Systemic Gregarine-like protozoa in juvenile Pacific oysters, Crassostrea gigas (Thunberg 1793)

During a detailed survey of Pacific oyster, Crassostrea gigas, juvenile health at intensive rearing Facilities during 1996 and 1997, systemic Gregarine-like protozoa were found in seed oysters planted on certain nursery beds. The organisms were never found in seed oysters from intensive production tank; facilities. Thr infections were not associated with morbidity and mortality and appeared to be of limited pathologic significance, The organisms occurred systemically in seed oysters and were most commonly found in vesicular connective tissue with minimal to no associated host response. Oyster seed planted on three nursery bed areas in Washington became infected but those planted in Arcata Bay, California never became infected. Infection intensity and prevalence initially increased after planting, then declined. The organisms are extremely rare in adult oysters and the data suggest that juvenile oysters essentially outgrow the infections without detectable clinical pathologic effects. Spore stages were never observed and a definitive taxonomic affinity could not be established.