Ocean Acidification

Coralline algae, calcifying red algae of the orders Corallinales, Hapalidiales and Sporolithales, are important components of many marine ecosystems, providing stability and protection from high energy waves in coral reefs, and providing shelter, substrate and nutrients for many organisms across a range of marine environments. Corallines are thought to be amongst the most vulnerable marine organisms in the face of climate change, as their high-Mg calcite mineralogy makes them especially vulnerable to ocean acidification. Corallines also require light to perform photosynthesis and are therefore vulnerable to siliciclastic sedimentation which can reduce illumination and bury corallines. Lab based acidification experiments show most coralline species respond negatively to elevated CO2 conditions, but the nature and magnitude of effects vary between species, and the rate of experimental CO2 increase is much faster than natural rates, making it difficult to use experimental data to predict how corallines respond to acidification on time scales of thousands to millions of years. While most coralline groups are not directly affected by temperature changes, global warming can alter the hydrosphere, increasing precipitation intensity and therefore terrestrial runoff in some coastal areas. Increased runoff can directly lower illumination in the water column and can also cause phytoplankton blooms that significantly reduce illumination. The earth’s climate and ocean conditions have changed greatly throughout the Cenozoic, with significant effects on many marine organisms including corals, benthic foraminifera, bivalves, sponges, and many other groups. The response of coralline algae to Cenozoic climate change, however, has received less research attention despite their importance in marine ecosystems.

This dissertation uses the fossil record to assess how coralline algae responded to different types of Cenozoic environmental change at three different scales. Chapter 1 looks at coralline response to rapid warming, acidification and sediment influxduring the Paleocene-Eocene Thermal Maximum (PETM) in northeastern India and Tibet. The PETM is the largest hyperthermal of the Cenozoic, where a massive release of CO2 caused widespread ocean acidification and warming of ~5-8 °C over ~10,000 years. At both study locations, coralline abundance is high in the pre-PETM and remains high into the PETM onset. Corallines then disappear abruptly as carbonate sedimentation rapidly ends under heavy siliciclastic input. The results of this chapter suggest that coralline algae may be more tolerant of acidification, but less tolerant of sedimentation, than previously thought.

Chapter 2 uses the early Eocene Sierra Blanca Limestone in Santa Barbara County, California, as a case study for rhodolith bed construction in the early Eocene greenhouse. Elevated temperatures and CO2 in the early Eocene prevented rhodolith beds from developing in most areas, so the Sierra Blanca Limestone provides rare insight into the environmental conditions allowing a robust coralline deposit to form in a seemingly hostile ocean. The lithology, taxonomic assemblages, and characteristics of Sierra Blanca rhodoliths suggest they developed on an offshore uplifted block along an upwelling coastline, allowing rhodoliths to thrive in relatively cool water beyond the reach of siliciclastic sedimentation.

Chapter 3 examines what controls the partitioning of coralline algae into reef and non-reef habitats on carbonate platforms during the Paleogene and Neogene. Coral reefs are significantly impacted by climate change during this interval, but coralline algae are more resilient. Coralline algae occupy non-reef settings through the studied interval and their abundance in non-reef environments shows little variation. As coral reefs develop, corallines persist in non-reef environments and expand into reefs where they play a crucial role in cementing the framework. When coral reefs collapse, corallines do not take over reef building, but persist and even expand in non-reef settings, often constructing rhodolith beds that serve some of the same ecological roles as coral reefs.

Together, these chapters highlight the ability of coralline algae to adapt to changing environmental conditions despite their assumed vulnerability. The discrepancies between lab-based experiments and paleontological observations suggest that the rate of change is vitally important: corallines can adapt to even significant change if it occurs slowly enough but are less able to adapt to rapid environmental change.

Coastal and marine systems are some of the most biologically productive and economically important systems on Earth, yet they are being compromised at an accelerating pace by climate change. Ocean warming, marine heatwaves, sea-level rise, ocean acidification and deoxygenation along with increasing extreme events are interacting to change the structure, function and resilience of ecosystems among coral reefs, mangroves seagrass meadows and salt marshes. Although effort has been devoted to understanding individual ecosystem effects of climatic change, we know less about comparative mechanistic synthesis and cross-ecosystem vulnerability trajectories. This synthesis explores recent understanding of the biophysical determinants of ecosystem decline, with particular reference to thresholds, regime shifts and feedbacks that limit capacity to adapt. We explore different sensitivities among these biogenic coastal systems, and explain why coral reefs are confronted with sharp thermal and carbonate chemistry thresholds, while mangroves and tidal wetlands have threshold-based resilience contingent on sediment supply and geomorphologic setting. Beyond ecological effects, we compile social–environmental responses including fishery reduction, coastal defense loss and blue carbon release, and rising human vulnerability especially in small island and developing coastal regions. The review also examines ecosystem-based adaptation and nature-based solutions, considering their co-benefits and implementation challenges in a high-emission world. By synthesizing climate drivers, ecosystem thresholds, and social–ecological tipping points within a unified conceptual framework, this review integrates mechanisms of change and alternative stable-state transitions to improve understanding of coastal ecosystem resilience under accelerating climate change.

Highlights

• Thermal stress was the most studied natural driver of seagrass decline.
• Study frequency reflects research effort, not ecological importance.
• Most multi-stressor studies assessed only two interacting drivers.
• Warming–acidification effects varied among species and exposure contexts.
• Indonesia remains underrepresented in long-term seagrass research.

Abstract

Seagrass ecosystems are exposed to natural environmental variability and climate-modulated extremes that can interact with anthropogenic pressures and accelerate ecological decline. This systematic review synthesized 60 peer-reviewed publications published between 2010 and 2025 to examine how temperature and marine heatwaves, ocean acidification and pH, salinity, drought and flooding, hydrodynamics, sea-level change, turbidity and light limitation, sediment conditions, and oxygen availability affect seagrass ecosystems. Thermal variables were the most frequently investigated category; however, publication frequency was treated as an indicator of research effort rather than a ranking of ecological importance. Experimental and field evidence showed that warming and marine heatwaves often reduce growth, photosynthetic performance, biomass, and recovery, although outcomes depend on species, exposure duration, light, nutrients, and prior stress history. Evidence for warming–acidification interactions was contrasting: acidification impaired plant performance during thermal stress in some studies, whereas enhanced inorganic carbon availability supported biomass or carbon metabolism in others. Most multi-stressor experiments evaluated only two drivers, and relatively few examined three or more simultaneous or successive stressors. The most commonly reported ecosystem consequences included habitat contraction, canopy and biomass loss, physiological impairment, altered carbon and nutrient cycling, sediment destabilization, community reorganization, and reduced resilience. Geographic evidence remained concentrated in Europe, the Mediterranean, Australia, China, and North America. Indonesia and much of Southeast Asia were comparatively underrepresented despite high seagrass diversity and documented declines associated with interacting environmental change. Future research should prioritize standardized long-term monitoring and multifactorial experiments that distinguish research frequency from ecological effect and support regionally calibrated risk assessment.

Brooding organisms can modify the chemical environment experienced by developing offspring, potentially altering their exposure to climate change. In marine systems, however, the carbonate chemistry of brood chambers, and the mechanisms governing it, remain poorly resolved. Here, we combine high-resolution pH and CO₂ measurements of the empty brood chamber (i.e., mantle cavity above gills) in the European flat oyster (Ostrea edulis) while also tracking valve gape to quantify how warming, ocean acidification, and ventilation interact to shape the internal environment experienced by larvae during incubation. Brood chamber pH was consistently lower than that of the surrounding seawater and responded to external conditions in a state-dependent manner governed by ventilation and respiration. When oysters were actively ventilating, brood chamber chemistry tracked ambient seawater with a persistent offset, whereas valve closure led to rapid CO₂ accumulation and pronounced declines in pH. These dynamics generated highly variable and behaviorally mediated exposure regimes, in which metabolic processes distorted ambient chemical signals over short timescales. Paired pH and CO₂ measurements provided preliminary constraints on aragonite saturation state (Ωar), revealing that brood chambers can remain undersaturated (Ωar < 1) for prolonged periods even in ventilating oysters and while overlying seawater remains favorable. Together, these results demonstrate that brood chambers function as dynamic microenvironments in which larval exposure to ocean acidification is governed not only by external conditions, but also by behavior. Because warming enhances metabolic CO₂ production and intensifies acidification within the chamber, future climate change is likely to amplify both the magnitude and variability of larval exposure. These findings help explain the apparent resilience of brooding species to ocean acidification, while also highlighting potential limits as environmental conditions move beyond historical bounds.

Highlights

• Ocean acidification disrupts calcium homeostasis in the mantle of mussels.
• Ocean acidification and N-NPs together intensify the inhibition of calcification.
• Surface charge determines distinct, mechanism-specific response pathways.
• Both omics reveal key roles for calcium transport, CA, and ECM synthesis.

Abstract

Ocean acidification (OA) and nanoplastics (NPs) increasingly co-occur in coastal ecosystems, yet their combined mechanistic impacts on calcifying invertebrates remain poorly resolved. Here, the mussel Mytilus coruscus was exposed for 30 days to factorial combinations of OA (pH 7.7 vs 8.1), surface-modified polystyrene NPs at a concentration of 100 μg/L (positively and negatively charged), and experimental shell damage to assess effects on shell repair, ion homeostasis, and energy metabolism. OA reduced shell repair quality by thinning repair layers, increasing porosity, and lowering calcium content, effects that were further enhanced by co-exposure to NPs, particularly negatively charged particles. OA and NPs jointly altered Ca²⁺ and Mg²⁺ levels, alkaline phosphatase activity, Ca²⁺Mg²⁺-ATPase activity, and key indicators of energy metabolism, including ATP content and cellular energy allocation. Transcriptomic and proteomic analyses revealed enrichment of ion transport, extracellular matrix, TGF-β signaling and other pathways, with divergent patterns linked to NPs surface charge. Together, these results suggest that nanoplastic surface charge under OA may impair mussel shell repair through associated alterations in ion homeostasis and energy metabolism. These findings highlight that particle surface properties shape how organisms respond to combined environmental stressors in acidifying marine environments.

This study applies the Forgotten Effects Theory (FET), a fuzzy-logic-based modelling framework, to analyse how construction life cycle processes generate direct and indirect pressures on multiple planetary boundaries. The approach captures multistage and nonlinear causal relationships derived from expert-based incidence matrices. Results show that mineral extraction, material distribution, and waste disposal strongly influence ocean acidification; construction operations and building use affect biosphere integrity; material production drives land-use change; and demolition contributes to climate change. The analysis revealed substantial hidden effects: the influence of mineral extraction on ocean acidification increased from 0.4 to 0.9 when material production was included as a mediator; the relationship between building use and biosphere integrity increased from 0 to 0.8 through indirect pathways; and the influence of demolition on climate change increased from 0 to 0.8 through waste-disposal mediation. A ±10% sensitivity analysis confirmed the stability of the main incidence hierarchy, with changes below 0.05 for the most relevant coefficients. These findings demonstrate that indirect (“forgotten”) effects can be as influential as direct impacts, significantly intensifying pressures across planetary boundaries. The construction sector is therefore characterised as a cumulative and interconnected system in which impacts propagate and reinforce each other across life cycle stages. These results highlight the limitations of silo-based environmental assessments and underscore the need for integrated, life cycle-oriented sustainability strategies. Policies prioritising upstream interventions, material efficiency and reduced demolition can generate broader systemic benefits. The study demonstrates the potential of FET as a transparent and reproducible modelling approach for linking local construction decisions with global environmental processes, supporting more integrated circular economy strategies and contributing to the achievement of Sustainable Development Goals 11, 12 and 13.

Coral reefs are especially sensitive to ocean warming and acidification caused by climate change and can have a major impact on reef-associated fish species and reef stability. This study analyzes the diversity of coral reef fishes over time using data from the National Coral Reef Monitoring Program (NCRMP) of NOAA from the year 2014 to 2024 from the Hawaiian Archipelago. The impacts of environmental factors such as sea surface temperature, ocean pH, and coral cover were evaluated on fish population structure. Shannon Diversity Index as the main statistical method to quantify the changes in biodiversity between monitoring years. The mean sea surface temperature (MST) ranged from 26.8°C in 2014 to 29.1°C in 2024, with a steady increase over the years, whereas the mean pH decreased from 8.12 to 7.95 over the years. During the same time frame, there was significant loss of coral cover, from 62% to 36%. The Shannon Diversity Index was then found to have decreased from 3.45 to 2.51, representing a loss of some 27.2% of biodiversity. The results indicate that the diversity of coral reef fishes is negatively correlated with environmental stress. Fish functional groups were further simplified, with habitat specialist fishes being more affected than generalist fishes under climate stress. The rising temperature and acidification of the oceans are important factors in the decline of coral reef fish. The continued monitoring and adaptive conservation strategies will be critical to ensure that reef biodiversity will persist into the future under different climate change scenarios.

The coastal zone is an epicenter of climate change impacts, including rising sea levels, an increase in sea temperature, and a decrease in salinity through precipitation and ice melt. In addition, changes in carbon chemistry will result in the “acidification” of seawater. Direct physiological effects of extreme weather events will likely cause some sub-optimal changes in Manila clam abundance, with implications for aquaculture production, especially in warmer waters such as the Adriatic and the west coast of the Korean penisula. Indirect effects on food and predator species and synergistic impacts of multiple pollutants may also prove influential. Currently predicted acidification will, in itself, be within the tolerances of R. philippinarum, although it may contribute to increased oxidative stress and energetic demand, especially when in combination with other challenges. Overall, while Manila clam distributions, locally, regionally, and globally, will adjust, the species is considered resilient within temperate conditions and is expected to extend its global distribution northwards in Alaska, Europe, and probably Siberia.

The Pacific Small Island Developing States (Pacific SIDS) are at the forefront of climate change impacts, yet these effects remain understudied and therefore poorly understood. Ocean acidification (OA) research in Samoa is in its infancy, with a new OA project established to monitor seawater carbon chemistry in coastal ecosystems. The study measured seawater carbon chemistry—including pH, dissolved carbon dioxide and total alkalinity—within the marine protected area (MPA) of Savaia, Lefaga, on Upolu Island, Samoa. Sampling was conducted over twelve months at four locations within the MPA (Site 1, Site 2, Site 3, Site 4), focusing on spatio-temporal variability in pH, temperature and total alkalinity. Measurements were obtained using water samples and an iSAMI pH sensor. The mean pH recorded was 8.06 ± 0.07, while the mean total alkalinity was 1955.1 ± 25 µmol∙kg⁻¹, which is lower than the typical value of 2305 µmol∙kg⁻¹ observed in tropical environments. This decline in total alkalinity and the corresponding low pH could be a potential source of acidification to downstream coastal ecosystems with potential implications for coral reefs, biodiversity and fishery livelihoods.

Anthropogenic CO2 emissions are expected to increase ocean acidity and temperature over the coming century. Under the Shared Socioeconomic Pathway (SSP) 5–8.5 scenario, mean surface ocean pH is projected to decrease by approximately 0.39 units, while sea surface temperature is expected to rise by 2–4 °C between 2010 and 2100. Sea pens (Octocorallia) are important bioindicators of soft-sediment habitat quality and associated benthic communities; however, their responses to these stressors remain considerably less studied than those of their hexacoral counterparts.

This thesis investigates the individual and combined effects of ocean acidification and warming, based on SSP5–8.5 projections, on the sea pen Pennatula phosphorea (Linnaeus, 1758), collected from the Swedish Gullmarsfjord. During winter and summer 2025, four pH treatments (8.0, control; 7.8, present-day low value; 7.4, projected low value for 2100; and 7.0, extreme scenario) and four temperature treatments (7.7 °C, winter control; 14 °C, summer control; 17 °C, present-day high value; and 20 °C, projected high value for 2100) were assessed. Response variables included behavioural traits (colony burrowing, colony inflation, polyp opening and bioluminescence emission), survival, and bioluminescence substrate levels (coelenterazine concentration and maximum light intensity, Lmax). Three hypotheses were tested: (1) ocean acidification and warming would negatively affect all response variables, with stronger effects under combined exposure; (2) prolonged exposure would result in either progressive deterioration or acclimation of behavioural traits; and (3) larger individuals would be less affected by stressors than smaller individuals.

The effects of acidification and warming varied among response variables, seasons, and whether stressors were applied individually or in combination. Overall, pH 7.8 produced few significant effects, except on burrowing behaviour, whereas pH 7.4 and 7.0 generated effects ranging from negligible (p > 0.1) to highly significant (p < 0.01), generally with greater impacts at pH 7.0. Similarly, temperatures of 17 and 20 °C elicited responses ranging from negligible to highly significant, with the strongest effects observed at 20 °C for quantitative measures of bioluminescence substrates. Combined exposure generally amplified the observed effects, suggesting that tolerance to one stressor may be compromised by the presence of another. No consistent evidence of behavioural acclimation or progressive deterioration was detected over time. Contrary to the initial hypothesis, larger individuals appeared more susceptible to environmental stressors than smaller individuals.

Molluscan shells are increasingly threatened by a complex array of environmental stressors—climate-related drivers, changes in seawater chemistry, contaminants, and biotic cues. This review provides a mechanistic synthesis of how these stressors disrupt the biomineralization process across life stages. We develop a conditional hierarchical response framework from the reviewed evidence. The framework links ionoregulatory and acid-base disturbances with molecular, microstructural, and shell-level responses, while allowing for compensatory, nonlinear, species-specific, and life-stage-specific outcomes. This molecular disruption manifests as defects in calcium carbonate (CaCO₃) crystalline microstructure, which may in turn compromise shell macro-scale physical properties (strength, hardness, growth). We synthesize combined and context-dependent effects of co-occurring stressors and distinguish formally tested interactions from qualitative comparisons of combined treatments. Future research must leverage integrated multi-omics, advanced in vivo imaging, and multi-stressor experimental designs to unravel candidate molecular and physiological thresholds and predict adaptive potential. This framework may help identify testable mechanisms, evidence gaps, and context-dependent vulnerabilities relevant to conservation.

Highlights

• A conditional framework links molecular, microstructural, and shell-level responses.
• Early stages often show high sensitivity, although direct ontogenetic comparisons remain limited.
• Energy limitation is a recurrent, but often indirectly inferred, physiological constraint.

CO₂-driven ocean acidification (OA) threatens marine calcifiers, while coastal ecosystems face increasing trace-metal contamination; their combined effects, and the modulating role of parental environmental history, remain poorly understood. We used the sea urchin Arbacia lixula from a naturally acidified CO₂ vent and a nearby ambient site at Ischia (Tyrrhenian Sea) to test how chronic adult exposure to low pH shapes larval responses to copper and low pH. Offspring from ambient (pH ~ 8.1) and vent (pH ~ 7.7) adults were reared for 48 h in a fully crossed design (two pH levels × three copper concentrations: 0, 5, 20 µg L⁻¹). At 24 hpf, development was dominated by parental history: ambient-derived larvae advanced rapidly, whereas vent-derived larvae showed higher arrest and slower progression, especially under low pH and high copper. By 48 hpf these effects diminished and pH and its interaction with copper became dominant, with both origins converging toward the echinopluteus stage. Nonetheless, malformations rose sharply under low pH regardless of copper, and skeletal-rod morphometrics revealed additional copper-related effects invisible to binary abnormality classifications. Chronic parental exposure did not enhance larval tolerance and sometimes increased sensitivity, underscoring the need to integrate multiple stressors and parental legacy.

Diatoms are a major phytoplankton group that plays a critical role in aquatic biogeochemical processes. Their metabolism relies on trace metals as cofactors for numerous enzymes, and changes in seawater pH may therefore influence metal sorption. Here, we investigated the effects of pH variability on diatom growth and trace-metal sorption using neutron activation analysis (NAA). Although NAA has rarely been applied to phytoplankton, we demonstrate its suitability for marine diatom samples. Overall, diatoms exposed to CO₂ treatment exhibited higher cell abundance but lower intracellular elemental concentrations. In particular, Nitzschia navis-varingica showed significantly lower concentrations of Al, Ce, Co, Cr, Fe, Mg, Mn and Zn under the CO₂ treatment. These trends are consistent with previous studies. For example, Zinc, which is a cofactor in many enzymes, plays a role in inorganic carbon acquisition; under lower pH, reduced enzymatic metal requirements likely explain the lower Zn concentrations observed. Nitzschia navis-varingica also had significantly lower levels of Co under lower pH. Both Thalassiosira pseudonana and Nitzschia navis-varingica showed lower Fe concentrations under CO₂ treatment. As Iron is required for photosynthetic and respiratory processes that support the carbon-concentrating mechanism (CCM), reduced CCM activity at lower pH may decrease Fe demand. Overall, this study highlights NAA as a robust approach for quantifying metal sorption in marine organisms and provides new insight into the effects of ocean acidification on the growth and elemental composition of diatoms.

Phytoplankton are primary producers in marine ecosystems and play a central role in biogeochemical cycles, yet their physiological responses to altered seawater pH vary among taxa and over time. Here, we examined the effects of sustained pH manipulation on pigment composition, growth, and dark respiration in three ecologically important phytoplankton taxa – a diatom (Pseudo-nitzschia spp.), a dinoflagellate (Heterocapsa pygmaea), and a haptophyte (Emiliania huxleyi) – during a 15-day controlled laboratory experiment. Cultures were maintained at pH 8.1, 7.8, and 7.5, representing present-day and enhanced acidification conditions. Responses to pH were species-specific and strongly time-dependent. Growth and cell-specific respiration rates showed relatively small and often transient differences among pH treatments, suggesting short-term metabolic adjustment under altered pH. In contrast, pigment composition exhibited clearer and more consistent pH-related responses, primarily expressed as shifts in temporal patterns rather than uniform directional changes. In Pseudo-nitzschia spp. and H. pygmaea, several key pigments displayed pronounced pH-dependent trajectories, whereas E. huxleyi showed greater temporal variability and weaker separation among pH treatments. Overall, these results demonstrate that photophysiological traits respond sensitively to sustained pH changes even when population-level growth and respiration remain comparatively stable, highlighting pigment composition as a potentially sensitive indicator of short-term phytoplankton acclimation to ocean acidification.

Increasing atmospheric carbon dioxide (CO₂) is driving global ocean acidification (OA). This process may threaten the persistence of bed-forming mussels by weakening the byssal system that anchors them to the seafloor. Here, blue mussels (Mytilus edulis) were exposed to present-day (∼460 ppm pCO₂) normocapnic and projected end-century (∼1200 ppm pCO₂) hypercapnic conditions for four weeks at 12 °C. Byssus production, thread morphology, whole-byssus mechanics and the underlying physiological condition index (CI) were quantified. Unlike previous studies, this study analysed the byssus as an intact functional unit. This approach better reflects its mechanical performance in situ. Median thread production fell by 50% under elevated pCO₂ and the number of individuals producing no threads at all increased from 3% to 23% of the population. Thread diameter and plaque area were unaffected. Whole byssus tensile testing revealed a distinctive mechanical pattern (elastic loading, force plateau, and structural failure) regardless of environmental CO₂ concentration. Whole byssus attachment strength scaled linearly with thread number in both treatments, and the mechanical work required to detach mussels under hypercapnia dropped by 42%. Elevated pCO₂ reduced mussel condition index by 19% relative to the control, indicating an energetic burden. These results show that near-future ocean acidification weakens mussel attachment primarily by lowering individual physiological condition, which directly drives the reduction in total attachment energy, rather than by lowering individual thread quality. This likely results from a shift in energy use away from thread production, as seen in poorer mussel condition. As a result, mussels may become more prone to being dislodged by waves or predators. The findings of this study indicate that ocean acidification can reduce the overall strength of M. edulis beds, with important effects on rocky shore ecosystems and the viability of mussel farming in a changing climate.

Highlights

• Sharks maintain acid-base balance under elevated CO₂ via effective buffering.
• Physiological compensation carries energetic costs that constrain aerobic scope.
• Olfactory-mediated foraging is more OA-sensitive than baseline locomotor function.
• Multi-stressor effects (warming + OA) amplify biological impacts across life stages.
• Evidence is biased toward benthic species; pelagic sharks remain understudied.

Abstract

Ocean acidification (OA), driven by increasing atmospheric carbon dioxide (CO₂), is a major component of global ocean change with widespread implications for marine organisms. Sharks (elasmobranchs) are often assumed to be relatively resilient to OA due to their distinctive physiology, including strong acid–base regulation and urea-based osmoconformation. However, empirical evidence evaluating this assumption remains limited and fragmented. This review synthesizes current knowledge on the physiological, behavioural, and sensory responses of sharks to OA within a comparative framework that incorporates insights from teleost fishes. A systematic literature search following PRISMA guidelines identified studies examining OA effects across shark species and life stages. Available evidence indicates that sharks generally maintain extracellular acid-base balance under elevated CO₂, demonstrating effective physiological buffering. However, this compensation is not without cost. Energetic trade-offs, reduced aerobic scope under multi-stressor conditions, and alterations in metabolic and oxidative responses have been reported. Behavioural and sensory-mediated processes, particularly olfactory-driven foraging, appear more sensitive to OA, with impairments emerging even in the absence of obvious physiological failure. Responses are highly species-specific and often amplified by co-occurring stressors such as warming. Despite these findings, current data are strongly biased toward small, benthic species, with limited representation of pelagic taxa and long-term responses. Sharks cannot be considered uniformly resilient to OA; rather, their responses are context-dependent, energetically constrained, and potentially consequential at population and ecosystem levels. Future research integrating long-term, multi-stressor, and mechanistic approaches will be critical for improving predictions of shark responses under ongoing ocean change.