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Description (résumé)

Brown Ring disease is caused by Vibrio tapetis, which, by adhering to newly secreted shell matrices, interferes with the biomineralization process. Colonization and alteration of the matrices induce the deposit of conchiolin on the inner surface of the valves, forming a characteristic brown deposit instead of contributing to shell growth. This defense mechanism consists firstly in coating the bacteria within the organic layers of melanized matrices, and secondly in covering the brown deposit with shell layers to achieve complete healing of the shell. This defense phenotype varies greatly depending on, (1) the host physiology, its immune response and its microbiota, (2) the pathogen virulence and its capacity to modulate shell fluid pH, and (3) the environmental conditions. The first barriers V. tapetis faces, the mantle epithelium and the extrapallial fluids, react rapidly and hemocytosis and the activation of enzymes. This early immune response does not induce a decrease of host physiological parameters, suggesting a commensal relationship. On the other hand, in advanced stages of disease or incomplete recovery, the microparasite induces immunodepression and interrupted shell growth, clearly showing trade-offs between immune defense and biomineralization processes. Thus, in the context of Brown Ring Disease, immunity and shell repair processes interact closely either in synergy or as trade-offs depending on their position along the mutualism-parasitism interaction gradient.  

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