A coccidian infection of the kidney of the native littleneck clam, Protothaca staminea

During the summer of 1980, numerous exposed native littleneck clams (Protothaca staminea) were observed in Sequim Bay, Washington. The kidneys of these clams were found to be parasitized by a coccidian. Heavily parasitized kidney was characterized by massive epithelial destruction and an intense inflammatory response. In addition, large numbers of “brown cells” were present throughout the tissues of infected clams.Infected kidneys harbored several stages of the life history of the parasite. The present coccidian differs markedly in several respects from other coccidians in bivalves. First, the entire life history apparently occurs in the kidney of the clam, in contrast to the presumed involvement of an alternative host in other known coccidian parasites of bivalves. Second, a thick-walled spore is produced in addition to a thin-walled oocyst. The apparent presence of two types of dissemination stages is unknown in the Coccidia. Finally, the sporocysts are tetrazoic as opposed to dizoic or multiple sporocysts in previously described species. This parasite, therefore, appears to represent an undescribed genus and species of Coccidia.