Haemocyte parameters associated with resistance to brown ring disease in Ruditapes spp. clams

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.

Proteomic and Transcriptomic Responses Enable Clams to Correct the pH of Calcifying Fluids and Sustain Biomineralization in Acidified Environments

Seawater pH and carbonate saturation are predicted to decrease dramatically by the end of the century. This process, designated ocean acidification (OA), threatens economically and ecologically important marine calcifiers, including the northern quahog (Mercenaria mercenaria). While many studies have demonstrated the adverse impacts of OA on bivalves, much less is known about mechanisms of resilience and adaptive strategies. Here, we examined clam responses to OA by evaluating cellular (hemocyte activities) and molecular (high-throughput proteomics, RNASeq) changes in hemolymph and extrapallial fluid (EPF-the site of biomineralization located between the mantle and the shell) in M. mercenaria continuously exposed to acidified (pH ~7.3; pCO₂ ~2700 ppm) and normal conditions (pH ~8.1; pCO₂ ~600 ppm) for one year. The extracellular pH of EPF and hemolymph (~7.5) was significantly higher than that of the external acidified seawater (~7.3). Under OA conditions, granulocytes (a sub-population of hemocytes important for biomineralization) were able to increase intracellular pH (by 54% in EPF and 79% in hemolymph) and calcium content (by 56% in hemolymph). The increased pH of EPF and hemolymph from clams exposed to high pCO₂ was associated with the overexpression of genes (at both the mRNA and protein levels) related to biomineralization, acid-base balance, and calcium homeostasis, suggesting that clams can use corrective mechanisms to mitigate the negative impact of OA.

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.

Depuration processes affect the Vibrio community in the microbiota of the Manila clam, Ruditapes philippinarum

As filter-feeders, bivalve molluscs accumulate Vibrio into edible tissues. Consequently, an accurate assessment of depuration procedures and the characterization of the persistent Vibrio community in depurated shellfish represent a key issue to guarantee food safety in shellfish products. The present study investigated changes in the natural Vibrio community composition of the Ruditapes philippinarum microbiota with specific focus on human pathogenic species. For this purpose, the study proposed a MLSA-NGS approach (rRNA 16S, recA and pyrH) for the detection and identification of Vibrio species. Clam microbiota were analysed before and after depuration procedures performed in four depuration plants, using culture-dependent and independent approaches. Microbiological counts and NGS data revealed differences in terms of both contamination load and Vibrio community between depuration plants. The novel MLSA-NGS approach allowed for a clear definition of the Vibrio species specific to each depuration plant. Specifically, depurated clam microbiota showed presence of human pathogenic species. Ozone treatments and the density of clams in the depuration tank probably influenced the level of contamination and the Vibrio community composition. The composition of Vibrio community specific to each plant should be carefully evaluated during the risk assessment to guarantee a food-safe shellfish-product for the consumer.

Shell repair and the potential microbial causal in a shell disease of the pearl oyster Pinctada fucata

The pearl oyster Pinctada fucata is famous for producing luxurious pearls. As filter feeders, they are confronted with various infectious microorganisms. Despite a long history of aquaculture, diseases in P. fucata are not well studied, which limits the development of the pearl industry. We report here a shell disease in P. fucata and a study of the shell repair processes. Scanning electron microscopy (SEM) revealed that the nacreous layer gradually recovered from disordered CaCO₃ deposition, accompanied by a polymorphic transition from a calcite-aragonite mixture to an aragonite-dominant composition, as revealed by X-ray diffraction analysis. SEM also showed that numerous microbes were embedded in the abnormal shell layers. Similar indications were induced by a high concentration of microbes injected into the extrapallial space, suggesting the potential pathogenic effect of uncontrolled microbes. Furthermore, hemocytes were found to participate in pathogens resistance and might promote shell repair. These results further our understanding of pathogen-host interactions in pearl oysters and have implications for biotic control in pearl aquaculture.

Transepithelial migration of mucosal hemocytes in Crassostrea virginica and potential role in Perkinsus marinus pathogenesis

We have recently described the presence of hemocytes associated with mucus covering the pallial organs (mantle, gills, and body wall) 3 of the eastern oyster Crassostrea virginica. These hemocytes, hereby designated "pallial hemocytes" share common general characteristics with circulating hemocytes but also display significant differences particularly in their cell surface epitopes. The specific location of pallial hemocytes as peripheral cells exposed directly to the marine environment confers them a putative sentinel role. The purpose of this study was to gain a better understanding of the source of these pallial hemocytes by evaluating possible exchanges between circulatory and pallial hemocyte populations and whether these exchanges are regulated by pathogen exposure. Bi-directional transepithelial migrations of hemocytes between pallial surfaces and the circulatory system were monitored using standard cell tracking approaches after staining with the vital fluorescent dye carboxyfluorescein diacetate succinimidyl ester (CFSE) in conjunction with fluorescent microscopy and flow cytometry. Results showed bi-directional migration of hemocytes between both compartments and suggest that hemocyte migration from the pallial mucus layer to the circulatory system may occur at a greater rate compared to migration from the circulatory system to the pallial mucus layer, further supporting the role of pallial hemocytes as sentinel cells. Subsequently, the effect of the obligate parasite Perkinsus marinus and the opportunistic pathogen Vibrio alginolyticus on transepithelial migration of oyster hemocytes was investigated. Results showed an increase in hemocyte migration in response to P. marinus exposure. Furthermore, P. marinus cells were acquired by pallial hemocytes before being visible in underlying tissues and the circulatory system suggesting that this parasite could use pallial hemocytes as a vehicle facilitating its access to oyster tissues. These results are discussed in light of new evidence highlighting the role of oyster pallial organs as a portal for the initiation of P. marinus infections in oysters.

Vibrio tapetis Displays an Original Type IV Secretion System in Strains Pathogenic for Bivalve Molluscs

The Brown Ring Disease (BRD) caused high mortality rates since 1986 in the Manila clam Venerupis philippinarum introduced and cultured in Western Europe from the 1970s. The causative agent of BRD is a Gram-Negative bacterium, Vibrio tapetis, which is also pathogenic to fish. Here we report the first assembly of the complete genome of V. tapetis CECT4600^T, together with the genome sequences of 16 additional strains isolated across a broad host and geographic range. Our extensive genome dataset allowed us to describe the pathogen pan- and core genomes and to identify putative virulence factors. The V. tapetis core genome consists of 3,352 genes, including multiple potential virulence factors represented by haemolysins, transcriptional regulators, Type I restriction modification system, GGDEF domain proteins, several conjugative plasmids, and a Type IV secretion system. Future research on the coevolutionary arms race between V. tapetis virulence factors and host resistance mechanisms will improve our understanding of how pathogenicity develops in this emerging pathogen.

Hybridization improved bacteria resistance in abalone: Evidence from physiological and molecular responses

Hybridization is an effective way of improving germplasm in abalone, as it often generates benign traits in the hybrids. The hybrids of Haliotis discus hannai and H. gigantea have shown heterosis in terms of disease resistance than one or both parental species. In the present study, to elucidate the physiological and molecular mechanism of this heterosis, we analyzed the dynamic changes of several immune indexes including survival rate, total circulating haemocyte count (THC), phagocytic activity, reactive oxygen species level (ROS) and phenoloxidase activity (PO) in two parental species, H. discus hannai (DD) and H. gigantea (GG), and their reciprocal hybrids H. discus hannai ♀ × H. gigantea ♂ (DG), H. gigantea ♀ × H. discus hannai ♂ (GD) challenged with a mixture of Vibrio harveyi, V. alginolyticus and V. parahaemolyticus (which have been demonstrated to be pathogenic to abalone). Besides, we cloned and analyzed three important immune genes: heat shock protein 70 (hsp70), ferritin and cold shock domain protein (csdp) in H. discus hannai and H. gigantea, then further investigated their mRNA level changes in the four abalone genotypes after bacterial challenge. Results showed that these physiological and molecular parameters were significantly induced by bacterial exposure, and their changing patterns were obviously different between the four genotypes: (1) Survival rates of the two hybrids were higher than both parental species after bacterial exposure; (2) DG had higher THC than the other three genotypes; (3) Phagocytosis responded slower in the hybrids than in the parental species; (4) DD's ROS level was lower than the other three genotypes at 48 h post infection; (5) Phenoloxidase activity was lower in DD during the infection compared to the other genotypes; (6) mRNA levels of hsp70 and csdp, were always lower in at least one parental species (DD) than in the hybrids after the bacterial exposure. Results from this study indicate that the hybrids are more active or efficient in immune system function, hence they could effectively defense against a bacterial invasion, leading to higher survival rates after challenge. This study provides physiological and molecular evidences for interpreting the disease resistant heterosis in this abalone hybrid system, which could help us in a better understanding and utilization of heterosis in abalone aquaculture.

Characterization of hemocytes from different body fluids of the eastern oyster Crassostrea virginica

Bivalve hemocytes are involved in a variety of physiological and immunological functions. Circulating hemocytes in the hemolymph represent the main component of the internal self-defense system while hemocytes present in the extrapallial space (between the mantle and the shell) are actively involved in biomineralization and shell formation. This study focused on the characterization of hemocytes from different body fluids of the eastern oyster Crassostrea virginica. Hemocytes present in the hemolymph were compared to those contained in the extrapallial fluid. Hemocytes associated with the mucus layer covering pallial organs (mantle, gills, body wall) were also investigated because of their potential role as sentinel cells. Hemocytes were characterized using flow cytometry in conjunction with fluorescent epitope markers (clusters of differentiation, lectins) as well as functional assays (i.e. phagocytosis and reactive oxygen species -ROS). Compared with the hemolymph, there was a significantly greater percentage of granulocytes and agranulocytes among extrapallial and pallial hemocytes, respectively. Accounting for the different percentages of hemocyte sub-populations, significant differences in surface carbohydrate and clusters of differentiation signatures were also revealed between the different fluids. Most informative epitope markers included concanavalin A, peanut agglutinin, soybean agglutinin, CD11b and CD14. Functional assays revealed significant differences in phagocytic activity and ROS production between hemocytes from the extrapallial fluid and hemolymph; however, less robust differences were observed between hemolymph cells and hemocytes associated with the pallial mucus. Findings from this study suggest that there are markedly different hemocyte populations in the three body fluids. The role of peripheral cells, particularly those associated with the pallial mucus, requires further investigations.

A glutamic acid decarboxylase (CgGAD) highly expressed in hemocytes of Pacific oyster Crassostrea gigas

Glutamic acid decarboxylase (GAD), a rate-limiting enzyme to catalyze the reaction converting the excitatory neurotransmitter glutamate to inhibitory neurotransmitter γ-aminobutyric acid (GABA), not only functions in nervous system, but also plays important roles in immunomodulation in vertebrates. However, GAD has rarely been reported in invertebrates, and never in molluscs. In the present study, one GAD homologue (designed as CgGAD) was identified from Pacific oyster Crassostrea gigas. The full length cDNA of CgGAD was 1689 bp encoding a polypeptide of 562 amino acids containing a conserved pyridoxal-dependent decarboxylase domain. CgGAD mRNA and protein could be detected in ganglion and hemocytes of oysters, and their abundance in hemocytes was unexpectedly much higher than those in ganglion. More importantly, CgGAD was mostly located in those granulocytes without phagocytic capacity in oysters, and could dynamically respond to LPS stimulation. Further, after being transfected into HEK293 cells, CgGAD could promote the production of GABA. Collectively, these findings suggested that CgGAD, as a GABA synthase and molecular marker of GABAergic system, was mainly distributed in hemocytes and ganglion and involved in neuroendocrine-immune regulation network in oysters, which also provided a novel insight to the co-evolution between nervous system and immune system.

Pathogenic marine microbes influence the effects of climate change on a commercially important tropical bivalve

There is growing evidence that climate change will increase the prevalence of toxic algae and harmful bacteria, which can accumulate in marine bivalves. However, we know little about any possible interactions between exposure to these microorganisms and the effects of climate change on bivalve health, or about how this may affect the bivalve toxin-pathogen load. In mesocosm experiments, mussels, Perna viridis, were subjected to simulated climate change (warming and/or hyposalinity) and exposed to harmful bacteria and/or toxin-producing dinoflagellates. We found significant interactions between climate change and these microbes on metabolic and/or immunobiological function and toxin-pathogen load in mussels. Surprisingly, however, these effects were virtually eliminated when mussels were exposed to both harmful microorganisms simultaneously. This study is the first to examine the effects of climate change on determining mussel toxin-pathogen load in an ecologically relevant, multi-trophic context. The results may have considerable implications for seafood safety.

Interactive effects of copper exposure and environmental hypercapnia on immune functions of marine bivalves Crassostrea virginica and Mercenaria mercenaria

Estuarine organisms such as bivalves are commonly exposed to trace metals such as copper (Cu) and hypercapnia (elevated CO2 levels) in their habitats, which may affect their physiology and immune function. This study investigated the combined effects of elevated CO2 levels (∼800-2000 μatm PCO2, such as predicted by the near-future scenarios of global climate change) and Cu (50 μg l(-1)) on immune functions of the sediment dwelling hard clams Mercenaria mercenaria and an epifaunal bivalve, the eastern oyster Crassostrea virginica. Clams and oysters were exposed for 4 weeks to different CO2 and Cu levels, and tissue Cu burdens and immune parameters were assessed to test the hypothesis that hypercapnia will enhance Cu uptake due to the higher bioavailability of free Cu(2+) and increase the immunomodulatory effects of Cu. Exposure to Cu stimulated key immune parameters of clams and oysters leading to increased number of circulating hemocytes, higher phagocytosis and adhesion ability of hemocytes, as well as enhanced antiparasitic and antibacterial properties of the hemolymph reflected in higher activities of lysozyme and inhibitors of cysteine proteases. Lysozyme activation by Cu exposure was most prominent in normocapnia (∼400 μatm PCO2) and an increase in the levels of the protease inhibitors was strongest in hypercapnia (∼800-2000 μatm PCO2), but other immunostimulatory effects of Cu were evident in all PCO2 exposures. Metabolic activity of hemocytes of clams and oysters (measured as routine and mitochondrial oxygen consumption rates) was suppressed by Cu exposure likely reflecting lower rates of ATP synthesis and/or turnover. However, this metabolic suppression had no negative effects of the studied immune functions of hemocytes such as phagocytosis or adhesion capacity. Hypercapnia (∼800-2000 μatm PCO2) slightly but significantly enhanced accumulation of Cu in hemocytes, consistent with higher Cu(2+) bioavailability in CO2-acidified water, but had little effect on cellular and humoral immune traits of clams and oysters. These findings indicate that low levels of Cu contamination may enhance immunity of estuarine bivalves while moderate hypercapnia (such as predicted by the near future scenarios of the global climate change) does not strongly affect their immune parameters.

Effects of a municipal effluent on the freshwater mussel Elliptio complanata following challenge with Vibrio anguillarum

The purpose of this study was to examine the cumulative effects of exposure to a pathogenic bacteria and municipal effluent in the freshwater mussel Elliptio complanata. Mussels were exposed to increasing concentrations of an ozone-treated effluent at 15°C for 7days. A sub-group of mussels was inoculated with Vibrio anguillarum and exposed to the same conditions as above. After the exposure period, mussels were collected to assess hemocyte count and viability, immunocompetence (phagocytosis and nitrite production), oxidative stress/inflammation (cyclooxygenase and lipid peroxidation) and oxygen radical/xenobiotic scavenging activity (metallothioniens, glutathione S-transferase). The results showed that mussels exposed to municipal effluent had increased hemocyte counts, phagocytosis, nitrites, lipid peroxidation and metallothioneins. In the inoculated mussels, the same responses were observed, in addition to cyclooxygenase and glutathione S-transferase activities. Multivariate analyses revealed that (1) the response pattern changed with effluent concentration, where increased responses observed at low effluent concentrations (>10%, V/V) were attenuated at higher effluent concentrations, (2) the effluent produced more pronounced changes in lipid peroxidation, metallothionein and hemocyte viability, and (3) the simultaneous presence of V. anguillarum led to more important changes in hemocyte count and viability and nitrite levels. In conclusion, the presence of V. anguillarum could alter the response of mussels to municipal effluent, which could lead to increased inflammation in mussels.

Morphology and classification of hemocytes in Pinctada fucata and their responses to ocean acidification and warming

Hemocytes play important roles in the innate immune response and biomineralization of bivalve mollusks. However, the hemocytes in pearl oysters are poorly understood. In the present study, we investigated the morphology and classification of hemocytes in the pearl oyster, Pinctada fucata. Three types of hemocytes were successfully obtained by light microscopy, electron microscopy and flow cytometry methods: small hyalinocytes, large hyalinocytes and granulocytes. The small hyalinocytes are the major hemocyte population. Morphological analyses indicated that these hemocytes have species-specific characterizations. In addition, we assessed the potential effects of ocean acidification (OA) and ocean warming (OW) on the immune parameters and calcium homeostasis of the hemocytes. OA and OW (31 °C) altered pH value of hemolymph, increased the total hemocyte count, total protein content, and percentage of large hyalinocytes and granulocytes, while it decreased the neutral red uptake ability, suggesting active stress responses of P. fucata to these stressors. Exposure to OW (25 °C) resulted in no significant differences, indicating an excellent immune defense to heat stress at this level. The outflow of calcium from hemocytes to hemolymph was also determined, implying the potential impact of OA and OW on hemocyte-mediated biomineralization. This study, therefore, provides insight into the classification and characterization of hemocyte in the pearl oyster, P. fucata, and also reveals the immune responses of hemocytes to OA and OW, which are helpful for a comprehensive understanding of the effects of global climate change on pearl oysters.

Immune responses to infectious diseases in bivalves

Many species of bivalve mollusks (phylum Mollusca, class Bivalvia) are important in fisheries and aquaculture, whilst others are critical to ecosystem structure and function. These crucial roles mean that considerable attention has been paid to the immune responses of bivalves such as oysters, clams and mussels against infectious diseases that can threaten the viability of entire populations. As with many invertebrates, bivalves have a comprehensive repertoire of immune cells, genes and proteins. Hemocytes represent the backbone of the bivalve immune system. However, it is clear that mucosal tissues at the interface with the environment also play a critical role in host defense. Bivalve immune cells express a range of pattern recognition receptors and are highly responsive to the recognition of microbe-associated molecular patterns. Their responses to infection include chemotaxis, phagolysosomal activity, encapsulation, complex intracellular signaling and transcriptional activity, apoptosis, and the induction of anti-viral states. Bivalves also express a range of inducible extracellular recognition and effector proteins, such as lectins, peptidoglycan-recognition proteins, thioester bearing proteins, lipopolysaccharide and β1,3-glucan-binding proteins, fibrinogen-related proteins (FREPs) and antimicrobial proteins. The identification of FREPs and other highly diversified gene families in bivalves leaves open the possibility that some of their responses to infection may involve a high degree of pathogen specificity and immune priming. The current review article provides a comprehensive, but not exhaustive, description of these factors and how they are regulated by infectious agents. It concludes that one of the remaining challenges is to use new "omics" technologies to understand how this diverse array of factors is integrated and controlled during infection.

Transcriptional changes in Manila clam (Ruditapes philippinarum) in response to Brown Ring Disease

Brown Ring Disease (BRD) is a bacterial infection affecting the economically-important clam Ruditapes philippinarum. The disease is caused by a bacterium, Vibrio tapetis, that colonizes the edge of the mantle, altering the biomineralization process and normal shell growth. Altered organic shell matrices accumulate on the inner face of the shell leading to the formation of the typical brown ring in the extrapallial space (between the mantle and the shell). Even though structural and functional changes have been described in solid (mantle) and fluid (hemolymph and extrapallial fluids) tissues from infected clams, the underlying molecular alterations and responses remain largely unknown. This study was designed to gather information on clam molecular responses to the disease and to compare focal responses at the site of the infection (mantle and extrapallial fluid) with systemic (hemolymph) responses. To do so, we designed and produced a Manila clam expression oligoarray (15K Agilent) using transcriptomic data available in public databases and used this platform to comparatively assess transcriptomic changes in mantle, hemolymph and extrapallial fluid of infected clams. Results showed significant regulation in diseased clams of molecules involved in pathogen recognition (e.g. lectins, C1q domain-containing proteins) and killing (defensin), apoptosis regulation (death-associated protein, bcl-2) and in biomineralization (shell matrix proteins, perlucin, galaxin, chitin- and calcium-binding proteins). While most changes in response to the disease were tissue-specific, systemic alterations included co-regulation in all 3 tested tissues of molecules involved in microbe recognition and killing (complement-related factors, defensin). These results provide a first glance at molecular alterations and responses caused by BRD and identify targets for future functional investigations.

The role of hybridization in improving the immune response and thermal tolerance of abalone

Recently, frequent death of cultured abalone drew our attention to the stress tolerance of abalone. Hybridization is an effective way of genetic improvement in aquaculture, which can introduce improved traits to the hybrids. In this study, we challenged the hybrids between Haliotis discus hannai and Haliotis gigantea, and their parents with bacteria (vibrio harveyi, vibrio alginolyticus and vibrio parahemolyticus), then held them at 20 °C and 28 °C, survival rates of the parental populations and hybrid populations were recorded. Then we tested the immune responses and thermal-induced responses of the four populations at different temperatures. Total hemocyte count (THC), respiratory burst, superoxide dismutase activity (SOD), acid phosphatase activity (ACP), alkaline phosphatase activity (AKP), myeloperoxidase activity (MPO), and HSP70 expression were determined on day 1 and day 7 of the temperature exposure. Results showed higher survival rates of the hybrids than their parents against bacteria challenge. For immune parameters, THCs were evaluated at 28 °C, while increased THC was also observed in H. discus hannai ♀ × H. gigantea ♂ (DG) and H. discus hannai ♀ × H. discus hannai ♂ (DD) at 12 °C (day 7); at 28 °C, respiratory burst was activated (day 1 and 7), while SOD activity first rose then fell over 7-days exposure; AKP activity was elevated at 12 °C and 28 °C (day 1), most notably in DG, and an increased level of ACP was observed in DG at 28 °C (day 7); MPO activity was suppressed at 12 °C and 28 °C on day 1, but recovered on day 7. For HSP70, increased HSP70 levels were observed in all populations at 28 °C (day 1), and DD got the lowest HSP70 level after 7-days exposure at 28 °C. Overall, the results suggest that temperature changes could significantly affect the physiological status of abalone, and hybrids may be more resistant to disease and thermal stresses than their parents.

Transcriptomic analysis of Ruditapes philippinarum hemocytes reveals cytoskeleton disruption after in vitro Vibrio tapetis challenge

The Manila clam, Ruditapes philippinarum, is an economically-important, commercial shellfish; harvests are diminished in some European waters by a pathogenic bacterium, Vibrio tapetis, that causes Brown Ring disease. To identify molecular characteristics associated with susceptibility or resistance to Brown Ring disease, Suppression Subtractive Hybridization (SSH) analyzes were performed to construct cDNA libraries enriched in up- or down-regulated transcripts from clam immune cells, hemocytes, after a 3-h in vitro challenge with cultured V. tapetis. Nine hundred and ninety eight sequences from the two libraries were sequenced, and an in silico analysis identified 235 unique genes. BLAST and "Gene ontology" classification analyzes revealed that 60.4% of the Expressed Sequence Tags (ESTs) have high similarities with genes involved in various physiological functions, such as immunity, apoptosis and cytoskeleton organization; whereas, 39.6% remain unidentified. From the 235 unique genes, we selected 22 candidates based upon physiological function and redundancy in the libraries. Then, Real-Time PCR analysis identified 3 genes related to cytoskeleton organization showing significant variation in expression attributable to V. tapetis exposure. Disruption in regulation of these genes is consistent with the etiologic agent of Brown Ring disease in Manila clams.

Gender differences in hemocyte immune parameters of bivalves: the Sydney rock oyster Saccostrea glomerata and the pearl oyster Pinctada fucata

Many authors have highlighted a high inter-individual variability in immune parameters of marine bivalves. A high number of studies have reported the impact of external factors on hemocytes immune parameters such as temperature, salinity, pollutants or pathogens. However, only a few of them considered the impact of intrinsic parameters such as sex. Therefore, the present study assessed the impact of gender on hemocytes functions on two marine bivalves. Our results led to the conclusion that the gender contributes to this inter-individual variability. When studying the impact of an environmental variable, a pathogen or a pollutant, the sex of each animal should be determined and taken into account in the analysis and interpretation of immune parameters.

Involvement of nitric oxide in the in vitro interaction between Manila clam, Ruditapes philippinarum, hemocytes and the bacterium Vibrio tapetis

The Manila clam, Ruditapes philippinarum can become infected by the bacterium Vibrio tapetis which causing the Brown Ring Disease along North European Atlantic coasts. Variations in clam immune parameters have been reported in clam challenged with V. tapetis but no studies have been done on Nitric Oxide (NO) production. NO is a toxic agent to pathogens produced mostly by immune cells such as hemocytes in invertebrates. In this study, we demonstrated that NO production in hemolymph and extrapallial fluid of clams is dose dependent and increases with incubation time with V. tapetis. Moreover, the augmentation of NO production seems to be directly correlated to cell rounding and to the loss of pseudopods-forming capacity of hemocytes during the infection process.

Effect of acclimatization on hemocyte functional characteristics of the Pacific oyster (Crassostrea gigas) and carpet shell clam (Ruditapes decussatus)

Most experimental procedures on molluscs are done after acclimatization of wild animals to lab conditions. Similarly, short-term acclimation is often unavoidable in a field survey when biological analysis cannot be done within the day of sample collection. However, acclimatization can affect the general physiological condition and particularly the immune cell responses of molluscs. Our aim was to study the changes in the hemocyte characteristics of the Pacific oyster Crassostrea gigas and the carpet shell clam Ruditapes decussatus acclimated 1 or 2 days under emersed conditions at 14 ± 1 °C and for 1, 2, 7, or 10 days to flowing seawater conditions (submerged) at 9 ± 1 °C, when compared to hemolymph withdrawn from organisms sampled in the field and immediately analyzed in the laboratory (unacclimated). The hemocyte characteristics assessed by flow cytometry were the total (THC) and differential hemocyte count, percentage of dead cells, phagocytosis, and reactive oxygen species (ROS) production. Dead hemocytes were lower in oysters acclimated both in emersed and submerged conditions (1%-5%) compared to those sampled in the field (7%). Compared to oysters, the percentage of dead hemocytes was lower in clams (0.4% vs. 1.1%) and showed a tendency to decrease during acclimatization in both emersed and submerged conditions. In comparison to organisms not acclimated, the phagocytosis of hemocytes decreased in both oysters and clams acclimated under submerged conditions, but was similar in those acclimated in emersed conditions. The ROS production remained stable in both oysters and clams acclimated in emersed conditions, whereas in submerged conditions ROS production did not change in both the hyalinocytes and granulocytes of oysters, but increased in clams. In oysters, the THC decreased when they were acclimated 1 and 2 days in submerged conditions and was mainly caused by a decrease in granulocytes, but the decrease in THC in oysters acclimated 2 days in emersed conditions was caused by a decrease in hyalinocytes and small agranular cells. In clams, the THC was significantly lower in comparison to those not acclimated, regardless of the conditions of the acclimatization. These findings demonstrate that hemocyte characteristics were differentially affected in both species by the tested conditions of acclimatization. The phagocytosis and ROS production in clams and phagocytosis in oysters were not different in those acclimated for 1 day under both conditions, i.e. emersed and submerged, and those sampled in the field (unacclimated). The THC was significantly affected by acclimatization conditions, so the differences between clams and oysters should be considered in studies where important concentrations of hemocytes are required. The difference in the immune response between both species could be related to their habitat (epifaunal vs. infaunal) and their ability of resilience to manipulation and adaptation to captivity. Our results suggest that functional characteristics of hemocytes should be analyzed in both oysters and clams during the first 1 or 2 days, preferably acclimated under emersed rather than submerged conditions.

Development of a monoclonal antibody specific to granulocytes and its application for variation of granulocytes in scallop Chlamys farreri after acute viral necrobiotic virus (AVNV) infection

A monoclonal antibody (MAb 6H7) specific to granulocytes of scallop Chlamys farreri was produced by immunising mice with separated granulocytes as an antigen. Characterised using a flow cytometric immunofluorescence assay, MAb 6H7 reacted to granulocytes by 87.1% of total positive haemocytes. At the ultrastructural level, MAb 6H7 demonstrated epitope in cytoplasmic granules of granulocytes. Western blotting analysis indicated that a peptide of 155 kDa was recognised by MAb 6H7. It was therefore used to investigate granulocyte variation in C. farreri after acute viral necrobiotic virus (AVNV) infection using an enzyme-linked immunosorbent assay. The result illustrated that granulocytes varied greatly by AVNV infection, and their amount significantly increased on day 1 post-injection, then decreased on days 2, 3 and 4, thereafter, rebounded and approached to a second peak on day 6, finally went down gradually to the control level on day 8.

Individual sequence variability and functional activities of fibrinogen-related proteins (FREPs) in the Mediterranean mussel (Mytilus galloprovincialis) suggest ancient and complex immune recognition models in invertebrates

In this paper, we describe sequences of fibrinogen-related proteins (FREPs) in the Mediterranean mussel Mytilus galloprovincialis (MuFREPs) with the fibrinogen domain probably involved in the antigen recognition, but without the additional collagen-like domain of ficolins, molecules responsible for complement activation by the lectin pathway. Although they do not seem to be true or primive ficolins since the phylogenetic analysis are not conclusive enough, their expression is increased after bacterial infection or PAMPs treatment and they present opsonic activities similar to mammalian ficolins. The most remarkable aspect of these sequences was the existence of a very diverse set of FREP sequences among and within individuals (different mussels do not share any identical sequence) which parallels the extraordinary complexity of the immune system, suggesting the existence of a primitive system with a potential capacity to recognize and eliminate different kind of pathogens.

Mineral phase in shell repair of Manila clam Venerupis philippinarum affected by brown ring disease

The mineral phase of shell repair in the Manila clam Venerupis philippinarum affected by brown ring disease (BRD) was characterised at various scales and at various stages of shell repair by confocal Raman microspectrometry and scanning electron microscopy. Spherulitic and quadrangular aragonite microstructures associated with polyene pigments were clearly observed. Von Kossa staining showed that at the beginning of shell repair, hemocytes are filled with insoluble calcium carbonate salts in all fluids and then are transported toward the extrapallial fluids and the repair sites. Our analyses suggest that after a Vibrio tapetis attack and BRD deposit some clams rapidly cover the deposit, resulting in a modification in the microstructure, which could be produced by the participation of both the mantle and hemocytes.

Interactive effects of metal contamination and pathogenic organisms on the introduced marine bivalve Ruditapes philippinarum in European populations

In natural environment, marine organisms are concomitantly exposed to pollutants and multiple disease agents resulting in detrimental interactions. The present study evaluated interactive effects of metal contamination (cadmium) and pathogenic organisms (trematode parasites Himasthla elongata and pathogenic bacteria Vibrio tapetis) singularly and in combination on the bivalve Ruditapes philippinarum, an introduced species to Europe, under laboratory controlled conditions. After 7 days, metal bioaccumulation and pathogen load were analyzed as well as metallothionein (MT) response and hemocyte concentrations and activities. Results showed that infection by opportunistic pathogens affects metal accumulation, leading to maximal Cd accumulation in co-infected clams. Among stressors only V. tapetis induced significant effects on immune parameters whereas a particular interaction "trematode-bacteria" was shown on MT responses. Despite low trematode infection in agreement with the resistant status of R. philippinarum to these macroparasites, significant interaction with bacteria and metal occurred. Such results highlight the necessity of taking pathogens into account in ecotoxicological studies.

Bivalve population health: multistress to identify hot spots

This study investigated some stress (metals, parasites) and response (immunity, metallothionein) factors in two cockle and two Manila clam populations. Data from eight seasons were averaged to obtain global baseline values. Stress/response characteristics of each population were compared to population health status that was determined through population dynamics parameters. Four different scenarios were discussed: (1) a lightly stressed cockle population with correct population health but with a risk of deterioration (hot spot); (2) a lightly stressed introduced cockle population threatened of extinction. In this case ecological factors were suspected; (3) a moderately stressed clam population with moderate adaptative response. The population was sustainable but the level of stress should not increase (hotspot); and (4) a stressed clam population and unfavourable ecological conditions preventing clam settlement. This monitoring highlighted that the discrepancy between population health and stress levels could be due to insufficient response by bivalves and/or by unfavourable ecological factors.

Combined effects of a parasite, QPX, and the harmful-alga, Prorocentrum minimum on northern quahogs, Mercenaria mercenaria

Northern quahogs, Mercenaria mercenaria (L.), frequently are infected with the parasite Quahog Parasite Unknown (QPX, Labyrintohomorpha, Thraustochytriales), which can cause morbidity and mortality of the quahogs. Possible interactions between this parasitic disease and exposure to the harmful dinoflagellate Prorocentrum minimum in M. mercenaria were studied experimentally. Quahogs from Massachusetts with variable intensity of QPX infection were exposed, under controlled laboratory conditions, to cultured P. minimum added to the natural plankton at a cell density equivalent to a natural bloom. After 5 days of exposure, individual clams were diagnosed histologically to assess prevalence and intensity of parasitic infection, as well as other pathological conditions. Further, cellular defense status of clams was evaluated by analyzing hemocyte parameters (morphological and functional) using flow-cytometry. Exposure of quahogs to P. minimum resulted in: a lower percentage of phagocytic hemocytes, higher production of reactive oxygen species (ROS), larger hemocyte size, more-numerous hemocytic aggregates, and increased numbers of hemocytes in gills accompanied by vacuolation and hyperplasia of the water-tubular epithelial cells of the gills. Quahogs had a low prevalence of QPX; by chance, the parasite was present only in quahogs exposed to P. minimum. Thus, the effect of QPX alone on the hemocyte parameters of quahogs could not be assessed in this experiment, but it was possible to assess different responses of infected versus non-infected quahogs to P. minimum. QPX-infected quahogs exposed to P. minimum had repressed percentage of phagocytic hemocytes, consistent with immuno-modulating effect of P. minimum upon several molluscan species, as well as smaller hemocytes and increased hemocyte infiltration throughout the soft tissues. This experiment demonstrates the importance of considering interactive effects of different factors on the immunology and histopathology of bivalve shellfish, and highlights the importance of considering the presence of parasites when bivalves are subjected to harmful-algal blooms.

Resistance to Brown Ring Disease in the Manila clam, Ruditapes philippinarum: a study of selected stocks showing a recovery process by shell repair

European stocks of the Manila clam Ruditapes philippinarum are affected by the Brown Ring Disease (BRD), which is caused by Vibrio tapetis. BRD is characterized by an accumulation of a brown organic matrix on the inner face of the shell. Clams that recover from BRD develop a white mineralized layer covering the brown matrix. Stocks of clams that showed resistance to BRD development, as enhanced recovery, have been monitored since 2000. We have examined two selected stocks: a Low Susceptibility (LS) stock and a High Susceptibility stock (HS), over three generations. The LS stock showed less evidence of the BRD symptoms, and more evidence of total shell repair, both in the field and following experimental challenge with V. tapetis, indicating that some clams may be less vulnerable to a V. tapetis attack than others. The inner face of the valves of the LS and HS clams of the two last generations were analysed with scanning electron microscopy. Examination of shells from BRD-affected clams showed that during the repair process, calcium crystals were progressively laid down until the affected zone was entirely covered. By the end of the shell repair process, a final organic layer covered the calcium crystal mounds. This layer seemed essential in the recovery process. The results indicate that the shell repair capability of the clams is the principal mechanism implicated in the development of BRD resistance in the Manila clam stocks. However, this resistance did not increase with generation because the broodstock was maintained at a site where selection pressure was low, due to a low prevalence of V. tapetis.

Diversity and pathogenecity of Vibrio species in cultured bivalve molluscs

Shellfish production is seriously affected by bacterial pathogens that cause high losses in hatcheries and in the aquaculture sector. A number of Vibrio species are considered important pathogens and have provoked severe mortality outbreaks. The pathologies caused by vibrios in bivalves have been described since the 1960s; however, over recent years, successive episodes of high mortality have been recorded due to these microorganisms. The present work provides an updated overview of the different studies performed in relation with the diversity of Vibrio spp. associated to bivalves. Special attention is given to the main Vibrio diseases and implicated species affecting the different life stages of cultured bivalves.

Evidence of an inflammatory-like response in non-normally pigmented tissues of two scleractinian corals

Increasing evidence of links between climate change, anthropogenic stress and coral disease underscores the importance of understanding the mechanisms by which reef-building corals resist infection and recover from injury. Cellular inflammation and melanin-producing signalling pathway are two mechanisms employed by invertebrates to remove foreign organisms such as pathogens, but they have not been recorded previously in scleractinian corals. This study demonstrates the presence of the phenoloxidase (PO) activating melanin pathway in two species of coral, Acropora millepora and a massive species of Porites, which both develop local pigmentation in response to interactions with a variety of organisms. L-DOPA (3-(3,4-dihydroxyphenyl)-L-alanine) substrate-based enzyme activation assays demonstrated PO activity in healthy tissues of both species and upregulation in pigmented tissues of A. millepora. Histological staining conclusively identified the presence of melanin in Porites tissues. These results demonstrate that the PO pathway is active in both coral species. Moreover, the upregulation of PO activity in areas of non-normal pigmentation in A. millepora and increased melanin production in pigmented Porites tissues suggest the presence of a generalized defence response to localized stress. Interspecific differences in the usage of pathways involved in innate immunity may underlie the comparative success of massive Porites sp. as long-lived stress tolerators.

DjlA, a membrane-anchored DnaJ-like protein, is required for cytotoxicity of clam pathogen Vibrio tapetis to hemocytes

DjlA is an inner membrane cochaperone belonging to the DnaJ family, which has been shown to be involved in Legionella sp. pathogenesis. In this study, we explored the role of this protein in the physiology and virulence of Vibrio tapetis, the etiological agent of brown ring disease (BRD) in Manila clam (Ruditapes philippinarum). Analysis of the djlA locus in V. tapetis revealed a putative organization in an operon with a downstream gene that we designated duf924(Vt), which encodes a conserved protein with an unknown function and has homologues in bacteria and eukaryotes. djlA mutants displayed a reduced growth rate and showed an important loss of cytotoxic activity against R. philippinarum hemocytes in vitro, which could be restored by extrachromosomal expression of wild-type djlA(Vt) but not duf924(Vt). These results are in keeping with the potential importance of DjlA for bacterial pathogenicity and open new perspectives for understanding the mechanism of action of this protein in the novel V. tapetis-R. philippinarum interaction model.

Phenoloxidase-associated cellular defence in the Sydney rock oyster, Saccostrea glomerata, provides resistance against QX disease infections

The enzyme phenoloxidase is a critical component of the immunological defence of invertebrates. Previously, we have shown that the activity of phenoloxidase in Sydney rock oysters (Saccostrea glomerata) correlates with the severity of QX disease outbreaks. The aetiological agent of QX disease is the opportunistic protozoan parasite, Marteilia sydneyi. In this study, we examined the response of oyster haemocytes to challenge with M. sydneyi. Granular haemocytes were able to rapidly phagocytose parasite sporonts. Phagocytosis stimulated intracellular associated phenoloxidase activity that led to the complete melanisation of phagosomes. Significant differences in phagocytic indexes and phenoloxidase activities were observed between oysters selected for resistance to QX disease (QXR) and non-selected wild-type oysters. The data suggest that phagocytosis and cellular melanisation are critical defensive responses of Sydney rock oysters infected with M. sydneyi.

Repeated sampling of individual bivalve mollusks I: intraindividual variability and consequences for haemolymph constituents of the Manila clam, Ruditapes philippinarum

Components of the haemolymph are understood to constitute the internal defense system of bivalve mollusks and their levels are often considered to be indicators of "health"; however, relatively little proof exists of the role that these elements play in the success or failure of defense against a pathogen. A change associated with infection may be the consequence of disease rather than a measure of the capacity to respond effectively to a pathogen. One way to assess whether haemocyte or serum-component concentrations are related to resistance to microbial infection is to sample individuals over time, both before and after they are experimentally or naturally infected. But sampling itself may alter the parameter being assessed. In addition, interindividual variation is large and the degree of intraindividual variation over time is largely unknown. To evaluate intra- vs interindividual variability measured over time and to assess the effects of repeated sampling, we subjected Manila clams, Ruditapes philippinarum, to multiple haemolymph samplings during both field and laboratory experiments, and measured four parameters: haemocyte density, protein concentration, and the activities of leucine amino peptidase and DOPA-oxidase. A repeated-measures ANOVA indicated that individuals with high or low levels at one sampling, tended to have high or low levels, respectively, at the other sampling times. Furthermore, the index of individuality, which is the ratio of intra- to interindividual variability, for these four parameters was comparable to that for human serum components. Repeated sampling had no measured effect on field-deployed clams, which were sampled at intervals of 1-3 months, but significantly depressed values in laboratory-held clams sampled at 1-month intervals. Results demonstrated relative intraindividual constancy in the measured variables and suggested that minimizing sample frequency and volume, and maintaining animals in a comparatively natural environment should all facilitate repeated sampling with minimum injury to experimental mollusks.

The susceptibility of Irish-grown and Galician-grown Manila clams, Ruditapes philippinarum, to Vibrio tapetis and Brown Ring Disease

Brown Ring Disease (BRD), which affects the Manila clam in Europe, is caused by the bacterium, Vibrio tapetis. BRD has been diagnosed in Ireland on only one occasion (1997) although the aetiological agent has recently been detected in apparently healthy Manila clams from a number of sites around the Irish coast. The present work investigated the susceptibilities to BRD of two stocks of Manila clams, one from Ireland and the second from Galicia, north-western Spain, where BRD has been reported on a number of occasions. Exposure of the clams was by addition of V. tapetis to the holding waters. Development of BRD was assessed by the appearance of brown ring signs on the host shells, by bacterial isolation and characterization, and by detection of the bacterium by PCR. The pathogen was recovered from infected individuals and confirmed as V. tapetis by biochemical tests and a slide agglutination test. Galician clams experienced significantly higher mortalities, BRD prevalences and V. tapetis levels than Irish clams. Background infection with V. tapetis in the control stocks prevented conclusions being drawn on comparative susceptibility of the two stocks. Irish clams were significantly affected by the experimental challenge, as demonstrated by the development of BRD and an increase in V. tapetis levels. Results illustrate the vulnerability of Irish clams to BRD and have implications for the movement and transfer of clam seed in Ireland.

Cytologic diagnosis of diseases of invertebrates

Invertebrate medicine in the context of an exotic or zoo animal veterinary practice is in its infancy. Establishment of species-specific reference values and evaluation of the effectiveness of cytology for diagnosis of specific diseases are necessary. Despite the lack of normal reference parameters for most species encountered in clinical practice, important information may be obtained from cytologic examination of tissue imprints, aspirates, scrapings, and hemolymph. This information may be essential to establish a specific diagnosis, focus investigations, and influence treatments. It is hoped that this article stimulates veterinarians who work with invertebrates to use diagnostic cytology and disseminate the results of their experience.

Effects of the pathogenic Vibrio tapetis on defence factors of susceptible and non-susceptible bivalve species: II. Cellular and biochemical changes following in vivo challenge

This work compared the effect of challenge with Vibrio tapetis, the etiologic agent of brown ring disease (BRD) in clams, and other bacterial strains on defence-related factors in four bivalve species: Ruditapes philippinarum (highly susceptible to BRD), R. decussatus (slightly susceptible to BRD), Mercenaria mercenaria and Crassostrea virginica (both non-susceptible to BRD). Results show that bacterial challenge modulated defence-related factors, namely total and differential haemocyte counts, percentage of viable haemocytes, and lysozyme activity, both in haemolymph and extrapallial fluid. Injection with bacteria induced a response that was dependent upon the bacterial and bivalve species investigated, and upon the site of inoculation: external (pallial cavity), pseudo-internal (extrapallial space), or internal compartment (adductor muscle). The most conspicuous changes were systematically measured in R. philippinarum injected with V. tapetis, indicating a bacterial pathogenicity particular to the host in which it causes a specific disease syndrome. Alterations of defence-related factors were maximal in haemolymph of clams injected with V. tapetis in the muscle, and in the extrapallial fluid when the bacteria were injected into the pallial or the extrapallial cavity. Resistance to the development of the BRD symptom was not related to the extent of the haemocyte reaction measured following in vivo challenge.

Effects of the pathogenic Vibrio tapetis on defence factors of susceptible and non-susceptible bivalve species: I. Haemocyte changes following in vitro challenge

In microbial infections, the interaction between microorganisms and phagocytic cells is a crucial determinant in the outcome of the disease process. We used flow cytometry to study the in vitro interactions between Vibrio tapetis, the bacterium responsible for Brown Ring Disease (BRD) in the Manila clam Ruditapes philippinarum, and haemocytes from three bivalve species: the Manila clam (susceptible to BRD), the hard clam Mercenaria mercenaria and the eastern oyster Crassostrea virginica (both non-susceptible to BRD). Results demonstrated that V. tapetis cells and extracellular products elicit major changes in the haemocytes of R. philippinarum, including decreased viability and phagocytic activity, and altered size and internal structure. V. tapetis was able to kill haemocytes from M. mercenaria and C. virginica but to a far lesser extent than those of R. philippinarum. These results suggest that disease resistance is not solely dependent on a host activity against the pathogen, but is also a function and magnitude of the injury to the host cell by a given pathogen.

Persistence of vibrios in marine bivalves: the role of interactions with haemolymph components

Marine bivalves are widespread in coastal environments and, due to their filter-feeding habit, they can accumulate large numbers of bacteria thus acting as passive carriers of human pathogens. Bivalves possess both humoral and cellular defence mechanisms that operate in a co-ordinated way to kill and eliminate infecting bacteria. Vibrio species are very abundant in coastal waters and are commonly isolated from edible bivalves tissues where they can persist after depuration processes in controlled waters. Such observations indicate that vibrios are regular components of bivalve microflora and that the molluscs can represent an important ecological niche for these bacteria. Here we tried to summarize data on the interactions between vibrios and bivalve haemolymph; the available evidence supports the hypothesis that persistence of bacteria in bivalve tissues depends, at least in part, on their sensitivity to the bactericidal activity of the haemolymph. Results obtained with an in vitro model of Vibrio cholerae challenged against Mytilus galloprovincialis haemocytes indicate that bacterial surface components, soluble haemolymph factors and the signalling pathways of the haemocyte host are involved in determining the result of vibrio-haemolymph interactions.