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Quahog Parasite Unknown (QPX) is a facultative parasite of the hard clam, Mercenaria mercenaria. Although it has been observed in clams since the 1960's and cultivated since the 1990's, conflicting reports on important aspects of its biology have prevented its formal description. 18S rRNA gene sequences identify QPX as a thraustochytrid, but its production of copious mucus is atypical for this group. There are also conflicting reports about whether QPX shares common features of thraustochytrids, such as the production of an ectoplasmic net and biflagellate zoospores. This study reaffirms the previous descriptions of zoospore production by QPX in culture, in multiple strains from several geographic locations, and provides detail on how to maintain QPX cultures under conditions that promote the production of zoospores. Furthermore, we describe new aspects of the life cycle not previously observed. Finally, we erect Mucochytrium quahogii gen. nov., sp. nov. to accommodate this unusual thraustochytrid.