Various aspects of bivalve mollusk immunity

The outcomes of marine bivalve farming are not secure, principally due to i nfectious diseases. All micro-organisms are involved, from bacteria to viru ses, protozoans and fungi. To recover from infections, invertebrates do not possess acquired immunity, but they are capable of potent non specific rea ctions, both humoral and cellular, belonging to the so-called innate immuni ty. Also present in vertebrates, innate immune reactions an considered its complementary to the acquired immune ones. Mollusk hemocytes include several sub-populations according to morphologica l criteria. The percentages of these sub-populations were modified by the i nfection, putting in evidence the central role of these immune competent ce lls. As a general phenomenon, phagocytosis involve intra-hemocytic generati on of reactive intermediate oxygen radicals which participated to the invad er destruction. Meanwhile, some micro-organisms were capable of inhibiting the generation of radicals, escaping their toxicity and consequently succee ding in their pathogenicity. Stress proteins are involved in preserving the protein structures. Heal sho ck induced the production of specific proteins into bivalve hemocytes. Such phenomenon would normally be used in the field according to the season or in relationship with recovery during tidal emersion. Mollusks also possessed a large panel of molecular weapons, some of them be ing cytotoxic, others displaying antimicrobial activity. For instance, the mussel plasma contained a potent cytolytic activity due to a molecular comp lex of 320 kDa, acting by oligomerization at the cell membrane level, leadi ng to the Formation of transmembrane pores. The mussel antibiotic activity involved numerous peptides belonging to seve ral molecular families. Stored in the hemocyte granules, their release was triggered by pathogen entry but not by a sham injection. Closed to the inse ct cyclic peptides, the mussel peptides were originals as possessing eight cysteines instead of six, which argued in favour of a more compact molecula r structure. Another characteristic was the constant existence of two exten sion sequences as reported only for Limulus tachiplesin. Until recently, the bivalve immune capacities were largely unknown. In addi tion to the growing data obtained on gastropods and leeches, they contribut e to the identification of the position of the mollusk immune system among the evolution of the fundamental biological functions.