The distribution, prevalence, and morphological features of the cystic stage of an apicomplexan parasite of native littleneck clams (Protothaca staminea) in British Columbia.
Sixty-nine of 98 native littleneck clams (Protothaca staminea) collected from Cooper's Cove, Sooke Basin, British Columbia, Canada, during November 1995 contained apicomplexan cysts. The cysts, which measured 20-150 µm in diameter, occurred in several tissues, particularly in the kidney and in connective tissue surrounding the intestine and contained closely packed, banana-shaped zoites that measured about 25 × 4 µm. A pronounced fibrillar layer underlain by labyrinthine structures separated the host tissues from the cyst wall. The apical region of the zoites was reinforced and appeared as an electron-dense, caplike structure. It is considered that the cysts probably represent a stage of a heteroxenous coccidian life cycle, with a predator of clams serving as the definitive host, in which gamogony and sporogony occur.