Marteilia
Marteilia is a protozoan genus of organisms that are parasites of bivalves. It causes QX disease in Sydney rock oysters and Aber disease in European flat oysters. After being infected by "Marteilia", bivalves lose pigmentation in their visceral tissue and become emaciated (Carrasco, Green, & Itoh, 2015). History. In the late 1960s and early 1970s, there was a huge decline in European flat oyster ("Ostrea edulis") population in Brittany, France. This had a huge socioeconomic impact in Europe. "Marteilia refringens" was discovered to be the cause of this decline in oyster (Grizel et al., 1974). Around the same time, "Marteilia  sydneyi" was also found to be causing mortalities in Sydney rock oyster ("Saccostrea glomerate") in Australia (Perkins & Wolf, 1976). There has been some success in breeding strains of Sydney rock oyster that are resistant to Marteilia ('QX disease'). Morphology. "Marteilia" has a very peculiar morphology. The outermost cell is the primary cell. Within the primary cell, there is a nucleus and between 3 and 16 secondary cells. Within a secondary cell, there is a nucleus and between 1 and 6 spores. Within each spore, there is a nucleus and another spore, which has yet another nucleus and spore within. This  spore within a spore within a spore is termed a tricellular spore. "Marteilia" has tricellular spores where as the similar genera "Paramarteilia" and "Paramyxa" have bicellular and tetracellular spores respectively (Feist, Hine, Bateman, Stentiford, & Longshaw, 2009). Cell Cycle. "Marteilia"’s morphology is derived from its unique cell cycle. The primary cell undergoes mitosis and produces the secondary cell within the primary cell rather than outside the primary cell. The secondary cell then undergoes mitosis to produce more secondary cells. After reaching a certain number of secondary cells, each secondary cell then undergoes mitosis to produce a spore within itself. The  spores undergo a series of endogenous mitosis until it becomes a tricellular spore (Feist, Hine, Bateman, Stentiford, & Longshaw, 2009). Life Cycle. "Marteilia" begins its life cycle by infecting the gills of bivalves. At the gills, it undergoes sporogony where it replicates endogenously, producing secondary cells. "Marteilia" then enters the haemolymph and is transported then to the host's digestive tubule. Once there, it attaches itself to the digestive tubule epithelium and undergoes sporulation. After producing many spores, "Marteilia" enters its final stage and ruptures, releasing the spores. Currently, changes to "Marteilia" spores after release are unknown but it is assumed  that some eventually reach another host's gills and repeat its cycle in its new host. (Kleeman, Adlard, & Lester, 2002) References. Specific