Studies on Marteilia sydneyi, agent of QX disease in the Sydney rock oyster, Saccostrea commercialis, with implications for its life cycle

An indirect fluorescent antibody test (IFAT) was developed to test for mature and immature stages of Marteilia sydneyi in the digestive gland of the Sydney rock oyster, Saccostrea commercialis. Immunogold labelling of sections for electron microscopy showed that the sporont membrane, refringent granules, spore wall and haplosporosomes were particularly antigenic. The antibody did not react with any myxosporidean parasite found in local fish. Large numbers of sporonts were shed by infected oysters before oyster death. Lightly infected oysters were apparently able to shed all of their parasites and recover. Refringent granules were proteinaceous and an unlikely energy source for the shed parasite. Growth of the sporont was associated with the enlargement and production of refringent granules. The findings implicate filter-feeding or detritivorous invertebrates rather than scavenging invertebrates or fish in the life cycle of Marteilia parasites.