Climate change, including seawater warming and salinity fluctuations, is increasingly affecting marine ecosystems worldwide. The blue mussel, Mytilus edulis, widely distributed along the temperate coasts of the Northern Hemisphere, thrives in environments characterized by temperature fluctuations and salinity gradients. In particular, populations in the Baltic and North Seas are exposed to significant variation in these factors, which can affect the reproductive capacity of blue mussels, essential for sustainability of their populations. This study assessed the effects of varying temperature and salinity on the reproductive performance of blue mussels from the Baltic and North Seas, focusing on sperm motility, ATP content and fertilization success. Additionally, sperm mitochondrial function in Baltic Sea mussels was examined under different temperature and osmolarity conditions. The results showed that mussels from both populations tolerated seawater warming, but were sensitive to cold and low salinity, with sperm motility and fertilization success significantly impaired under these conditions. The salinity window for sperm motility and fertilization was population specific: optimal ranges were a salinity of 13-17 for Baltic Sea mussels and 21-35 for North Sea mussels. Notably, North Sea mussels were unable to reproduce at salinity 9, whereas Baltic Sea mussels were severely impaired at salinity 5. High temperature (25°C) reduced mitochondrial respiratory efficiency and increased reactive oxygen species (ROS) production, while osmolarity did not appear to be a key factor. These findings highlight population-specific reproductive traits in M. edulis and link sperm performance to mitochondrial function, providing new insights into benthic adaptation to changing coastal environments.

Irregular heat spikes associated with marine heat waves may either induce thermal hardening and enhance resilience or exceed physiological limits and compromise future stress tolerance in marine ectotherms. Understanding the mechanisms underlying these contrasting responses is critical for predicting the impacts of increasing thermal variability. We investigated whether prior heat exposure induces protective responses in Baltic blue mussels (Mytilus spp.). Mussels were exposed to repeated heat stress (RHS; four cycles of 2.5 h at 27°C followed by 21.5 h recovery at 15°C) and subsequently allowed a 4-day recovery period. Cardiac performance during acute warming (15-35°C) was assessed using heart rate and Arrhenius breakpoint temperature (ABT). In addition, RHS-exposed and naïve mussels were subjected to prolonged heat exposure (27°C, 5 days), during which energy status, mitochondrial activity, and transcriptional responses associated with proteostasis, oxidative stress, immunity, and apoptosis were evaluated. RHS induced a transient proteostatic and immune response, characterized by increased expression of hsp70A and immune-related genes (MyD88a, MyD88b, MyD88c, C1, and TLRb), which was not maintained during recovery. During prolonged heat exposure, RHS-exposed mussels exhibited 100% mortality by day 5, indicating the absence of a beneficial hardening response. Prolonged heat exposure induced chaperone responses (hsp60 and hsp90) in both groups, reflecting activation of stress response pathways. In contrast, RHS-exposed mussels showed reduced expression of antioxidant genes, including glutathione peroxidase (GPx) and manganese superoxide dismutase (MnSOD), suggesting compromised antioxidant defence and increased vulnerability to cellular damage. Despite maintaining high ATP levels and stable mitochondrial electron transport system activity, RHS-exposed mussels appeared unable to effectively allocate energy towards repair and stress mitigation. Overall, repeated heat stress and recovery did not induce thermal hardening of Baltic blue mussels under the tested conditions but instead reduced tolerance to prolonged heat exposure. This outcome may reflect the combined effects of the high prior heat exposure temperature, low salinity, and population-specific characteristics influencing adaptive thermal responses.

The European flat oyster (Ostrea edulis) is a foundational ecosystem engineer that has suffered widespread population declines due to overfishing, habitat degradation, and disease. Effective restoration requires understanding how environmental stressors impact juvenile oysters, a critical life stage for survival and recruitment. While trace metals such as zinc (Zn) and copper (Cu) are essential micronutrients, elevated concentrations can disrupt cellular physiology and immune function, particularly under temperature fluctuations associated with climate change. In this study, we investigated the combined effects of dissolved Zn (100 and 1000 μg L-1) and Cu (10 and 100 μg L-1) at three temperatures (5 °C, 15 °C, 22 °C) on immune defenses in juvenile O. edulis. Baseline tissue concentrations in controls were 73.0 ± 9.5 μg g-1 Cu and 1240.5 ± 113.7 μg g-1 Zn and were unaffected by temperature. Low metal exposures did not alter tissue concentrations, whereas high exposures induced temperature-dependent accumulation, peaking at 22 °C (CuH: 381.9 ± 78.6 μg g-1; ZnH: 4573.6 ± 603.8 μg g-1). Temperature strongly modulated cellular immunity: hemocyte abundance was highest at 15 °C, phagocytosis and acid phosphatase activity increased at 22 °C, while lipid peroxidation levels were elevated at 5 °C and 22 °C, indicating stress-induced responses at thermal extremes. Zn stimulated acid phosphatase but suppressed phenoloxidase activity, whereas Cu increased hemocyte mortality and modestly stimulated phenoloxidase. Lysozyme activity was elevated at cold exposure (5 °C), suggesting enhanced antibacterial defenses. Multivariate PLS-DA analyses revealed clearer separation of immune profiles by temperature than by metal exposure. These results demonstrate that temperature is the dominant factor shaping immune competence in juvenile O. edulis, while metal contamination at environmentally realistic levels exerts only minor effects, with implications for restoration and management under climate change.

Environmental salinity strongly shapes the physiology of osmoconforming marine invertebrates, yet population-level metabolic responses remain poorly understood. The ocean quahog, Arctica islandica, exhibits extreme geographic variation in growth and maximum lifespan, ranging from several decades in brackish environments to more than four centuries in fully marine habitats. The physiological mechanisms underlying these differences are unknown, although there are indications that habitat salinity may influence metabolic processes underlying life-history traits, including lifespan. We compared A. islandica populations from the fully marine Barents Sea and the brackish White Sea, acclimating individuals to three salinity regimes (17, 25, 33 PSU) spanning natural and experimentally altered conditions. Survival, tissue energy status and whole-body metabolomic profiles were assessed. Quahogs from the White Sea and Barents Sea showed pronounced population-specific metabolomic signatures and distinct responses to salinity acclimation. White Sea individuals showed higher levels of glycine, proline, tyrosine and tryptophan, as well as elevated oxidative stress markers (methionine sulfoxide, cystine) and glutathione metabolism at habitat salinity, indicating greater exposure to reactive oxygen species, probably due to higher metabolic activity. Salinity shifts triggered differential regulation of amino acids, osmolytes and urea cycle intermediates, with White Sea quahogs showing more pronounced osmolyte adjustments and tricarboxylic acid cycle flexibility. Despite substantial metabolomic remodelling, gross tissue bioenergetics, including proteins, carbohydrates, lipids and electron transport system activity, remained stable. These results reveal population-specific strategies for salinity tolerance, highlighting the metabolic flexibility of White Sea quahogs relative to the more conservative metabolism of Barents Sea individuals.

Eutrophication and chemical pollution often co-occur in coastal ecosystems, where their combined effects impact model sentinel sessile organisms like bivalves. We investigated the interactive effects of the serotonin re-uptake inhibitor fluoxetine (FLX) and hypoxia on metabolic and reproductive processes in the blue mussel Mytilus edulis. Given the central role of serotonin in regulating bivalve metabolism and reproduction, we hypothesized that FLX, alone and combined with hypoxia, will disrupt these functions. Mussels were exposed for three weeks to environmentally relevant FLX concentrations (targets 20 and 200 ng/L, "low" and "high") under normoxia (8.8 mg O2/L) or hypoxia (0.9 mg O2/L). Metabolic and reproductive status were evaluated using histological indicators of tissue condition and gonadal development, supported by biochemical and molecular markers. FLX exposure reduced egg density and promoted digestive gland atrophy. Hypoxia accelerated gonadal development, reduced sperm density and stimulated the electron transport system activity, indicating metabolic compensation for reduced oxygen availability, while energy reserves remained stable. Molecular and metabolite responses to FLX were dependent on concentration and oxygen regime: FLX-induced changes in catecholamine (low FLX) and nucleotide metabolism (high FLX) in normoxia were absent under hypoxia condition. Hypoxia also impacted catecholamine and tricarboxylic acid cycle metabolism, but only at the low FLX concentration. Low FLX reduced mussel β-catenin expression in the gonad, but expression patterns of genes related to gonad development or apoptosis did not consistently align with histological changes, reflecting complex interactions between FLX, oxygen regime, and sex. These findings show that pharmaceutical pollution exerts context-dependent effects through interactions with hypoxia in a habitat-forming marine species.

Zinc oxide nanoparticles (nZnO) are increasingly utilized in industrial, medical, and personal care products, particularly as the main ingredient in sunscreens, raising concerns about their environmental impact, especially in coastal ecosystems. The Baltic Sea, experiencing severe eutrophication, faces persistent hypoxia due to excessive nutrient runoff and limited water exchange. Simultaneously, coastal pollution from industrial and urban activities introduces nZnO, a highly biotoxic nanopollutant. The combined effects of hypoxia and nZnO contamination may amplify environmental stress, yet their interactions remain insufficiently studied. This study investigates the combined effects of nZnO exposure and fluctuating dissolved oxygen regimes (specifically short- and long-term hypoxia and subsequent reoxygenation) on Mytilus edulis, a sentinel species in these ecosystems. By assessing a range of cellular and molecular markers, including oxidative stress, oxygen sensing, protein quality control, stress response, apoptosis, and inflammation, we show that nZnO exacerbates hypoxia-induced oxidative stress, delaying redox recovery and prolonging oxidative damage during reoxygenation. Specifically, nZnO exposure maintains elevated LPO and PC levels after reoxygenation, indicating prolonged oxidative imbalance. While M. edulis typically recovers from hypoxia-induced stress, nZnO disrupts this process by impairing antioxidant defenses, prolonging HIF-1α activation, and dysregulating p53, JNK, and p38 expression, thereby interfering with normal hypoxia-reoxygenation response. Additionally, nZnO alters HSP70, Lon protease, and caspase-3 regulation, disrupting protein-folding and apoptotic pathways. These findings suggest a synergistic interaction between nZnO and hypoxia, heightening the organism's vulnerability to environmental stress and suggesting risks for marine organisms in nanoparticle-polluted, hypoxia-prone coastal regions.

Lipid-lowering drugs such as gemfibrozil (GFB) are widely used and highly biologically active, contributing to their persistence in wastewater and subsequent release into aquatic ecosystems. However, the potential impacts and toxic mechanisms of these emerging pollutants on non-target marine organisms, particularly keystone bivalves like Mytilus edulis, remain poorly understood. To address this knowledge gap, we investigated the effects of environmentally relevant concentrations of GFB (25 mu g l- 1) on oxidative, nitrosative, and dicarbonyl stress in M. edulis, and explored how abiotic stressors such as elevated temperature and air exposure modulate these effects. Our results indicated that GFB and temperature interact to significantly influence oxidative stress markers, including lipid peroxidation (LPO) and protein carbonylation (PC) levels in mussels. Notably, the combination of GFB and warming exhibited antagonistic effects, leading to reduced LPO levels in both submerged and airexposed mussels. Air exposure alone elevated PC levels across all groups, while warming reduced these levels. Total antioxidant capacity increased during air exposure, with GFB exerting minimal influence on this parameter. Nitrosative stress, as indicated by nitric oxide levels, was significantly affected by GFB only under air exposure conditions. The glutathione system underwent notable alterations, with glutathione reductase activity stimulated during immersion and suppressed during air exposure. Dicarbonyl stress markers, including methylglyoxal and glyoxalase enzyme activities, generally intensified in response to GFB during air exposure. Overall, environmentally relevant concentrations of GFB induced oxidative and dicarbonyl stress in M. edulis, suggesting a shift toward glycolytic metabolism that could impair energy-dependent processes like reproduction. Combined stressor scenarios involving GFB and warming typically exhibited antagonistic rather than synergistic effects. Despite these biochemical disruptions, the mussels demonstrated resilience, particularly during air exposure, highlighting the complexity of environmental stress interactions. These findings emphasize the importance of considering multiple stressors in pollution risk assessments for aquatic ecosystems.

Coastal hypoxia poses a growing threat to marine biodiversity, significantly impacting the survival and reproductive success of sessile benthic organisms such as mussels. While much research has focused on the physiological responses of adult mussels to hypoxia, the effects on their reproductive processes remain underexplored. This study investigates how hypoxia (2 mg O2 l-1, with one-week and two-week exposures) and short-term anoxia (<0.1 mg O2 l-1, with 24-h and 72-h exposures) affect the reproductive parameters (including sperm motility and metabolism, egg size and number, and fertilization success) of the Baltic Sea blue mussel, Mytilus edulis. Our findings show that prolonged (two-week) hypoxia significantly decreases the spawning response rate of adult mussels and leads to a reduction in sperm velocity by ∼50 % six hours post-spawning. Short-term (24-72 h) anoxia had no effect on the spawning rate or the sperm velocity. However, sperm mitochondrial ATP synthesis capacity and coupling efficiency diminished after 72 h of anoxic exposure, while hypoxia exposure had no effect on these traits. Increased reactive oxygen species production in sperm mitochondria was noted under both hypoxic and anoxic conditions. Maternal investment strategies, as indicated by the relationship between egg size and number, did not show significant changes following exposure to hypoxia and anoxia. However, one week of hypoxia exposure was associated with the release of larger eggs by female mussels. Despite the negative impact of low oxygen conditions on spawning rates and sperm quality, no decrease in fertilization success was observed, possibly due to fertilization being conducted under normoxic conditions. Our data suggest that a reduced spawning rate from prolonged hypoxia could disrupt spawning synchronization, challenging natural reproduction. While sperm from males pre-exposed to hypoxia and anoxia maintained their ability to fertilize eggs under normoxia, impaired sperm quality may affect fertilization success under low oxygen conditions, warranting further investigation.

Perfluorooctanoic acid (PFOA) and nano-titanium dioxide (nano-TiO₂) are widely used in industrial applications such as manufacturing and textiles, and can be released into the environment, causing toxicity to marine organisms. To study the effects of these pollutants on the gonadal development, we exposed the males of Mytilus coruscus to varying PFOA concentrations (2 and 200 μg/L) alone or combined with nano-TiO2 (0.1 mg/L, size: 25 nm) for 14 days. Co-exposure to PFOA and nano-TiO₂ resulted in a short-term (7 days) decrease in the gonadosomatic index (GSI), which recovered to baseline levels. In contrast, long-term (14 days) exposure induced changes in the testes, including increased protein content, decreased lipid content, reductions in spermatic area and sperm count, and elevated apoptotic cell levels. Furthermore, key genes essential for gonadal maturation were significantly upregulated after long-term exposure. PFOA and nano-TiO2 can disrupt the gonadal function in the male mussels by interfering with Wnt family signaling pathways, modulation of steroid and lipid metabolism and induction of apoptosis. Therefore, PFOA and nanoparticle pollutants may pose a significant risk to the reproductive capacity of mussels’ populations from polluted coastal environments.

Lake Baikal harbors freshwater profundal amphipods, including the eurybathic species Ommatogammarus flavus and O. albinus. O. flavus occupies shallower habitats, including the littoral zone, which is subject to greater temperature variability. Consequently, O. flavus may demonstrate a higher degree of tolerance to temperature fluctuations in comparison with O. albinus. We compared the metabolic responses of these two amphipod species to thermal ramping. Specimens were collected from a range of depths and acclimated to a temperature of 4 °C. They were then subjected to controlled temperature up- and downshifts to assess the parameters of tissue energy status, antioxidant enzyme activity, and survival. O. flavus showed a higher degree of tolerance to warming than the deeper-dwelling O. albinus, while both species were resilient to temperature decreases. The depth of sampling had a minor effect on glycogen levels in O. flavus as well as catalase and glutathione S-transferase activity in O. albinus but had no effect on survival during experimental warming. Glucose level was shown to be the most sensitive biochemical marker to temperature variations, indicating that it could be used as a stress indicator for Baikal deep-water amphipods. This finding might provide insight into their adaptability to ambient temperature fluctuations.

Mitochondria generate up to 90% of cellular ATP, making it critical to understand how abiotic factors affect mitochondrial function under varying conditions. Using clones of the rotifer Lecane inermis with known thermal preferences, we investigated mitochondrial bioenergetic responses to four thermal regimes: standard temperature, optimal temperature, low suboptimal temperature, and high suboptimal temperature. The study aimed to determine how mitochondrial parameters in intact organisms vary with temperature shifts and whether these responses differ across experimental populations. We assessed key bioenergetic parameters: routine respiration (representing overall metabolic rate), electron transport system (ETS) capacity (indicative of oxidative phosphorylation potential), and proton leak rates (reflecting the energetic costs of maintaining mitochondrial membrane potential). Our results showed that populations with different thermal preferences displayed distinct mitochondrial responses to temperature changes, particularly at suboptimal temperatures. In contrast, responses were more uniform under standard and optimal conditions. Our findings demonstrated that metabolic plasticity in changing environments often involves trade-offs between mitochondrial efficiency and maintenance. By studying mitochondrial respiration at the whole-organism level, we revealed the complex temperature dependence of bioenergetic traits, providing insights beyond isolated mitochondria studies. This research highlights how a cascade of plastic responses spanning from mitochondrial responses to overall growth patterns is triggered by temperature changes, offering a valuable perspective in the context of global warming and organismal adaptation.

The oxygen inventory of the global ocean is declining. This phenomenon, known as ocean deoxygenation, has emerged as a fundamental pathway for climate change to alter marine ecosystems. An important concern is how this global oxygen decline will manifest in coastal and oceanic systems that are already subject to low oxygen, or hypoxic conditions. There is also a clear need to understand how the intensification and/or expansion of hypoxia will affect ocean food webs and biogeochemical cycles. Building a predictive understanding of ocean hypoxia is a multi-scaled and multi-disciplinary research endeavor. Recent advances in ocean observation, experimental biology, and ecosystem modeling are being applied to ocean hypoxia research to reshape our understanding of the future ocean.

The lucinid clam Lucinoma capensis thrives at the oxygen minimum zone margins in the Benguela Upwelling System, where oxygen levels fluctuate dramatically. Understanding its adaptation to such extreme conditions provides key insights into survival strategies under fluctuating oxygen availability. We investigated the transcriptomic and metabolomic responses of L. capensis under normoxia, hypoxia, and recovery, focusing on the gills and digestive gland. Our findings highlight distinct organ-specific responses, with the gills showing strong transcriptional changes to oxygen fluctuations, in contrast to the more stable profile observed in the digestive gland. Under hypoxic conditions, the gills exhibited coordinated downregulation of protein synthesis, transposable element activity, and immune function, suggesting a tightly regulated energy conservation strategy and mechanisms to preserve symbiont stability and genomic integrity. Activation of prokaryotic metabolism in the gills supports the symbionts' role in host energy acquisition and sulfide detoxification during hypoxia. In contrast, the digestive gland showed minimal transcriptional shifts during anoxia, with upregulation of pathways supporting structural maintenance. Upon reoxygenation, the gills displayed an active and asymmetric recovery, characterised by rapid restoration of protein synthesis and gradual normalisation of protein degradation and immune functions. Despite significant transcriptomic changes, the metabolome remained largely stable, reflecting L. capensis's resilience to oxygen fluctuations. However, an overshoot in TCA cycle intermediates and derepression of previously downregulated pathways indicate that reoxygenation involves active metabolic reprogramming, not merely a return to baseline. This study highlights the specialised tissue responses and symbiotic contributions that enable L. capensis to thrive in one of the ocean's most challenging environments.

Marine protected area (MPA) networks are important for supporting biodiversity, enhancing ecosystem resilience, and facilitating species recovery. For the effectiveness of conservation and restoration, functional connectivity plays a vital role. The dispersal, movement, and successful establishment of organisms between suitable habitats and MPAs ensure long-term sustainability of the populations. Despite its importance, functional connectivity is rarely integrated into restoration planning, which limits the effectiveness of species reintroductions, habitat connectivity, and adaptation to environmental changes. In this study, we applied an integrative approach combining molecular detections (environmental DNA [eDNA] and meroplankton metabarcoding) with biophysical modeling to explore the functional connectivity between two Natura 2000 MPAs in the North Sea: Borkum Reef Ground (BRG) and Sylt Outer Reef (SOR). We focused on the European flat oyster (Ostrea edulis), a reef-building species that once provided vast reef habitats but is now functionally extinct in the German Bight and is therefore the subject of recent restoration measures at BRG. Our results showed partial but informative correspondence between molecular detections of oyster genetic traces and the modeled larval pathways during the June-July 2022 sampling period. We further explored larval dispersal across entire spawning seasons in 2022 and 2023. Connectivity between BRG and SOR was highly dependent on larval drift depth. Surface-drifting larvae showed strong interannual variability, with 3% reaching SOR in 2022 when northwesterly winds dominated, increasing to 22% in 2023 under westerly and southwesterly winds. Larvae drifting at depth, however, exhibited near-zero connectivity, leading to high self-recruitment rates, with over 25% settling near the original restoration sites. Our results demonstrate that wind-driven currents are a key driver of interannual variability in larval retention and dispersal. Additionally, they highlight the role of biological traits, such as vertical positioning and pelagic larval duration, in shaping connectivity between MPAs and oyster restoration sites. These findings emphasize the need to integrate connectivity assessments into MPA management and the restoration planning of reef-building benthic species. The interdisciplinary approach presented here provides a quantitative framework for assessing connectivity under species- and site-specific conditions, offering a transferable tool to evaluate the restoration potential of other species and enhance the functional network between MPAs.

Ocean warming and metal pollution pose a threat to coastal ecosystems worldwide. In the German Bight, efforts to restore biogenic reefs using the native European flat oyster (Ostrea edulis) face challenges due to environmental conditions and potential pollutants of the North Sea. Besides O. edulis, the non-native Pacific oyster (Crassostrea gigas) inhabits the North Sea. Larval stages of bivalves are known to be sensitive to pollution. In this study, we investigate the effect of the trace metals copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb) in combination with water temperatures of 18° and 24 °C on the embryo-larval development of C. gigas and acute mortality of C. gigas and O. edulis D-larvae. This multi-stressor approach revealed that Cu was the most toxic metal, regardless of temperature, species or life stage. While elevated temperatures mitigated the negative effects of metal exposure on embryo-larval development, larval mortality was species- and metal-dependent at the tested temperatures. O. edulis D-larvae demonstrated a greater absolute tolerance to metal exposure at both temperatures, but a species comparison showed that O. edulis D-larvae had lower relative tolerance to the combined stress of warming and metal exposure than C. gigas. Based on the resulting toxicity thresholds, an environmental risk assessment for Cu was conducted to identify potentially hazardous areas for O. edulis restoration to be included in future habitat suitability studies and site selection for restoration. The identified areas may also indicate problematic environmental conditions for larval stages of other invertebrate species or fish.

Oxidative stress is a prevalent mechanism of physiological stress caused by exposure to pollutants or environmental hypoxia in aquatic organisms. The hyporheic zone (HZ), a transitional area between surface and groundwater with distinct ecological conditions, is highly susceptible to pollution and hypoxia, but the physiological responses of its aquatic inhabitants to these stressors remain poorly understood. The aim of this study was to investigate the effect of wastewater treatment plant (WWTP) effluent as a point source of contamination on the battery of biomarkers of antioxidant defence (catalase, CAT; glutathione reductase, GR), xenobiotic biotransformation (glutathione-S-transferase, GST), and aerobic/anaerobic energy metabolism (pyruvate kinase, PK; phosphoenolpyruvate carboxykinase, PEPCK; lactate dehydrogenase, LDH) in a stygophilous freshwater amphipod Synurella ambulans from the Sava River HZ. The samplings were conducted in relation to the Sava River's hydrologic regime after low river discharge (in September 2020) and after high river discharge (in April 2021) at one site (Medsave) upstream of the WWTP outflow and at two sites downstream (Podsused and Jarun). S. ambulans populations at sites downstream of the WWTP showed higher CAT, GR, GST and LDH activity than population at the upstream site. PK/PEPCK ratio was significantly lower in the S. ambulans populations at Podsused and Jarun sites compared to Medsave population after low river discharge, indicating lower aerobic capacity and greater reliance on anaerobic metabolic pathways. This study improves the understanding of the physiological responses of crustaceans exposed to chemical pollution and environmental stressors, thus contributing to the ecological assessment of groundwater connected ecosystems.

Climate change and eutrophication intensify marine heat-waves (MHWs) and cause fluctuations in oxygen and pH levels (FH) in coastal ecosystems. A temperate zone ecosystem engineer, the blue mussel (Mytilus edulis), provides a valuable model to investigate the cumulative effects of these stressors on the performance of keystone benthic organisms. To assess whether FH exposure alters mussels' responses to MHWs, mussels were exposed to either well-oxygenated or FH conditions (daytime ∼8.5 mg L-1 O2, pH 8.0, night-time ∼0.6 mg L-1 O2, pH 7.3) for 4-weeks before temperature increases (18 °C-26 °C). Physiological responses including respiration (RR), clearance (CR) and ammonia excretion rate (ER), byssus production, biomarkers of energy status and of oxidative stress were measured before and after 1-4, 7-10, and 14-17 days of MHW. Energy consumption rate increased and mussels' CR decreased but cellular energy reserves remained stable during MHW. Total antioxidant capacity overall increased and lipid peroxidation (LPO) decreased after one day of MHW, except in FH digestive gland, where LPO remained at baseline, suggesting redox balance adjustments post-FH acclimation and warming. RR increased and ER transiently increased during the MHW, with FH exposure preventing ER return to baseline and RR stabilization after two weeks post-MHW. A parallel increase of the protein concentrations between 4 and 10 days of MHW in gills indicate protein turnover shifts linked to the thermal acclimation. These findings suggest that FH acclimation modifies mussels' physiological responses to MHW, potentially reducing their resilience to the intensifying coastal hypoxia and extreme weather events predicted for future Baltic Sea coasts.

Climate change is causing extreme short-term warming with greater intensity and more frequent occurrence. Reproduction and subsequent recruitment of coastal ecosystem engineers, such as the blue mussel, may be impacted by the extreme temperatures because these vital functions are sensitive to the timing of short-term changes in abiotic factors. We exposed intertidal blue mussels, Mytilus edulis, to a thermal challenge from 10 to 29 degrees C using an ecologically relevant heating rate of 4 degrees C/h. We assessed their reproductive status by observing spawning activity and by analyzing histological sections of their gonad tissue. In addition, we monitored their heart rates and valve gaping behavior to determine their thermal performance. We identified three spawning groups: non-spawners that had mature gonads but did not release gametes, post-spawners that released mature gametes prior to the thermal challenge, and active spawners that released mature gametes during the thermal challenge. Across temperatures, active spawners had significantly higher heart rates and their heart rate peaked at the temperatures 3.9 and 3.2 degrees C higher compared to those of non-spawners or post-spawners, respectively. Post-spawners had significantly narrower valve gapes across temperatures compared to both other spawning groups. Hence, the metabolic response to warming strongly depends on the reproductive status, with active spawners experiencing increased thermal stress due to heightened metabolism, non-spawners showing heat-induced metabolic depression, and post-spawners adopting an energy-conserving strategy indicated by reduced gaping. Considered together, spawning during recurring short-term warming events may elevate mortality risk with potential consequences for the local biodiversity in a future climate.

The king scallop (Pecten maximus) is a highly aerobic subtidal bivalve species vulnerable to fluctuations in oxygen availability. This study investigated the effects of short-term (15 min) and long-term (90 min) hypoxia-reoxygenation (H/R) stress on substrate-specific mitochondrial functions in the gill and digestive gland tissues of P. maximus, oxidizing substrates that engage mitochondrial Complex I (pyruvate, palmitate) and Complex II (succinate). Under normoxic conditions, scallop mitochondria preferentially oxidized pyruvate. H/R stress induced a significant decline in Complex I-driven ATP synthesis, increased proton leak and dysregulated fatty acid oxidation, indicating mitochondrial vulnerability to H/R stress. Following H/R, both tissues demonstrated a greater capacity for succinate oxidation than for Complex I substrates; however, long-term H/R exposure led to a reduction in respiratory coupling efficiency across all substrates. Notably, gill mitochondria exhibited more effective regulation of reactive oxygen species efflux and electron leak compared with digestive gland mitochondria under H/R stress. Despite these physiological changes, no evidence of oxidative damage was detected, suggesting the presence of a robust mitochondrial antioxidant defense. Collectively, these findings suggest that succinate oxidation plays an important role in stress recovery in P. maximus, providing insights into mitochondrial resilience and the management of oxidative stress during intermittent hypoxia.