Transcriptomic responses reveal impaired physiological performance of the pearl oyster following repeated exposure to marine heatwaves

Long-term thermal stress reshapes the tolerance of head kidney of Hong Kong catfish (Clarias fuscus) to acute heat shock by regulating energy metabolism and immune response

Elevated water temperatures caused by climate warming can affect fish survival. However, fish can maintain normal physiological functions through physiological plasticity. When temperature fluctuations exceed their tolerance range, even stress-resistant species like Siluriformes are affected. It is known that fish have adaptive regulation mechanisms to reshape their tolerance to temperature stress, but the ability to respond to acute thermal shock and recover after adaptive remodeling remains unclear. This study investigated the effects of different culture temperatures on the ability of Hong Kong catfish (Clarias fuscus) to respond to acute heat stress and stress recovery. C. fuscus were cultured at normal temperature (NT, 26 °C) or high temperature (HT, 34 °C) for 90 days, and then their head kidney transcriptome was analyzed after acute heat stress (34 °C) and subsequent recovery (26 °C). The results revealed 8165 differentially expressed genes (DEGs) in the NT group and 8537 DEGs in the HT group during the entire temperature treatment process, with each group responding differently to various stages of temperature treatment. Enrichment analysis showed that both NT and HT groups had enriched pathways related to energy metabolism and immune response during acute heat stress. However, acute heat stress disrupted the energy supply and oxidative metabolism in the NT group, while enhancing the HT group's ability to respond to repeated heat stress. This experiment demonstrated that high-temperature culture reshaped the energy metabolism balance in the head kidney tissue, improving anti-stress and stress recovery abilities. These findings lay a foundation for further research on the plasticity of fish in coping with acute temperature changes.

Growth disturbances in bivalve shell: Implications for past and future intra-annual scale climate change

Growth disturbances in bivalve shells are widely observed in both fossil and modern species, yet the conditions and mechanisms driving their formation remain unclear. Through controlled experiments on Mimachlamys nobilis, we demonstrate that abrupt intra-annual temperature fluctuations (≥5 °C) induce shell growth disturbances by altering the energy budget, diverting resources from shell growth to stress responses. Under ≥5 °C fluctuations (Groups C and D), 67 % of the individuals exhibited shell thinning, reduced growth rates, and formed growth disturbance lines (Groups C and D), whereas <5 °C fluctuations (Groups A and B) caused disturbances in only 12 % of the cases. These bivalve growth disturbances serve as a sensitive bio-indicator of short-term temperature fluctuations, providing a novel tool for reconstructing intra-annual level marine climatic fluctuations in deep time.

Physiological energetics of selectively bred oysters (Crassostrea hongkongensis) under marine heatwaves

Marine heatwaves (MHWs) have become more frequent and intense in the context of rapid climate change, causing detrimental effects on marine bivalves and ecosystems they sustain. While selective breeding programs for bivalves can substantially enhance growth performance, their ability to improve thermal stress tolerance remains largely unexplored. Here, we compared physiological energetics of wild and selectively bred Hongkong oysters (Guihao No. 1) under intensifying MHWs conditions. Following two consecutive events of MHWs, selectively bred oysters exhibited around 10% higher survival rate than that of wild oysters. Throughout the course of the experiment, the clearance rate of selectively bred oysters was significantly increased in comparison to wild oysters showing significantly depressed ability to feed. Nevertheless, exposure of selectively bred oysters to MHWs elicited significantly increased oxygen assumption and ammonia excretion rates, which in turn enhanced their O:N ratio. When couched into energetic terms, while MHWs inhibited the individual scope for growth, selectively bred oysters displayed better thermal tolerance than wild oysters. Taken together, our findings highlight the potential of new varieties of selectively bred oysters (such as Guihao No. 2) in coping with intensifying MHWs and guide the future development of selective breeding strategies to enhance the oyster thermal resilience in this era of unprecedented climate change.

Do you remember? Within-generation and transgenerational heat stress memory of recurring marine heatwaves in threespine stickleback

Marine heatwaves can have major and lasting effects on organism physiology and species persistence. Such temperature extremes are increasing in frequency, with consecutive heatwave events already occurring within the lifetime of many organisms. Heat stress memory (thermal priming) by individuals is a potential within-generation response to cope with recurring marine heatwaves. However, whether this form of biological memory can be inherited across generations is not well known. We used a three-generation experiment to investigate individual and transgenerational effects of single and recurring marine heatwaves on fitness-related traits using stickleback (Gasterosteus aculeatus) as a model species. We exposed adults (both sexes) to heatwaves and assessed female reproductive output in both the parent and offspring generation, and offspring (both sexes) survival, growth and behaviour to establish a holistic picture of potential heatwave effects on ectothermic fish. Exposure to single, extreme heatwaves lowered reproductive output, decreased offspring exploratory behaviour, impeded capacity to respond to further thermal stress and reduced long-term survival. However, prior experience of heatwaves (heat stress memory) mitigated some of these effects at both an individual (growth) and transgenerational (fecundity) level, indicating that species experiencing increasing heatwave frequency as part of ongoing climate change may cope better than previously thought.

Behavioral responses of clams to recurrent marine heatwaves

Marine heatwaves (MHWs) have increased strikingly in past decades, leading to dramatic changes in global marine ecosystems. As dominant infaunal species in coastal ecosystems, clams play a critical ecological role, but little is known about their behavioral responses to intensifying MHWs. Here, we investigated behavioral performances and associated gene expressions of an ecologically and economically important clam species, Ruditapes philippinarum, under recurrent scenarios of MHWs. While burrowing behaviors of R. philippinarum were not significantly affected by low-intensity MHWs, its burrowing ability decreased significantly when acutely exposed to MHWs occurring at high-intensity. Virtually unaffected behavioral performances, yet, were shown in clams under repeated scenarios of MHWs, in line with significantly increased expressions of genes closely associated with energy metabolism and behavioral neuroscience. These findings contribute to a better understanding of behavioral responses of infaunal organisms to MHWs and make a leap forward in linking climate change to bioturbation in marine ecosystems.

Extreme heat event influences the toxic impacts of nano-TiO2 with different crystal structures in mussel Mytilus coruscus

The wide use of nano‑titanium dioxide (nano-TiO2) and its ubiquitous emission into aquatic environments are threatening environmental health. Ambient temperature can affect the aggregation state of nano-TiO2 in seawater, thus influencing the intake and physiological effects on marine species. We studied the physiological effects of mixed nano-TiO2 (a mixture of anatase and rutile crystals with an average particle size of 25 nm, P25) on mussels. Subsequently, we investigated the oxidative stress, immunotoxicity, neurotoxicity, and detoxification in Mytilus coruscus exposed to two different crystal structures of nano-TiO2 (anatase and rutile) at 100 μg/L concentration under marine heatwaves (MHWs, 28 °C). MHWs and nano-TiO2 exposure induced neurotoxicity and immune damage and caused dysregulation of redox balance in the gills. Moreover, MHWs exposure disturbed the glutathione system and detoxification function of mussels, resulting in enhanced toxicity of nano-TiO2 under co-exposure. Anatase exposure significantly impaired the antioxidant system and downregulated the relative expression of antioxidant-related genes (Nrf2 and Bcl-2), HSP-90, and immune parameters under MHWs, while producing higher ROS levels compared to rutile. Based on integrated biomarker response (IBR), mussels co-exposed to anatase and MHW showed the highest value (19.29). However, there was no significant difference in bioaccumulation of titanium between anatase (6.07 ± 0.47 μg/g) and rutile (5.3 ± 0.44 μg/g) exposures under MHWs. These results indicate that MHWs would elevate the potential hazard of nanoparticles to marine organisms.

Novel Ser74 of NF-κB/IκBα phosphorylated by MAPK/ERK regulates temperature adaptation in oysters

Phosphorylation of Ser32 and Ser36 controls the degradation of IκBα is the conserved cascade mechanisms of immune core signaling pathway, NF-κB pathway in metazoans, but it's response to abiotic stress and the presence of novel phosphorylation mechanisms in other species remain unclear. Herein, we reported a novel heat-induced phosphorylation site (Ser74) at oysters' major IκBα, which independently regulated ubiquitination-proteasome degradation without the requirement of phosphorylation at S32 and S36. And this site was phosphorylated by ERK/MAPK pathway, which then promoted REL nuclear translocation to activate cell survival related genes to defend heat-stress. The MAPK-NF-κB cascade exhibited divergent thermal responses and adaptation patterns between two congeneric oyster species with differential habitat temperatures, indicating its involvement in shaping temperature adaptation. This study demonstrated that the existence of complex and unique phosphorylation-mediated signaling transduction mechanism in marine invertebrates, and expanded our understanding of the evolution and function of established classical pathway crosstalk mechanisms.

Bivalves under extreme weather events: A comparative study of five economically important species in the South China sea during marine heatwaves

Marine heatwaves (MHWs) are increasing in frequency and intensity, threatening marine organisms and ecosystems they support. Yet, little is known about impacts of intensifying MHWs on ecologically and economically important bivalves cultured in the South China Sea. Here, we compared survival and physiological responses of five bivalve species, Pinctada fucata, Crassostrea angulata, Perna viridis, Argopecten irradians and Paphia undulata, to two consecutive MHWs events (3 days of thermal exposure to + 4 °C or + 8 °C, following 3 days of recovery under ambient conditions). While P. fucata, P. viridis, and P. undulata are native to the South China Sea region, C. angulata and A. irradians are not. Individuals of P. fucata, C. angulata and P. viridis had higher stress tolerance to MHWs than A. irradians and P. undulata, the latter already experiencing 100% mortality under +8 °C conditions during the first event. With increasing intensity of MHWs, standard metabolic rates of all five species increased significantly, in line with significant depressions of function-related energy-metabolizing enzymes (CMA, NKA, and T-ATP). Likewise, activities of antioxidant enzymes (SOD, CAT, and MDA) and shell mineralization-related enzymes (AKP and ACP) responded significantly to MHWs, despite species-specific performances observed. These findings demonstrate that some bivalve species can likely fail to accommodate intensifying MHWs events in the South China Sea, but some may persist. If this is the case, then one would expect substantial loss of fitness in bivalve aquaculture in the South China Sea under intensifying MHWs conditions.

Characterization of MKK family genes and their responses to temperature stress and Vibrio parahaemolyticus infection in noble scallop Chlamys nobilis

Mitogen-activated protein kinase kinase (MKK), the key element of the Mitogen-activated protein kinase (MAPK) signaling pathway, is crucial for the immune response to adverse environments in aquatic animals. Nevertheless, there is limited information regarding the role of the MKK gene family in mollusks. In our study, genome data and transcriptome were used to identify four MKK genes (CnMKK4, CnMKK5, CnMKK6, and CnMKK7) in the noble scallop. The result of the gene structure, motif analysis, and phylogenetic tree revealed that MKK genes are relatively conserved in bivalves. Moreover, four CnMKK genes were significantly highly expressed in immune-related tissues, suggesting that CnMKKs may related to bivalve immunity. Furthermore, CnMKK6 and CgMKK4 were significantly differentially expressed (P < 0.05) under 24 h of temperature stress, and all CnMKKs were significantly differentially expressed (P < 0.05) under 24 h of Vibrio parahaemolyticus infection. These results showed that the CnMKKs may have a significant impact under biotic and abiotic stresses. In conclusion, the result of the CnMKKs provides valuable insights into comprehending the function of MKK genes in mollusks.

Metabolic dysfunctions in pearl oysters following recurrent marine heatwaves

Marine heatwaves (MHWs) have become more frequent, intense and extreme in oceanic systems in the past decade, resulting in mass mortality events of marine invertebrates and devastating coastal marine ecosystems. While metabolic homeostasis is a fundamental requirement in stress tolerance, little is known about its role under intensifying MHWs conditions. Here, we investigated impacts of MHWs on the metabolism in pearl oysters (Pinctada maxima) - an ecologically and economically significant bivalve species in tropical ecosystems. Activities of digestive enzymes (gastric proteases, lipases, and amylases) did not significantly respond to various scenario of recurrent MHWs varying from 24 °C to 28 °C (moderate) and 32 °C (severe). The metabolomics analysis revealed nine and five key metabolism pathways under both MHWs scenarios. Specifically, pathways associated with energy metabolism were impaired by moderate MHWs, manifesting in downregulation of differential metabolite (The nicotinic acid and N-acetyl-glutamic acid). The content of CDP-ethanolamine was significantly decrease, and the perturbations of oxidative stress caused by the decreased of content of D-glutamine. Metabolites related to a suite of body functions (e.g., the lipid metabolism, biomineralization, and antioxidant defenses) showed significantly negative responses by severe MHWs. These findings reveal the metabolic impairments of marine bivalves when subjected to MHWs varying in intensity and frequency, implying cascading consequences which deserve further investigation.

MAPK/ERK-PK(Ser11) pathway regulates divergent thermal metabolism of two congeneric oyster species

Pyruvate kinase (PK), as a key rate-limiting enzyme in glycolysis, has been widely used to assess the stress tolerance and sensitivity of organisms. However, its phosphorylation regulatory mechanisms mainly focused on human cancer research, with no reports in marine organisms. In this study, we firstly reported a conserved PK Ser11 phosphorylation site in mollusks, which enhanced enzyme activity by promoting substrate binding, thereby regulating divergent thermal metabolism of two allopatric congeneric oyster species with differential habitat temperature. It was phosphorylated by ERK kinase, and regulated by the classical MAPK pathway. The MAPK/ERK-PK signaling cascade responded to increased environmental temperature and exhibited stronger activation pattern in the relatively thermotolerant species (Crassostrea angulata), indicating its involvement in shaping temperature adaptation. These findings highlight the presence of complex and unique phosphorylation-mediated signaling transduction mechanisms in marine organisms, and provide new insights into the evolution and function of the crosstalk between classical pathways.

Reductive stress and cytotoxicity in the swollen river mussel (Unio tumidus) exposed to microplastics and salinomycin

Multistress effects lead to unpredicted consequences in aquatic ecotoxicology and are extremely concerning. The goal of this study was to trace how specific effects of the antibiotic salinomycin (Sal) and microplastics (MP) on the bivalve molluscs are manifested in the combined environmentally relevant exposures. Unio tumidus specimens were treated with Sal (0.6 μg L-1), MP (1 mg L-1, 2 μm size), and both at 18 °C (Mix) and 25 °C (MixT) for 14 days. The redox stress and apoptotic enzyme responses and the balance of Zn/Cu in the digestive gland were analyzed. The shared signs of stress included a decrease in NAD+/NADH and Zn/Cu ratios and lysosomal integrity and an increase in Zn-metallothioneins and cholinesterase levels. MP caused a decrease in the glutathione (GSH) concentration and redox state, total antioxidant capacity, and Zn levels. MP and Mix induced coordinated apoptotic/autophagy activities, increasing caspase-3 and cathepsin D (CtD) total and extralysosomal levels. Sal activated caspase-3 only and increased by five times Cu level in the tissue. Due to the discriminant analysis, the cumulative effect was evident in the combined exposure at 18 °C. However, under heating, the levels of NAD+, NADH, GSH, GSH/GSSG and metallothionein-related thiols were decreased, and coordination of the cytosolic and lysosomal death stimuli was distorted, confirming that heating and pollution could exert unexpected synergistic effects on aquatic life.

Molecular and behavioural responses of the mussel Mytilus edulis exposed to a marine heatwave

Marine heatwaves (MHW) threaten marine organisms and tend to increase in frequency and intensity. We exposed the blue mussel Mytilus edulis to a MHW lasting 23 days, including two 10-d periods of thermal intensity increase of +5 °C (20 °C-25 °C) interspersed by 1 day back to 20 °C, followed by a 4-d recovery period. We investigated behaviour responses of mussels and gene expression changes relative to the circadian rhythm (Per), oxidative stress (SOD), cellular apoptosis (CASP3), energy production (ATPs), and general stress response (hsp70). Results showed that the MHW disturbed the valve activity of mussels. Particularly, mussels increased the number of valve micro-closures, showing a stressful state of organisms. Mussels also decreased Per, CASP3, ATPs, and Hsp70 gene expression. Some behavioural and molecular effects persisted after the MHW, suggesting a limited recovery capacity of individuals. This work highlighted the vulnerability of M. edulis to a realistic MHW.

Ocean acidification stunts molluscan growth at CO2 seeps

Ocean acidification can severely affect bivalve molluscs, especially their shell calcification. Assessing the fate of this vulnerable group in a rapidly acidifying ocean is therefore a pressing challenge. Volcanic CO₂ seeps are natural analogues of future ocean conditions that offer unique insights into the scope of marine bivalves to cope with acidification. Here, we used a 2-month reciprocal transplantation of the coastal mussel Septifer bilocularis collected from reference and elevated pCO₂ habitats to explore how they calcify and grow at CO₂ seeps on the Pacific coast of Japan. We found significant decreases in condition index (an indication of tissue energy reserves) and shell growth of mussels living under elevated pCO₂ conditions. These negative responses in their physiological performance under acidified conditions were closely associated with changes in their food sources (shown by changes to the soft tissue δ¹³C and δ¹⁵N ratios) and changes in their calcifying fluid carbonate chemistry (based on shell carbonate isotopic and elemental signatures). The reduced shell growth rate during the transplantation experiment was further supported by shell δ¹³C records along their incremental growth layers, as well as their smaller shell size despite being of comparable ontogenetic ages (5-7 years old, based on shell δ¹⁸O records). Taken together, these findings demonstrate how ocean acidification at CO₂ seeps affects mussel growth and reveal that lowered shell growth helps them survive stressful conditions.

Survival and physiological energetics of highly invasive mussels exposed to heatwaves

The South American mussel, Mytella strigata, is a highly invasive fouling species of great concern along intertidal shores in East and Southeast Asia, posing serious threats to native biodiversity and ecosystems. Intertidal areas, being increasingly attacked by heatwaves over the last decade, are among the most thermally challenging habitats, yet the fate of this highly invasive mussel under scenarios of hotter heatwaves remains unknown. Here, we investigated how M. strigata responded to intensifying heatwaves frequently occurring in the South China Sea. Over 97% of individuals survived the five-day-lasting heatwaves, suggesting their high ability to cope with short-term heatwaves. Virtually unaffected clearance rate and absorption efficiency throughout the course of heatwaves indicate the maintenance of energy acquisition, and significantly decreased respiration rate implies the depression of energy metabolism, generating significant decreases in the O:N ratio when heatwaves occurred. Scope for growth of heatwaves-stressed mussels significantly decreased during initial exposure and then increased over time. These findings indicate the remarkable ability of M. strigata to cope with heatwaves recorded in its invasive habitats and call the attention for the rapid spread of this highly invasive fouling species in the context of climate change.

Differential gene expression analysis in the scallop Argopecten purpuratus exposed to altered pH and temperature conditions in an upwelling-influenced farming area

Increased carbon dioxide in the atmosphere and its absorption across the ocean surface will alter natural variations in pH and temperature levels, occurring in coastal upwelling ecosystems. The scallop Argopecten purpuratus, one of the most economically important species farmed in northern Chile, has been shown to be vulnerable to these environmental drivers. However, the regulatory responses at the gene-level of scallops to these climate stressors remain almost unknown. Consequently, we used an orthogonal experimental design and RNAseq approach to analyze the acute effects of variability in pH and temperature on gene expression in the muscle tissue of A. purpuratus. In respect to control conditions (pH ~ 8.0/ 14 °C), the influence of low pH (~ 7.7) and temperature (14 °C) induced the activation of several genes associated with apoptotic signaling pathways and protein localization to plasma membrane. Elevated temperature (18 °C) and pH (~8.0) conditions increased the expression of transcripts associated with the activation of muscle contraction, regulation, and sarcomere organization effects on muscle tissue. In scallops exposed to low pH and elevated temperature, the genes expressed were differentially associated with the oxidation-reduction process, signal translation, and positive regulation of GTPase activity. These results indicated that the differentially expressed genes under the experimental conditions tested are mainly related to the mitigation of cellular damage and homeostasis control. Our results add knowledge about the function of the adductor muscle in response to stressors in scallops. Furthermore, these results could help in the identification of molecular biomarkers of stress necessary to be integrated into the aquaculture programs for the mitigation of climate change.

Impacts of marine heatwaves on byssus production in highly invasive fouling mussels

Marine heatwaves (MHWs) are projected to increase in their frequency, intensity, and duration, causing irreversible and catastrophic consequences for intertidal ecosystems around the world. The highly invasive fouling mussel, Arcuatula senhousia, can cause marked habitat alteration by constructing extremely intense byssal mats, devastating the biodiversity of many intertidal systems, yet very little is known about its fate under conditions of more frequent, hotter and longer MHWs. Here, we assessed impacts of two scenarios of MHWs (low-intensity with 4 °C rise of seawater temperature and high-intensity with 8 °C rise, respectively) on the byssal production of A. senhousia. Mussels exposed to low-intensity MHWs did not show any significant differences in the number, length and diameter of byssal threads, compared with those not thermally stressed. Under high-intensity scenario, the byssus production was significantly depressed, and byssal threads became fewer, shorter and finer, in line with significant decreases in cumulative length and volume. These findings provide a better understanding of responses of invasive fouling mussels such as A. senhousia to MHWs and make a leap forward in linking climate change and biological fouling in marine ecosystems.

Responses of digestive metabolism to marine heatwaves in pearl oysters

Marine heatwaves (MHWs) have increased in intensity and frequency in global oceans, causing deleterious effects on many marine organisms and ecosystems they support. Bivalves are among the most vulnerable taxonomic groups to intensifying MHWs, yet little is known about the underlying mechanisms. Here, we investigated the impact of MHWs on the digestive metabolism of pearl oysters (Pinctada maxima). Two moderate and severe scenarios of MHWs were performed by increasing seawater temperature respectively from 24 °C to 28 °C and 32 °C for 3 days. When subjected to MHWs and with increasing intensity, pearl oysters significantly enhanced their digestive enzymatic activities, such as lipase and amylase. LC-MS-based metabolomics revealed negative responses in the lipid metabolism (e.g., steroid biosynthesis, glycerophospholipid metabolism, and sphingolipid metabolism), the amino acid metabolism (e.g., glutamate, histidine, arginine, and proline), and the B-vitamins metabolism. These findings indicate that the digestive metabolism of marine bivalves can likely succumb to intensifying MHWs events.

Post-responses of intertidal bivalves to recurrent heatwaves

Heatwaves are becoming hotter, longer and more frequent, threatening the survival of intertidal bivalves and devastating their ecosystems. Yet, substantially overlooked are heatwave-induced post-responses, which are important to assess cascading consequences. Here, we investigated responses of intertidal bivalves, Ruditapes philippinarum, to recurrent heatwaves. Physiological and gene expression analyses demonstrated that the mantle tissue of R. philippinarum did not sensitively respond to heatwaves, but revealed post-responses under recovery scenarios. Of 20 genes related to essential physiology and fitness, 18 were down-regulated during the 1st recovery period, but following repeated exposure, 13 genes were up-regulated, in line with significantly increased activities of energy-metabolizing enzymes, and antioxidant and nonspecific enzymes. The down-regulation of genes involved in biomineralization, nevertheless, was observed under recovery scenarios, implying the trade-off between essential physiological and fitness-related functions. These findings pave the way for understanding the physiological plasticity of marine bivalves in response to intensifying heatwaves.