QPX (Quahaug parasite X), a pathogen of northern quahaug Mercenaria mercenaria from the gulf of St. Lawrence, Canada

In 1989, an unidentified protistan parasite, QPX (Quahaug Parasite X), was found in quahaugs Mercenaria mercenaria (Linnaeus, 1758) from a hatchery on Prince Edward Island, Canada, which was suffering extensive mortalities. The parasite was identical to one reported from mass mortalities of wild populations of quahaugs in the late 1960's. QPX elicits a massive inflammatory response characterized by extensive infiltration of haemocytes with necrosis of the connective tissues in the digestive gland and within the musculature of the foot. Light microscopy revealed a range of parasite stages including thick-walled cyst-like stages. The majority of QPX tissue stages were enclosed within a translucent 'halo' indicative of possible host tissue lysis. The same feature was observed in QPX isolated from quahaug tissue cultured on potato dextrose agar. Culture in sterile artificial seawater and on potato dextrose agar resulted in production of a biflagellate stage. Features of all developmental stages observed are described and compared with those of the Thraustochytriales and Labyrinthulales.