Quahog parasite unknown (QPX) is an opportunistic thraustochytrid parasite of the hard clam, Mercenaria mercenaria, that has been attributed to millions of dollars in losses due to numerous mass mortality events. Despite QPX being recognized for over 50 years and maintained in culture for over 20 years, the complete life cycle of this parasite has remained elusive precluding thorough understanding of its biology. In particular, presence or absence of a zoospore stage (motile dispersal stage) similar to that of other thraustochytrids remained unclear. By modulating culture conditions, QPX zoospores have been successfully and reliably produced from lines maintained in culture for extended periods, as well as newly isolated lines. In addition to the confirmation of zoospores, new stages in QPX replication were also observed with a possible life cycle proposed. The factors that primarily influence zoosporulation in QPX include the removal of mucus from cultured cells, culture age, and temperature. Preliminary infection trials using bath exposure demonstrated no direct infectivity of zoospores, implying that additional stressors are required to produce infections, however, this requires further testing. These findings provide valuable information on the basic biology of the parasite, enabling a better understanding of mechanisms controlling QPX dynamics in the environment and in clams.