The Immune Responses of the Pacific Oyster Magallana gigas to Infestation by the Boring Sponge Pione vastifica

The Pacific oyster, Magallana gigas (Thunberg, 1793), is one of the most widely cultured species in the world. Despite the resistance of this species to most pathogens, oyster farms increasingly face problems associated with infestations by various organisms. One of the most dangerous epibiont pests is the boring sponge, Pione vastifica (Hancock, 1849). The aim of this study was to determine the effect of infestation by the sponge P. vastifica on the immune system of the oyster M. gigas. The key parameters of the nonspecific immune response, including the cellular composition of the hemolymph, the production of reactive oxygen species, and the phagocytic activity, were studied in hemolymph samples using flow cytometry and fluorescence microscopy. Oysters with signs of shell perforation showed activation of the immune system expressed as an increase in the production of reactive oxygen species by granulocytes, the main immune cells, and an increase in the phagocytic activity of granulocytes and hyalinocytes. The relative number of granulocytes in oysters infested by boring sponges was significantly lower than that in healthy individuals. Since hemocytes of bivalves are involved in biomineralization processes, this result may indicate hemocyte infiltration into the mantle, which is the main tissue responsible for shell repair. This is the first study to focus on the immune system of Pacific oysters infested by boring sponges. The results we obtained provide insights into how the oyster responds to the presence of such dangerous epibiont pests as the boring sponge.