QPX (Quahog Parasite Unknown), a thraustochytrid, is the source of morbidity and mortality in hard clams, Mercenaria mercenaria along the northeast coast of North America. The QPX organism is a facultative pathogen that results in disease in clams held at high densities and salinities. The disease most commonly causes losses in two-year-old aquacultured clams. In some locations, nodules and swellings containing QPX can be identified in the mantle often in areas adjacent to the siphon. Histologically, the organism is surrounded by thick mucus that inhibits phagocytosis by the clam's inflammatory cells especially during the spring and fall when the clam's immune system is less active. Variability in severity of infection and morbidity has been associated with origin of the brood stock with juvenile clams from southern U.S. brood stock being the most affected.