Abstract
QX disease is a fatal disease in Sydney rock oysters caused by the protozoan parasite Marteilia sydneyi. The current study investigates the phagocytosis of M. sydneyi by Sydney rock oyster hemocytes. It also compares the in vitro phagocytic activities of hemocytes from oysters bred for QX disease resistance (QXR) with those of wild-type oysters. After ingestion of M. sydneyi, hemocyte granules fused with phagosome membranes and the pH of phagosomes decreased. Significantly (p=<0.05) more phagosomes in QXR hemocytes showed obvious changes in pH within 40 min of phagocytosis, when compared with wild-type hemocytes. Phenoloxidase deposition was also evident in phagosomes after in vitro phagocytosis. Most importantly, ingested and melanised M. sydneyi were detected in vivo among hemocytes from infected oysters. Overall, the data suggest that Sydney rock oyster hemocytes can recognise and phagocytose M. sydneyi, and that resistance against QX disease may be associated with enhanced phagolysosomal activity in QXR oysters.