Amoeboflagellate Disease of Larval Geoduck Clams


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Amoeboflagellate disease, Isonema-like disease.

Scientific name or taxonomic affiliation

Isonema-like flagellate.

Geographic distribution

Larvae from the only geoduck clam hatchery in Washington State.

Host species

Panopea abrupta.

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 vicinity 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., McGladdery, S.E., Price, I.M. (1994): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Amoeboflagellate Disease of Larval Geoduck Clams

Date last revised: September 1994
Comments to Susan Bower

Date modified: