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 h atchery on Prince Edward Island, Canada, which was suffering extensive mortalities. The parasite was identical to one reported from mass mor talities of wild populations of quahaugs in the late 1960's. QPX elici ts a massive inflammatory response characterized by extensive infiltra tion of haemocytes with necrosis of the connective tissues in the dige stive gland and within the musculature of the foot. Light microscopy r evealed a range of parasite stages including thick-walled cyst-like st ages. The majority of QPX tissue stages were enclosed within a translu cent 'halo' indicative of possible host tissue lysis. The same feature was observed in QPX isolated from quahaug tissue cultured on potato d extrose agar. Culture in sterile artificial seawater and on potato dex trose agar resulted in production of a biflagellate stage. Features of all developmental stages observed are described and compared with tho se of the Thraustochytriales and Labyrinthulales.