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Introduction Marteilia refringens and Bonamia ostreae are protozoan parasites responsible for mortalities of farmed and wild flat oysters Ostrea edulis in Europe since 1968 and 1979, respectively. Despite almost 40 years of research, the life-cycle of these parasites is still poorly known, especially regarding their environmental distribution. Methods We carried out an integrated field study to investigate the dynamics of M. refringens and B. ostreae in Rade of Brest, where both parasites are known to be present. We used real-time PCR to monitor seasonally over four years the presence of both parasites in flat oysters. In addition, we used previously developed eDNA based-approaches to detect parasites in planktonic and benthic compartments for the last two years of the survey. Results M. refringens was detected in flat oysters over the whole sampling period, sometimes with a prevalence exceeding 90%. It was also detected in all the sampled environmental compartments, suggesting their involvement in parasite transmission and overwintering. In contrast, B. ostreae prevalence in flat oysters was low and the parasite was almost never detected in planktonic and benthic compartments. Finally, the analysis of environmental data allowed describing the seasonal dynamics of both parasites in Rade of Brest: M. refringens was more detected in summer and fall than in winter and spring, contrary to B. ostreae which showed higher prevalence in winter and spring. Discussion The present study emphasizes the difference between M. refringens and B. ostreae ecology, the former presenting a wider environmental distribution than the latter, which seems closely associated to flat oysters. Our findings highlight the key role of planktonic and benthic compartments in M. refringens transmission and storage or potential overwintering, respectively. More generally, we provide here a method that could be useful not only to further investigate non cultivable pathogens life-cycle, but also to support the design of more integrated surveillance programs.

The “International symposium on ecology and evolution of marine parasites and diseases” is a unique meeting combining marine ecology and parasitology. Considering the need to integrate parasitology in ecosystem studies and global change scenarios, this symposium provides a great opportunity to present the latest research findings regarding marines parasites, to gather international experts from various disciplines and to identify next research priorities in this area, both at the European and international levels. The symposium included seven sessions spread over 3 days on parasites affecting marine organisms: parasite adaptation and evolution, biogeopgraphy and macroecology, parasite detection, disease impact and management, epidemiology, environmental parasitology. A workshop focusing on micro-parasites of bivalves was also organised on the 4th day and was opened to interested participants. The symposium was held at "Espace Encan" in La Rochelle, on the 15th-18th of November 2022. The 7 sessions were also available remotely through the Imagina application: https://app.imagina.com/marine-parasitology-symposium A total of 123 participants from 22 differents countries attended the Symposium. The scientific committee : - Dr. Isabelle Arzul, IFREMER, France - Dr. David W. Thieltges, Royal Netherlands Institute for Sea Research, The Netherlands - Dr. Mathias Wegner, Alfred Wegener Institute, Germany - Pr. Xavier de Montaudouin, University of Bordeaux, France The organizing committee: IFREMER, Adaptation and health of marine invertabrates Unit (ASIM), La Tremblade, France - Isabelle Arzul, Isabelle.Arzul@ifremer.fr - Lydie Canier,Lydie.Canier@ifremer.fr - Véronique Betto, Veronique.Betto@ifremer.fr The Sponsors: This event has received funding from Ifremer, the Aquitaine Region, the Communauté d'Agglomération de La Rochelle, UMR EPOC/University of Bordeaux, and the European Union Reference Laboratory for Molluscs Diseases.

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International audience

L’huitre plate, la seule huitre native des côtes européennes, est une espèce emblématique de Bretagne. Convoitée depuis l'Antiquité, sur-pêchée au 19ème siècle, fleuron de l'ostréiculture au 20ème, elle est désormais menacée de disparition à l'aube du 21ème. Sous l'impulsion de la commission OSPAR mais aussi dans un contexte grandissant de crise écologique et d'érosion de la biodiversité imposant une nécessaire conservation et/ou restauration de nos patrimoines naturels, une alliance européenne réunissant biologistes et gestionnaires a été créée en novembre 2017 à Berlin sous l'égide de l'Agence Allemande de Conservation de la Nature (BFN) avec un objectif commun: favoriser le retour de l'huitre plate sur les côtes européennes. Cette alliance pour la restauration de l'huitre plate (NORA) regroupe désormais plus d'une centaine de scientifiques et de gestionnaires, tous au chevet de cette espèce. En France, cette démarche est assurée nationalement par le programme FOREVER ("Flat Oyster Recovery"), démarré en 2018 et financé par le FEAMP pour sa première phase (2018-2020) focalisée en Bretagne. Le présent rapport dresse le bilan du projet. En trois ans, ce projet pionnier a permis : (1) de proposer un premier inventaire des dernières populations relictuelles d'huitres plates encore présentes en Bretagne, en fournissant un diagnostic de leur état écologique, parasitaire et génétique et en affinant la description de son habitat actuel. (2) d'étudier plus précisément les mécanismes écologiques impliqués dans la dynamique d'un banc d'huitres plates : abondance et dispersion larvaire, dynamique du recrutement, facteurs environnementaux, effet de la prédation, impact des maladies parasitaires. L'ensemble de ces processus est étudié à la fois en rade de Brest et en baie de Quiberon. (3) d'initier une démarche unique en France et proposer une méthode adaptée permettant la conservation et la restauration écologique des derniers bancs. Cette démarche inclue des opérations de communication et de sensibilisation actives associées à des actions pilotes concrètes et facilement transférables de restauration écologique : enrichissement coquillier, semis de naissain et développement de récifs artificiels adaptés à l'espèce. Ce rapport apporte aussi de nombreux nouveaux éléments de gestion, utiles aux professionnels de l'ostréiculture, de la pêche, mais aussi aux gestionnaires de l'environnement, notamment aux animateurs du réseau des sites Natura 2000, à l'Office Français de la Biodiversité ainsi qu'à différentes associations impliquées. A titre d'exemple, ci-joint quelques résultats marquants : Pour la profession ostréicole, la meilleure compréhension du recrutement de l'espèce ouvre les voies pour assurer un captage "régulier" pour les années à venir. Les cartes de dispersion expliquent la distribution spatiale du recrutement dans les deux seuls sites de captage français. Les tendances temporelles démontrent l'importance de gérer les populations sauvages, seul moyen d'assurer un captage lorsque des étés frais moins favorables à la reproduction. Le seuil des 18°C/19°C constitue une température critique des masses d'eau en deçà de laquelle le recrutement se fait mal, surtout quand les stocks sont faibles. Pour le secteur de la pêche professionnel (mais aussi récréative), les résultats montrent l'extrême fragilité des bancs résiduels bretons et insistent sur le fait qu'aucune exploitation durable de cette espèce ne pourra être menée sans une gestion très précise des populations résiduelles incluant conservation, restauration et suivis scientifiques. Pour les gestionnaires de l'environnement, ce rapport est l'occasion de mettre en lumière cette espèce-ingénieur, qui après trois siècles de surexploitation, est menacée de disparition. Pourtant une huitrière en bonne santé apporte de nombreux services écosystémiques, comme lutter contre l'érosion de la biodiversité et la dégradation de la qualité de l'eau. De nombreux éléments précis sont fournis pour permettre aux services de l'état, aux gestionnaires Natura 2000 et à l'OFB de protéger et restaurer les dernières populations présentes sur leurs périmètres d'intervention. En savoir plus sur le programme FOREVER et les autres projets européens de conservation et restauration de l'huitre plate : https://noraeurope.eu

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The Pacific oyster, Crassostrea gigas, is a mollusk bivalve commercially important as a food source. Pacific oysters are subjected to stress and diseases during culture. The autophagy pathway is involved in numerous cellular processes, including responses to starvation, cell death, and microorganism elimination. Autophagy also exists in C. gigas, and plays a role in the immune response against infections. Although this process is well-documented and conserved in most animals, it is still poorly understood in mollusks. To date, no study has provided a complete overview of the molecular mechanism of autophagy in mollusk bivalves. In this study, human and yeast ATG protein sequences and public databases (Uniprot and NCBI) were used to identify protein members of the C. gigas autophagy pathway. A total of 35 autophagy related proteins were found in the Pacific oyster. RACE-PCR was performed on several genes. Using molecular (real-time PCR) and protein-based (western blot and immunohistochemistry) approaches, the expression and localization of ATG12, ATG9, BECN1, MAP1LC3, MTOR, and SQSTM1, was investigated in different tissues of the Pacific oyster. Comparison with human and yeast counterparts demonstrated a high homology with the human autophagy pathway. The results also demonstrated that the key autophagy genes and their protein products were expressed in all the analyzed tissues of C. gigas. This study allows the characterization of the complete C. gigas autophagy pathway for the first time. Abbreviations: ATG: autophagy related; Atg1/ULK: unc-51 like autophagy activating kinase; ATG7: autophagy related 7; ATG9: autophagy related 9; ATG12: autophagy related 12; BECN1: beclin 1; BSA: bovine serum albumin; cDNA: complementary deoxyribonucleic acid; DNA: deoxyribonucleic acid; GABARAP: GABA type A receptor-associated protein; IHC: immunohistochemistry; MAP1LC3/LC3/Atg8: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; NCBI: national center for biotechnology information; ORF: open reading frame; PBS: phosphate-buffered saline; PCR: polymerase chain reaction; PtdIns3K: class III phosphatidylinositol 3-kinase; RACE-PCR: rapid amplification of cDNA-ends by polymerase chain reaction; RNA: ribonucleic acid; SQSTM1: sequestosome 1; Uniprot: universal protein resource; WIPI: WD repeat domain, phosphoinositide interacting.

Macroautophagy is a mechanism that is involved in various cellular processes, including cellular homeostasis and innate immunity. This pathway has been described in organisms ranging in complexity from yeasts to mammals, and recent results indicate that it occurs in the mantle of the Pacific oyster, Crassostrea gigas. However, the autophagy pathway has never been explored in the hemocytes of C. gigas, which are the main effectors of its immune system and thus play a key role in the defence of the Pacific oyster against pathogens. To investigate autophagy in oyster hemocytes, tools currently used to monitor this mechanism in mammals, including flow cytometry, fluorescent microscopy and transmission electron microscopy, were adapted and applied to the hemocytes of the Pacific oyster. Oysters were exposed for 24 and 48 h to either an autophagy inducer (carbamazepine, which increases the production of autophagosomes) or an autophagy inhibitor (ammonium chloride, which prevents the degradation of autophagosomes). Autophagy was monitored in fresh hemocytes withdrawn from the adductor muscles of oysters using a combination of the three aforementioned methods. We successfully labelled autophagosomes and observed them by flow cytometry and fluorescence microscopy, and then used electron microscopy to observe ultrastructural modifications related to autophagy, including the presence of double-membrane-bound vacuoles. Our results demonstrated that autophagy occurs in hemocytes of C. gigas and can be modulated by molecules known to modulate autophagy in other organisms. This study describes an integrated approach that can be applied to investigate autophagy in marine bivalves at the cellular level.

Emerging pathogens threaten aquaculture industries worldwide. There is urgency to become better prepared and meet these challenges, as aquaculture is growing rapidly and will contribute increasingly to food security. There are important Pacific oyster production areas today as yet unchallenged by emergent OsHV-1 microvariants, notably in the Americas, and to these areas the viruses are an immediate threat.The ICES Workshop on Emerging Mollusc Pathogens addressed fundamental questions regarding capabilities (expertise, infrastructure) and communication (among the pathology community, regulation, and industry). We sought to identify areas warranting investment and support to manage emerging diseases in shellfish aquaculture industries, and to provide justification for resources to be directed to these areas for more effective management of shellfish health. Thirty-six priorities were identified spanning four areas: i) improving communication and frameworks; ii) essential infrastructure and expertise; iii) key research priorities; and iv) the central role of husbandry in shellfish aquaculture health management. Improving communication in every regard was recognized as the greatest need. Better coordinating efforts, improving data sharing, and improving trust on the part of industry to create buy-in and improve reporting were among identified priorities. Regulatory frameworks were recognized as needing to be flexible, given the often urgent need to act with incomplete information when a disease emerges.For essential infrastructure and expertise the workshop emphasized the continued importance of traditional fields such as pathology, bacteriology and virology, which remain foundational to aquaculture health management. Maintaining laboratory capacity is critical, in distribution as well as technologically: there is value in maintaining not just national or pan-national laboratories but provincial governmental and academic labs too, which can be more intimately engaged with local producers, and familiar with local environments. Disease challenge models and facilities were additionally identified as essential for testing hypotheses, as was expertise in epidemiology, which is sharply limited for marine systems.Discussion of key research priorities highlighted the value of long-term studies for understanding infection patterns; for appreciating dynamics like resistance and tolerance evolution versus virulence evolution; and for understanding effects of changing environments. The importance of improving ecological and evolutionary perspective in general was emphasized, as was the need to develop strategies for monitoring the diversity of potential pathogens. Establishment of mol-luscan cell lines for viral work would be a particularly valuable tool.Aquatic animal husbandry offers myriad opportunities to control pathogens. Understanding inter-sections of disease with aquaculture production for different host-pathogen systems requires continued research investment. Focused investment is also required to pursue strategies emerging from this research, including disease resistance breeding. Tighter collaboration among practical aquaculture scientists and breeders and pathologists should be promoted.Finally, funding was recognised as a continuing challenge. Priorities of donors and funding agencies do not always match commercial and health management priorities; basic surveillance is a key area difficult to support. Short funding timelines challenge our ability to address key questions and challenges, including breeding to control diseases. And there is little funding for truly trans-national, collaborative research. Managing diseases to support aquaculture growth is a global problem that demands science that is equally global, which requires the capacity to build broad international collaborations. It would be immensely beneficial for sustainable aquaculture development if trans-national research initiatives in aquatic animal health and other aquaculture related fields could be established to support innovation in this area in the future.

Alignment of Mikrocytos small subunit ribosomal gene sequences obtained in the present study. (DOCX 197 kb)

Transmission electron microscopy of Mikrocytos veneroïdes n. sp. at different stages. (DOCX 1814 kb)

The overarching goal of VIVALDI is to increase the sustainability and competitiveness of the European shellfish industry by improving the understanding of bivalve diseases and by developing innovative solutions and tools for the prevention, control and mitigation of the major pathogens affecting the main European farmed shellfish species: Pacific oyster (Crassostrea gigas), mussels (Mytilus edulis and M. galloprovincialis), European flat oyster (Ostrea edulis), clams (Venerupis philipinarum) and scallops (Pecten maximus ). The project addresses the most harmful pathogens affecting either one or more of these shellfish species: the virus OsHV-1, Vibrio species including V. aestuarianus, V. splendidus, V. harveyi and V. tapetis, as well as the parasite Bonamia ostreae. The project is committed to provide practical solutions based on the most advanced knowledge. VIVALDI will dissect the disease mechanisms associated with pathogen virulence and pathogenesis and host immune responses, develop in vivo and in vitro models, and apply "omic" approaches that will help the development of diagnostic tools and drugs against pathogen targets, and breeding programmes in a collaborative effort with industrial partners. The proposal will include a global shellfish health approach, recognising that cultured bivalves are often exposed to several pathogens simultaneously, and that disease outbreaks can be due to the combined effect of two or more pathogens. The proposal will also investigate advantages and risks of the used of disease-resistant selected animals in order to improve consumer confidence and safety.

Parue en abstract dans Frontiers in Veterinary Science; Animal movements are one of the main ways to introduce and spread pathogens. In French oyster farming, many stakeholders and premises are heterogeneously divided over the country and a highly dynamic flow of oysters exists among them. In the context of animal disease surveillance and control, analysis of the animal movement’ network can provide useful information to build adapted surveillance strategies or to develop risk management. Movement network analysis has been widely used in terrestrial production, to evaluate the vulnerability of animal movement network to the spread of a specific disease.In France, since 2008, Pacific oyster spat (Crassostrea gigas) has been affected by massive mortality outbreaks associated with the detection of a newly reported variant of ostreid herpesvirus type 1 (OsHV-1). These mortality events have a direct economic impact causing considerable concern to oyster farmers. A previous epidemiological study has highlighted the potential role of oyster transfers in the spread of these outbreaks. However, neither mandatory database nor reliable data is publicly available concerning oyster transfers in France.In this context, a field study was carried out in the main oyster production area in France, Charente-Maritime bay, to map oyster movements and to characterize the corresponding network structure related to potential disease spread. Seventy-five oyster farmers were randomly selected in Charente-Maritime bay between July and September 2010, according to a stratified sampling design based on farm category regarding production type and location (i.e. spat producers, local farmers, beyond farmers, local farmer-senders, beyond farmer-senders). Data related to the farm characteristics and activities, routine rearing scheme and potential changes in husbandry practices were collected during a face-to-face interview of the oyster farmer, using a standardized questionnaire and a land register. Movement data were spatialized and analyzed using social network analysis.A 85.3% participation rate to the study was obtained. Results showed a seasonal variation of oyster transfers in all the sampling strata, with peaks of transfers in spring and autumn, and different geographical patterns of oyster transfers between strata. Growing sites were shared by farms belonging to different farm category which have different behaviour related to oyster movements in time. These results show heterogeneity of contacts between strata both in time and space. Network measures allowed identifying the growing sites having a high risk of becoming infected, of transmitting diseases to other sites or controlling the flow of oysters from one part of the network to another part. The growing sites at high risk of becoming infected could be targeted for surveillance activities (Figure 1). The growing sites at high risk of transmitting diseases to other sites could be the first ones where implementation of early or enhanced control measures (e.g. quarantine, depopulation) could be focused. The growing sites controlling the flow of oysters could be considered as key players in the initial spread of a disease. If these sites would be removed from the network by early movement restriction, this may contribute to contain the disease spread in a smaller part of the area, providing that the propagation by the marine currents and the effect of environmental factors are considered all together.As a conclusion, movement network analysis may provide valuable information to policy makers in preparedness (surveillance) and response (control) to an epidemic.

International audience; In 1972, France underwent an oyster (Crassostrea angulata) crisis and urgently needed to diversify its aquaculture. Thus, Asari clam (Ruditapes philippinarum) was introduced at that time for aquaculture purpose, concomitantly with the introduction of Pacific oyster (Crassostrea gigas). All Asari clam adults and spat originated from the same area (Puget Sound, WA, USA). After a promising start and the implementation of a national Research & Development program, Asari clam culture rapidly faced a series of concomitant handicaps: firstly, in spite the fact that cultural practices were optimized and locking points addressed, leasing ground availability was limited during the 1980s’ due to certain reluctance from oyster farmers to share their leases and/or diversify their activity; secondly, mortality events in the parks probably due to the spat quality, diseases and/or zootechnical errors and/or predation by triggerfish; and thirdly economical competition with Italian production rapidly increased, exacerbated by the occurrence of neonaturalized Asari clam populations and the resulting professional fishing. Meanwhile, European vs national regulations, concerning minimum legal shell length of clams devoted to the market, were unfavourable to France due to contrasted rules between the Mediterranean Sea and the Atlantic Ocean; eventually, several diseases impacted drastically clam populations. At the end of the 80’s in Northern part of Brittany (France), mass mortality occurred due to brown ring disease, later related to a prokaryote (Vibrio tapetis). Presently, the French production remains limited to 2 - 3000 metric tons, mainly based upon professional fishing on neonaturalized populations located in two sites (Arcachon bay and Morbihan Gulf). In Arcachon bay, a comprehensive population dynamics study demonstrated the concomitant effects of fishing activity and environmental characteristics on the population dynamics. A management model was developed to assess various scenarios mainly based on conservation measures (i.e. fishing area, and/or fishing licences number, and/or fishing period). Implementation of those recommendations has provided some encouraging results. However, Asari clam fitness remains poor: a genetic impoverishment due to population isolation was argued by fishermen, but transplant experiments demonstrated that these bivalves kept their plasticity, at least in terms of growth and condition index. Thus, several environmental factors were investigated as possible key parameters explaining low clam performances. Again, pathologies were pointed out and particularly the high pressure exerted by the protist Perkinsus sp. (perkinsosis). Moreover, a new pathology was discovered in 2005 in Arcachon bay, the brown ring disease. Although the etiological factor has not been confirmed, viral origin is suspected. A meta-analysis comparing Asari clam characteristics in Arcachon Bay with the international literature pointed out that their reduced condition index in this bay was likely resulting from combined unfavorable factors (e.g., diseases, trace elements). However, 30% of the condition index variability among sites at the worldwide level was explained by food availability (chlorophyll a concentration). A comparative morphometric studies on four populations of the French Atlantic coasts, using conventional shape analysis, also revealed significant relationships between morphometric ratios and environmental parameters (chlorophyll a concentrations and seawater temperature). Eventually, marine ecologists and state managers as well, must deal with conflictual goals: on one hand Asari clam is an important exploited bivalve in France, on the other hand, the species remains an exotic species which needs careful attention (e.g., European regulations on invasive species)

Depuis 2008, la filière ostréicole française doit faire face à des surmortalités exceptionnelles des naissains d’huîtres creuses, Crassostrea gigas, (huîtres de moins d’un an) comprises entre 60 et 90% dans tous les sites d’élevage. Des cas de mortalité similaires ont été décrits en Irlande, dans les îles anglo-normandes, au Portugal en 2009 et au Royaume-Uni en 2010. Récemment deux épisodes de mortalité, associés à la présence d’un virus OsHV1 μvar (ou un variant très proche) ont été également décrits en Nouvelle Zélande (décembre 2010) et en Australie (janvier 2011) soulignant la vulnérabilité de la filière ostréicole dans le monde. L’objectif de ce document est de faire un point sur les recherches menées depuis l’apparition de ce phénomène et de présenter les acquis scientifiques et techniques obtenus suite aux études menées en 2010 par différents acteurs de la filière (Laboratoires Ifremer et Centres techniques régionaux) dans le cadre du projet « surmortalité des huîtres creuses, Crassostrea gigas ». Ces résultats ont été présentés au cours de deux journées en décembre 2010 à l’Ifremer de Nantes, aux quelles participaient la DPMA, la DGAl, l’ANSES, le CNC, certaines CRCs et l’ensemble des scientifiques impliqués.

En 2008, des mortalités massives d'huîtres creuses (Crassostrea gigas) ont été recensées en France pendant l'été. Ce phénomène, observé de façon récurrente sur les côtés françaises depuis le milieu des années 70, a présenté un caractère exceptionnel en 2008 car il a été observé simultanément dans tous les bassins ostréicoles. De plus, les taux de mortalité calculés ont été particulièrement élevés (80 à 100 % pour de nombreux lots). La détection, fréquente et souvent simultanée, d'herpèsvirus OsHV1 et de plusieurs bactéries de la famille des Vibrionacées dans le cadre de la surveillance menée par le réseau national Repamo (réseau de pathologie des mollusques) a mis en évidence le caractère infectieux de l'épisode épidémique 2008. Une étude épidémiologique a été mise en place à l'automne 2008 afin d'identifier la (ou les) sources et facteurs de risque impliqués dans ces mortalités. La première partie de ce travail consiste en une enquête épidémiologique descriptive dont les premiers résultats font l'objet de cette présentation. Les objectifs spécifiques de celle-ci sont d'identifier la population affectée au sein de la population générale d'huîtres creuses et de décrire la distribution des mortalités dans le temps et dans l'espace. Les données de mortalité, basées sur les déclarations des ostréiculteurs, ont été recueillies auprès des Directions départementales des Affaires Maritimes et auprès des Sections Régionales Conchylicoles ; elles ont été complétées par les données de réseaux d'observation locaux et nationaux tels que le REPAMO et le REMORA. Plus de 7000 données, comportant des informations sur le lieu, la date et le taux de mortalité, l'âge et l'origine des individus et certaines caractéristiques culturales (transferts et manipulation) ont ainsi été collectées. Une première analyse descriptive de ces données montre que le phénomène épidémique n'a pas été identique dans tous les bassins conchylicoles. En Normandie le pic épidémique a été centré fin juin-début juillet ; en Bretagne et en Pays de Loire un premier pic épidémique a été observé fin mai-début juin, suivi d'un second pic de plus grande amplitude fin juin-début-juillet. En Méditerranée et en Poitou-Charentes, le phénomène est apparu graduellement et a culminé fin juin-début juillet ; en Aquitaine, le phénomène a été plus tardif, décalé d'un mois. Une analyse cartographique sous SIG permet un descriptif spatio-temporel plus fin du phénomène à l'échelle de chaque bassin producteur. Toutes les classes d'âge en élevage ont été touchées, cependant le naissain (

The Ifremer Repamo network (REseau de PAthologie des MOllusques) is the national surveillance network in charge of monitoring marine mollusc health on French coastlines. It is commissioned to enforce the official regulations and to provide a public service, under the supervision of the competent authority (DGAL Veterinary Directorate) in the Ministry.of Agriculture and Fisheries. Its goals are to prevent the introduction and the dissemination of infectious agents, in particular pathogens involved in reportable diseases, and to monitor the changes in the pathogens already present on the French coasts., Le réseau Ifremer Repamo (REseau de PAthologie des MOllusques) est le réseau national de surveillance de la santé des mollusques marins du littoral français. Il assure une mission réglementaire et une activité de service public déléguée par le Ministère de l'agriculture et de la pêche à travers la DGAL (Direction générale de l'alimentation). Ses objectifs sont de prévenir l'introduction et la propagation d'agents pathogènes, en particulier ceux à déclaration obligatoire et de surveiller l'évolution de ceux déjà présents sur le territoire national.

A monitoring of the natural and high density populations of commercial molluscs is performed on an annual basis within the scope of the Repamo mollusc pathology monitoring network The aim of this surveillance is (1) to monitor the health status of the molluscs along the French coast and to produce a reference picture, and (2) to study the evolution of the noticeable pathogenic agents existing on the national territory. Different sampling strategies have been introduced for the past 15 years; each strategy is based on an active monitoring including the systematic study of natural mollusc deposits. Indeed, the natural and high density mollusc populations are interrelated and pathogenic agents can be and have been exchanged between these populations with dramatic consequences for shellfish culture. Mollusc farming is performed in the natural milieu, next to natural populations and, very often, natural individuals are collected and kept in farming conditions (spawners, spats); as a matter of fact, the distinction between natural and cultivated populations is really difficult to make especially as farming is mostly performed in an extensive way. These natural populations are also the reflection of a production sector as far as diseases (reservoir, sentinel) are concerned, since mollusc farming is mostly based on transfers. The main natural species studied were the flat oyster Ostrea edulis, the abalone Haliotis tuberculata, the cockle Ceratoderma edule, the clam Ruditapes decussatus and R. philippinarum and the mussel Mytilus edulis and M. galloprovincialis. Different pathogenic agents have been observed and mostly noticeable agents like Bonamia ostreae and Marteilia refringens. The collected data enable the gradual mapping of the French coast regarding the different pathogenic agents. This map could then be used to improve security during transfers, the preservation of pathogen free zones and to better understand a certain amount of abnormal mortality cases, Dans le cadre du réseau de surveillance de pathologie des mollusques Repamo, un suivi des populations élevées et sauvages de mollusques d'intérêt commercial est effectué annuellement. Ce suivi a pour objectif (1) de surveiller l'état de santé des mollusques du littoral français et d'en dresser une image de référence et (2) de surveiller l'évolution des agents pathogènes à déclaration obligatoires présents sur le territoire national. Différentes stratégies d'échantillonnage ont été mises en place au cours des 15 dernières années, chacune reposant sur une surveillance active intégrant systématiquement l'étude des gisements naturels de mollusques. En effet, les populations élevées et sauvages de mollusques sont interdépendantes et des échanges d'agents pathogènes entre ces populations peuvent et ont déjà eu lieu avec des conséquences dramatiques pour la conchyliculture. L'élevage de mollusques se déroule dans le milieu naturel à proximité des populations sauvages et très souvent, des individus sauvages sont collectés et placés en condition d'élevage (géniteurs, naissain) ; ainsi, la distinction entre élevage et population sauvage est souvent délicate d'autant plus que les méthodes d'élevage sont en majeure partie extensives. Ces populations sauvages sont également le reflet d'un secteur de production en terme de maladie (réservoir, sentinelle) puisque l'élevage de mollusques repose en grande partie sur des transferts. Les principales espèces sauvages étudiées ont été l'huître plate Ostrea edulis, l'ormeau Haliotis tuberculata, la coque Ceratoderma edule, les palourdes Ruditapes decussatus et R. philippinarum et les moules Mytilus edulis et M. galloprovincialis. Différents agents pathogènes ont été observés notamment des agents à déclaration obligatoire tels que Bonamia ostreae et Marteilia refringens. Les données collectées permettent de dresser progressivement une cartographie du littoral français vis-à-vis de différents agents pathogènes, cartographie qui pourrait permettre d'améliorer la sécurité des transferts, la préservation de zones indemnes de pathogène et une meilleure compréhension de certains cas de mortalités anormales de mollusques.