by elevated CO2. A potential explanation is the remarkable adaptation of H. ocellatum to low environmental oxygen conditions (hypoxia) and diel fluctuations in CO2 encountered in their shallow reef habitat. This ability translates into behavioural tolerance of near-future ocean acidification, suggesting that behavioural tolerance and subsequent adaptation to projected future CO2 levels might be possible in some other fish, if adaptation can keep pace with the rate of rising CO2 levels. [ABSTRACT FROM AUTHOR]

Ocean acidification (OA) is predicted to affect the physiology of some fishes. To date, most studies have investigated this issue using stable p CO 2 levels based on open ocean projections. Yet, most shallow, nearshore systems experience temporal and spatial p CO 2 fluctuations. For example, p CO 2 on coral reefs is highest at night and lowest during the day, but as OA progresses, both the average p CO 2 and magnitude of fluctuations are expected to increase. We exposed four coral reef fishes – Lutjanus fulviflamma, Caesio cuning, Abudefduf whitleyi, and Cheilodipterus quinquelineatus – to ambient, stable elevated, or fluctuating elevated p CO 2 conditions for 9–11 days. Then, we measured swimming performance, oxygen uptake rates, and haematological parameters during the day and at night. When compared to ambient p CO 2 conditions, L. fulviflamma, C. cuning, and A. whitleyi exposed to fluctuating elevated p CO 2 increased swimming performance, maximum oxygen uptake rates, and aerobic scope, regardless of time of day; whereas, the only nocturnal species studied, C. quinquelineatus , decreased maximum oxygen uptake rates and aerobic scope. Our findings suggest that exposure to fluctuating or stable elevated p CO 2 can physiologically benefit some coral reef fishes; however, other species, such as the cardinalfish examined here, may be more sensitive to future OA conditions. • Coral reefs exhibit natural CO 2 fluctuations expected to increase with climate change. • We measured swimming performance, O 2 uptake rates, aerobic scope, and blood variables. • Three of the four species benefitted under elevated fluctuating CO 2 conditions. • The nocturnal cardinalfish studied may be more sensitive to elevated fluctuating CO 2. • Studies should use ecologically-relevant CO 2 when predicting climate change impacts. [ABSTRACT FROM AUTHOR]