Marteiliosis is caused by two protistan parasites of the genus Marteilia, Marteilia refringens and M. sydneyi (phylum Paramyxea). The known susceptible species for infection with Marteilia refringens are the European flat oyster (Ostrea edulis), Australian mud oyster (O. angasi), Argentinean oyster (O. puelchana) and Chilean flat oyster (O. chilensis), blue mussel (Mytilus edulis) and the Mediterranean mussel (M. galloprovincialis).
New infections are seasonal, generally associated with high mortality and close proximity to infected oysters is believed to aid transmission.
The visceral tissues of infected oysters may lose their pigmentation, becoming pale yellow. In some cases the mantle become translucent and shell growth may cease. Affected shellfish can become emaciated and in heavy infections tissues appears shrunken and slimy. Mortalities seem to be associated with tissue damage following sporulation of the parasite.
Marteilia refringens infects the digestive and connective tissues of the oyster and forms mature spores within the epithelium of the digestive tubules. Earlier stages occur in the epithelia of the digestive ducts and possibly the gills. Breakdown of the tissues of the digestive gland occurs when the spores of the parasite are released. Cellular accumulations, due to the infiltration of haemocytes, also occur to varying degrees in infected oysters. Cross-sections of the digestive gland show the parasite in the epithelial cells of the digestive ducts and the epithelial cells of the digestive tubules. The unique feature of internal cleavage to produce cells within cells during sporulation differentiates Marteilia spp. from all other Protista.