Juvenile Atlantic cod behavior appears robust to near-future CO2 levels

Effects of ocean acidification on dopamine-mediated behavioral responses of a coral reef damselfish

We investigated whether CO₂-induced ocean acidification (OA) affects dopamine receptor-dependent behavior in bicolor damselfish (Stegastes partitus). Damselfish were kept in aquaria receiving flow through control (pH ~ 8.03; pCO₂ ~ 384 μatm) or OA (pH ~ 7.64; CO₂ ~ 1100 μatm) seawater at a rate of 1 L min^-1. Despite this relatively fast flow rate, fish respiration further acidified the seawater in both control (pH ~7.88; pCO₂ ~ 595 μatm) and OA (pH ~7.55; pCO₂ ~ 1450 μatm) fish-holding aquaria. After five days of exposure, damselfish locomotion, boldness, anxiety, and aggression were assessed using a battery of behavioral tests using automated video analysis. Two days later, these tests were repeated following application of the dopamine D₁ receptor agonist SKF 38393. OA-exposure induced ceiling anxiety levels that were significantly higher than in control damselfish, and SKF 38393 increased anxiety in control damselfish to a level not significantly different than that of OA-exposed damselfish. Additionally, SKF 38393 decreased locomotion and increased boldness in control damselfish but had no effect in OA-exposed damselfish, suggesting an alteration in activity of dopaminergic pathways that regulate behavior under OA conditions. These results indicate that changes in dopamine D₁ receptor function affects fish behavior during exposure to OA. However, subsequent measurements of seawater sampled using syringes during the daytime (~3-4 pm local time) from crevasses in coral reef colonies, which are used as shelter by damselfish, revealed an average pH of 7.73 ± 0.03 and pCO₂ of 925.8 ± 62.2 μatm; levels which are comparable to Representative Concentration Pathway (RCP) 8.5 predicted end-of-century mean OA levels in the open ocean. Further studies considering the immediate environmental conditions experienced by fish as well as individual variability and effect size are required to understand potential implications of the observed OA-induced behavioral effects on damselfish fitness in the wild.

A Systematic Review of the Behavioural Changes and Physiological Adjustments of Elasmobranchs and Teleost’s to Ocean Acidification with a Focus on Sharks

In recent years, much attention has been focused on the impact of climate change, particularly via ocean acidification (OA), on marine organisms. Studying the impact of OA on long-living organisms, such as sharks, is especially challenging. When the ocean waters absorb anthropogenic carbon dioxide (CO2), slow-growing shark species with long generation times may be subjected to stress, leading to a decrease in functionality. Our goal was to examine the behavioral and physiological responses of sharks to OA and the possible impacts on their fitness and resilience. We conducted a systematic review in line with PRISMA-Analyses, of previously reported scientific experiments. We found that most studies used CO2 partial pressures (pCO2) that reflect representative concentration pathways for the year 2100 (e.g., pH ~7.8, pCO2 ~1000 μatm). Since there is a considerable knowledge gap on the effect of OA on sharks, we utilized existing data on bony fish to synthesize the available knowledge. Given the similarities between the behaviors and physiology of these two superclasses’ to changes in CO2 and pH levels, there is merit in including the available information on bony fish as well. Several studies indicated a decrease in shark fitness in relation to increased OA and CO2 levels. However, the decrease was species-specific and influenced by the intensity of the change in atmospheric CO2 concentration and other anthropogenic and environmental factors (e.g., fishing, temperature). Most studies involved only limited exposure to future environmental conditions and were conducted on benthic shark species studied in the laboratory rather than on apex predator species. While knowledge gaps exist, and more research is required, we conclude that anthropogenic factors are likely contributing to shark species’ vulnerability worldwide. However, the impact of OA on the long-term stability of shark populations is not unequivocal.

Six common behavioral trials and their relevance for perch performance in natural lakes

Behavioral traits measured in laboratory settings are commonly used when predicting ecological effects and evolutionary outcomes in natural systems. However, uncertainties regarding the relevance of simplified lab-based behavioral tests for complex natural environments have created doubts about the use of these tests within aquatic ecology and ecotoxicology. In this study, we scrutinize the assumption that fish performance in six commonly applied behavioral assays has relevance for in situ behavior, by comparing individual behavior tracked in both artificial laboratory settings as well as in two natural lakes. We show that: i) commonly measured behavioral traits of individual fish (Perca fluviatilis) have low predictive power for within-lake behaviors if interpreted alone, but that; ii) composite variables synthesized from several (six) behavioral assays explain important in situ measures such as swimming activity, dispersion, home-range size, and habitat preference. While our findings support recent criticisms against the use of single behavioral tests for predicting environmental effects, we provide empirical evidences suggesting that fish performances in multiple laboratory assays are highly relevant for fish behavior in nature.

Elevated CO2 affects anxiety but not a range of other behaviours in juvenile yellowtail kingfish

Elevated seawater CO₂ can cause a range of behavioural impairments in marine fishes. However, most studies to date have been conducted on small benthic species and very little is known about how higher oceanic CO₂ levels could affect the behaviour of large pelagic species. Here, we tested the effects of elevated CO₂, and where possible the interacting effects of high temperature, on a range of ecologically important behaviours (anxiety, routine activity, behavioural lateralization and visual acuity) in juvenile yellowtail kingfish, Seriola lalandi. Kingfish were reared from the egg stage to 25 days post-hatch in a full factorial design of ambient and elevated CO₂ (~500 and ~1000 μatm pCO₂) and temperature (21 °C and 25 °C). The effects of elevated CO₂ were trait-specific with anxiety the only behaviour significantly affected. Juvenile S. lalandi reared at elevated CO₂ spent more time in the dark zone during a standard black-white test, which is indicative of increased anxiety. Exposure to high temperature had no significant effect on any of the behaviours tested. Overall, our results suggest that juvenile S. lalandi are largely behaviourally tolerant to future ocean acidification and warming. Given the ecological and economic importance of large pelagic fish species more studies investigating the effect of future climate change are urgently needed.

Brain and Behavioral Asymmetry: A Lesson From Fish

It is widely acknowledged that the left and right hemispheres of human brains display both anatomical and functional asymmetries. For more than a century, brain and behavioral lateralization have been considered a uniquely human feature linked to language and handedness. However, over the past decades this idea has been challenged by an increasing number of studies describing structural asymmetries and lateralized behaviors in non-human species extending from primates to fish. Evidence suggesting that a similar pattern of brain lateralization occurs in all vertebrates, humans included, has allowed the emergence of different model systems to investigate the development of brain asymmetries and their impact on behavior. Among animal models, fish have contributed much to the research on lateralization as several fish species exhibit lateralized behaviors. For instance, behavioral studies have shown that the advantages of having an asymmetric brain, such as the ability of simultaneously processing different information and perform parallel tasks compensate the potential costs associated with poor integration of information between the two hemispheres thus helping to better understand the possible evolutionary significance of lateralization. However, these studies inferred how the two sides of the brains are differentially specialized by measuring the differences in the behavioral responses but did not allow to directly investigate the relation between anatomical and functional asymmetries. With respect to this issue, in recent years zebrafish has become a powerful model to address lateralization at different level of complexity, from genes to neural circuitry and behavior. The possibility of combining genetic manipulation of brain asymmetries with cutting-edge in vivo imaging technique and behavioral tests makes the zebrafish a valuable model to investigate the phylogeny and ontogeny of brain lateralization and its relevance for normal brain function and behavior.

Neural effects of elevated CO2 in fish may be amplified by a vicious cycle

Maladaptive behavioural disturbances have been reported in some fishes and aquatic invertebrates exposed to projected future CO₂ levels. These disturbances have been linked to altered ion gradients and neurotransmitter function in the brain. Still, it seems surprising that the relatively small ionic changes induced by near-future CO₂ levels can have such profound neural effects. Based on recent transcriptomics data, we propose that a vicious cycle can be triggered that amplifies the initial disturbance, explaining how small pH regulatory adjustments in response to ocean acidification can lead to major behavioural alterations in fish and other water-breathing animals. The proposed cycle is initiated by a reversal of the function of some inhibitory GABA_A receptors in the direction of neural excitation and then amplified by adjustments in gene expression aimed at suppressing the excitation but in reality increasing it. In addition, the increased metabolic production of CO₂ by overexcited neurons will feed into the cycle by elevating intracellular bicarbonate levels that will lead to increased excitatory ion fluxes through GABA_A receptors. We also discuss the possibility that an initiation of a vicious cycle could be one of the several factors underlying the differences in neural sensitivity to elevated CO₂ displayed by fishes.

Behavioural responses of fish groups exposed to a predatory threat under elevated CO2

Most of the studies dealing with the effects of ocean acidification (OA) on fish behaviour tested individuals in isolation, even when the examined species live in shoals in the wild. Here we evaluated the effects of elevated CO₂ concentrations (i.e. ∼900 μatm) on the shelter use and group cohesion of the gregarious damselfish Chromis viridis using groups of sub-adults exposed to a predatory threat. Results showed that, under predatory threat, fish reared at elevated CO₂ concentrations displayed a risky behaviour (i.e. decreased shelter use), whereas their group cohesion was unaffected. Our findings add on increasing evidence to account for social dynamics in OA experiments, as living in groups may compensate for CO₂-induced risky behaviour.

Elevated CO2 alters behavior, growth, and lipid composition of Pacific cod larvae

High-latitude seas, which support a number of commercially important fisheries, are predicted to be most immediately impacted by ongoing ocean acidification (OA). Elevated CO₂ levels have been shown to induce a range of impacts on the physiology and behavior of marine fish larvae. However, these responses have yet to be characterized for most fishery species, including Pacific cod (Gadus macrocephalus). Based on laboratory experiments, we present a multi-faceted analysis of the sensitivity of Pacific cod larvae to elevated CO₂. Fish behavior in a horizontal light gradient was used to evaluate the sensitivity of behavioral phototaxis in 4-5 week old cod larvae. Fish at elevated CO₂ levels (∼1500 and 2250 μatm) exhibited a stronger phototaxis (moved more quickly to regions of higher light levels) than fish at ambient CO₂ levels (∼600 μatm). In an independent experiment, we examined the effects of elevated CO₂ levels on growth of larval Pacific cod over the first 5 weeks of life under two different feeding treatments. Fish exposed to elevated CO₂ levels (∼1700 μatm) were smaller and had lower lipid levels at 2 weeks of age than fish at low (ambient) CO₂ levels (∼500 μatm). However, by 5 weeks of age, this effect had reversed: fish reared at elevated CO₂ levels were slightly (but not significantly) larger and had higher total lipid levels and storage lipids than fish reared at low CO₂. Fatty acid composition differed significantly between fish reared at high and low CO₂ levels (p < 0.01) after 2 weeks of feeding, but this effect diminished by week 5. Effects of CO₂ on FA composition of the larvae differed between the two diets, an effect possibly related more to dietary equilibrium and differential lipid class storage than a fundamental effect of CO₂ on fatty acid metabolism. These experiments point to a stage-specific sensitivity of Pacific cod to the effects of OA. Further understanding of these effects will be required to predict the impacts on production of Pacific cod fisheries.

Elevated CO2 impairs olfactory-mediated neural and behavioral responses and gene expression in ocean-phase coho salmon (Oncorhynchus kisutch)

Elevated concentrations of CO₂ in seawater can disrupt numerous sensory systems in marine fish. This is of particular concern for Pacific salmon because they rely on olfaction during all aspects of their life including during their homing migrations from the ocean back to their natal streams. We investigated the effects of elevated seawater CO₂ on coho salmon (Oncorhynchus kisutch) olfactory-mediated behavior, neural signaling, and gene expression within the peripheral and central olfactory system. Ocean-phase coho salmon were exposed to three levels of CO₂ , ranging from those currently found in ambient marine water to projected future levels. Juvenile coho salmon exposed to elevated CO₂ levels for 2 weeks no longer avoided a skin extract odor that elicited avoidance responses in coho salmon maintained in ambient CO₂ seawater. Exposure to these elevated CO₂ levels did not alter odor signaling in the olfactory epithelium, but did induce significant changes in signaling within the olfactory bulb. RNA-Seq analysis of olfactory tissues revealed extensive disruption in expression of genes involved in neuronal signaling within the olfactory bulb of salmon exposed to elevated CO₂ , with lesser impacts on gene expression in the olfactory rosettes. The disruption in olfactory bulb gene pathways included genes associated with GABA signaling and maintenance of ion balance within bulbar neurons. Our results indicate that ocean-phase coho salmon exposed to elevated CO₂ can experience significant behavioral impairments likely driven by alteration in higher-order neural signal processing within the olfactory bulb. Our study demonstrates that anadromous fish such as salmon may share a sensitivity to rising CO₂ levels with obligate marine species suggesting a more wide-scale ecological impact of ocean acidification.

Acid-base physiology, neurobiology and behaviour in relation to CO2-induced ocean acidification

Experimental exposure to ocean and freshwater acidification affects the behaviour of multiple aquatic organisms in laboratory tests. One proposed cause involves an imbalance in plasma chloride and bicarbonate ion concentrations as a result of acid-base regulation, causing the reversal of ionic fluxes through GABA_A receptors, which leads to altered neuronal function. This model is exclusively based on differential effects of the GABA_A receptor antagonist gabazine on control animals and those exposed to elevated CO₂ However, direct measurements of actual chloride and bicarbonate concentrations in neurons and their extracellular fluids and of GABA_A receptor properties in aquatic organisms are largely lacking. Similarly, very little is known about potential compensatory mechanisms, and about alternative mechanisms that might lead to ocean acidification-induced behavioural changes. This article reviews the current knowledge on acid-base physiology, neurobiology, pharmacology and behaviour in relation to marine CO₂-induced acidification, and identifies important topics for future research that will help us to understand the potential effects of predicted levels of aquatic acidification on organisms.

Effects of elevated carbon dioxide on male and female behavioural lateralization in a temperate goby

Behavioural abnormality in fishes has been proposed as a significant consequence of the increasing levels of carbon dioxide occurring in the oceans. Negative effects of elevated CO₂ have been reported for behaviours such as predator-prey interactions, foraging, hearing and behavioural lateralization. Importantly, the effects vary greatly both within and between species, and some recent studies have shown minimal effects of CO₂ on behaviour. Whether the effect of CO₂ also varies between males and females is, however, virtually unexplored. According to resource allocation theory, females are expected to be more sensitive to elevated CO₂, meaning that non-sex-specific studies may overlook ecologically important differences between the sexes. In this study, we investigated the possible differences between males and females in their response to elevated CO₂ by performing behavioural lateralization tests in adult temperate two-spotted gobies, Gobiusculus flavescens. We found that the strength of the side bias (absolute lateralization) was unaffected by the CO₂ treatment, and there was no difference between males and females. The control fish were slightly right-biased in their behavioural asymmetry (mean relative lateralization of 14). Exposure to high CO₂ affected this pattern, such that treated fish were slightly left-biased (mean relative lateralization of -10), regardless of their sex. The same results were obtained yet again when the study was repeated during a second year. We discuss our results in light of the great variation in lateralization that has been reported to depend on variables such as species, ecological settings and environmental factors.

Growth performance and survival of larval Atlantic herring, under the combined effects of elevated temperatures and CO2

In the coming decades, environmental change like warming and acidification will affect life in the ocean. While data on single stressor effects on fish are accumulating rapidly, we still know relatively little about interactive effects of multiple drivers. Of particular concern in this context are the early life stages of fish, for which direct effects of increased CO₂ on growth and development have been observed. Whether these effects are further modified by elevated temperature was investigated here for the larvae of Atlantic herring (Clupea harengus), a commercially important fish species. Over a period of 32 days, larval survival, growth in size and weight, and instantaneous growth rate were assessed in a crossed experimental design of two temperatures (10°C and 12°C) with two CO₂ levels (400 μatm and 900 μatm CO₂) at food levels mimicking natural levels using natural prey. Elevated temperature alone led to increased swimming activity, as well as decreased survival and instantaneous growth rate (G_i). The comparatively high sensitivity to elevated temperature in this study may have been influenced by low food levels offered to the larvae. Larval size, G_i and swimming activity were not affected by CO₂, indicating tolerance of this species to projected "end of the century" CO₂ levels. A synergistic effect of elevated temperature and CO₂ was found for larval weight, where no effect of elevated CO₂ concentrations was detected in the 12°C treatment, but a negative CO₂ effect was found in the 10°C treatment. Contrasting CO₂ effects were found for survival between the two temperatures. Under ambient CO₂ conditions survival was increased at 12°C compared to 10°C. In general, CO₂ effects were minor and considered negligible compared to the effect of temperature under these mimicked natural food conditions. These findings emphasize the need to include biotic factors such as energy supply via prey availability in future studies on interactive effects of multiple stressors.

Ocean acidification does not impair predator recognition but increases juvenile growth in a temperate wrasse off CO2 seeps

Fish behavioural effects under Ocean Acidification (OA) rely on changes expected to occur in brain function, which can be reversed by gabazine, a GABA-A antagonist. Here, using standard two-channel choice flume, we assessed OA effects on the predator recognition ability of both gabazine-treated and -untreated Symphodus ocellatus post-settlers living off CO₂ seeps in the Mediterranean Sea. To estimate the post-settlers background predation risk we evaluated the density of their predator in the wild and through otolith aging techniques we assessed their post-settlement growth. Results showed that: 1) post-settlers predator recognition was unaffected under OA; 2) post-settlers living in elevated CO₂ were on average 15% bigger in size than those from ambient conditions. Our results support fish behavioural tolerance to OA, potentially mediated by pre-exposure to high-risk predation levels, and speculate that by increasing body size, juvenile fish might more efficiently avoid their predators.

Differences in neurochemical profiles of two gadid species under ocean warming and acidification

Background

Exposure to future ocean acidification scenarios may alter the behaviour of marine teleosts through interference with neuroreceptor functioning. So far, most studies investigated effects of ocean acidification on the behaviour of fish, either isolated or in combination with environmental temperature. However, only few physiological studies on this issue were conducted despite the putative neurophysiological origin of the CO₂-induced behavioural changes. Here, we present the metabolic consequences of long-term exposure to projected ocean acidification (396–548 μatm PCO₂ under control and 915–1272 μatm under treatment conditions) and parallel warming in the brain of two related fish species, polar cod (Boreogadus saida, exposed to 0 °C, 3 °C, 6 °C and 8 °C) and Atlantic cod (Gadus morhua, exposed to 3 °C, 8 °C, 12 °C and 16 °C). It has been shown that B. saida is behaviourally vulnerable to future ocean acidification scenarios, while G. morhua demonstrates behavioural resilience.

Results

We found that temperature alters brain osmolyte, amino acid, choline and neurotransmitter concentrations in both species indicating thermal responses particularly in osmoregulation and membrane structure. In B. saida, changes in amino acid and osmolyte metabolism at the highest temperature tested were also affected by CO₂, possibly emphasizing energetic limitations. We did not observe changes in neurotransmitters, energy metabolites, membrane components or osmolytes that might serve as a compensatory mechanism against CO₂ induced behavioural impairments. In contrast to B. saida, such temperature limitation was not detected in G. morhua; however, at 8 °C, CO₂ induced an increase in the levels of metabolites of the glutamate/GABA-glutamine cycle potentially indicating greater GABAergic activity in G.morhua. Further, increased availability of energy-rich substrates was detected under these conditions.

Conclusions

Our results indicate a change of GABAergic metabolism in the nervous system of Gadus morhua close to the optimum of the temperature range. Since a former study showed that juvenile G. morhua might be slightly more behaviourally resilient to CO₂ at this respective temperature, we conclude that the observed change of GABAergic metabolism could be involved in counteracting OA induced behavioural changes. This may serve as a fitness advantage of this respective species compared to B. saida in a future warmer, more acidified polar ocean.

Electronic supplementary material

The online version of this article (10.1186/s12983-017-0238-5) contains supplementary material, which is available to authorized users.

CO2-induced ocean acidification does not affect individual or group behaviour in a temperate damselfish

Open ocean surface CO₂ levels are projected to reach approximately 800 µatm, and ocean pH to decrease by approximately 0.3 units by the year 2100 due to anthropogenic CO₂ emissions and the subsequent process of ocean acidification (OA). When exposed to these CO₂/pH values, several fish species display abnormal behaviour in laboratory tests, an effect proposed to be linked to altered neuronal GABA_A- receptor function. Juvenile blacksmith (Chromis punctipinnis) are social fish that regularly experience CO₂/pH fluctuations through kelp forest diurnal primary production and upwelling events, so we hypothesized that they might be resilient to OA. Blacksmiths were exposed to control conditions (pH ∼ 7.92; pCO₂ ∼ 540 µatm), constant acidification (pH ∼ 7.71; pCO₂ ∼ 921 µatm) and oscillating acidification (pH ∼ 7.91, pCO₂ ∼ 560 µatm (day), pH ∼ 7.70, pCO₂ ∼ 955 µatm (night)), and caught and tested in two seasons of the year when the ocean temperature was different: winter (16.5 ± 0.1°C) and summer (23.1 ± 0.1°C). Neither constant nor oscillating CO₂-induced acidification affected blacksmith individual light/dark preference, inter-individual distance in a shoal or the shoal's response to a novel object, suggesting that blacksmiths are tolerant to projected future OA conditions. However, blacksmiths tested during the winter demonstrated significantly higher dark preference in the individual light/dark preference test, thus confirming season and/or water temperature as relevant factors to consider in behavioural tests.

Intertidal pool fish Girella laevifrons (Kyphosidae) shown strong physiological homeostasis but shy personality: The cost of living in hypercapnic habitats

Tide pools habitats are naturally exposed to a high degree of environmental variability. The consequences of living in these extreme habitats are not well established. In particular, little it is known about of the effects of hypercanic seawater (i.e. high pCO₂ levels) on marine vertebrates such as intertidal pool fish. The aim of this study was to evaluate the effects of increased pCO₂ on the physiology and behavior in juveniles of the intertidal pool fish Girella laevifrons. Two nominal pCO₂ concentrations (400 and 1600μatm) were used. We found that exposure to hypercapnic conditions did not affect oxygen consumption and absorption efficiency. However, the lateralization and boldness behavior was significantly disrupted in high pCO₂ conditions. In general, a predator-risk cost of boldness is assumed, thus the increased occurrence of shy personality in juvenile fishes may result in a change in the balance of this biological interaction, with significant ecological consequences.

Elevated carbon dioxide has limited acute effects on Lepomis macrochirus behaviour

The current study investigated the behavioural response of Lepomis macrochirus following exposures to elevated carbon dioxide (CO₂ ). For this, L. macrochirus were held at ambient pCO₂ (160 μatm pCO₂ ) for 7 days, then exposed to elevated pCO₂ (8300 μatm pCO₂ ) for 5 days, and then returned to ambient conditions for a further 5 days to recover. At the end of each exposure period, several behavioural metrics were quantified (boldness, lateralization and activity). Data showed no change in lateralization and most metrics associated with performance and boldness. During the boldness test, however, average velocity, velocity in the thigmotaxis (outer) zone and proportion of activity in the thigmotaxis zone increased with pCO₂ exposure. During post-exposure, average velocity of L. macrochirus decreased. In addition, individual rank was repeatable during the pre-exposure and post-exposure period in three of the 17 metrics investigated (average velocity in the middle zone, average velocity near object and total shuttles to the object zone), but not during the CO₂ exposure period, suggesting that elevated pCO₂ disrupted some behavioural performances. Overall, this study found elevated pCO₂ caused disruption to behaviours of freshwater fishes such as L. macrochirus and effects do not appear to be as serious as has been shown for marine fishes.

Painted Goby Larvae under High-CO2 Fail to Recognize Reef Sounds

Atmospheric CO₂ levels have been increasing at an unprecedented rate due to anthropogenic activity. Consequently, ocean pCO₂ is increasing and pH decreasing, affecting marine life, including fish. For many coastal marine fishes, selection of the adult habitat occurs at the end of the pelagic larval phase. Fish larvae use a range of sensory cues, including sound, for locating settlement habitat. This study tested the effect of elevated CO₂ on the ability of settlement-stage temperate fish to use auditory cues from adult coastal reef habitats. Wild late larval stages of painted goby (Pomatoschistus pictus) were exposed to control pCO₂ (532 μatm, pH 8.06) and high pCO₂ (1503 μatm, pH 7.66) conditions, likely to occur in nearshore regions subjected to upwelling events by the end of the century, and tested in an auditory choice chamber for their preference or avoidance to nighttime reef recordings. Fish reared in control pCO₂ conditions discriminated reef soundscapes and were attracted by reef recordings. This behaviour changed in fish reared in the high CO₂ conditions, with settlement-stage larvae strongly avoiding reef recordings. This study provides evidence that ocean acidification might affect the auditory responses of larval stages of temperate reef fish species, with potentially significant impacts on their survival.

Expression of genes involved in brain GABAergic neurotransmission in three-spined stickleback exposed to near-future CO2

Change in the activity of the main inhibitory receptor, GABA_A, has been suggested to be a general mechanism behind the behavioural alterations reported in ocean acidification studies on fish. It has been proposed that regulatory acid-base mechanisms in response to high CO₂ alter the neuronal Cl^- and HCO₃^- gradients that are important for GABA_A receptor function. Here, we report a comprehensive analysis of gene expression of GABA_A receptor subunits and of genes involved in GABAergic transmission in the brain of fish exposed to near-future CO₂. Altogether, 56 mRNA transcripts were quantified in brains of three-spined stickleback (Gasterosteus aculeatus) kept in control pCO₂ (333 ± 30 μatm CO₂) or at high pCO₂ levels (991 ± 57 μatm) for 43 days. The gene expression analysis included GABA_A receptor subunits (α1-6, β1-3, γ1-3, δ, π and ρ1-3), enzymes and transporters involved in GABA metabolism (GAD1-2, GABAT and GAT1-3), GABA_A receptor-associated proteins (GABARAP and GABARAPL), ion cotransporters (KCC1-4, NKCC1, ClC21-3, AE3 and NDAE) and carbonic anhydrase (CAII). Exposure to high CO₂ had only minor effects on the expression of genes involved in GABAergic neurotransmission. There were significant increases in the mRNA levels of α family subunits of the GABA_A receptor, with a more pronounced expression of α1₂, α3, α4 and α6b. No changes were detected in the expression of other GABA_A subunits or in genes related to receptor turnover, GABA metabolism or ion transport. Although the minor changes seen for mRNA levels might reflect compensatory mechanisms in the high-CO₂ conditions, these were apparently insufficient to restore normal neural function, because the behavioural changes persisted within the time frame studied.

Effect of elevated carbon dioxide on shoal familiarity and metabolism in a coral reef fish

Atmospheric CO₂ is expected to more than double by the end of the century. The resulting changes in ocean chemistry will affect the behaviour, sensory systems and physiology of a range of fish species. Although a number of past studies have examined effects of CO₂ in gregarious fishes, most have assessed individuals in social isolation, which can alter individual behaviour and metabolism in social species. Within social groups, a learned familiarity can develop following a prolonged period of interaction between individuals, with fishes preferentially associating with familiar conspecifics because of benefits such as improved social learning and greater foraging opportunities. However, social recognition occurs through detection of shoal-mate cues; hence, it may be disrupted by near-future CO₂ conditions. In the present study, we examined the influence of elevated CO₂ on shoal familiarity and the metabolic benefits of group living in the gregarious damselfish species the blue-green puller (Chromis viridis). Shoals were acclimated to one of three nominal CO₂ treatments: control (450 µatm), mid-CO₂ (750 µatm) or high-CO₂ (1000 µatm). After a 4-7 day acclimation period, familiarity was examined using a choice test, in which individuals were given the choice to associate with familiar shoal-mates or unfamiliar conspecifics. In control conditions, individuals preferentially associated with familiar shoal-mates. However, this association was lost in both elevated-CO₂ treatments. Elevated CO₂ did not impact the calming effect of shoaling on metabolism, as measured using an intermittent-flow respirometry methodology for social species following a 17-20 day acclimation period to CO₂ treatment. In all CO₂ treatments, individuals exhibited a significantly lower metabolic rate when measured in a shoal vs. alone, highlighting the complexity of shoal dynamics and the processes that influence the benefits of shoaling.

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?

Altered brain ion gradients following compensation for elevated CO2 are linked to behavioural alterations in a coral reef fish

Neurosensory and behavioural disruptions are some of the most consistently reported responses upon exposure to ocean acidification-relevant CO2 levels, especially in coral reef fishes. The underlying cause of these disruptions is thought to be altered current across the GABAA receptor in neuronal cells due to changes in ion gradients (HCO3(-) and/or Cl(-)) that occur in the body following compensation for elevated ambient CO2. Despite these widely-documented behavioural disruptions, the present study is the first to pair a behavioural assay with measurements of relevant intracellular and extracellular acid-base parameters in a coral reef fish exposed to elevated CO2. Spiny damselfish (Acanthochromis polyacanthus) exposed to 1900 μatm CO2 for 4 days exhibited significantly increased intracellular and extracellular HCO3(-) concentrations and elevated brain pHi compared to control fish, providing evidence of CO2 compensation. As expected, high CO2 exposed damselfish spent significantly more time in a chemical alarm cue (CAC) than control fish, supporting a potential link between behavioural disruption and CO2 compensation. Using HCO3(-) measurements from the damselfish, the reversal potential for GABAA (EGABA) was calculated, illustrating that biophysical properties of the brain during CO2 compensation could change GABAA receptor function and account for the behavioural disturbances noted during exposure to elevated CO2.

Intraspecific variation in physiological performance of a benthic elasmobranch challenged by ocean acidification and warming

Elucidating the combined effects of increasing temperature and ocean acidification on performance of fishes is central to our understanding of how species will respond to global climate change. Measuring the metabolic costs associated with intense and short activities, such as those required to escape predators, is key to quantifying changes in performance and estimating the potential effects of environmental stressors on survival. In this study, juvenile little skate Leucoraja erinacea from two neighboring locations (Gulf of Maine, or northern location, and Georges Bank, or southern location) were developmentally acclimatized and reared at current and projected temperatures (15, 18 or 20°C) and acidification conditions (pH 8.1 or 7.7), and their escape performance was tested by employing a chasing protocol. The results from this study suggest countergradient variation in growth between skates from the two locations, while the optimum for escape performance was at a lower temperature in individuals from the northern latitudes, which could be related to adaptation to the local thermal environment. Aerobic performance and scope declined in skates from the northern latitudes under simulated ocean warming and acidification conditions. Overall, the southern skates showed lower sensitivity to these climatic stressors. This study demonstrates that even mobile organisms from neighboring locations can exhibit substantial differences in energetic costs of exercise and that skates from the northern part of the geographic range may be more sensitive to the directional increase in temperature and acidification expected by the end of the century.

Are global warming and ocean acidification conspiring against marine ectotherms? A meta-analysis of the respiratory effects of elevated temperature, high CO2 and their interaction

With the occurrence of global change, research aimed at estimating the performance of marine ectotherms in a warmer and acidified future has intensified. The concept of oxygen- and capacity-limited thermal tolerance, which is inspired by the Fry paradigm of a bell-shaped increase-optimum-decrease-type response of aerobic scope to increasing temperature, but also includes proposed negative and synergistic effects of elevated CO2 levels, has been suggested as a unifying framework. The objectives of this meta-analysis were to assess the following: (i) the generality of a bell-shaped relationship between absolute aerobic scope (AAS) and temperature; (ii) to what extent elevated CO2 affects resting oxygen uptake MO2rest and AAS; and (iii) whether there is an interaction between elevated temperature and CO2. The behavioural effects of CO2 are also briefly discussed. In 31 out of 73 data sets (both acutely exposed and acclimated), AAS increased and remained above 90% of the maximum, whereas a clear thermal optimum was observed in the remaining 42 data sets. Carbon dioxide caused a significant rise in MO2rest in only 18 out of 125 data sets, and a decrease in 25, whereas it caused a decrease in AAS in four out of 18 data sets and an increase in two. The analysis did not reveal clear evidence for an overall correlation with temperature, CO2 regime or duration of CO2 treatment. When CO2 had an effect, additive rather than synergistic interactions with temperature were most common and, interestingly, they even interacted antagonistically on MO2rest and AAS. The behavioural effects of CO2 could complicate experimental determination of respiratory performance. Overall, this meta-analysis reveals heterogeneity in the responses to elevated temperature and CO2 that is not in accordance with the idea of a single unifying principle and which cannot be ignored in attempts to model and predict the impacts of global warming and ocean acidification on marine ectotherms.