An interplay between plasticity and parental phenotype determines impacts of ocean acidification on a reef fish

Impacts of climate-related stressors on social group cohesion and individual sociability in fish

Group-living in animals comes with a number of benefits associated with predator avoidance, foraging, and reproduction. A large proportion of fish species display grouping behaviour. Fish may also be particularly vulnerable to climate-related stressors including thermal variation, hypoxia, and acidification. As climate-related stressors are expected to increase in magnitude and frequency, any effects on fish behaviour may be increased and affect the ability of fish species to cope with changing conditions. Here we conduct a systematic review of the effects of temperature, hypoxia, and acidification on individual sociability and group cohesion in shoaling and schooling fishes. Searches of the published and grey literature were carried out, and studies were included or excluded based on selection criteria. Data from studies were then included in a meta-analysis to examine broad patterns of effects of climate-related stressors in the literature. Evidence was found for a reduction in group cohesion at low oxygen levels, which was stronger in smaller groups. While several studies reported effects of temperature and acidification, there was no consistent effect of either stressor on sociability or cohesion. There was some evidence that marine fishes are more strongly negatively affected by acidification compared with freshwater species, but results are similarly inconsistent and more studies are required. Additional studies of two or more stressors in combination are also needed, although one study found reduced sociability following exposure to acidification and high temperatures. Overall, there is some evidence that hypoxia, and potentially other climate-related environmental changes, impact sociability and group cohesion in fishes. This may reduce survival and adaptability in shoaling and schooling species and have further ecological implications for aquatic systems. However, this synthesis mainly highlights the need for more empirical studies examining the effects of climate-related factors on social behaviour in fishes.

Cross-talk between tissues is critical for intergenerational acclimation to environmental change in Acanthochromis polyacanthus

Organisms' responses to environmental changes involve complex, coordinated responses of multiple tissues and potential parental influences. Here using a multi-tissue approach we determine how variation in parental behavioural tolerance and exposure to elevated CO2 influences the developmental and intergenerational molecular responses of their offspring in the coral reef fish Acanthochromis polyacanthus to future ocean acidification (OA) conditions. Gills and liver showed the highest transcriptional response to OA in juvenile fish regardless of parental OA conditioning, while the brain and liver showed the greatest intergenerational acclimation signals. Developmentally induced signals of OA, such as altered neural function in the brain, were restored to control levels after intergenerational exposure. Intergenerational CO2 exposure also enabled the offspring to adjust their metabolic processes, potentially allowing them to better meet the energetic demands of a high CO2 environment. Furthermore, offspring of OA-exposed parents differentially expressed a new complement of genes, which may facilitate intergenerational acclimatory responses. A genetic component of intergenerational plasticity also played a crucial role, with the parental behavioural phenotype largely determining the offspring's transcriptional signals. Overall, our results reveal tissue-specific transcriptional changes underlying intergenerational plastic responses to elevated CO2 exposure, enhancing understanding of organismal acclimation to OA throughout the whole body.

Transcriptomic responses in the nervous system and correlated behavioural changes of a cephalopod exposed to ocean acidification

BACKGROUND

The nervous system is central to coordinating behavioural responses to environmental change, likely including ocean acidification (OA). However, a clear understanding of neurobiological responses to OA is lacking, especially for marine invertebrates.

RESULTS

We evaluated the transcriptomic response of the central nervous system (CNS) and eyes of the two-toned pygmy squid (Idiosepius pygmaeus) to OA conditions, using a de novo transcriptome assembly created with long read PacBio ISO-sequencing data. We then correlated patterns of gene expression with CO2 treatment levels and OA-affected behaviours in the same individuals. OA induced transcriptomic responses within the nervous system related to various different types of neurotransmission, neuroplasticity, immune function and oxidative stress. These molecular changes may contribute to OA-induced behavioural changes, as suggested by correlations among gene expression profiles, CO2 treatment and OA-affected behaviours.

CONCLUSIONS

This study provides the first molecular insights into the neurobiological effects of OA on a cephalopod and correlates molecular changes with whole animal behavioural responses, helping to bridge the gaps in our knowledge between environmental change and animal responses.

Neuromolecular mechanisms related to reflex behaviour in Aplysia are affected by ocean acidification

While ocean acidification (OA) impacts the behaviour of marine organisms, the complexity of neurosystems makes linking behavioural impairments to environmental change difficult. Using a simple model, we exposed Aplysia to ambient or elevated CO2 conditions (approx. 1500 µatm) and tested how OA affected the neuromolecular response of the pleural-pedal ganglia and caused tail withdrawal reflex (TWR) impairment. Under OA, Aplysia relax their tails faster with increased sensorin-A expression, an inhibitor of mechanosensory neurons. We further investigate how OA affects habituation training output, which produced a 'sensitization-like' behaviour and affected vesicle transport and stress response gene expression, revealing an influence of OA on learning. Finally, gabazine did not restore normal behaviour and elicited little molecular response with OA, instead, vesicular transport and cellular signalling link other neurotransmitter processes with TWR impairment. Our study shows the effects of OA on neurological tissue parts that control for behaviour.

Multistress Interplay: Time and Duration of Ocean Acidification Modulate the Toxicity of Mercury and Other Metals

The current understanding of multistress interplay assumes stresses occur in perfect synchrony, but this assumption is rarely met in the natural marine ecosystem. To understand the interplay between nonperfectly overlapped stresses in the ocean, we manipulated a multigenerational experiment (F0-F3) to explore how different temporal scenarios of ocean acidification will affect mercury toxicity in a marine copepod Pseudodiaptomus annandalei. We found that the scenario of past acidification aggravated mercury toxicity but current and persistent acidification mitigated its toxicity. We specifically performed a proteomics analysis for the copepods of F3. The results indicated that current and persistent acidification initiated the energy compensation for development and mercury efflux, whereas past acidification lacked the barrier of H+ and had dysfunction in the detoxification and efflux system, providing a mechanistic understanding of mercury toxicity under different acidification scenarios. Furthermore, we conducted a meta-analysis on marine animals, demonstrating that different acidification scenarios could alter the toxicity of several other metals, despite evidence from nonsynchronous scenarios remaining limited. Our study thus demonstrates that time and duration of ocean acidification modulate mercury toxicity in marine copepods and suggests that future studies should move beyond the oversimplified scenario of perfect synchrony in understanding multistress interaction.

Physiological and transcriptomic responses of Aurelia coerulea polyps to acidified seawater conditions

Scyphozoan jellyfish, known for their evolutionary position and ecological significance, are thought to exhibit relatively notable resilience to ocean acidification. However, knowledge regarding the molecular mechanisms underlying the scyphozoan jellyfish response to acidified seawater conditions is currently lacking. In this study, two independent experiments were conducted to determine the physiological and molecular responses of moon jellyfish (Aurelia coerulea) polyps to within- and trans-generational exposure to two reduced pH treatments (pH 7.8 and pH 7.6). The results revealed that the asexual reproduction of A. coerulea polyps significantly declined under acute exposure to pH 7.6 compared with that of polyps at ambient pH conditions. Transcriptomics revealed a notable upregulation of genes involved in immunity and cytoskeleton components. In contrast, genes associated with metabolism were downregulated in response to reduced pH treatments after 6 weeks of within-generational acidified conditions. However, reduced pH treatments had no significant influence on the asexual reproduction of A. coerulea polyps after exposure to acidified conditions over a total of five generations, suggesting that A. coerulea polyps may acclimate to low pH levels. Transcriptomics revealed distinct gene expression profiles between within- and trans-generational exposure groups to two reduced pH treatments. The offspring polyps of A. coerulea subjected to trans-generational acidified conditions exhibited both upregulated and downregulated expression of genes associated with metabolism. These physiological and transcriptomic characteristics of A. coerulea polyps in response to elevated CO2 levels suggest that polyps produced asexually under acidified conditions may be resilient to such conditions in the future.

Long non-coding RNAs mediate fish gene expression in response to ocean acidification

The majority of the transcribed genome does not have coding potential but these non-coding transcripts play crucial roles in transcriptional and post-transcriptional regulation of protein-coding genes. Regulation of gene expression is important in shaping an organism's response to environmental changes, ultimately impacting their survival and persistence as population or species face global change. However, the roles of long non-coding RNAs (lncRNAs), when confronted with environmental changes, remain largely unclear. To explore the potential role of lncRNAs in fish exposed to ocean acidification (OA), we analyzed publicly available brain RNA-seq data from a coral reef fish Acanthochromis polyacanthus. We annotated the lncRNAs in its genome and examined the expression changes of intergenic lncRNAs (lincRNAs) between A. polyacanthus samples from a natural CO2 seep and a nearby control site. We identified 4728 lncRNAs, including 3272 lincRNAs in this species. Remarkably, 93.03% of these lincRNAs were species-specific. Among the 125 highly expressed lincRNAs and 403 differentially expressed lincRNAs in response to elevated CO2, we observed that lincRNAs were either neighboring or potentially trans-regulating differentially expressed coding genes associated with pH regulation, neural signal transduction, and ion transport, which are known to be important in the response to OA in fish. In summary, lncRNAs may facilitate fish acclimation and mediate the responses of fish to OA by modulating the expression of crucial coding genes, which offers insight into the regulatory mechanisms underlying fish responses to environmental changes.

Mechanistic toxicity and growth abnormalities mediated by subacute exposure to environmentally relevant levels of benzophenone-3 in clown anemonefish (Amphiprion ocellaris)

Benzophenone-3 (BP-3) is a UV filter that is ubiquitously present in the environment due to its photostability and degradation resistance and has wide applications in personal care products. BP-3 will eventually be discharged into the ocean. Studies shows BP-3 interferes with endocrine system of aquatic organisms, especially fish. However, the toxicity and mechanisms of subacute exposure of the coral reef fish to BP-3 remain elusive. Here, we exposed the one-month-old clown anemonefish to BP-3 at 1 and 10 μg/L for 14 and 28 days, respectively. After chronic exposure, the effects of BP-3 on the growth of clown anemonefish were investigated in terms of growth-related hormones, immune enzyme activity, digestive enzyme activity, transcriptional profiling of feeding- and obesity-related genes and digital RNA sequencing. The body weight in the BP-3 groups were abnormally increased (1 μg/L group in 14 days treatment and all groups in 28 days treatment), altered insulin content (28 days exposure), immune-related and digestive-related enzymatic activities. At the molecular level, BP-3 interferes with the expression of feeding- and obesity-related genes. Digital RNA sequencing analysis showed that BP-3 interferes with Kyoto encyclopedia of genes and genomes (KEGG) pathways related to growth, social behavior (learning behavior), Mitogen-Activated Protein Kinase (MAPK) signaling pathway, PI3K-Akt signaling pathway, and insulin secretion. Notably, in the insulin secretion, BP-3 induced Ca2+ up-regulation that may damage β cells. Growth abnormalities and social behavior (learning behavior) KEGG pathway disturbances may have potential impacts on populations of clown anemonefish. Our results reveal the toxicological effects of subacute exposure to BP-3, and provides insight into the effects and mechanisms of BP-3 on clown anemonefish growth.

Neuromolecular responses in disrupted mutualistic cleaning interactions under future environmental conditions

BACKGROUND

Mutualistic interactions, which constitute some of the most advantageous interactions among fish species, are highly vulnerable to environmental changes. A key mutualistic interaction is the cleaning service rendered by the cleaner wrasse, Labroides dimidiatus, which involves intricate processes of social behaviour to remove ectoparasites from client fish and can be altered in near-future environmental conditions. Here, we evaluated the neuromolecular mechanisms behind the behavioural disruption of cleaning interactions in response to future environments. We subjected cleaner wrasses and surgeonfish (Acanthurus leucosternon, serving as clients) to elevated temperature (warming, 32 °C), increased levels of CO2 (high CO2, 1000 ppm), and a combined condition of elevated CO2 and temperature (warming and high CO2, 32 °C, and 1000 ppm) for 28 days.

RESULTS

Each of these conditions resulted in behavioural disruptions concerning the motivation to interact and the quality of interaction (high CO2 - 80.7%, warming - 92.6%, warming and high CO2 - 79.5%, p < 0.001). Using transcriptomics of the fore-, mid-, and hindbrain, we discovered that most transcriptional reprogramming in both species under warming conditions occurred primarily in the hind- and forebrain. The associated functions under warming were linked to stress, heat shock proteins, hypoxia, and behaviour. In contrast, elevated CO2 exposure affected a range of functions associated with GABA, behaviour, visual perception, thyroid hormones and circadian rhythm. Interestingly, in the combined warming and high CO2 condition, we did not observe any expression changes of behaviour. However, we did find signs of endoplasmic reticulum stress and apoptosis, suggesting not only an additive effect of the environmental conditions but also a trade-off between physiological performance and behaviour in the cleaner wrasse.

CONCLUSIONS

We show that impending environmental shifts can affect the behaviour and molecular processes that sustain mutualistic interactions between L. dimidiatus and its clients, which could have a cascading effect on their adaptation potential and possibly cause large-scale impacts on coral reef ecosystems.

Invited review - the effects of anthropogenic abiotic stressors on the sensory systems of fishes

Climate change is a growing global issue with many countries and institutions declaring a climate state of emergency. Excess CO₂ from anthropogenic sources and changes in land use practices are contributing to many detrimental changes, including increased global temperatures, ocean acidification and hypoxic zones along coastal habitats. All senses are important for aquatic animals, as it is how they can perceive and respond to their environment. Some of these environmental challenges have been shown to impair their sensory systems, including the olfactory, visual, and auditory systems. While most of the research is focused on how ocean acidification affects olfaction, there is also evidence that it negatively affects vision and hearing. The effects that temperature and hypoxia have on the senses have also been investigated, but to a much lesser extent in comparison to ocean acidification. This review assembles the known information on how these anthropogenic challenges affect the sensory systems of fishes, but also highlights what gaps in knowledge remain with suggestions for immediate action. Olfaction, vision, otolith, pH, freshwater, seawater, marine, central nervous system, electrophysiology, mechanism.

Rhythms and Clocks in Marine Organisms

The regular movements of waves and tides are obvious representations of the oceans' rhythmicity. But the rhythms of marine life span across ecological niches and timescales, including short (in the range of hours) and long (in the range of days and months) periods. These rhythms regulate the physiology and behavior of individuals, as well as their interactions with each other and with the environment. This review highlights examples of rhythmicity in marine animals and algae that represent important groups of marine life across different habitats. The examples cover ecologically highly relevant species and a growing number of laboratory model systems that are used to disentangle key mechanistic principles. The review introduces fundamental concepts of chronobiology, such as the distinction between rhythmic and endogenous oscillator-driven processes. It also addresses the relevance of studying diverse rhythms and oscillators, as well as their interconnection, for making better predictions of how species will respond to environmental perturbations, including climate change. As the review aims to address scientists from the diverse fields of marine biology, ecology, and molecular chronobiology, all of which have their own scientific terms, we provide definitions of key terms throughout the article.

Isotope systematics of subfossil, historical, and modern Nautilus macromphalus from New Caledonia

Cephalopod carbonate geochemistry underpins studies ranging from Phanerozoic, global-scale change to outcrop-scale paleoecological reconstructions. Interpreting these data hinges on assumed similarity to model organisms, such as Nautilus, and generalization from other molluscan biomineralization processes. Aquarium rearing and capture of wild Nautilus suggest shell carbonate precipitates quickly (35 μm/day) in oxygen isotope equilibrium with seawater. Other components of Nautilus shell chemistry are less well-studied but have potential to serve as proxies for paleobiology and paleoceanography. To calibrate the geochemical response of cephalopod δ15Norg, δ13Corg, δ13Ccarb, δ18Ocarb, and δ44/40Cacarb to modern anthropogenic environmental change, we analyzed modern, historical, and subfossil Nautilus macromphalus from New Caledonia. Samples span initial human habitation, colonialization, and industrial pCO2 increase. This sampling strategy is advantageous because it avoids the shock response that can affect geochemical change in aquarium experiments. Given the range of living depths and more complex ecology of Nautilus, however, some anthropogenic signals, such as ocean acidification, may not have propagated to their living depths. Our data suggest some environmental changes are more easily preserved than others given variability in cephalopod average living depth. Calculation of the percent respired carbon incorporated into the shell using δ13Corg, δ13Ccarb, and Suess-effect corrected δ13CDIC suggests an increase in the last 130 years that may have been caused by increasing carbon dioxide concentration or decreasing oxygen concentration at the depths these individuals inhabited. This pattern is consistent with increasing atmospheric CO2 and/or eutrophication offshore of New Caledonia. We find that δ44/40Ca remains stable across the last 130 years. The subfossil shell from a cenote may exhibit early δ44/40Ca diagenesis. Questions remain about the proportion of dietary vs ambient seawater calcium incorporation into the Nautilus shell. Values of δ15N do not indicate trophic level change in the last 130 years, and the subfossil shell may show diagenetic alteration of δ15N toward lower values. Future work using historical collections of Sepia and Spirula may provide additional calibration of fossil cephalopod geochemistry.

Multigenerational inspections of environmental thermal perturbations promote metabolic trade-offs in developmental stages of tropical fish

Global warming both reduces global temperature variance and increases the frequency of extreme weather events. In response to these ambient perturbations, animals may be subject to trans- or intra-generational phenotype modifications that help to maintain homeostasis and fitness. Here, we show how temperature-associated transgenerational plasticity in tilapia affects metabolic trade-offs during developmental stages under a global warming scenario. Tropical tilapia reared at a stable temperature of 27 °C for a decade were divided into two temperature-experience groups for four generations of breeding. Each generation of one group was exposed to a single 15 °C cold-shock experience during its lifetime (cold-experienced CE group), and the other group was kept stably at 27 °C throughout their lifetimes (cold-naïve CN group). The offspring at early life stages from the CE and CN tilapia were then assessed by metabolomics-based profiling, and the results implied that parental cold-experience might affect energy provision during reproduction. Furthermore, at early life stages, progeny may be endowed with metabolic traits that help the animals cope with ambient temperature perturbations. This study also applied the feature rescaling and Uniform Manifold Approximation and Projection (UMAP) to visualize metabolic dynamics, and the result could effectively decompose the complex omic-based datasets to represent the energy trade-off variability. For example, the carbohydrate to free amino acid conversion and enhanced compensatory features appeared to be hypothermic-responsive traits. These multigenerational metabolic effects suggest that the tropical ectothermic tilapia may exhibit transgenerational phenotype plasticity, which could optimize energy allocation under ambient temperature challenges. Knowledge about such metabolism-related transgenerational plasticity effects in ectothermic aquatic species may allow us to better predict how adaptive mechanisms will affect fish populations in a climate with narrow temperature variation and frequent extreme weather events.

The extensive transgenerational transcriptomic effects of ocean acidification on the olfactory epithelium of a marine fish are associated with a better viral resistance

Background

Progressive CO₂-induced ocean acidification (OA) impacts marine life in ways that are difficult to predict but are likely to become exacerbated over generations. Although marine fishes can balance acid–base homeostasis efficiently, indirect ionic regulation that alter neurosensory systems can result in behavioural abnormalities. In marine invertebrates, OA can also affect immune system function, but whether this is the case in marine fishes is not fully understood. Farmed fish are highly susceptible to disease outbreak, yet strategies for overcoming such threats in the wake of OA are wanting. Here, we exposed two generations of the European sea bass (Dicentrarchus labrax) to end-of-century predicted pH levels (IPCC RCP8.5), with parents (F1) being exposed for four years and their offspring (F2) for 18 months. Our design included a transcriptomic analysis of the olfactory rosette (collected from the F2) and a viral challenge (exposing F2 to betanodavirus) where we assessed survival rates.

Results

We discovered transcriptomic trade-offs in both sensory and immune systems after long-term transgenerational exposure to OA. Specifically, RNA-Seq analysis of the olfactory rosette, the peripheral olfactory organ, from 18-months-old F2 revealed extensive regulation in genes involved in ion transport and neuronal signalling, including GABAergic signalling. We also detected OA-induced up-regulation of genes associated with odour transduction, synaptic plasticity, neuron excitability and wiring and down-regulation of genes involved in energy metabolism. Furthermore, OA-exposure induced up-regulation of genes involved in innate antiviral immunity (pathogen recognition receptors and interferon-stimulated genes) in combination with down-regulation of the protein biosynthetic machinery. Consistently, OA-exposed F2 challenged with betanodavirus, which causes damage to the nervous system of marine fish, had acquired improved resistance.

Conclusion

F2 exposed to long-term transgenerational OA acclimation showed superior viral resistance, though as their metabolic and odour transduction programs were altered, odour-mediated behaviours might be consequently impacted. Although it is difficult to unveil how long-term OA impacts propagated between generations, our results reveal that, across generations, trade-offs in plastic responses is a core feature of the olfactory epithelium transcriptome in OA-exposed F2 offspring, and will have important consequences for how cultured and wild fish interacts with its environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08647-w.

Rapid evolution fuels transcriptional plasticity to ocean acidification

Ocean acidification (OA) is postulated to affect the physiology, behavior, and life-history of marine species, but potential for acclimation or adaptation to elevated pCO2 in wild populations remains largely untested. We measured brain transcriptomes of six coral reef fish species at a natural volcanic CO2  seep and an adjacent control reef in Papua New Guinea. We show that elevated pCO2 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 CO2 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 CO2 , 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.

CO2 induced seawater acidification impacts survival and development of European eel embryos

Fish embryos may be vulnerable to seawater acidification resulting from anthropogenic carbon dioxide (CO₂) emissions or from excessive biological CO₂ production in aquaculture systems. This study investigated CO₂ effects on embryos of the European eel (Anguilla anguilla), a catadromous fish that is considered at risk from climate change and that is targeted for hatchery production to sustain aquaculture of the species. Eel embryos were reared in three independent recirculation systems with different pH/CO₂ levels representing “control” (pH 8.1, 300 μatm CO₂), end-of-century climate change (“intermediate”, pH 7.6, 900 μatm CO₂) and “extreme” aquaculture conditions (pH 7.1, 3000 μatm CO₂). Sensitivity analyses were conducted at 4, 24, and 48 hours post-fertilization (hpf) by focusing on development, survival, and expression of genes related to acute stress response (crhr1, crfr2), stress/repair response (hsp70, hsp90), water and solute transport (aqp1, aqp3), acid-base regulation (nkcc1a, ncc, car15), and inhibitory neurotransmission (GABAAα6b, Gabra1). Results revealed that embryos developing at intermediate pH showed similar survival rates to the control, but egg swelling was impaired, resulting in a reduction in egg size with decreasing pH. Embryos exposed to extreme pH had 0.6-fold decrease in survival at 24 hpf and a 0.3-fold change at 48 compared to the control. These observed effects of acidification were not reflected by changes in expression of any of the here studied genes. On the contrary, differential expression was observed along embryonic development independent of treatment, indicating that the underlying regulating systems are under development and that embryos are limited in their ability to regulate molecular responses to acidification. In conclusion, exposure to predicted end-of-century ocean pCO₂ conditions may affect normal development of this species in nature during sensitive early life history stages with limited physiological response capacities, while extreme acidification will negatively influence embryonic survival and development under hatchery conditions.

Paths towards greater consensus building in experimental biology

In a recent editorial, the Editors-in-Chief of Journal of Experimental Biology argued that consensus building, data sharing, and better integration across disciplines are needed to address the urgent scientific challenges posed by climate change. We agree and expand on the importance of cross-disciplinary integration and transparency to improve consensus building and advance climate change research in experimental biology. We investigated reproducible research practices in experimental biology through a review of open data and analysis code associated with empirical studies on three debated paradigms and for unrelated studies published in leading journals in comparative physiology and behavioural ecology over the last 10 years. Nineteen per cent of studies on the three paradigms had open data, and 3.2% had open code. Similarly, 12.1% of studies in the journals we examined had open data, and 3.1% had open code. Previous research indicates that only 50% of shared datasets are complete and re-usable, suggesting that fewer than 10% of studies in experimental biology have usable open data. Encouragingly, our results indicate that reproducible research practices are increasing over time, with data sharing rates in some journals reaching 75% in recent years. Rigorous empirical research in experimental biology is key to understanding the mechanisms by which climate change affects organisms, and ultimately promotes evidence-based conservation policy and practice. We argue that a greater adoption of open science practices, with a particular focus on FAIR (Findable, Accessible, Interoperable, Re-usable) data and code, represents a much-needed paradigm shift towards improved transparency, cross-disciplinary integration, and consensus building to maximize the contributions of experimental biologists in addressing the impacts of environmental change on living organisms.

The Olfactory Tract: Basis for Future Evolution in Response to Rapidly Changing Ecological Niches

Within the forebrain the olfactory sensory system is unique from other sensory systems both in the projections of the olfactory tract and the ongoing neurogenic potential, characteristics conserved across vertebrates. Olfaction plays a crucial role in behaviors such as mate choice, food selection, homing, escape from predators, among others. The olfactory forebrain is intimately associated with the limbic system, the region of the brain involved in learning, memory, and emotions through interactions with the endocrine system and the autonomic nervous system. Previously thought to lack a limbic system, we now know that teleost fishes process emotions, have exceptional memories, and readily learn, behaviors that are often associated with olfactory cues. The association of neuromodulatory hormones, and more recently, the immune system, with odor cues underlies behaviors essential for maintenance and adaptation within natural ecological niches. Increasingly anthropogenic perturbations affecting ecosystems are impacting teleost fishes worldwide. Here we examine the role of the olfactory tract as the neural basis for the integration of environmental cues and resulting behaviors necessary for the regulation of biotic interactions that allow for future adaptation as the climate spins out of control.

Ocean acidification affects the expression of neuroplasticity and neuromodulation markers in seabream

A possible explanation for acidification-induced changes in fish behaviour is that acidification interferes with neurogenesis and modifies the plasticity of neuronal circuitry in the brain. We tested the effects on the olfactory system and brain of gilthead seabream (Sparus aurata) to 4 weeks' exposure to ocean acidification (OA). Olfactory epithelium (OE) morphology changed shortly after OA exposure and persisted over the 4 weeks. Expression of genes related to olfactory transduction, neuronal excitability, synaptic plasticity, GABAergic innervation, and cell proliferation were unchanged in the OE and olfactory bulb (OB) after 4 weeks' exposure. Short-term changes in the ionic content of plasma and extradural fluid (EDF) returned to control levels after 4 weeks' exposure, except for [Cl⁻], which remained elevated. This suggests that, in general, there is an early physiological response to OA and by 4 weeks a new homeostatic status is achieved. However, expression of genes involved in proliferation, differentiation and survival of undifferentiated neurons were modified in the brain. In the same brain areas, expression of thyroid hormone signalling genes was altered suggesting modifications in the thyroid-system may be linked to the changes in neuroplasticity and neurogenesis. Overall, the results of the current study are consistent with and effect of OA on neuroplasticity.

Summary: Ocean acidification alters fish behaviour. We show altered expression of genes involved in neuroplasticity and neuromodulation in fish exposed to high PCO₂, highlighting their possible roles in such behavioural alterations.

Meta-analysis reveals an extreme "decline effect" in the impacts of ocean acidification on fish behavior

Ocean acidification-decreasing oceanic pH resulting from the uptake of excess atmospheric CO2-has the potential to affect marine life in the future. Among the possible consequences, a series of studies on coral reef fish suggested that the direct effects of acidification on fish behavior may be extreme and have broad ecological ramifications. Recent studies documenting a lack of effect of experimental ocean acidification on fish behavior, however, call this prediction into question. Indeed, the phenomenon of decreasing effect sizes over time is not uncommon and is typically referred to as the "decline effect." Here, we explore the consistency and robustness of scientific evidence over the past decade regarding direct effects of ocean acidification on fish behavior. Using a systematic review and meta-analysis of 91 studies empirically testing effects of ocean acidification on fish behavior, we provide quantitative evidence that the research to date on this topic is characterized by a decline effect, where large effects in initial studies have all but disappeared in subsequent studies over a decade. The decline effect in this field cannot be explained by 3 likely biological explanations, including increasing proportions of studies examining (1) cold-water species; (2) nonolfactory-associated behaviors; and (3) nonlarval life stages. Furthermore, the vast majority of studies with large effect sizes in this field tend to be characterized by low sample sizes, yet are published in high-impact journals and have a disproportionate influence on the field in terms of citations. We contend that ocean acidification has a negligible direct impact on fish behavior, and we advocate for improved approaches to minimize the potential for a decline effect in future avenues of research.

Meta-analysis reveals an extreme "decline effect" in the impacts of ocean acidification on fish behavior

Ocean acidification-decreasing oceanic pH resulting from the uptake of excess atmospheric CO2-has the potential to affect marine life in the future. Among the possible consequences, a series of studies on coral reef fish suggested that the direct effects of acidification on fish behavior may be extreme and have broad ecological ramifications. Recent studies documenting a lack of effect of experimental ocean acidification on fish behavior, however, call this prediction into question. Indeed, the phenomenon of decreasing effect sizes over time is not uncommon and is typically referred to as the "decline effect." Here, we explore the consistency and robustness of scientific evidence over the past decade regarding direct effects of ocean acidification on fish behavior. Using a systematic review and meta-analysis of 91 studies empirically testing effects of ocean acidification on fish behavior, we provide quantitative evidence that the research to date on this topic is characterized by a decline effect, where large effects in initial studies have all but disappeared in subsequent studies over a decade. The decline effect in this field cannot be explained by 3 likely biological explanations, including increasing proportions of studies examining (1) cold-water species; (2) nonolfactory-associated behaviors; and (3) nonlarval life stages. Furthermore, the vast majority of studies with large effect sizes in this field tend to be characterized by low sample sizes, yet are published in high-impact journals and have a disproportionate influence on the field in terms of citations. We contend that ocean acidification has a negligible direct impact on fish behavior, and we advocate for improved approaches to minimize the potential for a decline effect in future avenues of research.

Molecular basis of parental contributions to the behavioural tolerance of elevated pCO2 in a coral reef fish

Knowledge of adaptive potential is crucial to predicting the impacts of ocean acidification (OA) on marine organisms. In the spiny damselfish, Acanthochromis polyacanthus, individual variation in behavioural tolerance to elevated pCO2 has been observed and is associated with offspring gene expression patterns in the brain. However, the maternal and paternal contributions of this variation are unknown. To investigate parental influence of behavioural pCO2 tolerance, we crossed pCO2-tolerant fathers with pCO2-sensitive mothers and vice versa, reared their offspring at control and elevated pCO2 levels, and compared the juveniles' brain transcriptional programme. We identified a large influence of parental phenotype on expression patterns of offspring, irrespective of environmental conditions. Circadian rhythm genes, associated with a tolerant parental phenotype, were uniquely expressed in tolerant mother offspring, while tolerant fathers had a greater role in expression of genes associated with histone binding. Expression changes in genes associated with neural plasticity were identified in both offspring types: the maternal line had a greater effect on genes related to neuron growth while paternal influence impacted the expression of synaptic development genes. Our results confirm cellular mechanisms involved in responses to varying lengths of OA exposure, while highlighting the parental phenotype's influence on offspring molecular phenotype.

Ion-transporting capacity and aerobic respiration of larval white seabass (Atractoscion nobilis) may be resilient to ocean acidification conditions

Ocean acidification (OA) has been proposed to increase the energetic demand for acid-base regulation at the expense of larval fish growth. Here, white seabass (Atractoscion nobilis) eggs and larvae were reared at control (542 ± 28 μatm) and elevated pCO₂ (1831 ± 105 μatm) until five days post-fertilization (dpf). Skin ionocytes were identified by immunodetection of the Na⁺/K⁺-ATPase (NKA) enzyme. Larvae exposed to elevated pCO₂ possessed significantly higher skin ionocyte number and density compared to control larvae. However, when ionocyte size was accounted for, the relative ionocyte area (a proxy for total ionoregulatory capacity) was unchanged. Similarly, there were no differences in relative NKA abundance, resting O₂ consumption rate, and total length between control and treatment larvae at 5 dpf, nor in the rate at which relative ionocyte area and total length changed between 2 and 5 dpf. Altogether, our results suggest that OA conditions projected for the next century do not significantly affect the ionoregulatory capacity or energy consumption of larval white seabass. Finally, a retroactive analysis of the water in the recirculating aquarium system that housed the broodstock revealed the parents had been exposed to average pCO₂ of ~1200 μatm for at least 3.5 years prior to this experiment. Future studies should investigate whether larval white seabass are naturally resilient to OA, or if this resilience is the result of parental chronic acclimation to OA, and/or from natural selection during spawning and fertilization in elevated pCO₂.

Diel pCO2 fluctuations alter the molecular response of coral reef fishes to ocean acidification conditions

Environmental partial pressure of CO₂ (pCO₂ ) variation can modify the responses of marine organisms to ocean acidification, yet the underlying mechanisms for this effect remain unclear. On coral reefs, environmental pCO₂  fluctuates on a regular day-night cycle. Effects of future ocean acidification on coral reef fishes might therefore depend on their response to this diel cycle of pCO₂ . To evaluate the effects on the brain molecular response, we exposed two common reef fishes (Acanthochromis polyacanthus and Amphiprion percula) to two projected future pCO₂  levels (750 and 1,000 µatm) under both stable and diel fluctuating conditions. We found a common signature to stable elevated pCO₂ for both species, which included the downregulation of immediate early genes, indicating lower brain activity. The transcriptional programme was more strongly affected by higher average pCO₂ in a stable treatment than for fluctuating treatments, but the largest difference in molecular response was between stable and fluctuating pCO₂ treatments. This indicates that a response to a change in environmental pCO₂ conditions is different for organisms living in a fluctuating than in stable environments. This differential regulation was related to steroid hormones and circadian rhythm (CR). Both species exhibited a marked difference in the expression of CR genes among pCO₂ treatments, possibly accommodating a more flexible adaptive approach in the response to environmental changes. Our results suggest that environmental pCO₂  fluctuations might enable reef fishes to phase-shift their clocks and anticipate pCO₂ changes, thereby avoiding impairments and more successfully adjust to ocean acidification conditions.

Acidification can directly affect olfaction in marine organisms

In the past decade, many studies have investigated the effects of low pH/high CO2 as a proxy for ocean acidification on olfactory-mediated behaviours of marine organisms. The effects of ocean acidification on the behaviour of fish vary from very large to none at all, and most of the maladaptive behaviours observed have been attributed to changes in acid-base regulation, leading to changes in ion distribution over neural membranes, and consequently affecting the functioning of gamma-aminobutyric acid-mediated (GABAergic) neurotransmission. Here, we highlight a possible additional mechanism by which ocean acidification might directly affect olfaction in marine fish and invertebrates. We propose that a decrease in pH can directly affect the protonation, and thereby, 3D conformation and charge distribution of odorants and/or their receptors in the olfactory organs of aquatic animals. This can sometimes enhance signalling, but most of the time the affinity of odorants for their receptors is reduced in high CO2/low pH; therefore, the activity of olfactory receptor neurons decreases as measured using electrophysiology. The reduced signal reception would translate into reduced activation of the olfactory bulb neurons, which are responsible for processing olfactory information in the brain. Over longer exposures of days to weeks, changes in gene expression in the olfactory receptors and olfactory bulb neurons cause these neurons to become less active, exacerbating the problem. A change in olfactory system functioning leads to inappropriate behavioural responses to odorants. We discuss gaps in the literature and suggest some changes to experimental design in order to improve our understanding of the underlying mechanisms and their effects on the associated behaviours to resolve some current controversy in the field regarding the extent of the effects of ocean acidification on marine fish.

Invasion and rapid adaptation of guppies (Poecilia reticulata) across the Hawaiian Archipelago

How much does natural selection, as opposed to genetic drift, admixture, and gene flow, contribute to the evolution of invasive species following introduction to a new environment? Here we assess how evolution can shape biological invasions by examining population genomic variation in non-native guppies (Poecilia reticulata) introduced to the Hawaiian Islands approximately a century ago. By examining 18 invasive populations from four Hawaiian islands and four populations from the native range in northern South America, we reconstructed the history of introductions and evaluated population structure as well as the extent of ongoing gene flow across watersheds and among islands. Patterns of differentiation indicate that guppies have developed significant population structure, with little natural or human-mediated gene flow having occurred among populations following introduction. Demographic modeling and admixture graph analyses together suggest that guppies were initially introduced to O'ahu and Maui and then translocated to Hawai'i and Kaua'i. We detected evidence for only one introduction event from the native range, implying that any adaptive evolution in introduced populations likely utilized the genetic variation present in the founding population. Environmental association tests accounting for population structure identified loci exhibiting signatures of adaptive variation related to predators and landscape characteristics but not nutrient regimes. When paired with high estimates of effective population sizes and detectable population structure, the presence of environment-associated loci supports the role of natural selection in shaping contemporary evolution of Hawaiian guppy populations. Our findings indicate that local adaptation may engender invasion success, particularly in species with life histories that facilitate rapid evolution. Finally, evidence of low gene flow between populations suggests that removal could be an effective approach to control invasive guppies across the Hawaiian archipelago.

Artificial light at night interacts with predatory threat to alter reef fish metabolite profiles

Light cycles and predatory threat define activity patterns (e.g. feeding/sleeping, activity/rest) in most diurnal fish species. Artificial light at night (ALAN) may disrupt natural cycles and biochemical processes, a mismatch which can eventually reduce condition and fitness. We evaluate the separate and joint effects of ALAN and predator threat on metabolism within brain, liver and muscle tissue of a common, wild caught damselfish, blue green chromis (Chromis viridis). The effects of ALAN varied according to tissue type and predator exposure. In all tissues we observed changes in metabolic pathways associated with increased activity under continuous light (despite provision of shelter), specifically those associated with energy metabolism, cell signalling, responses to oxidative stress and markers of cellular damage. In both the brain and liver tissues, predator threat served to moderate the influence of ALAN on metabolic change, likely due to increased sheltering behaviour. However, no interaction of predator threat with ALAN was observed in metabolism of the muscle tissue. Our results highlight complex sub-acute effects of ALAN exposure on tissue specific and whole organism energy metabolism. Collectively these effects indicate that ALAN has significant scope to reduce fitness of coastal fishes and potentially threaten ecosystem services, but that these changes are highly complex and may be altered by biotic drivers of activity.

Plasticity in parental effects confers rapid larval thermal tolerance in the estuarine anemone Nematostella vectensis

Parental effects can prepare offspring for different environments and facilitate survival across generations. We exposed parental populations of the estuarine anemone, Nematostella vectensis, from Massachusetts to elevated temperatures and quantified larval mortality across a temperature gradient. We found that parental exposure to elevated temperatures resulted in a consistent increase in larval thermal tolerance, as measured by the temperature at which 50% of larvae die (LT50), with a mean increase in LT50 of 0.3°C. Larvae from subsequent spawns returned to baseline thermal thresholds when parents were returned to normal temperatures, indicating plasticity in these parental effects. Histological analyses of gametogenesis in females suggested that these dynamic shifts in larval thermal tolerance may be facilitated by maternal effects in non-overlapping gametic cohorts. We also compared larvae from North Carolina (a genetically distinct population with higher baseline thermal tolerance) and Massachusetts parents, and observed that larvae from heat-exposed Massachusetts parents had thermal thresholds comparable to those of larvae from unexposed North Carolina parents. North Carolina parents also increased larval thermal tolerance under the same high-temperature regime, suggesting that plasticity in parental effects is an inherent trait for N. vectensis Overall, we find that larval thermal tolerance in N. vectensis shows a strong genetic basis and can be modulated by parental effects. Further understanding of the mechanisms behind these shifts can elucidate the fate of thermally sensitive ectotherms in a rapidly changing thermal environment.

Bigfin reef squid demonstrate capacity for conditional discrimination and projected future carbon dioxide levels have no effect on learning capabilities

Anthropogenic carbon dioxide (CO₂) emissions are being absorbed by the oceans, a process known as ocean acidification, and risks adversely affecting a variety of behaviours in a range of marine species, including inhibited learning in some fishes. However, the effects of elevated CO₂ on learning in advanced invertebrates such as cephalopods are unknown. Any impacts to the learning abilities of cephalopods could have far-reaching consequences for their populations and the communities they inhabit. Cephalopods have some of the most advanced cognitive abilities among invertebrates and are one of the few invertebrate taxa in which conditional discrimination has been demonstrated, though the trait has not been demonstrated in any species of squid. Here, we tested for the first time the capacity for conditional discrimination in a squid species (Sepioteuthis lessoniana). Furthermore, we investigated the effects of projected future CO₂ levels (1,084 µatm) on conditional discrimination and learning more generally. A three-task experiment within a two-choice arena was used to test learning and conditional discrimination. Learning was measured by improvements in task completion in repeated trials over time and the number of trials required to pass each task. Squid exhibited significant learning capabilities, with an increase in correct choices over successive trials and a decrease in the number of trials needed to complete the successive tasks. Six of the 12 squid tested successfully passed all three tasks indicating a capacity for conditional discrimination in the species. Elevated CO₂ had no effect on learning or on the capacity for conditional discrimination in squid. This study highlights the remarkable cognitive abilities of S. lessoniana, demonstrated by their capacity for conditional discrimination, and suggests that ocean acidification will not compromise learning abilities. However, other behavioural traits in the species have been shown to be altered at comparable elevated CO₂ conditions. It is not clear why some ecologically important behaviours are altered by elevated CO₂ whereas others are unaffected. Future research should focus on the physiological mechanism responsible for altered behaviours in squid at elevated CO₂.

Understanding 'Non-genetic' Inheritance: Insights from Molecular-Evolutionary Crosstalk

Understanding the evolutionary and ecological roles of 'non-genetic' inheritance (NGI) is daunting due to the complexity and diversity of epigenetic mechanisms. We draw on insights from molecular and evolutionary biology perspectives to identify three general features of 'non-genetic' inheritance systems: (i) they are functionally interdependent with, rather than separate from, DNA sequence; (ii) precise mechanisms vary phylogenetically and operationally; and (iii) epigenetic elements are probabilistic, interactive regulatory factors and not deterministic 'epialleles' with defined genomic locations and effects. We discuss each of these features and offer recommendations for future empirical and theoretical research that implements a unifying inherited gene regulation (IGR) approach to studies of 'non-genetic' inheritance.

Antagonistic Interactions and Clutch-Dependent Sensitivity Induce Variable Responses to Ocean Acidification and Warming in Squid (Doryteuthis pealeii) Embryos and Paralarvae

Ocean acidification (OA) and warming seas are significant concerns for coastal systems and species. The Atlantic longfin squid, Doryteuthis pealeii, a core component of the Northwest Atlantic trophic web, has demonstrated impacts, such as reduced growth and delayed development, under high chronic exposure to acidification (2200 ppm), but the combined effects of OA and warming have not been explored in this species. In this study, D. pealeii egg capsules were reared under a combination of several acidification levels (400, 2200, and 3500 ppm) and temperatures (20 and 27°C). Hatchlings were measured for a range of metrics [dorsal mantle length (DML), yolk sac volume (YV), malformation, and hatching success] in three trials over the 2016 breeding season (May – October). Although notable resistance to stressors was seen, highlighting variability within and between clutches, reduced DML and malformation of the embryos occurred at the highest OA exposure. Surprisingly, increased temperatures did not appear to exacerbate OA impacts, although responses were variable. Time to hatching, which increased with acidification, decreased much more drastically under warming and, further, decreased or removed delays caused by acidification. Hatching success, while variable by clutch, showed consistent patterns of greater late stage loss of embryos under acidification and greater early stage loss under warming, highlighting the potential difference in timing between these stressors for this system, i.e., that acidification stress builds up and causes impacts over time within the egg capsule as the embryos grow and respire. High OA-exposed hatchlings from the warmer conditions often showed reduced impacts compared to those reared in ambient temperatures. This may be due to the increased developmental rate and subsequently reduced OA exposure time of embryos in the higher temperature treatment. These results indicate a substantive potential plasticity to multiple stressors during the embryonic development of this species of squid, but do not predict how this species would fare under these future ocean scenarios.

Fish embryo vulnerability to combined acidification and warming coincides with a low capacity for homeostatic regulation

The vulnerability of fish embryos and larvae to environmental factors is often attributed to a lack of adult-like organ systems (gills) and thus insufficient homeostatic capacity. However, experimental data supporting this hypothesis are scarce. Here, by using Atlantic cod (Gadus morhua) as a model, the relationship between embryo vulnerability (to projected ocean acidification and warming) and homeostatic capacity was explored through parallel analyses of stage-specific mortality and in vitro activity and expression of major ion pumps (ATP-synthase, Na+/K+-ATPase, H+-ATPase) and co-transporters (NBC1, NKCC1). Immunolocalization of these transporters was used to study ionocyte morphology in newly hatched larvae. Treatment-related embryo mortality until hatching (+20% due to acidification and warming) occurred primarily during an early period (gastrulation) characterized by extremely low ion transport capacity. Thereafter, embryo mortality decreased in parallel with an exponential increase in activity and expression of all investigated ion transporters. Significant changes in transporter activity and expression in response to acidification (+15% activity) and warming (-30% expression) indicate some potential for short-term acclimatization, although this is probably associated with energetic trade-offs. Interestingly, whole-larvae enzyme activity (supported by abundant epidermal ionocytes) reached levels similar to those previously measured in gill tissue of adult cod, suggesting that early-life stages without functional gills are better equipped in terms of ion homeostasis than previously thought. This study implies that the gastrulation period represents a critical transition from inherited (maternal) defenses to active homeostatic regulation, which facilitates enhanced resilience of later stages to environmental factors.

Ocean acidification promotes broad transcriptomic responses in marine metazoans: a literature survey

For nearly a decade, the metazoan-focused research community has explored the impacts of ocean acidification (OA) on marine animals, noting that changes in ocean chemistry can impact calcification, metabolism, acid-base regulation, stress response and behavior in organisms that hold high ecological and economic value. Because OA interacts with several key physiological processes in marine organisms, transcriptomics has become a widely-used method to characterize whole organism responses on a molecular level as well as inform mechanisms that explain changes in phenotypes observed in response to OA. In the past decade, there has been a notable rise in studies that examine transcriptomic responses to OA in marine metazoans, and here we attempt to summarize key findings across these studies. We find that organisms vary dramatically in their transcriptomic responses to pH although common patterns are often observed, including shifts in acid-base ion regulation, metabolic processes, calcification and stress response mechanisms. We also see a rise in transcriptomic studies examining organismal response to OA in a multi-stressor context, often reporting synergistic effects of OA and temperature. In addition, there is an increase in studies that use transcriptomics to examine the evolutionary potential of organisms to adapt to OA conditions in the future through population and transgenerational experiments. Overall, the literature reveals complex organismal responses to OA, in which some organisms will face more dramatic consequences than others. This will have wide-reaching impacts on ocean communities and ecosystems as a whole.

Elevated CO2 and food ration affect growth but not the size-based hierarchy of a reef fish

Under projected levels of ocean acidification, shifts in energetic demands and food availability could interact to effect the growth and development of marine organisms. Changes to individual growth rates could then flow on to influence emergent properties of social groups, particularly in species that form size-based hierarchies. To test the potential interactive effects of (1) food availability, (2) elevated CO₂ during juvenile development, and (3) parental experience of elevated CO₂ on the growth, condition and size-based hierarchy of juvenile fish, we reared orange clownfish (Amphiprion percula) for 50 days post-hatching in a fully orthogonal design. Development in elevated CO₂ reduced standard length and weight of juveniles, by 9% and 11% respectively, compared to ambient. Development under low food availability reduced length and weight of juveniles by 7% and 15% respectively, compared to high food. Parental exposure to elevated CO₂ restored the length of juveniles to that of controls, but it did not restore weight, resulting in juveniles from elevated CO₂ parents exhibiting 33% lower body condition when reared in elevated CO₂. The body size ratios (relative size of a fish from the rank above) within juvenile groups were not affected by any treatment, suggesting relative robustness of group-level structure despite alterations in individual size and condition. This study demonstrates that both food availability and elevated CO₂ can influence the physical attributes of juvenile reef fish, but these changes may not disrupt the emergent group structure of this social species, at least amongst juveniles.

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.

The past, present and future of cleaner fish cognitive performance as a function of CO2 levels

Ocean acidification is one of the many consequences of climate change. Various studies suggest that marine organisms' behaviour will be impaired under high CO₂. Here, we show that the cognitive performance of the cleaner wrasse, Labroides dimidiatus, has not suffered from the increase of CO₂ from pre-industrial levels to today, and that the standing variation in CO₂ tolerance offers potential for adaptation to at least 750 µatm. We acclimated cleaners over 30 days to five levels of pCO_2, from pre-industrial to high future CO₂ scenarios, before testing them in an ecologically relevant task-the ability to learn to prioritize an ephemeral food source over a permanent one. Fish learning abilities remained stable from pre-industrial to present-day pCO₂. While performance was reduced under mid (750 µatm) and high CO₂ (980 µatm) scenarios, under the former 36% of cleaners still solved the task. The presence of tolerant individuals reveals potential for adaptation, as long as selection pressure on cognitive performance is strong. However, the apparent absence of high CO₂ tolerant fish, and potentially synergistic effects between various climate change stressors, renders the probability of further adaptation unlikely.

Neurobiological and behavioural responses of cleaning mutualisms to ocean warming and acidification

Cleaning interactions are textbook examples of mutualisms. On coral reefs, most fishes engage in cooperative interactions with cleaners fishes, where they benefit from ectoparasite reduction and ultimately stress relief. Furthermore, such interactions elicit beneficial effects on clients' ecophysiology. However, the potential effects of future ocean warming (OW) and acidification (OA) on these charismatic associations are unknown. Here we show that a 45-day acclimation period to OW (+3 °C) and OA (980 μatm pCO2) decreased interactions between cleaner wrasses (Labroides dimidiatus) and clients (Naso elegans). Cleaners also invested more in the interactions by providing tactile stimulation under OA. Although this form of investment is typically used by cleaners to prolong interactions and reconcile after cheating, interaction time and client jolt rate (a correlate of dishonesty) were not affected by any stressor. In both partners, the dopaminergic (in all brain regions) and serotoninergic (forebrain) systems were significantly altered by these stressors. On the other hand, in cleaners, the interaction with warming ameliorated dopaminergic and serotonergic responses to OA. Dopamine and serotonin correlated positively with motivation to interact and cleaners interaction investment (tactile stimulation). We advocate that such neurobiological changes associated with cleaning behaviour may affect the maintenance of community structures on coral reefs.

Conservation through the lens of (mal)adaptation: Concepts and meta-analysis

Evolutionary approaches are gaining popularity in conservation science, with diverse strategies applied in efforts to support adaptive population outcomes. Yet conservation strategies differ in the type of adaptive outcomes they promote as conservation goals. For instance, strategies based on genetic or demographic rescue implicitly target adaptive population states whereas strategies utilizing transgenerational plasticity or evolutionary rescue implicitly target adaptive processes. These two goals are somewhat polar: adaptive state strategies optimize current population fitness, which should reduce phenotypic and/or genetic variance, reducing adaptability in changing or uncertain environments; adaptive process strategies increase genetic variance, causing maladaptation in the short term, but increase adaptability over the long term. Maladaptation refers to suboptimal population fitness, adaptation refers to optimal population fitness, and (mal)adaptation refers to the continuum of fitness variation from maladaptation to adaptation. Here, we present a conceptual classification for conservation that implicitly considers (mal)adaptation in the short-term and long-term outcomes of conservation strategies. We describe cases of how (mal)adaptation is implicated in traditional conservation strategies, as well as strategies that have potential as a conservation tool but are relatively underutilized. We use a meta-analysis of a small number of available studies to evaluate whether the different conservation strategies employed are better suited toward increasing population fitness across multiple generations. We found weakly increasing adaptation over time for transgenerational plasticity, genetic rescue, and evolutionary rescue. Demographic rescue was generally maladaptive, both immediately after conservation intervention and after several generations. Interspecific hybridization was adaptive only in the F1 generation, but then rapidly leads to maladaptation. Management decisions that are made to support the process of adaptation must adequately account for (mal)adaptation as a potential outcome and even as a tool to bolster adaptive capacity to changing conditions.

Beyond buying time: the role of plasticity in phenotypic adaptation to rapid environmental change

How populations and species respond to modified environmental conditions is critical to their persistence both now and into the future, particularly given the increasing pace of environmental change. The process of adaptation to novel environmental conditions can occur via two mechanisms: (1) the expression of phenotypic plasticity (the ability of one genotype to express varying phenotypes when exposed to different environmental conditions), and (2) evolution via selection for particular phenotypes, resulting in the modification of genetic variation in the population. Plasticity, because it acts at the level of the individual, is often hailed as a rapid-response mechanism that will enable organisms to adapt and survive in our rapidly changing world. But plasticity can also retard adaptation by shifting the distribution of phenotypes in the population, shielding it from natural selection. In addition to which, not all plastic responses are adaptive-now well-documented in cases of ecological traps. In this theme issue, we aim to present a considered view of plasticity and the role it could play in facilitating or hindering adaption to environmental change. This introduction provides a re-examination of our current understanding of the role of phenotypic plasticity in adaptation and sets the theme issue's contributions in their broader context. Four key themes emerge: the need to measure plasticity across both space and time; the importance of the past in predicting the future; the importance of the link between plasticity and sexual selection; and the need to understand more about the nature of selection on plasticity itself. We conclude by advocating the need for cross-disciplinary collaborations to settle the question of whether plasticity will promote or retard species' rates of adaptation to ever-more stressful environmental conditions. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

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.

Ocean Acidification Impairs Foraging Behavior by Interfering With Olfactory Neural Signal Transduction in Black Sea Bream, Acanthopagrus schlegelii

In recent years, ocean acidification (OA) caused by oceanic absorption of anthropogenic carbon dioxide (CO₂) has drawn worldwide concern over its physiological and ecological effects on marine organisms. However, the behavioral impacts of OA and especially the underlying physiological mechanisms causing these impacts are still poorly understood in marine species. Therefore, in the present study, the effects of elevated pCO₂ on foraging behavior, in vivo contents of two important neurotransmitters, and the expression of genes encoding key modulatory enzymes from the olfactory transduction pathway were investigated in the larval black sea bream. The results showed that larval sea breams (length of 4.71 ± 0.45 cm) reared in pCO₂ acidified seawater (pH at 7.8 and 7.4) for 15 days tend to stall longer at their acclimated zone and swim with a significant slower velocity in a more zigzag manner toward food source, thereby taking twice the amount of time than control (pH at 8.1) to reach the food source. These findings indicate that the foraging behavior of the sea bream was significantly impaired by ocean acidification. In addition, compared to a control, significant reductions in the in vivo contents of γ-aminobutyric acid (GABA) and Acetylcholine (ACh) were detected in ocean acidification-treated sea breams. Furthermore, in the acidified experiment groups, the expression of genes encoding positive regulators, the olfaction-specific G protein (Golf) and the G-protein signaling 2 (RGS2) and negative regulators, the G protein-coupled receptor kinase (GRK) and arrestin in the olfactory transduction pathway were found to be significantly suppressed and up-regulated, respectively. Changes in neurotransmitter content and expression of olfactory transduction related genes indicate a significant disruptive effect caused by OA on olfactory neural signal transduction, which might reveal the underlying cause of the hampered foraging behavior.

Predatory strategies and behaviours in cephalopods are altered by elevated CO2

There is increasing evidence that projected near-future carbon dioxide (CO₂ ) levels can alter predator avoidance behaviour in marine invertebrates, yet little is known about the possible effects on predatory behaviours. Here we tested the effects of elevated CO₂ on the predatory behaviours of two ecologically distinct cephalopod species, the pygmy squid, Idiosepius pygmaeus, and the bigfin reef squid, Sepioteuthis lessoniana. Both species exhibited an increased latency to attack and altered body pattern choice during the attack sequence at elevated CO₂ . I. pygmaeus also exhibited a 20% decrease in predation rate, an increased striking distance, and reduced preference for attacking the posterior end of prey at elevated CO₂ . Elevated CO₂ increased activity levels of S. lessoniana comparable to those previously shown in I. pygmaeus, which could adversely affect their energy budget and increase their potential to be preyed upon. The effects of elevated CO₂ on predatory behaviours, predation strategies and activity levels of cephalopods reported here could have far-reaching consequences in marine ecosystems due to the ecological importance of cephalopods in the marine food web.

Aquatic acidification: a mechanism underpinning maintained oxygen transport and performance in fish experiencing elevated carbon dioxide conditions

Aquatic acidification, caused by elevating levels of atmospheric carbon dioxide (CO2), is increasing in both freshwater and marine ecosystems worldwide. However, few studies have examined how acidification will affect oxygen (O2) 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 CO2 at the level of O2 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 O2 uptake have yet to be verified. Here, we summarize current data related to one recently proposed mechanism underpinning the maintenance of O2 uptake during exposure to aquatic acidification, and reveal knowledge gaps that could be targeted for future research. Most studies have examined O2 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.