Impaired reproduction, energy reserves and dysbiosis: The overlooked consequences of heatwaves in a bivalve mollusc

Transcriptional responses of Mediterranean mussels (Mytilus galloprovincialis) under the 2022 Marine Heatwave: a trade-off of physiological regulation between metabolism, stress response, and shell biomineralization in a mixed exposure scenario

There has been a notable increase in occurrence and intensity of marine heatwaves (MHWs) over the past decades, with a consequent remarkable risk to vulnerable species as marine bivalves. This study examines the responses of farmed Mytilus galloprovincialis to the 2022 MHW that impacted the Northwestern (NW) Adriatic Sea. Expression of key transcripts involved in functions of digestive glands and mantles were investigated to explore the putative acclimatory processes contributing to mussel fitness. The 2022 MHW was characterized by persistent sea temperature anomalies, elevated salinity, and dramatically low chlorophyll-a levels. Despite the temporal trends of pH and the extreme seawater temperatures reached in July and August, the carbonate system never reached the undersaturation state, being favourable for bivalve biomineralization. Transcriptional profiles in digestive glands and mantles displayed a two-step temporal response. In digestive glands, metabolism and lysosomal response functional categories showed an initial decrease (late May), and a recovery in late August. Antioxidant and cytoprotective related gene products showed a February to August increased expression, with strong up-regulations in August. In mantles, transcripts involved in shell biomineralization were prompted in the initial stage of the MHW, likely to withstand the abrupt changes of seawater parameters and to maintain bivalve growth. At high MHW intensities, energy was diverted towards the strong stress response activation in digestive glands, with a relative decrease of mRNA levels for shell biomineralization transcripts. Results showed that a trade-off between core physiological processes may contribute to the acclimatory response of mussels to cope with the adverse conditions of the 2022 MHW in the NW Adriatic Sea.

Presence and characterization of the human pathogenic Vibrio species in the microbiota of Manila clams using cultural and molecular methods

The North Adriatic lagoons and the Po River Delta are important areas for farming Manila clams (Ruditapes philippinarum). These areas have been heavily impacted by climate change, reducing livestock numbers and increasing pathogen spread. Shellfish, particularly clams, are primary vectors for Vibrio pathogens affecting humans. In this study, the occurrence of human pathogenic Vibrio species on Manila clams was investigated using an integrated approach that combined culture-dependent and culture-independent techniques. Samples were collected over three years from farming areas in the northeastern Adriatic lagoons and the Po River Delta, regions seriously impacted by climate change and pollution. In this study, species of the human pathogen Vibrio were analyzed in the clam microbiota and characterized using recA-pyrH metabarcoding and shotgun metagenomics. Human pathogenic Vibrio species were widespread in the clam microbiota, especially in summer, demonstrating that the environmental conditions on the northern Adriatic coasts allowed the growth of these bacteria. V. parahaemolyticus and V. vulnificus were also quantified using qPCR in <50 % of summer samples Shotgun metagenomics revealed the similarity of V. parahaemolyticus strains to other worldwide genomes, enabling improved pathogen identification and tracking. In the future, climate change could cause these conditions to become even more favorable to these bacteria, potentially increasing pathogen spread. Consequently, enhanced monitoring and control of both the environment and seafood products should be planned to ensure food safety.

Multi-stressor dynamics: Effects of marine heatwave stress and harmful algal blooms on juvenile mussel (Perna canaliculus) survival and physiology

In New Zealand, the frequency and intensity of marine heatwaves (MHWs) and blooms of the harmful algal species, Alexandrium pacificum, are increasing in areas where there are natural reefs and commercial farms of the mussel, Perna canaliculus. In this study, we assessed the whole organism, tissue and molecular-level response of juvenile (spat) P. canaliculus exposed to these abiotic and biotic stressors, alone and together. Spat of P. canaliculus were held at a control temperature of 17 °C or a MHW temperature of 22 °C for the duration of the experiment and exposed to two environmentally relevant concentrations of the dinoflagellate A. pacificum for 4 days, followed by a recovery period of 4 days. The MHW temperature alone had no detrimental effect on mussel survival; however, exposure to A. pacificum at both temperatures led to reduced spat survival, especially at higher microalgal concentrations (< 90 % survival vs 100 % at the control temperature, no A. pacificum). The combination of both MHW temperature and A. pacificum exposure resulted in higher mortality than either stressor alone (as low as 59 % survival compared to 99 % at the MHW temperature alone). Spat exposed to A. pacificum alone showed up to a 4-fold reduction in byssal plaque production, crucial for spat attachment and therefore survival. Growth rate was >7.5 times slower, and there were increased histological alterations as compared to mussels in the control treatment. Similarly, spat exposed to A. pacificum showed significant changes in gene expression, particularly in pathways related to lipid metabolism and detoxification. Spat exposed to MHW temperatures alone had fewer differentially expressed genes, most being heat shock proteins. These findings emphasise the importance of understanding multi-stressor impacts in marine environments, particularly in the context of climate change and harmful algal blooms (HABs).

Transcriptional modulation in Mediterranean Mussel Mytilus galloprovincialis following exposure to four pharmaceuticals widely distributed in coastal areas

Ecotoxicological risk and the mode of action of human drugs on non-target marine animals remain unclear, keeping a gap of knowledge on risks related to ecosystem disruption and chemical contamination of food chains. Understanding these impacts is critical to developing proper waste management practices and regulatory frameworks to prevent long-term environmental and human health problems. This study investigates the impacts of Gemfibrozil, Metformin, Ramipril, and Venlafaxine, individually and combined on Mytilus galloprovincialis over 30 days and assesses persistent effects post-recovery using RNA-seq and 16S rRNA microbiota profiling. All pharmaceuticals caused few changes in the microbiota while gene expression analyses highlighted drug-specific alterations. Gemfibrozil exposure led to alterations in lipid and fatty acid metabolism, suggesting a similar mode of action to that observed in target species. Metformin significantly impacted the mussels' energy metabolism, with disruptions in specific genes and pathways potentially related to glucose uptake and insulin signaling. Metformin was also the treatment leading to the most significant changes in predicted functional profiles of the microbiota, suggesting that it may influence the microbiota's potential to interact with host glucose metabolism. Ramipril exposure resulted in the up-regulation of stress response and cell cycle regulation pathways and Venlafaxine induced changes in serotonin and synapse pathways, indicating potential similarities in mechanisms of action with target species. Mixture of the four pharmaceuticals severely impacted mussel physiology, including impairment of oxidative phosphorylation and compensatory activation of several pathways involved in energy metabolism. Despite recovery after depuration, changes in stress and energy related metabolism pathways suggests potential persistent effects from combined pharmaceutical exposure. Notably, the up-regulation of mTOR1 signaling in all treatments after 30 days underscores its key role in coordinating bivalve stress responses. The Transcriptomic Hazard Index (THI) calculated for each treatment indicates major/severe hazards after exposure that decreased to slight/moderate hazards after depuration.

Ecophysiological responses to heat waves in the marine intertidal zone

One notable consequence of climate change is an increase in the frequency, scale and severity of heat waves. Heat waves in terrestrial habitats (atmospheric heat waves, AHW) and marine habitats (marine heat waves, MHW) have received considerable attention as environmental forces that impact organisms, populations and whole ecosystems. Only one ecosystem, the intertidal zone, experiences both MHWs and AHWs. In this Review, we outline the range of responses that intertidal zone organisms exhibit in response to heat waves. We begin by examining the drivers of thermal maxima in intertidal zone ecosystems. We develop a simple model of intertidal zone daily maximum temperatures based on publicly available tide and solar radiation models, and compare it with logged, under-rock temperature data at an intertidal site. We then summarize experimental and ecological studies of how intertidal zone ecosystems and organisms respond to heat waves across dimensions of biotic response. Additional attention is paid to the impacts of extreme heat on cellular physiology, including oxidative stress responses to thermally induced mitochondrial overdrive and dysfunction. We examine the energetic consequences of these mechanisms and how they shift organismal traits, including growth, reproduction and immune function. We conclude by considering important future directions for improving studies of the impacts of heat waves on intertidal zone organisms.

"Groundbreaking study: Combined effect of marine heatwaves and polyethylene microplastics on Pacific oysters, Crassostrea gigas"

Microplastics (MPs) and rising marine seawater temperatures are one of the major environmental problems threatening the survival of marine organisms and biodiversity. However, interactions between such multiple stressors are virtually unexplored. This study aimed to assess the combined effect of two temperatures and polyethylene MPs on the Pacific oyster Crassostrea gigas, one of the most globalized mollusc species for aquaculture. Our work highlights the potential ecological risk posed by these two factors on marine bivalve molluscs. The experimental design was carried out following a 14-day exposure of oysters to environmental concentrations of polyethylene MPs (0.01 mg.L-1), and to two temperatures (15 °C and 22 °C). Sampling was performed on days 0, 7, and 14. The μ-FTIR analysis was applied to quantify MPs of interest and to check a potential environmental contamination. Tissue samples of digestive glands were collected from the oysters to evaluate the activity of biomarkers including superoxide dismutase, glutathione-s-transferase, malondialdehyde and laccase through protein levels. We note that the combination of MPs and high water temperature (HWT, 22 °C) had a significant impact both on the survival of animals and on stress markers, by modifying lipid peroxidation and immune responses. This original study gave the first innovative results on this topic and provides us with knowledge of the combined effects of MPs pollution and HWT (simulating marine heatwaves situation) on C. gigas. There remains a lack of information on the toxicity and the potential environmental hazard of plastics in the marine environment.

Physiological plasticity and life history traits affect Chamelea gallina acclimatory responses during a marine heatwave

The striped venus clam (Chamelea gallina) is a relevant economic resource in the Adriatic Sea. This study explored the physiological status of C. gallina at four sites selected along a gradient from high to low incidence of recorded historical mortality events and low to high productivity in the Northwestern Adriatic Sea. Investigations were performed during the marine heatwave in 2022 (from July to November). The optimal temperature range for C. gallina was exceeded in July and September, exacerbating stress conditions and a poor nutritional status, particularly at the low productivity sites. Transcriptional profiles assessed in digestive glands showed that clams from the low productivity sites up-regulated transcripts related to feeding/digestive functions as a possible compensatory mechanism to withstand adverse environmental conditions. Clams from the high productivity sites, that in a previous study showed enrichment of health-promoting microbiome components, displayed a healthier metabolic makeup (IDH up-regulation) and induction of protective antioxidant and immune responses. These features are hallmarks of putative enhanced resilience of the species towards environmental stress. Despite the well-known high sensitivity of C. gallina to environmental variations and its narrow window of acclimatory potential, results highlight that local conditions may influence physiological plasticity of this clam species and shape either positively or negatively its response capabilities to environmental changes. The identification of health-promoting endogenous mechanisms both from the animal (this study) and from its associated microbiome may provide the foundation for developing novel tools and strategies to improve clam health and production in low productivity areas or under adverse environmental conditions.

TORC1 Regulates Thermotolerance via Modulating Metabolic Rate and Antioxidant Capacity in Scallop Argopecten irradians irradians

Target of rapamycin complex 1 (TORC1) is a key regulator of metabolism in eukaryotes across multiple pathways. Although TORC1 has been extensively studied in vertebrates and some invertebrates, research on this complex in scallops is limited. In this study, we identified the genes encoding TORC1 complex subunits in the scallop Argopecten irradians irradians through genome-wide in silico scanning. Five genes, including TOR, RAPTOR, LST8, DEPTOR, and PRAS40, that encode the subunits of TORC1 complex were identified in the bay scallop. We then conducted structural characterization and phylogenetic analysis of the A. i. irradians TORC1 (AiTORC1) subunits to determine their structural features and evolutionary relationships. Next, we analyzed the spatiotemporal expressions of AiTORC1-coding genes during various embryo/larvae developmental stages and across different tissues in healthy adult scallops. The results revealed stage- and tissue-specific expression patterns, suggesting diverse roles in development and growth. Furthermore, the regulation of AiTORC1-coding genes was examined in temperature-sensitive tissues (the mantle, gill, hemocyte, and heart) of bay scallops exposed to high-temperature (32 °C) stress over different durations (0 h, 6 h, 12 h, 24 h, 3 d, 6 d, and 10 d). The expression of AiTORC1-coding genes was predominantly suppressed in the hemocyte but was generally activated in the mantle, gill, and heart, indicating a tissue-specific response to heat stress. Finally, functional validation was performed using the TOR inhibitor rapamycin to suppress AiTORC1, leading to an enhanced catabolism, a decreased antioxidant capacity, and a significant reduction in thermotolerance in bay scallops. Collectively, this study elucidates the presence, structural features, evolutional relationships, expression profiles, and roles in antioxidant capacity and metabolism regulation of AiTORC1 in the bay scallop, providing a preliminary understanding of its versatile functions in response to high-temperature challenges in marine mollusks.

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.

Comparative Analyses of Dynamic Transcriptome Profile of Heart Highlight the Key Response Genes for Heat Stress in Zhikong Scallop Chlamys farreri

Heat stress resulting from global climate change has been demonstrated to adversely affect growth, development, and reproduction of marine organisms. The Zhikong scallop (Chlamys farreri), an important economical mollusk in China, faces increasing risks of summer mortality due to the prolonged heat waves. The heart, responsible for transporting gas and nutrients, is vital in maintaining homeostasis and physiological status in response to environmental changes. In this study, the effect of heat stress on the cardiac function of C. farreri was investigated during the continuous 30-day heat stress at 27 °C. The results showed the heart rate of scallops increased due to stress in the initial phase of high temperature exposure, peaking at 12 h, and then gradually recovered, indicating an acclimatization at the end of the experiment. In addition, the levels of catalase (CAT), superoxide dismutase (SOD), and total antioxidant capacity (T-AOC) exhibited an initial increase followed by recovery in response to heat stress. Furthermore, transcriptome analysis of the heart identified 3541 differentially expressed genes (DEGs) in response to heat stress. Subsequent GO and KEGG enrichment analysis showed that these genes were primarily related to signal transduction and oxidative stress, such as the phosphatidylinositol signaling system, regulation of actin cytoskeleton, MAPK signaling pathway, FoxO signaling pathway, etc. In addition, two modules were identified as significant responsive modules according to the weighted gene co-expression network analysis (WGCNA). The upregulation of key enzymes within the base excision repair and gap junction pathways indicated that the heart of C. farreri under heat stress enhanced DNA repair and maintained cellular integrity. In addition, the variable expression of essential signaling molecules and cytoskeletal regulators suggested that the heart of C. farreri modulated cardiomyocyte contraction, intracellular signaling, and heart rate through complex regulation of phosphorylation and calcium dynamics in response to heat stress. Collectively, this study enhances our understanding of cardiac function and provides novel evidence for unraveling the mechanism underlying the thermal response in mollusks.

Transcriptomic insights into cessation of clam embryonic development following transgenerational exposure to ocean acidity extreme

Ocean acidity extremes (OAX) events are becoming more frequent and intense in coastal areas in the context of climate change, generating widespread consequences on marine calcifying organisms and ecosystems they support. While transgenerational exposure to end-of-century scenario of ocean acidification (i.e., at pH 7.7) can confer calcifiers resilience, whether and to what extent such resilience holds true under OAX conditions is still poorly understood. Here, we found that transgenerational exposure of Ruditapes philippinarum to OAX resulted in cessation of embryonic development at the trochophore stage, implying devastating consequences of OAX on marine bivalves. We identified a large number of differentially expressed genes in embryos following transgenerationally exposed to OAX, which were mainly significantly enriched in KEGG pathways related to energy metabolism, immunity and apoptosis. These pathways were significantly activated, and genes involved in these processes were up-regulated, indicating strong cellular stress responses to OAX. These findings demonstrate that transgenerational exposure to OAX can result in embryonic developmental cessation by severe cellular damages, implying that transgenerational acclimation maybe not a panacea for marine bivalves to cope with OAX, and hence urgent efforts are required to understand consequences of intensifying OAX events in coastal ecosystems.

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.

Hiding from heat: The transcriptomic response of two clam species is modulated by behaviour and habitat

Rising occurrence of extreme warming events are profoundly impacting ecosystems, altering their functioning and services with significant socio-economic consequences. Particularly susceptible to heatwaves are intertidal shellfish beds, located in estuarine areas already stressed by factors such as rainfall events, red tides, eutrophication, and pollution. In Galicia, Northwestern Spain, these beds support vital shellfisheries, featuring the native clam Ruditapes decussatus and the non-indigenous R. philippinarum. Over recent decades, these populations have experienced notable abundance shifts due to various anthropogenic impacts, including climate change. In this habitat, patches of the seagrass Zostera noltei that coexist with bare sand can act as thermal refuges for benthic organisms such as clams. To assess the impact of heatwaves on these ecosystems, a mesocosm experiment was conducted. Juveniles of both clam species in two habitat types-bare sand and sand with Z. noltei-were exposed to simulated atmospheric heatwaves during diurnal low tide for four consecutive days. Subsequent transcriptomic analysis revealed that high temperatures had a more pronounced impact on the transcriptome of R. philippinarum compared to R. decussatus. The habitat type played a crucial role in mitigating heat stress in R. philippinarum, with the presence of Z. noltei notably ameliorating the transcriptomic response. These findings have direct applications in shellfishery management, emphasizing the importance of preserving undisturbed patches of Z. noltei as thermal refuges, contributing to the mitigation of heatwave effects on shellfish populations.