Invasive Ciliates of Juvenile Oysters

Category

Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Invasive ciliate infections, Invasive orchitophryid ciliate infection.

Scientific name or taxonomic affiliation

Invasive ciliates have not yet been identified to species. However, they are believed to belong in the family Orchitophryidae of the order Scuticociliatida which includes species that are facultative or obligate parasites and histophages such as that isolated from sea stars. In the United States, morphological features observed in histology suggested that the ciliate may belong to genera of either Anophryoides or Paranophrys. In Australia, mixed ciliate infections were common but the primary and most severe invasion appeared to involve a species of Uronema based on light microscopic observations and a positive immunofluorescent antibody test reaction using antibodies for a species of Uronema pathogenic to sea-caged southern bluefin tuna (Thunnus maccoyii) (Elston 1999).

Geographic distribution

Western USA; New South Wales, Australia.

Host species

Crassostrea gigas and Crassostrea sikamea.

Impact on the host

Ciliate infections were considered primary but opportunistic. They were invasive causing morbidity and mortality in juvenile (seed) oysters less than about 3 mm in shell height with the disease most common in oysters with shell heights of 0.5 to 1.5 mm. The ciliates breaches the barrier formed between the outer lobe of the mantle and the inner surface of the shell. Once within the extrapallial space, minimal to moderate mantle tissue damage as well as effects on shell deposition may occur. Significant damage leading to mortality occurs when the ciliate penetrates the thin mantle tissue that separates the coelomic cavity from the extrapallial space. On entering the coelomic cavity, the ciliates replicate rapidly and usually invade the vascular spaces and connective tissue between fibers of the adductor muscle. Cumulative mortalities in affected cultures of oyster seed usually exceed 50%, and the infection was considered as serious although sporadic for nursery production of the early seed stage in Washington state, USA. Apparently normal cultures of oyster seed can degenerate to heavily infected terminal cultures in 1 to 2 days. However, subclinical infections occur for several days before the dramatic culture failure ensues.

Diagnostic techniques

Whole Mounts: Highly motile ciliates observed in affected oysters. However, caution is required when using this method for diagnosis because bacteriophagous and saprophagous ciliates are usually common in juvenile oyster cultures. Observation of ciliate activity within the shells of living juvenile oysters (characterized by mantle movement, motility of the digestive tract and active ciliary movement in the gills and digestive tract organs) indicates a presumptive diagnosis that should be confirmed histologically

Histology: Ciliates observed in the extrapallial space and/or in the coelomic cavity and other vascular spaces such as between fibers of the adductor muscle. Ciliates observed in oysters from the USA were ovoid with a tapered anterior end, round in cross section, and holotrichous with about 13 somatic kineties. Their body averaged 32 µm (25 to 39 µm) in length, 18 µm (14 to 25 µm) in width and had an obvious anterior oral cavity and cytostome with at least two oral polykinetids (one triangular and one rectangular) and an elongated paroral (Elston et al. 1999). The infection can cause rupture of the digestive tract and gut contents may be observed in the coelomic cavity along with the ciliates.

Methods of control

Infections may be managed by sanitary means to prevent the entry of the ciliate into the culture system. Methods include: water filtration, maintenance of adequate water flow and clean culture conditions especially from the time of settlement until the seed is greater than 1.5 µm in length. In Australia, apparently the sand filters used in the nursery water system were a reservoir for ciliates (Elston 1999).

References

Elston, R.A. 1999. Health management, development and histology of seed oysters. Chapter 17. Invasive ciliate infections of juvenile oysters. pp. 83-85. World Aquaculture Society, Baton Rouge, Louisiana, USA. 110 pp.

Elston, R.A., D. Cheney, P. Frelier and D. Lynn. 1999. Invasive orchitophryid ciliate infections in juvenile Pacific and Kumomoto oysters, Crassostrea gigas and Crassostrea sikamea. Aquaculture 174: 1-14.

Citation Information

Bower, S.M. (2001): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Invasive Ciliates of Juvenile Oysters.

Date last revised: May 2001
Comments to Susan Bower

Date modified: