Bivalve immunity and response to infections: Are we looking at the right place?

Histological, biochemical and immunohistochemical assessments of Roundup®, atrazine, and 2,4-D mixtures on tissue architecture, body fluid conditions, nitrotyrosine protein and Na+/K+-ATPase expressions in the American oyster, Crassostera virginica

Pesticides are widely used to control weeds and pests in agricultural settings but harm non-target aquatic organisms. In this study, our objective was to evaluate the effect of short-term exposure (one week) to environmentally relevant concentrations of pesticides mixture (low concentration: 0.4 μg/l atrazine, 0.5 μg/l Roundup®, and 0.5 μg/l 2,4-D; high concentration: 0.8 μg/l atrazine, 1 μg/l Roundup®, and 1 μg/l 2,4-D) on tissue architecture, body fluid conditions, and 3-nitrotyrosine protein (NTP) and Na+/K+-ATPase, expressions in tissues of American oyster (Crassostrea virginica) under controlled laboratory conditions. Histological analysis demonstrated the atrophy in the gills and digestive glands of oysters exposed to pesticides mixture. Periodic acid-Schiff (PAS) staining showed the number of hemocytes in connective tissue increased in low- and high-concentration pesticides exposure groups. However, pesticides treatment significantly (P < 0.05) decreased the amount of mucous secretion in the gills and digestive glands of oysters. The extrapallial fluid (i.e., body fluid) protein concentrations and glucose levels were dropped significantly (P < 0.05) in oysters exposed to high-concentration pesticides exposure groups. Moreover, immunohistochemical analysis showed significant upregulations of NTP and Na+/K+-ATPase expressions in the gills and digestive glands in pesticides exposure groups. Our results suggest that exposure to environmentally relevant pesticides mixture causes morphological changes in tissues and alters body fluid conditions and NTP and Na+/K+-ATPase expressions in tissues, which may lead to impaired physiological functions in oysters.

Potential threats of microplastics and pathogenic bacteria to the immune system of the mussels Mytilus galloprovincialis

As one of the main components of marine pollution, microplastics (MPs) inevitably enter the mussel aquaculture environment. At the same time, pathogenic bacteria, especially pathogens such as Vibrio, can cause illness outbreaks, leading to large-scale death of mussels. The potential harm of MPs and pathogenic bacteria to bivalve remains unclear. This study designed two experiments (1) mussels (Mytilus galloprovincialis) were exposed to 100 particles/L or 1,000 particles/L polymethyl methacrylate (PMMA, 17.01 ± 6.74 μm) MPs and 1 × 107 CFU/mL Vibrio parahaemolyticus at the same time (14 days), and (2) mussels were exposed to 100 particles/L or 1,000 particles/L MPs for a long time (30 days) and then exposed to 1 × 107 CFU/mL V. parahaemolyticus to explore the effects of these two stresses on the mussel immune system. The results showed that after the combined exposure of V. parahaemolyticus and MPs, the lysosomal membrane stability of hemocytes decreased, lysozyme activity was inhibited, and hemocytes were induced to produce more lectins and defensins to fight pathogenic invasion. Long-term exposure to MPs caused a large amount of energy consumption in mussels, inhibited most of the functions of humoral immunity, increased the risk of mussel infection with pathogenic bacteria, and negatively affected mussel condition factor, the number of hemocytes, and the number of byssuses. Mussels may allocate more energy to deal with MPs and pathogenic bacterial infections rather than for growth. Above all, MPs exposure can affect mussel immune function or reduce its stress resistance, which in turn has an impact on mollusk farming.

Comparative transcriptomics revealed parallel evolution and innovation of photosymbiosis molecular mechanisms in a marine bivalve

Photosymbioses between heterotrophic hosts and autotrophic symbionts are evolutionarily prevalent and ecologically significant. However, the molecular mechanisms behind such symbioses remain less elucidated, which hinders our understanding of their origin and adaptive evolution. This study compared gene expression patterns in a photosymbiotic bivalve (Fragum sueziense) and a closely related non-symbiotic species (Trigoniocardia granifera) under different light conditions to detect potential molecular pathways involved in mollusc photosymbiosis. We discovered that the presence of algal symbionts greatly impacted host gene expression in symbiont-containing tissues. We found that the host immune functions were suppressed under normal light compared with those in the dark. In addition, we found that cilia in the symbiont-containing tissues play important roles in symbiont regulation or photoreception. Interestingly, many potential photosymbiosis genes could not be annotated or do not exhibit orthologues in T. granifera transcriptomes, indicating unique molecular functions in photosymbiotic bivalves. Overall, we found both novel and known molecular mechanisms involved in animal-algal photosymbiosis within bivalves. Given that many of the molecular pathways are shared among distantly related host lineages, such as molluscs and cnidarians, it indicates that parallel and/or convergent evolution is instrumental in shaping host-symbiont interactions and responses in these organisms.

Transcriptomics, proteomics, and physiological assays reveal immunosuppression in the eastern oyster Crassostrea virginica exposed to acidification stress

Ocean acidification (OA) is recognized as a major stressor for a broad range of marine organisms, particularly shell-building invertebrates. OA can cause alterations in various physiological processes such as growth and metabolism, although its effect on host-pathogen interactions remains largely unexplored. In this study, we used transcriptomics, proteomics, and physiological assays to evaluate changes in immunity of the eastern oyster Crassostrea virginica exposed to OA conditions (pH = 7.5 vs pH = 7.9) at various life stages. The susceptibility of oyster larvae to Vibrio infection increased significantly (131 % increase in mortality) under OA conditions, and was associated with significant changes in their transcriptomes. The significantly higher mortality of larvae exposed to pathogens and acidification stress could be the outcome of an increased metabolic demand to cope with acidification stress (as seen by upregulation of metabolic genes) at the cost of immune function (downregulation of immune genes). While larvae were particularly vulnerable, juveniles appeared more robust to the stressors and there were no differences in mortality after pathogen (Aliiroseovarius crassostrea and Vibrio spp.) exposure. Proteomic investigations in adult oysters revealed that acidification stress resulted in a significant downregulation of mucosal immune proteins including those involved in pathogen recognition and microbe neutralization, suggesting weakened mucosal immunity. Hemocyte function in adults was also impaired by high pCO2, with a marked reduction in phagocytosis (67 % decrease in phagocytosis) in OA conditions. Together, results suggest that OA impairs immune function in the eastern oyster making them more susceptible to pathogen-induced mortality outbreaks. Understanding the effect of multiple stressors such as OA and disease is important for accurate predictions of how oysters will respond to future climate regimes.

A Microcosm Experiment Reveals the Temperature-Sensitive Release of Mucochytrium quahogii (=QPX) from Hard Clams and Pallial Fluid as a Stable QPX Reservoir

Mucochytrium quahogii, also known as QPX or Quahog Parasite Unknown, is the causative agent of QPX disease in the hard clam (Mercenaria mercenaria). Host-pathogen-environment interactions between M. quahogii, the hard clam, and temperature were explored in a microcosm experiment. Hard clams were housed in individual tanks with sterile seawater under two temperature regimes: low (13 °C) temperature, which is thought to be optimal for QPX disease development, and high (20 °C) temperature, which has been shown to promote "healing" of QPX-infected clams. Hard clam tissue, pallial fluid, seawater, and shell biofilms were collected and assayed for M. quahogii. The release of M. quahogii from naturally infected live hard clams into seawater was detected only in the low temperature treatment, suggesting that temperature influences the release of potentially infectious cells. M. quahogii was commonly found in hard clam pallial fluid, even after 9 weeks in the lab, suggesting pallial fluid is a stable reservoir of M. quahogii within its primary host and that M. quahogii is not a transient component of the hard clam microbiota. Overall, results support a host-specific relationship and that M. quahogii is a commensal member of the hard clam microbiota, supporting its classification as an opportunistic pathogen.

GST-Mu of Cristaria plicata is regulated by Nrf2/Keap1 pathway in detoxification microcystin and has antioxidant function

Glutathione S-transferase is a crucial phase II metabolic enzyme involved in detoxification and metabolism in aquatic organisms. This study aimed to investigate the regulation of Nrf2/Keap1 pathway on microcystin-induced CpGST-Mu expression and CpGST-Mu resistance to hydrogen peroxide. A mu class GST from Cristaria plicata (CpGST-Mu) was identified. The full-length cDNA was 1026 bp, with an open reading frame of 558 bp. Subcellular localization revealed that CpGST-Mu was localized in cytoplasm. The optimum pH and temperature for the catalytic activity of CpGST-Mu protein was pH 6 and 40 °C, respectively. The results of Real-time quantitative PCR showed that CpGST-Mu mRNA was constitutively expressed in tissues, with the highest expression level in hepatopancreas and the lowest expression level in gill. The mRNA level of CpGST-Mu was significantly increased under the stress of microcystins and hydrogen peroxide. CpGST-Mu had an antagonistic effect on hydrogen peroxide. In the knockdown experiments, the mRNA levels of CpGST-Mu exhibited corresponding changes while Nrf2 and Keap1 genes were individually knocked down. These findings indicated that GST-Mu exhibited antioxidant properties and its expression was regulated by Nrf2/Keap1 signaling pathway. The study provided new information on the function of GST-Mu and could contribute to future studies on how to excrete microcystins in molluscs.

Pathology and infectious agents of unionid mussels: A primer for pathologists in disease surveillance and investigation of mortality events

Freshwater mussels are one of the most imperiled groups of organisms in the world, and more than 30 species have gone extinct in the last century. While habitat alteration and destruction have contributed to the declines, the role of disease in mortality events is unclear. In an effort to involve veterinary pathologists in disease surveillance and the investigation of freshwater mussel mortality events, we provide information on the conservation status of unionids, sample collection and processing techniques, and unique and confounding anatomical and physiological differences. We review the published accounts of pathology and infectious agents described in freshwater mussels including neoplasms, viruses, bacteria, fungi, fungal-like agents, ciliated protists, Aspidogastrea, Digenea, Nematoda, Acari, Diptera, and Odonata. Of the identified infectious agents, a single viral disease, Hyriopsis cumingii plague disease, that occurs only in cultured mussels is known to cause high mortality. Parasites including ciliates, trematodes, nematodes, mites, and insects may decrease host fitness, but are not known to cause mortality. Many of the published reports identify infectious agents at the light or ultrastructural microscopy level with no lesion or molecular characterization. Although metagenomic analyses provide sequence information for infectious agents, studies often fail to link the agents to tissue changes at the light or ultrastructural level or confirm their role in disease. Pathologists can bridge this gap between identification of infectious agents and confirmation of disease, participate in disease surveillance to ensure successful propagation programs necessary to restore decimated populations, and investigate mussel mortality events to document pathology and identify causality.

Triploid animals, a potential model for ETosis research: Influence of polyploidy on the formation and efficacy of extracellular traps in the eastern oyster

Decondensation and the subsequent release of chromatin from specific immune cells in response to inflammatory stimuli is a highly conserved aspect of the innate immune system and leads to the formation of extracellular traps, observable in nearly all forms of multicellular life. This process is known as ETosis, with the release of DNA and its associated antimicrobial proteins physically capturing and neutralizing pathogens following an infection or tissue damage. Despite the universality of this response, data concerning extracellular traps in non-model organisms is limited, with most invertebrate studies doing little more than proving their existence due to difficulties in stimulation and high interindividual variability in trap production. This study provides a novel, simple, and inexpensive method for the consistent stimulation of extracellular traps in eastern oyster (Crassostrea virginica) hemocytes. Using the methods described in this study, we compared how ploidy impacts the rate, size, and efficacy of extracellular traps. Findings demonstrated that hemocyte extracellular traps were potent antimicrobials against both Gram-positive and Gram-negative bacteria. Furthermore, we provide evidence to suggest that agranulocytes may be the primary ETosis effector cells in C. virginica. This study is the first to describe extracellular traps in C. virginica and highlights the possible benefits of using triploid animals to gain a further understanding of ETosis and the factors that regulate its induction and efficacy.

Antimicrobial Resistance Risk Assessment of Vibrio parahaemolyticus Isolated from Farmed Green Mussels in Singapore

Vibrio parahaemolyticus, commonly found in seafood products, is responsible for gastroenteritis resulting from the consumption of undercooked seafood. Hence, there is a need to characterize and quantify the risk involved from this pathogen. However, there has been no study reporting the quantification of hemolytic antimicrobial-resistant (AMR) Vibrio parahaemolyticus in locally farmed shellfish in Singapore. In this study, ampicillin, penicillin G, tetracycline resistant, and non-AMR hemolytic V. parahaemolyticus were surveyed and quantified in green mussel samples from different premises in the food chain (farm and retail). The occurrence data showed that 31/45 (68.9%) of farmed green mussel samples, 6/6 (100%) farm water samples, and 41/45 (91.1%) retail shellfish samples detected the presence of hemolytic V. parahaemolyticus. V. parahaemolyticus counts ranged from 1.6-5.9 Log CFU/g in the retail shellfish samples and 1.0-2.9 Log CFU/g in the farm water samples. AMR risk assessments (ARRA), specifically for ampicillin, penicillin G, tetracycline, and hemolytic (non-AMR) scenarios were conducted for the full farm-to-home and partial retail-to-home chains. The hemolytic ARRA scenario estimated an average probability of illness of 5.7 × 10^-3 and 1.2 × 10^-2 per serving for the full and partial chains, respectively, translating to 165 and 355 annual cases per total population or 2.9 and 6.2 cases per 100,000 population, respectively. The average probability of illness per year ratios for the three ARRAs to the hemolytic ARRA were 0.82, 0.81, and 0.47 (ampicillin, penicillin G, and tetracycline, respectively) for the full chain and 0.54, 0.39, and 0.09 (ampicillin, penicillin G, and tetracycline, respectively) for the partial chain. The sensitivity analysis showed that the overall cooking effect, initial concentrations of pathogenic V. parahaemolyticus, and harvest duration and harvest temperature were key variables influencing the risk estimates in all of the modelled ARRAs. The study findings can be used by relevant stakeholders to make informed decisions for risk management that improve food safety.

Hemocytes of Yesso scallop characterized by cytological, molecular marker, and functional analyses

Bivalve hemocytes have pivotal role as cellular biodefense. However, no information is available for cytological parameters, marker gene and function of the hemocytes in Yesso scallop, a commercially important aquaculture species worldwide. Due to their extremely strong cell aggregation ability, the scallop hemocytes were not able to assess as a single cell so far. In the present study, we established methodologies for studying the hemocytes of Yesso scallop, assessed cell morphology, measured seasonal fluctuation, and analyzed transcriptomes and cellular behavior during the immune response. Our results showed that the Yesso scallop possesses a single type of leukocyte-type hemocytes similar to other bivalve granulocytes circulating at an average of 1 × 10⁷ cells/ml throughout the year. In addition, we identified five molecular marker genes specific to the scallop hemocytes. These hemocyte markers enabled us to precisely detect the hemocyte localization. Using these markers, we confirmed that tissue transplantation can experimentally induce an immune response, leading to the mobilization of circulating hemocytes for encapsulation. This study provides a comprehensive understanding of scallop hemocytes and their role in the cellular biodefense system of bivalves and various methods for cytological analysis.

Mytilus galloprovincialis releases immunologically functional haemocytes to the intervalvar space in response to tissue injury and infection

Haemocytes of Mytilus galloprovincialis represent the main component of the internal self-defence system. Although haemocytes from haemolymph are usually studied to analyse these animals' immune response, the presence of haemocytes in the intervalvar liquid, which is essentially sea water, led us to characterize them. Several functional (ROS production, phagocytosis, gene expression, travel velocity and distance) and morphological (area, size and granularity) assays were performed by applying different stimuli to the mussels (waterborne infection, shell injury and their combination). Our results revealed that intervalvar liquid haemocytes share common characteristics with haemolymph haemocytes (for instance, the cell morphology and the cell population structure divided in three main groups) but also show significant differences in size (usually smaller in the intervalvar liquid), mobility (commonly faster in the intervalvar liquid), ROS production (higher in non-stimulated intervalvar liquid cells) and gene expression (IL17, Myd88 and CathL are over expressed in liquid intervalvar cells compared to haemolymph cells). Moreover, differences were observed when mussels were subjected to the mentioned treatments. These free intervalvar haemocytes could constitute the first line of defence as external sentinels extending the immunological alert system outside of the mussel body.

Unveiling the Impact of Gene Presence/Absence Variation in Driving Inter-Individual Sequence Diversity within the CRP-I Gene Family in Mytilus spp

Mussels (Mytilus spp.) tolerate infections much better than other species living in the same marine coastal environment thanks to a highly efficient innate immune system, which exploits a remarkable diversification of effector molecules involved in mucosal and humoral responses. Among these, antimicrobial peptides (AMPs) are subjected to massive gene presence/absence variation (PAV), endowing each individual with a potentially unique repertoire of defense molecules. The unavailability of a chromosome-scale assembly has so far prevented a comprehensive evaluation of the genomic arrangement of AMP-encoding loci, preventing an accurate ascertainment of the orthology/paralogy relationships among sequence variants. Here, we characterized the CRP-I gene cluster in the blue mussel Mytilus edulis, which includes about 50 paralogous genes and pseudogenes, mostly packed in a small genomic region within chromosome 5. We further reported the occurrence of widespread PAV within this family in the Mytilus species complex and provided evidence that CRP-I peptides likely adopt a knottin fold. We functionally characterized the synthetic peptide sCRP-I H1, assessing the presence of biological activities consistent with other knottins, revealing that mussel CRP-I peptides are unlikely to act as antimicrobial agents or protease inhibitors, even though they may be used as defense molecules against infections from eukaryotic parasites.

Microbiota of the Digestive Glands and Extrapallial Fluids of Clams Evolve Differently Over Time Depending on the Intertidal Position

The Manila clam (Ruditapes philippinarum) is the second most exploited bivalve in the world but remains threatened by diseases and global changes. Their associated microbiota play a key role in their fitness and acclimation capacities. This study aimed at better understanding the behavior of clam digestive glands and extrapallial fluids microbiota at small, but contrasting spatial and temporal scales. Results showed that environmental variations impacted clam microbiota differently according to the considered tissue. Each clam tissue presented its own microbiota and showed different dynamics according to the intertidal position and sampling period. Extrapallial fluids microbiota was modified more rapidly than digestive glands microbiota, for clams placed on the upper and lower intertidal position, respectively. Clam tissues could be considered as different microhabitats for bacteria as they presented different responses to small-scale temporal and spatial variabilities in natural conditions. These differences underlined a more stringent environmental filter capacity of the digestive glands.

RNA-Seq comparative study reveals molecular effectors linked to the resistance of Pinna nobilis to Haplosporidium pinnae parasite

With the intensification of maritime traffic, recently emerged infectious diseases have become major drivers in the decline and extinction of species. Since 2016, mass mortality events have decimated the endemic Mediterranean Sea bivalve Pinna nobilis, affecting ca. 100% of individuals. These events have largely been driven by Haplosporidium pinnae's infection, an invasive species which was likely introduced by shipping. While monitoring wild populations of P. nobilis, we observed individuals that survived such a mass mortality event during the summer of 2018 (France). We considered these individuals resistant, as they did not show any symptoms of the disease, while the rest of the population in the area was devastated. Furthermore, the parasite was not detected when we conducted a PCR amplification of a species-specific fragment of the small subunit ribosomal DNA. In parallel, the transcriptomic analysis showed evidence of some parasite RNA indicating that the resistant individuals had been exposed to the parasite without proliferating. To understand the underlying mechanisms of resistance in these individuals, we compared their gene expression with that of susceptible individuals. We performed de novo transcriptome assembly and annotated the expressed genes. A comparison of the transcriptomes in resistant and susceptible individuals highlighted a gene expression signature of the resistant phenotype. We found significant differential expressions of genes involved in immunity and cell architecture. This data provides the first insights into how individuals escape the pathogenicity associated with infection.

A protein inhibitor of activated STAT (CgPIAS) negatively regulates the expression of ISGs by inhibiting STAT activation in oyster Crassostrea gigas

The protein inhibitor of activated STAT (PIAS) family proteins act as the important negative regulators in janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathway, which can be also involved in regulating the expression of interferon-stimulated genes (ISGs). In the present study, a PIAS homologue (designated as CgPIAS) was identified from oyster Crassostrea gigas. The open reading frame (ORF) of CgPIAS cDNA was of 1887 bp encoding a peptide of 628 amino acid residues. The CgPIAS protein contains a conserved scaffold attachment factor A/B/acinus/PIAS (SAP) domain, a Pro-Ile-Asn-Ile-Thr (PINIT) motif, a RING-finger-like zinc-binding domain (RLD) and two SUMO-interaction Motifs (SIMs). The deduced amino acid sequence of CgPIAS shared 74.58-81.36% similarity with other PIAS family members in the RLD domain. The mRNA transcripts of CgPIAS were detected in all the tested tissues with highest level in haemocytes (32.98-fold of mantles, p < 0.001). After poly (I:C) and recombinant Interferon-like protein (rCgIFNLP) stimulation, the mRNA expression of CgPIAS in haemocytes significantly up-regulated to the highest level at 48 h (7.38-fold, p < 0.001) and at 24 h (13.08-fold, p < 0.01), respectively. Moreover, the nuclear translocation of CgPIAS was observed in haemocytes after poly (I:C) stimulation. Biolayer Interferometry (BLI) assay revealed that the recombinant protein CgPIAS-RLD could interact with the recombinant protein CgSTAT in vitro with the K_D value of 3.88 × 10^-8 M. In the CgPIAS-RNAi oysters, the green signals of CgSTAT protein in nucleus of haemocytes increased compared with that in NC-RNAi group, and the mRNA expression of myxovirus resistance (CgMx1), oligoadenylate synthase-like proteins (CgOASL), CgViperin and IFN-induced protein 44-like (CgIFI44L-1) in haemocytes significantly increased at 12 h after poly (I:C) stimulation, which were 2.39-fold (p < 0.05), 2.18-fold (p < 0.001), 1.74-fold (p < 0.05), and 2.89-fold (p < 0.01) of that in control group, respectively. The above results indicated that CgPIAS negatively regulated the ISG expression by inhibiting STAT activation in oyster C. gigas.

β-1,3-Glucan/chitin unmasking in the Saccharomyces cerevisiae mutant, Δmnn9, promotes immune response and resistance of the Pacific oyster (Crassostrea gigas) to Vibrio coralliilyticus infection

Yeast cells can play a crucial role in immune activation in fish and shellfish predominantly due to the cell wall component β-1,3-glucan, providing protection against bacterial or viral infections. However, the immunostimulatory capacity of dietary yeast cells remains poorly studied in bivalves. To understand the role of yeast cell wall components (mannan, β-glucan and chitin) as immune activators, this study characterized the surface carbohydrate exposure of the wild-type baker's yeast Saccharomyces cerevisiae (WT) and its Δmnn9 mutant, which presents a defective mannan structure, and compared these profiles with that of β-glucan particles, using fluorescein isothiocyanate (FITC)-labeled lectin binding analysis. Then, a first trial evaluated the immunological response in Crassostrea gigas juveniles after being fed for 24 h with an algae-based diet (100A) and its 50% substituted version (based on dry weight) with WT (50A50WT) and Δmnn9 (50A50Y), and the posterior resistance of the juveniles against Vibrio coralliilyticus infection (trial 1). The mRNA expression was measured for β-glucan-binding protein (CgβGBP), Toll-like receptor 4 (CgTLR4), C-type lectin receptor 3 (CgCLec-3), myeloid differentiation factor 88 (CgMyD88), nuclear factor-kappa B (CgNFκB), lysozyme (CgLys), interleukin 17-5 (CgIL17-5), and superoxide dismutase (CgSOD), in oysters, before and 24 h after the bacterial inoculation. A second trial tested the effect of incorporating Δmnn9 into the 100A diet for 24 h at different substitution levels: 0, 5, 10, 25, and 50% (100A, 95A5Y, 90A10Y, 75A25Y, and 50A50Y), followed by the bacterial challenge with V. coralliilyticus (trial 2). Our findings showed that the outer cell wall surface of WT is largely composed of mannan, while Δmnn9 presents high exposure of β-glucan and chitin, exhibiting similar FITC-lectin binding profiles (fluorescence intensity) to β-glucan particles. A significantly higher survival after the bacterial challenge was observed in oysters fed on 50A50Y compared to those fed 50A50WT and 100A in trial 1. This better performance of 50A50Y was supported by significantly higher gene expressions of CgLys, CgSOD, CgMyD88, and CgβGBP compared to 100A, and CgSOD and CgNFκB in relation to those fed on 50A50WT, prior to the bacterial inoculation. Furthermore, improved survival was observed in oysters fed 50A50Y compared to those offered lower Δmnn9 levels and 100A in trial 2. The superior performance of Δmnn9-fed oysters is mostly associated with the elevated presence of unmasked β-glucans on Δmnn9 cell wall surface, facilitating their interactions with oyster hemocytes. Further studies are needed to evaluate administration dose and frequency of Δmnn9 to develop strategies for long-term feeding.

The Gill Microbiota of Argopecten purpuratus Scallop Is Dominated by Symbiotic Campylobacterota and Upwelling Intensification Differentially Affects Their Abundance

Despite the great importance of gills for bivalve mollusks (respiration, feeding, immunity), the microbiota associated with this tissue has barely been characterized in scallops. The scallop Argopecten purpuratus is an important economic resource that is cultivated in areas where coastal upwelling is intensifying by climate change, potentially affecting host-microbiota interactions. Thus, we first characterized the bacterial community present in gills from cultivated scallops (by 16S rRNA gene amplicon sequencing) and assessed their stability and functional potential in animals under farm and laboratory conditions. Results showed that under both conditions the gill bacterial community is dominated by the phylum Campylobacterota (57%), which displays a chemoautotrophic potential that could contribute to scallop nutrition. Within this phylum, two phylotypes, namely symbionts A and B, were the most abundant; being, respectively, taxonomically affiliated to symbionts with nutritional functions in mussel gills, and to uncultured bacteria present in coral mucus. Additionally, in situ hybridization and scanning electron microscopy analyses allowed us to detect these symbionts in the gills of A. purpuratus. Given that shifts in upwelling phenology can cause disturbances to ecosystems, affecting bacteria that provide beneficial functions to the host, we further assessed the changes in the abundance of the two symbionts (via qPCR) in response to a simulated upwelling intensification. The exposure to combined decreasing values in the temperature, pH, and oxygen levels (upwelling conditions) favored the dominance of symbiont B over symbiont A; suggesting that symbiont abundances are modulated by these environmental changes. Overall, results showed that changes in the main Campylobacterota phylotypes in response to upwelling intensification could affect its symbiotic function in A. purpuratus under future climate change scenarios. These results provide the first insight into understanding how scallop gill-microbial systems adapt and respond to climate change stressors, which could be critical for managing health, nutrition, and scallop aquaculture productivity.

Hemocytes of bivalve mollusks as cellular models in toxicological studies of metals and metal-based nanomaterials

Understanding the impacts of environmental pollutants on immune systems is indispensable in ecological and health risk assessments due to the significance of normal immunological functions in all living organisms. Bivalves as sentinel organisms with vital ecological importance are widely distributed in aquatic environments and their innate immune systems are the sensitive targets of environmental pollutants. As the central component of innate immunity, bivalve hemocytes are endowed with specialized endolysosomal systems for particle internalization and metal detoxification. These intrinsic biological features make them a unique cellular model for metal- and nano-immunotoxicology research. In this review, we firstly provided a general overview of bivalve's innate immunity and the classification and immune functions of hemocytes. We then summarized the recent progress on the interactions of metals and nanoparticles with bivalve hemocytes, with emphasis on the involvement of hemocytes in metal regulation and detoxification, the interactions of hemocytes and nanoparticles at eco/bio-nano interface and hemocyte-mediated immune responses to the exposure of metals and nanoparticles. Finally, we proposed the key knowledge gaps and future research priorities in deciphering the fundamental biological processes of the interactions of environmental pollutants with the innate immune system of bivalves as well as in developing bivalve hemocytes into a promising cellular model for nano-immuno-safety assessment.

Immune Defense in Hypoxic Waters: Impacts of CO2 Acidification

AbstractPeriodic episodes of low oxygen (hypoxia) and elevated CO₂ (hypercapnia) accompanied by low pH occur naturally in estuarine environments. Under the influence of climate change, the geographic range and intensity of hypoxia and hypercapnic hypoxia are predicted to increase, potentially jeopardizing the survival of economically and ecologically important organisms that use estuaries as habitat and nursery grounds. In this review we synthesize data from published studies that evaluate the impact of hypoxia and hypercapnic hypoxia on the ability of crustaceans and bivalve molluscs to defend themselves against potential microbial pathogens. Available data indicate that hypoxia generally has suppressive effects on host immunity against bacterial pathogens as measured by in vitro and in vivo assays. Few studies have documented the effects of hypercapnic hypoxia on crustaceans or bivalve immune defense, with a range of outcomes suggesting that added CO₂ might have additive, negative, or no interactions with the effects of hypoxia alone. This synthesis points to the need for more partial pressure of O₂ × low pH factorial design experiments and recommends the development of new host∶pathogen challenge models incorporating natural transmission of a wide range of viruses, bacteria, and parasites, along with novel in vivo tracking systems that better quantify how pathogens interact with their hosts in real time under laboratory and field conditions.

Effects of seawater temperature and acute Vibriosp. challenge on the haemolymph immune and metabolic responses of adult mussels (Perna canaliculus)

The New Zealand Greenshell™ mussel (Perna canaliculus) is an endemic bivalve species with cultural importance, that is harvested recreationally and commercially. However, production is currently hampered by increasing incidences of summer mortality in farmed and wild populations. While the causative factors for these mortality events are still unknown, it is believed that increasing seawater temperatures and pathogen loads are potentially at play. To improve our understanding of these processes, challenge experiments were conducted to investigate the combined effects of increased seawater temperature and Vibrio infection on the immune and metabolic responses of adult mussels. Biomarkers that measure the physiological response of mussels to multiple-stressors can be utilised to study resilience in a changing environment, and support efforts to strengthen biosecurity management. Mussels acclimated to two temperatures (16 °C and 24 °C) were injected with either autoclaved, filtered seawater (control) or Vibriosp. DO1 (infected). Then, haemolymph was sampled 24 h post-injection and analysed to quantify haemocyte immune responses (via flow-cytometry), antioxidant capacity (measured electrochemically) and metabolic responses (via gas chromatography-mass spectrometry) to bacterial infection. Both seawater temperature and injection type significantly influenced the immune and metabolite status of mussels. A lack of interaction effects between temperature and injection type indicated that the effects of Vibrio sp. 24 h post-infection were similar between seawater temperatures. Infected mussels had a higher proportion of dead haemocytes and lower overall haemocyte counts than uninfected controls. The proportion of haemocytes showing evidence of apoptosis was higher in mussels held at 24 °C compared with those held at 16 °C. The proportion of haemocytes producing reactive oxygen species did not differ between temperatures or injection treatments. Mussels held at 24 °C exhibited elevated levels of metabolites linked to the glycolysis pathway to support energy production. The saccharopin-lysine pathway metabolites were also increased in these mussels, indicating the role of lysine metabolism. A decrease in metabolic activity (decreases in BCAAs, GABA, urea cycle metabolites, oxidative stress metabolites) was largely seen in mussels injected with Vibrio sp. Itaconate increased as seen in previous studies, suggesting that antimicrobial activity may have been activated in infected mussels. This study highlights the complex nature of immune and metabolic responses in mussels exposed to multiple stressors and gives an insight into Vibrio sp. infection mechanisms at different seawater temperatures.

First subcellular localization of the amnesic shellfish toxin, domoic acid, in bivalve tissues: Deciphering the physiological mechanisms involved in its long-retention in the king scallop Pecten maximus

Domoic acid (DA), the phycotoxin responsible for amnesic shellfish poisoning (ASP), is an excitatory amino acid naturally produced by at least twenty-eight species of the bloom-forming marine diatoms Pseudo-nitzschia spp. Suspension feeders, such as bivalve mollusks, can accumulate and lengthy retain high amounts of DA in their tissues, threatening human health and leading to extensive-prolonged fishery closures, and severe economic losses. This is particularly problematic for the king scallop Pecten maximus, which retains high burdens of DA from months to years compared to other fast-depurator bivalves. Nonetheless, the physiological and cellular processes responsible for this retention are still unknown. In this work, for the first time, a novel immunohistochemical techniques based on the use of an anti-DA antibody was successfully developed and applied for DA-detection in bivalve tissues at a subcellular level. Our results show that in naturally contaminated P. maximus following a Pseudo-nitzschia australis outbreak, DA is visualized mainly within small membrane-bounded vesicles (1 - 2.5 µm) within the digestive gland cells, identified as autophagosomic structures by means of immune-electron microscopy, as well as in the mucus-producing cells, particularly those from gonad ducts and digestive tract. Trapping of DA in autophagososomes may be a key mechanism in the long retention of DA in scallops. These results and the development of DA-immunodetection are essential to provide a better understanding of the fate of DA, and further characterize DA contamination-decontamination kinetics in marine bivalves, as well as the main mechanisms involved in the long retention of this toxin in P. maximus.

Proteomic changes associated with predator-induced morphological defenses in oysters

Inducible prey defenses occur when organisms undergo plastic changes in phenotype to reduce predation risk. When predation pressure varies persistently over space or time, such as when predator and prey co-occur over only part of their biogeographic ranges, prey populations can become locally adapted in their inducible defenses. In California estuaries, native Olympia oyster (Ostrea lurida) populations have evolved disparate phenotypic responses to an invasive predator, the Atlantic oyster drill (Urosalpinx cinerea). In this study, oysters from an estuary with drills, and oysters from an estuary without drills, were reared for two generations in a laboratory common garden, and subsequently exposed to cues from Atlantic drills. Comparative proteomics was then used to investigate molecular mechanisms underlying conserved and divergent aspects of their inducible defenses. Both populations developed smaller, thicker, and harder shells after drill exposure, and these changes in shell phenotype were associated with up-regulation of calcium transport proteins that could influence biomineralization. Inducible defenses evolve in part because defended phenotypes incur fitness costs when predation risk is low. Immune proteins were down-regulated by both oyster populations after exposure to drills, implying a trade-off between biomineralization and immune function. Following drill exposure, oysters from the population that co-occurs with drills grew smaller shells than oysters inhabiting the estuary not yet invaded by the predator. Variation in the response to drills between populations was associated with isoform-specific protein expression. This trend suggests that a stronger inducible defense response evolved in oysters that co-occur with drills through modification of an existing mechanism.

Species- and site-specific circulating bacterial DNA in Subantarctic sentinel mussels Aulacomya atra and Mytilus platensis

Impacts of climate changes are particularly severe in polar regions where warmer temperatures and reductions in sea-ice covers threaten the ecological integrity of marine coastal ecosystems. Because of their wide distribution and their ecological importance, mussels are currently used as sentinel organisms in monitoring programs of coastal ecosystems around the world. In the present study, we exploited the concept of liquid biopsy combined to a logistically friendly sampling method to study the hemolymphatic bacterial microbiome in two mussel species (Aulacomya atra and Mytilus platensis) in Kerguelen Islands, a remote Subantarctic volcanic archipelago. We found that the circulating microbiome signatures of both species differ significantly even though their share the same mussel beds. We also found that the microbiome differs significantly between sampling sites, often correlating with the particularity of the ecosystem. Predictive models also revealed that both species have distinct functional microbiota, and that the circulating microbiome of Aulacomya atra was more sensitive to changes induced by acute thermal stress when compared to Mytilus platensis. Taken together, our study suggests that defining circulating microbiome is a useful tool to assess the health status of marine ecosystems and to better understand the interactions between the sentinel species and their habitat.

Transcriptomic, Proteomic, and Functional Assays Underline the Dual Role of Extrapallial Hemocytes in Immunity and Biomineralization in the Hard Clam Mercenaria mercenaria

Circulating hemocytes in the hemolymph represent the backbone of innate immunity in bivalves. Hemocytes are also found in the extrapallial fluid (EPF), the space delimited between the shell and the mantle, which is the site of shell biomineralization. This study investigated the transcriptome, proteome, and function of EPF and hemolymph in the hard clam Mercenaria mercenaria. Total and differential hemocyte counts were similar between EPF and hemolymph. Overexpressed genes in the EPF were found to have domains previously identified as being part of the "biomineralization toolkit" and involved in bivalve shell formation. Biomineralization related genes included chitin-metabolism genes, carbonic anhydrase, perlucin, and insoluble shell matrix protein genes. Overexpressed genes in the EPF encoded proteins present at higher abundances in the EPF proteome, specifically those related to shell formation such as carbonic anhydrase and insoluble shell matrix proteins. Genes coding for bicarbonate and ion transporters were also overexpressed, suggesting that EPF hemocytes are involved in regulating the availability of ions critical for biomineralization. Functional assays also showed that Ca²⁺ content of hemocytes in the EPF were significantly higher than those in hemolymph, supporting the idea that hemocytes serve as a source of Ca²⁺ during biomineralization. Overexpressed genes and proteins also contained domains such as C1q that have dual functions in biomineralization and immune response. The percent of phagocytic granulocytes was not significantly different between EPF and hemolymph. Together, these findings suggest that hemocytes in EPF play a central role in both biomineralization and immunity.

Characterization of a novel activating protein-1 (AP-1) gene and the association of its single nucleotide polymorphisms with vibrio resistance in Tegillarca granosa

The blood clam Tegillarca granosa is a commercial marine bivalve of economic value, accounting for approximately 50% of clam production in China. In recent years, the yield of blood clams has been threatened by bacterial infections caused by marine Vibrio species that thrive under a rising sea temperature. The transcription factor activating protein-1 (AP-1) is emerging as an important player in the innate immunity of marine bivalves against viral or bacterial infections. In this study, the full-length cDNA of a novel T. granosa AP-1 (TgAP-1) was cloned for the first time. The 1591-bp cDNA encoded a protein of 292 amino acid residues with a calculated molecular weight of 32.8 kDa. The TgAP-1 protein contained an N-terminal Jun domain and a C-terminal basic region leucine zipper domain typically found in Jun proteins (a subfamily of AP-1 proteins). TgAP-1 was ubiquitously expressed in T. granosa, with the highest expression detected in the gill and foot, followed by the mantle, hemolymph, and hepatopancreas. Exposure to Vibrio harveyi induced TgAP-1 expression in gill tissues and the expression levels of TgAP-1 of resistant blood clams were always lower than that of control population whether Vibro infection or not. A total of 18 single nucleotide polymorphisms (SNPs) of TgAP-1 were detected in T. granosa. SNP-typing and haplotyping of resistant and susceptible populations revealed that six SNPs (AG type of TgSNP-1, GA type of TgSNP-2, TG type of TgSNP-4, CT type of TgSNP-7, AG type of TgSNP-11, and GA type of TgSNP-12) and four haplotypes (fHap2, fHap3, fHap6, and fHap7) were significantly associated with V. harveyi resistance. Risk assessment showed that fHap2 (CG) and fHap7 (GA) were associated with an increased resistance, while fHap3 (CT) and fHap6 (AG) were associated with an increased susceptibility. The results from this study supported a potential role of TgAp-1 in the anti-Vibro immunity of T. granosa. The discovery of the genetic molecular markers and haplotypes related to Vibrio resistance can provide guidance for selective breeding of T. granosa in the future.

Methodological Approaches To Assess Innate Immunity and Innate Memory in Marine Invertebrates and Humans

Assessing the impact of drugs and contaminants on immune responses requires methodological approaches able to represent real-life conditions and predict long-term effects. Innate immunity/inflammation is the evolutionarily most widespread and conserved defensive mechanism in living organisms, and therefore we will focus here on immunotoxicological methods that specifically target such processes. By exploiting the conserved mechanisms of innate immunity, we have examined the most representative immunotoxicity methodological approaches across living species, to identify common features and human proxy models/assays. Three marine invertebrate organisms are examined in comparison with humans, i.e., bivalve molluscs, tunicates and sea urchins. In vivo and in vitro approaches are compared, highlighting common mechanisms and species-specific endpoints, to be applied in predictive human and environmental immunotoxicity assessment. Emphasis is given to the 3R principle of Replacement, Refinement and Reduction of Animals in Research and to the application of the ARRIVE guidelines on reporting animal research, in order to strengthen the quality and usability of immunotoxicology research data.

Immunotoxicity of Perfluorooctanoic Acid to the Marine Bivalve Species Ruditapes philippinarum

Polyfluorinated alkylated substances are recognized as an important class of pollutants in marine environments. Bivalves are good model organisms for evaluating the toxicity of pollutants and monitoring marine environments. In the present study, immunotoxicity of perfluorooctanoic acid (PFOA) was investigated by measuring biomarkers of the immune profile of Ruditapes philippinarum. In bivalves, hemocytes are an important component of the immune system. Thus, hemocyte proliferation, phagocytosis, cell viability, and immune enzyme activities, which have been applied as marine pollution bioindicators, were identified and observed for changes after exposure to PFOA in R. philippinarum. Based on the integrated biomarker responses method, we selected five biomarkers to evaluate PFOA risk at the multibiomarker level. In addition, the histopathological alterations of hemocytes in bivalves were used as indexes of the response to environmental stress. The subcellular structure of the hemocytes in R. philippinarum changed significantly with PFOA exposure, including hemocyte and nucleus morphological changes, organelle dissolution, cytomembrane and karyotheca swelling, and cytoplasm vacuolization. The present study verifies PFOA immunotoxicity to R. philippinarum at different levels and the integrated assessment of stress levels caused by PFOA in marine environment. Our results will provide new insights into evaluating adverse effects of PFOA and monitoring marine ecosystem. Environ Toxicol Chem 2022;41:426-436. © 2021 SETAC.

Histopathology of a threatened surf clam, toheroa (Paphies ventricosa) from Aotearoa New Zealand

The toheroa (Paphies ventricosa) is endemic to Aotearoa (New Zealand). Following decades of overfishing in the 1900 s, commercial and recreational fishing of toheroa is now prohibited. For unknown reasons, protective measures in place for over 40 years have not ensured the recovery of toheroa populations. For the first time, a systematic pathology survey was undertaken to provide a baseline of toheroa health in remaining major populations. Using histopathology, parasites and pathologies in a range of tissues are assessed and quantified spatio-temporally. Particular focus is placed on intracellular microcolonies of bacteria (IMCs). Bayesian ordinal logistic regression is used to model IMC infection and several facets of toheroa health. Model outputs show condition to be the most important predictor of IMC intensity in toheroa tissues. The precarious state of many toheroa populations around Aotearoa should warrant greater attention from scientists, conservationists, and regulators. It is hoped that this study will provide some insight into the current health status of a treasured and iconic constituent of several expansive surf beaches in Aotearoa.

Polyamine Putrescine Regulates Oxidative Stress and Autophagy of Hemocytes Induced by Lipopolysaccharides in Pearl Oyster Pinctada fucata martensii

The polyamine putrescine (Put) is a ubiquitous small cationic amine. It plays an essential role in controlling the innate immune response. However, little is known about its function in mollusks. In this study, the Put content was observed to increase in the serum of pearl oyster Pinctada fucata martensii after 6 and 24 h of lipopolysaccharide (LPS) stimulation. Activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) increased, and nitric oxide synthase was downregulated in the Put group (i.e., combined treatment with Put and LPS) compared with that in the LPS group (i.e., combined treatment with phosphate-buffered saline and LPS). Furthermore, activities of alkaline phosphatase and acid phosphatase were inhibited after 6 h of LPS stimulation. The expression levels of the nuclear factor kappa B, IκB kinase, Janus kinase, and signal transducer and activator of transcription proteins genes were all significantly suppressed at 12 and 24 h in the Put group. Pseudomonas aeruginosa and Bacillus subtilis grew better after being incubated with the serum from the Put group than that from the LPS group. Additionally, the Put treatment remarkably inhibited the autophagy of hemocytes mediated by the AMP-activated protein kinase-mammalian target of rapamycin-Beclin-1 pathway. This study demonstrated that Put can effectively inhibit the inflammatory response induced by LPS in pearl oysters. These results provide useful information for further exploration of the immunoregulatory functions of polyamines in bivalves and contribute to the development of immunosuppressive agents.

Mussel Mass Mortality and the Microbiome: Evidence for Shifts in the Bacterial Microbiome of a Declining Freshwater Bivalve

Freshwater mussels (Unionida) are suffering mass mortality events worldwide, but the causes remain enigmatic. Here, we describe an analysis of bacterial loads, community structure, and inferred metabolic pathways in the hemolymph of pheasantshells (Actinonaias pectorosa) from the Clinch River, USA, during a multi-year mass mortality event. Bacterial loads were approximately 2 logs higher in moribund mussels (cases) than in apparently healthy mussels (controls). Bacterial communities also differed between cases and controls, with fewer sequence variants (SVs) and higher relative abundances of the proteobacteria Yokenella regensburgei and Aeromonas salmonicida in cases than in controls. Inferred bacterial metabolic pathways demonstrated a predominance of degradation, utilization, and assimilation pathways in cases and a predominance of biosynthesis pathways in controls. Only two SVs correlated with Clinch densovirus 1, a virus previously shown to be strongly associated with mortality in this system: Deinococcota and Actinobacteriota, which were associated with densovirus-positive and densovirus-negative mussels, respectively. Overall, our results suggest that bacterial invasion and shifts in the bacterial microbiome during unionid mass mortality events may result from primary insults such as viral infection or environmental stressors. If so, bacterial communities in mussel hemolymph may be sensitive, if generalized, indicators of declining mussel health.

The proline-rich myticalins from Mytilus galloprovincialis display a membrane-permeabilizing antimicrobial mode of action

Bivalve mollusks are continuously exposed to potentially pathogenic microorganisms living in the marine environment. Not surprisingly, these filter-feeders developed a robust innate immunity to protect themselves, which includes a broad panel of antimicrobial peptides. Among these, myticalins represent a recently discovered family of linear cationic peptides expressed in the gills of Mytilus galloprovincialis. Even though myticalins and insect and mammalian proline-rich antimicrobial peptides (PrAMPs) share a similar amino acid composition, we here show that none of the tested mussel peptides use a non-lytic mode of action relying on the bacterial transporter SbmA. On the other hand, all the tested myticalins perturbed and permeabilized the membranes of E. coli BW25113, as shown by flow-cytometry and atomic force microscopy. Circular dichroism spectra revealed that most myticalins did not adopt recognizable secondary structures in the presence of amphipathic environments, such as biological membranes. To explore possible uses of myticalins for biotech, we assessed their biocompatibility with a human cell line. Non-negligible cytotoxic effects displayed by myticalins indicate that their optimization would be required before their further use as lead compounds in the development of new antibiotics.

Quantification of the inflammatory responses to pro-and anti-inflammatory agents in Manila clam, Ruditapes philippinarum

Inflammation is a form of innate immune response of living organisms to harmful stimuli. In marine bivalves, inflammation is a common defense mechanism. Several studies have investigated the morphological features of inflammation in bivalves, such as hemocyte infiltration. However, the molecular and biochemical responses associated with inflammation in marine bivalves remain unexplored. Here, we investigated changes in nitric oxide (NO) levels, cyclooxygenase 2 (COX-2) activity, and allograft inflammatory factor-1 (AIF-1) gene expression levels in hemolymph samples collected from Manila clam (Ruditapes philippinarum) exposed to pro- and anti-inflammatory substances. These included the pro-inflammatory agent lipopolysaccharide (LPS), and the nonsteroidal anti-inflammatory drugs (NSAIDs) ibuprofen and diclofenac, all widely used in vertebrates. Our study showed that NO levels, COX-2 activity, and AIF-1 expression increased in response to the treatments with LPS and decreased in response to the treatments with NSAIDs in a concentration-dependent manner. These results suggest that the mechanism of inflammatory responses in bivalves is very similar to that of vertebrates, and we propose that inflammatory responses can be quantified using these techniques and used to determine the physiological status of marine bivalves exposed to biotic or abiotic stresses.

Microplastics alter digestive enzyme activities in the marine bivalve, Mytilus galloprovincialis

Microplastics are eaten by many invertebrates, particularly filter-feeding organisms like mussels. Since microplastics can be retained in the digestive system for extended periods, there is ample opportunity for them to interact with the functions of digestive enzymes. This study determined how the polymer type, size and concentration of ingested spherical microplastics affects the activities of seven key digestive enzymes in the digestive gland of Mytilus galloprovincialis, a common marine mussel. Polymer type significantly affected the activities of carbohydrase enzymes: polystyrene reduced amylase and xylanase activities, and increased cellulase activity. High concentrations of microplastics (5 × 10⁴ microplastics L^-1) caused a 2.5-fold increase in total protease activity. The activities of laminarinase, lipases and lipolytic esterases were unaffected by the polymer type, size or concentration of microplastics. Microplastics-induced changes to digestive enzyme activities can affect mussels' ability to acquire energy from food and reduce their energy reserves.

Silver nanoparticles' impact on the gene expression of the cytosolic adaptor MyD-88 and the interferon regulatory factor IRF in the gills and digestive gland of mytilus galloprovincialis

Silver nanoparticles (AgNPs) have been reported as stressors for the bivalves' immune system at different regulatory levels, impacting the detection step and receptors, and other mediators, as well as effector molecules. However, studies on how AgNPs impact the transmission of signals from receptors and whether they have an effect on mediators and transcription factors are still scarce. This study aims to investigate the effect of 12 hours of in vivo exposure to 100 µg/L of AgNPs on the gene expression of the cytosolic adaptor Myeloid, the differentiation protein 88 (MgMyD88-b), and the interferon regulatory factor (Me4-IRF) in the gills and digestive gland of Mytilus galloprovincialis, before and after blocking two major uptake pathways of nanoparticles (clathrin- and caveolae-mediated endocytosis). The results illustrate a tissue-specific gene expression of the MgMyD88-b and the Me4-IRF in the gills and digestive gland of M. galloprovincialis. In the gills, AgNPs did not significantly impact the expression of the two genes. However, blocking the caveolae-mediated endocytosis decreased the expression of Me4-IRF. However, inhibition of clathrin-mediated endocytosis in the digestive gland recorded a significant decrease in the expression of MgMyD88-b. Overall, the inhibition of the AgNPs' uptake routes have highlighted their potential interference with the immune response through the studied mediators' genes, which need to be studied further in future investigations.

The new PFAS C6O4 and its effects on marine invertebrates: First evidence of transcriptional and microbiota changes in the Manila clam Ruditapes philippinarum

There is growing concern for the wide use ofperfluorooctanoic acid (PFOA) because of its toxic effects on the environment and on human health. A new compound - the so called C6O4 (perfluoro ([5-methoxy-1,3-dioxolan-4-yl]oxy) acetic acid) - was recently introduced as one of the alternative to traditional PFOA, however this was done without any scientific evidence of the effects of C6O4 when dispersed into the environment. Recently, the Regional Agency for the Protection of the Environment of Veneto (Italy) detected high levels of C6O4 in groundwater and in the Po river, increasing the alarm for the potential effects of this chemical into the natural environment. The present study investigates for the first time the effects of C6O4 on the Manila clam Ruditapes philippinarum exposed to environmental realistic concentrations of C6O4 (0.1 µg/L and 1 µg/L) for 7 and 21 days. Furthermore, in order to better understand if C6O4 is a valid and less hazardous alternative to its substitute, microbial and transcriptomic alterations were also investigated in clams exposed to 1 µg/L ofPFOA. Results indicate that C6O4 may cause significant perturbations to the digestive gland microbiota, likely determining the impairment of host physiological homeostasis. Despite chemical analyses suggest a 5 times lower accumulation potential of C604 as compared to PFOA in clam soft tissues, transcriptional analyses reveal several alterations of gene expression profile. A large part of the altered pathways, including immune response, apoptosis regulation, nervous system development, lipid metabolism and cell membrane is the same in C6O4 and PFOA exposed clams. In addition, clams exposed to C6O4 showed dose-dependent responses as well as possible narcotic or neurotoxic effects and reduced activation of genes involved in xenobiotic metabolism. Overall, the present study suggests that the potential risks for marine organism following environmental contamination are not reduced by replacing PFOA with C6O4. In addition, the detection of both C6O4 and PFOA into tissues of clams inhabiting the Lagoon of Venice - where there are no point sources of either compounds - recommends a similar capacity to spread throughout the environment. These results prompt the urgent need to re-evaluate the use of C6O4 as it may represent not only an environmental hazard but also a potential risk for human health.

The symbiotic 'all-rounders': Partnerships between marine animals and chemosynthetic nitrogen-fixing bacteria

Nitrogen fixation is a widespread metabolic trait in certain types of microorganisms called diazotrophs. Bioavailable nitrogen is limited in various habitats on land and in the sea, and accordingly, a range of plant, animal, and single-celled eukaryotes have evolved symbioses with diverse diazotrophic bacteria, with enormous economic and ecological benefits. Until recently, all known nitrogen-fixing symbionts were heterotrophs such as nodulating rhizobia, or photoautotrophs such as cyanobacteria. In 2016, the first chemoautotrophic nitrogen-fixing symbionts were discovered in a common family of marine clams, the Lucinidae. Chemosynthetic nitrogen-fixing symbionts use the chemical energy stored in reduced sulfur compounds to power carbon and nitrogen fixation, making them metabolic 'all-rounders' with multiple functions in the symbiosis. This distinguishes them from heterotrophic symbionts that require a source of carbon from their host, and their chemosynthetic metabolism distinguishes them from photoautotrophic symbionts that produce oxygen, a potent inhibitor of nitrogenase. In this review, we consider evolutionary aspects of this discovery, by comparing strategies that have evolved for hosting intracellular nitrogen-fixing symbionts in plants and animals. The symbiosis between lucinid clams and chemosynthetic nitrogen-fixing bacteria also has important ecological impacts, as they form a nested symbiosis with endangered marine seagrasses. Notably, nitrogen fixation by lucinid symbionts may help support seagrass health by providing a source of nitrogen in seagrass habitats. These discoveries were enabled by new techniques for understanding the activity of microbial populations in natural environments. However, an animal (or plant) host represents a diverse landscape of microbial niches due to its structural, chemical, immune and behavioural properties. In future, methods that resolve microbial activity at the single cell level will provide radical new insights into the regulation of nitrogen fixation in chemosynthetic symbionts, shedding new light on the evolution of nitrogen-fixing symbioses in contrasting hosts and environments.

Comparative Proteomics of Ostreid Herpesvirus 1 and Pacific Oyster Interactions With Two Families Exhibiting Contrasted Susceptibility to Viral Infection

Massive mortality outbreaks affecting Pacific oysters (Crassostrea gigas) spat/juveniles are often associated with the detection of a herpesvirus called ostreid herpesvirus type 1 (OsHV-1). In this work, experimental infection trials of C. gigas spat with OsHV-1 were conducted using two contrasted Pacific oyster families for their susceptibility to viral infection. Live oysters were sampled at 12, 26, and 144 h post infection (hpi) to analyze host-pathogen interactions using comparative proteomics. Shotgun proteomics allowed the detection of seven viral proteins in infected oysters, some of them with potential immunomodulatoy functions. Viral proteins were mainly detected in susceptible oysters sampled at 26 hpi, which correlates with the mortality and viral load observed in this oyster family. Concerning the Pacific oyster proteome, more than 3,000 proteins were identified and contrasted proteomic responses were observed between infected A- and P-oysters, sampled at different post-injection times. Gene ontology (GO) and KEGG pathway enrichment analysis performed on significantly modulated proteins uncover the main immune processes (such as RNA interference, interferon-like pathway, antioxidant defense) which contribute to the defense and resistance of Pacific oysters to viral infection. In the more susceptible Pacific oysters, results suggest that OsHV-1 manipulate the molecular machinery of host immune response, in particular the autophagy system. This immunomodulation may lead to weakening and consecutively triggering death of Pacific oysters. The identification of several highly modulated and defense-related Pacific oyster proteins from the most resistant oysters supports the crucial role played by the innate immune system against OsHV-1 and the viral infection. Our results confirm the implication of proteins involved in an interferon-like pathway for efficient antiviral defenses and suggest that proteins involved in RNA interference process prevent viral replication in C. gigas. Overall, this study shows the interest of multi-omic approaches applied on groups of animals with differing sensitivities and provides novel insight into the interaction between Pacific oyster and OsHV-1 with key proteins involved in viral infection resistance.

Evolution and Potential Function in Molluscs of Neuropeptide and Receptor Homologues of the Insect Allatostatins

The allatostatins (ASTs), AST-A, AST-B and AST-C, have mainly been investigated in insects. They are a large group of small pleotropic alloregulatory neuropeptides that are unrelated in sequence and activate receptors of the rhodopsin G-protein coupled receptor family (GPCRs). The characteristics and functions of the homologue systems in the molluscs (Buccalin, MIP and AST-C-like), the second most diverse group of protostomes after the arthropods, and of high interest for evolutionary studies due to their less rearranged genomes remains to be explored. In the present study their evolution is deciphered in molluscs and putative functions assigned in bivalves through meta-analysis of transcriptomes and experiments. Homologues of the three arthropod AST-type peptide precursors were identified in molluscs and produce a larger number of mature peptides than in insects. The number of putative receptors were also distinct across mollusc species due to lineage and species-specific duplications. Our evolutionary analysis of the receptors identified for the first time in a mollusc, the cephalopod, GALR-like genes, which challenges the accepted paradigm that AST-AR/buccalin-Rs are the orthologues of vertebrate GALRs in protostomes. Tissue transcriptomes revealed the peptides, and their putative receptors have a widespread distribution in bivalves and in the bivalve Mytilus galloprovincialis, elements of the three peptide-receptor systems are highly abundant in the mantle an innate immune barrier tissue. Exposure of M. galloprovincialis to lipopolysaccharide or a marine pathogenic bacterium, Vibrio harveyi, provoked significant modifications in the expression of genes of the peptide precursor and receptors of the AST-C-like system in the mantle suggesting involvement in the immune response. Overall, our study reveals that homologues of the arthropod AST-systems in molluscs are potentially more complex due to the greater number of putative mature peptides and receptor genes. In bivalves they have a broad and varying tissue distribution and abundance, and the elements of the AST-C-like family may have a putative function in the immune response.

Cellular and molecular complementary immune stress markers for the model species Dreissena polymorpha

This study aimed to combine cellular and molecular analyses for better detail the effects of various stresses on a sentinel species of freshwater invertebrate. For this purpose, the hemocytes of the zebra mussel, Dreissena polymorpha, were exposed to different stresses at two different intensities, high or low: chemical (cadmium and ionomycin), physical (ultraviolet B), or biological ones (Cryptosporidium parvum and Toxoplasma gondii). After exposure, flow cytometry and droplet digital PCR analyses were performed on the same pools of hemocytes. Several responses related to necrosis, apoptosis, phagocytosis, production of nitric oxide and expression level of several genes related to the antioxidant, detoxification and immune systems were evaluated. Results showed that hemocyte integrity was compromised by both chemical and physical stress, and cellular markers of phagocytosis reacted to ionomycin and protozoa. While cadmium induced oxidative stress and necrosis, ionomycin tends to modulate the immune response of hemocytes. Although both biological stresses led to a similar immune response, C. parvum oocysts induced more effects than T. gondii, notably through the expression of effector caspases gene and an increase in hemocyte necrosis. This suggests different management of the two protozoa by the cell. This work provides new knowledge of biomarkers in the zebra mussel, at both cellular and molecular levels, and contributes to elucidate the mechanisms of action of different kinds of stress in this species.

Host Defense Effectors Expressed by Hemocytes Shape the Bacterial Microbiota From the Scallop Hemolymph

The interaction between host immune response and the associated microbiota has recently become a fundamental aspect of vertebrate and invertebrate animal health. This interaction allows the specific association of microbial communities, which participate in a variety of processes in the host including protection against pathogens. Marine aquatic invertebrates such as scallops are also colonized by diverse microbial communities. Scallops remain healthy most of the time, and in general, only a few species are fatally affected on adult stage by viral and bacterial pathogens. Still, high mortalities at larval stages are widely reported and they are associated with pathogenic Vibrio. Thus, to give new insights into the interaction between scallop immune response and its associated microbiota, we assessed the involvement of two host antimicrobial effectors in shaping the abundances of bacterial communities present in the scallop Argopecten purpuratus hemolymph. To do this, we first characterized the microbiota composition in the hemolymph from non-stimulated scallops, finding both common and distinct bacterial communities dominated by the Proteobacteria, Spirochaetes and Bacteroidetes phyla. Next, we identified dynamic shifts of certain bacterial communities in the scallop hemolymph along immune response progression, where host antimicrobial effectors were expressed at basal level and early induced after a bacterial challenge. Finally, the transcript silencing of the antimicrobial peptide big defensin ApBD1 and the bactericidal/permeability-increasing protein ApLBP/BPI1 by RNA interference led to an imbalance of target bacterial groups from scallop hemolymph. Specifically, a significant increase in the class Gammaproteobacteria and the proliferation of Vibrio spp. was observed in scallops silenced for each antimicrobial. Overall, our results strongly suggest that scallop antimicrobial peptides and proteins are implicated in the maintenance of microbial homeostasis and are key molecules in orchestrating host-microbiota interactions. This new evidence depicts the delicate balance that exists between the immune response of A. purpuratus and the hemolymph microbiota.

Hemocytes released in seawater act as Trojan horses for spreading of bacterial infections in mussels

Global warming has been associated with increased episodes of mass mortality events in invertebrates, most notably in bivalves. Although the spread of pathogens is one of multiple factors that contribute to such mass mortality events, we don’t fully understand the pathophysiological consequences of sea warming on invertebrates. In this work, we show that in temperature stress conditions, circulating hemocytes in mussels leave the hemolymph to gain access to the intervalvar fluid before being released in seawater. External hemocytes can survive for several hours in seawater before entering other mussels. When infected by bacteria, externally-infected hemocytes can enter naive mussels and promote bacterial dissemination in the host. These results reveal the existence of a new opportunistic mechanism used by pathogens to disseminate in marine ecosystems. Such mechanisms may explain how thermal anomalies triggered by global warming can favor episodic mass mortality observed in recent years in marine ecosystem.

Unraveling concordant and varying responses of oyster species to Ostreid Herpesvirus 1 variants

The Ostreid herpesvirus 1 (OsHV-1) and variants, particularly the microvariants (μVars), are virulent and economically devastating viruses impacting oysters. Since 2008 OsHV-1 μVars have emerged rapidly having particularly damaging effects on aquaculture industries in Europe, Australia and New Zealand. We conducted field trials in Tomales Bay (TB), California where a non-μVar strain of OsHV-1 is established and demonstrated differential mortality of naturally exposed seed of three stocks of Pacific oyster, Crassostrea gigas, and one stock of Kumamoto oyster, C. sikamea. Oysters exposed in the field experienced differential mortality that ranged from 64 to 99% in Pacific oysters (Tasmania>Midori = Willapa stocks), which was much higher than that of Kumamoto oysters (25%). Injection trials were done using French (FRA) and Australian (AUS) μVars with the same oyster stocks as planted in the field and, in addition, two stocks of the Eastern oyster, C. virginica. No mortality was observed in control oysters. One C. virginica stock suffered ~10% mortality when challenged with both μVars tested. Two Pacific oyster stocks suffered 75 to 90% mortality, while one C. gigas stock had relatively low mortality when challenged with the AUS μVar (~22%) and higher mortality when challenged with the French μVar (~72%). Conversely, C. sikamea suffered lower mortality when challenged with the French μVar (~22%) and higher mortality with the AUS μVar (~44%). All dead oysters had higher viral loads (~1000×) as measured by quantitative PCR relative to those that survived. However, some survivors had high levels of virus, including those from species with lower mortality. Field mortality in TB correlated with laboratory mortality of the FRA μVar (69% correlation) but not with that of the AUS μVar, which also lacked correlation with the FRA μVar. The variation in response to OsHV-1 variant challenges by oyster species and stocks demonstrates the need for empirical assessment of multiple OsHV-1 variants.

Contrasting Immunomodulatory Effects of Probiotic and Pathogenic Bacteria on Eastern Oyster, Crassostrea Virginica, Larvae

Several Vibrio spp. cause acute and severe mortality events in hatcheries where larvae of bivalve mollusks are reared, potentially leading to subsequent shortage of bivalve seed for the grow-out industry. In particular, strains of Vibrio coralliilyticus have been identified as a major cause of disease in Pacific, Crassostrea gigas, and eastern, C. virginica, oyster hatcheries in the United States of America. Probiotic bacteria are an inexpensive, practical, and natural method of disease control. Previous research shows that pretreatment of larval oysters with probiotic bacteria Bacillus pumilus RI06-95 (RI) and Phaeobacter inhibens S4 (S4) significantly decreases mortality caused by experimental challenge with the bacterial pathogen V. coralliilyticus RE22 (RE22). This study aims to characterize the immune response of 6-10-day-old eastern oyster larvae to experimental challenge with pathogen V. coralliilyticus RE22 and probionts RI and S4. Treatments included (a) pathogen and probiont exposure at a concentration of 5 × 10⁴ CFU per mL (~2500 bacterial cells per larva) for a duration of 6 h, (b) probiont exposure at the same concentration for a duration of 24 h, and (c) probiont RI daily treatment of larvae in the hatchery for 4, 11, and 15 days. Differential gene expression analysis compared pathogen or probiotic-treated transcriptomes to unexposed controls. Probiotic and pathogen treatment led to upregulation of transcripts coding for several immune pattern recognition receptors (PRRs) involved in environmental sensing and detection of microbes in oyster larvae. Larval oyster responses to pathogen RE22 suggested suppression of expression of genes in immune signaling pathways (myd88, tak1, nkap), failure in upregulation of immune effector genes, high metabolic demand, and oxidative stress that potentially contributed to mortality. On the other hand, the transcriptomic response to probiotic bacteria RI and S4 suggested activation of immune signaling pathways and expression of immune effectors (e.g., Cv-spi2, mucins and perforin-2). These key features of the host immune response to probiotic bacteria were shared despite the length of probiotic exposure, probiotic species, and the type of environment in which exposures were conducted. This study suggests that pre-exposure of eastern oyster larvae to probiotics for 6-24 h prior to pathogenic challenge leads to a robust and effective immune response that may contribute to protecting larvae from subsequent challenge with V. coralliilyticus RE22. This research provides new insights into host-microbe interactions in larval oysters that could be applied in the management of vibriosis in bivalve hatcheries.

Effects of high stocking density on the galectin gene expression in noble scallop Chalmys nobilis under bacterial infection

High stocking densities have been shown to have adverse effects on the physiology of bivalves. The noble scallop Chlamys nobilis is one of the most important cultured shellfish in Southern China. However, the effects of scallop stocking density on its immunity is not well understood. In this context, this study was conducted to assess the effect of high stocking density on the galectin (an important protein in innate immunity) gene expression of C. nobilis during bacterial infection. A full-length galectin (CnGal) gene was cloned. The ORF of the CnGal cDNA encodes a predicted protein containing 549 aa with four CRDs and no signal peptide. Our results reveal that high stocking density in the scallop not only led to high mortality and slow growth, but also changed tissue distribution of the CnGal expression. The individuals from the high stocking density group exhibited more differences among tissues than those from the control group, but the highest expression were both recorded in hemolymph. After the Vibrio parahaeomlyticus challenge, the gene's expression levels were all significantly up-regulated in the hemolymph and gill, but the time up to peak was different between the two tissues. The findings of this study could fill a gap in knowledge about how high stocking density affect scallop immunity at the molecular level.

Listeria monocytogenes induced dysbiosis in snails and rebiosis achieved by administration of the gut commensal Lactobacillus plantarum Sgs14 strain

Listeria monocytogenes strains were isolated from Cornu aspersum maxima snails from farm units experiencing high mortalities and were characterized by phenotypic, molecular and biochemical criteria. A high heterogeneity was observed in the pulsed-field gel electrophoresis (PFGE) pulsotypes as well as in the virulence (13-100% mortality) among the fifteen L. monocytogenes strains. One strain was characterized as non-virulent while three strains exhibited hypervirulent phenotype. Hypervirulence activity was associated with cell surface properties such as hydrophobicity, autoaggregation and biofilm formation, with increased tolerance to snail's gut barriers such as pedal mucus, gastric mucus, gastric juices, and acidic pH as well as with increased capacity to resist the antibacterial activity of snail haemolymph and modulate immune cell populations and functions such as chemotaxis and phagocytoses. L. monocytogenes dysbiosis was characterized by a clinicopathological phenotype including immobilization of snails' headfoot outside the shell, increased mucus-secreting cells in the intestinal epithelium and feces, alteration of intestinal ridges morphology and excessive increase of haemolymph immune cells and cell death. Rebiosis in L. monocytogenes SN3 strain infected snails was achieved by dietary supplementation of the snail-gut commensal probiotic L. plantarum Sgs14 strain by exhibiting anti-Listeria activity, reducing mortality and clinicopathological manifestations as well as exhibiting immunomodulatory activity.

Comparative genomic analysis of Vibrios yields insights into genes associated with virulence towards C. gigas larvae

BACKGROUND

Vibriosis has been implicated in major losses of larvae at shellfish hatcheries. However, the species of Vibrio responsible for disease in aquaculture settings and their associated virulence genes are often variable or undefined. Knowledge of the specific nature of these factors is essential to developing a better understanding of the environmental and biological conditions that lead to larvae mortality events in hatcheries. We tested the virulence of 51 Vibrio strains towards Pacific Oyster (Crassostreae gigas) larvae and sequenced draft genomes of 42 hatchery-associated vibrios to determine groups of orthologous genes associated with virulence and to determine the phylogenetic relationships among pathogens and non-pathogens of C. gigas larvae.

RESULTS

V. coralliilyticus strains were the most prevalent pathogenic isolates. A phylogenetic logistic regression model identified over 500 protein-coding genes correlated with pathogenicity. Many of these genes had straightforward links to disease mechanisms, including predicted hemolysins, proteases, and multiple Type 3 Secretion System genes, while others appear to have possible indirect roles in pathogenesis and may be more important for general survival in the host environment. Multiple metabolism and nutrient acquisition genes were also identified to correlate with pathogenicity, highlighting specific features that may enable pathogen survival within C. gigas larvae.

CONCLUSIONS

These findings have important implications on the range of pathogenic Vibrio spp. found in oyster-rearing environments and the genetic determinants of virulence in these populations.

Survival and infectivity of Toxoplasma gondii and Cryptosporidium parvum oocysts bioaccumulated by Dreissena polymorpha

AIMS

The study was aimed to understand the depuration process of Cryptosporidium parvum and Toxoplasma gondii oocysts by zebra mussel (Dreissena polymorpha), to consider the use of the zebra mussel as a bioremediation tool.

MATERIALS AND METHODS

Two experiments were performed: (i) individual exposure of mussel to investigate oocyst transfers between bivalves and water and (ii) in vivo exposure to assess the ability of the zebra mussel to degrade oocysts.

RESULTS

(i) Our results highlighted a transfer of oocysts from the mussels to the water after 3 and 7 days of depuration; however, some oocysts were still bioaccumulated in mussel tissue. (ii) Between 7 days of exposure at 1000 or 10 000 oocysts/mussel/day and 7 days of depuration, the number of bioaccumulated oocysts did not vary but the number of infectious oocysts decreased.

CONCLUSION

Results show that D. polymorpha can release oocysts in water via (pseudo)faeces in depuration period. Oocysts remain bioaccumulated and infectious oocyst number decreases during the depuration period in zebra mussel tissues. Results suggest a degradation of bioaccumulated C. parvum and T. gondii oocysts.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlighted the potential use of D. polymorpha as a bioremediation tool to mitigate of protozoan contamination in water resources.

Genome to phenome tools: In vivo and in vitro transfection of Crassostrea virginica hemocytes

The sequencing of the Crassostrea virginica genome has brought back the interest for gene delivery and editing methodologies. Here, we report the expression in oyster hemocytes of two heterologous expression vectors under the CMV promoter delivered with dendrimers. Expression was monitored using confocal microscopy, flow cytometry, and immunofluorescence assay. C. virginica hemocytes were able to express the green fluorescence protein and Crassostrea gigas vascular endothelial growth factor under CMV viral promoter both in vivo and in vitro. These results provide the bases for interrogating the genome and adapting genome editing methodologies.