Immune responses during the larval stages of Mytilus galloprovincialis: metamorphosis alters immunocompetence, body shape and behavior.

We investigated the development of the immune system during the larval stages of the mussel Mytilus galloprovincialis. The ability of trochophore and veliger larvae to phagocytose foreign particles (Escherichia coli and zymosan) was measured. Phagocytosis was detected as early as 24 h post-fertilization (hpf) using flow cytometry and fluorescence microscopy. However, although there was a high basal production of reactive oxygen and nitrogen species (ROS and NRS), the phagocytosis of zymosan did not trigger an associated increase in radical production. In addition, a panel of immune-related mussel genes (Myticin B, Myticin C, Mytilin B, Mytimycin precursor 1, Macrophage migration inhibition factor, lysozyme, C1q, membrane attack complex protein and fibrinogen-related protein) was selected for expression profile analysis throughout the different developmental stages (trochophore, veliger, metamorphosis, post-settlement and spat). The expression of these genes increased during the transition from trochophore to spat, and the level of expression was higher in oocytes than in trochophores, suggesting that gene expression during the first larval stages might be maternal in origin. Metamorphosis was identified as a crucial stage when larvae increased the expression of immune-related genes and responded to environmental signals. Whole-mount in situ hybridization studies showed the mantle edge as an important area in the development of immunocompetence in bivalve larvae. Larvae responded to both live and heat-inactivated bacteria by modulating expression of immune-related genes. Altogether, our results support that during the early stages of M. galloprovincialis development, immune mechanisms emerge to aid larvae in managing infections.