Invasive orchitophryid ciliate infections in juvenile Pacific and Kumomoto oysters, Crassostrea gigas and Crassostrea sikamea

During an extensive survey of juvenile oyster health from shellfish nursery facilities in Washington state, USA, a primary invasive ciliate disease was discovered. Ciliates invaded the extrapallial space and coelomic cavity of juvenile Pacific and Kumomoto oysters (Crassostrea gigas and Crassostrea sikamea) which had a shell height of from 0.5 mm to 2.0 mm. Both diploid and triploid oysters were affected but no infections were found in oysters larger than 3.0 mm shell height. The invasion was initiated when ciliates were able to bypass the outer mantle lobe and gain entry to the extrapallial space. Subsequently, the thin mantle tissue separating the extrapallial space from the coelomic cavity was breached. Once the extrapallial space was invaded in these small shellfish seed, the infection appeared to be irreversible. Cumulative mortality in affected cultures usually exceeded 50%, and the infection was considered a serious although sporadically occurring problem for nursery production of early stage seed. The ciliates were ovoid with a tapered anterior end and round in cross section with an estimated maximum body width averaging 18 micrometer and body length averaging 32 micrometer. The ciliates were holotrichously ciliated with a modal number of somatic kineties for these samples of 13. The cytoplasm of the ciliates was typically full of food vacuoles, which appeared to contain debris and in several cases what appeared to be entire nucleated cells. The invasive ciliates have an obvious anterior oral cavity and cytostome with at least two oral polykinetids and a paroral. Based on the shape of the oral cavity, they belong in the Order Scuticociliatida and can be most easily placed in the Family Orchitophryidae. In contrast to a previous report describing ciliates in oyster seed cultures, this infection was considered primary but opportunistic. Strategies for preventing and managing the infection are discussed.