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Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts

Amoeboflagellate Disease of Larval Geoduck Clams

An Isonema-like amoeboflagellate was associated with disease and mortalities among cultured larval geoduck clams (Panopea abrupta) in the only geoduck clam experimental hatchery in operation in Washington State, USA during the 1980s. In subsequent years, this parasite has not been encountered during the commercial hatchery production of geoduck seed in Washington nor in British Columbia.

Impact on the host

The flagellate penetrates the mantle and proliferates within the coelom resulting in the death of heavily infected geoduck clam larvae. There have been no other reports of invasive, pathogenic Isonema sp. and it was suggested that inclement conditions within the culture system may have predisposed the geoduck clam larvae to infection and the resulting mortalities. This flagellate is not known to infect juvenile or adult geoduck clams nor oyster larvae grown in the same hatchery facility as infected larval geoduck clams.

Diagnostic techniques

Wet Mounts: Examine moribund larvae (Nomarski differential interference contrast microscopy) for ovoid protozoa (about 19 µm in diameter with a prominent cytoplasmic vesicle, pronounced metaboly, and a nucleus with a distinct nucleolus) within the mantle tissue or coelomic cavity. Organisms free in the water, on the shell surface, or in the mantle cavity of larvae with advanced, terminal infections were often pyriform, motile with euglenoid movement (presence of a short anterior flagellum) and had a diminished or absent cytoplasmic vacuole.

Histology: Infection should be confirmed by finding the protozoan within the mantle and coelomic cavity of the geoduck clam larvae.

Electron Microscopy: Diagnostic characteristics are: distinctive ingestion apparatus composed of a microtubule complex, large peripherally oriented mitochondria with sparse cristae, subplasmalemmal microtubules, and the lack of a pellicle.

Methods of control

No known methods of prevention or control.


Elston, R.A. 1990. Mollusc diseases: guide for the shellfish farmer. University of Washington Press, Seattle, p. 37-39.

Kent, M.L., R.A. Elston, T.A. Nerad and T.K. Sawyer. 1987. An Isonema-like flagellate (Protozoa: Mastigophora) infection in larval geoduck clams, Panope abrupta. Journal of Invertebrate Pathology 50: 221-229.

Citation Information

Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Amoeboflagellate Disease of Larval Geoduck Clams.

Date last revised: August 2020
Comments to Susan Bower

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