Pathogens, especially vibrios, are largely responsible for larval diseases in shellfish aquaculture. In order to understand how the infection process occurs in Pacific oyster (Crassostrea gigas) and blue mussel (Mytilus edulis) larvae, it is of great importance that the anatomy of the animals is well understood. In such a way, the sequence in which the internal structures are destroyed by the invading bacteria can be unraveled. In this study, three techniques were used in combination in order to have the full picture of what is happening during the infection process. After exposure of the larvae to different pathogens during challenge tests, clinical signs such as abnormal swimming behavior and destruction of the velum were observed under the inverted microscope. At the same time, by labeling Vibrio hemicentroti (ME09) and V. anguillarum (NB10) with the Green-Fluorescence Protein (GFP), we found a proliferation of the bacteria within the visceral cavity and subsequent necrosis of digestive organs, until a completed occupied body cavity. In addition, histological sectioning confirmed the damage on the level of individual organs and cells in function of exposure time. Interestingly, ME09 and NB10 had different lethality risk in Pacific oyster and blue mussel larvae but with the same progression in the infection process. This is the first time that the invasive pathways and infection dynamics of these two Vibrio pathogens have been investigated in Pacific oyster and blue mussel larvae. Understanding the infection process will help improve bio-control strategies and enhance the prospects of viable larviculture for oysters and mussels.
Keywords: Vibriosis, Pacific oyster, blue mussel, larvae, Green-Fluorescence Protein (GFP), histology