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

CLPX - Clam Protozoan Unknown of Geoduck Clams

CLPX (Clam Protozoan Unknown) was observed in the wall of the gonad, in the musculature of the siphon, mantle and foot, and under the epithelium lining of the water channels and mantle cavity of geoduck clams. Infected geoduck clams were detected in all Fisheries Management Areas sampled but CLPX always occurred at very low prevalence and intensity of infection. Only two morphological stages have been found. Both stages are relatively large for protozoa (about 25 to 30 µm in diameter); one stage has a single large vacuole that compresses the nucleus to the periphery of the cell (Fig. 1), and the other stage has multiple vacuoles (Fig. 2). There was no evidence of a host reaction to their presence.

Figure 1 CLPX (arrow) with a large single vacuole, located in the musculature of the siphon.

Figure 2 CLPX (arrow) with many vacuoles, located in the connective tissue of the mantle.

Figures 1 and 2. Histological section through CLPX, clam protozoan unknown in the tissues of geoduck clams. Haematoxylin and eosin stain.

CLPX resembles an unidentified protozoan that is infrequently observed in the Pacific littleneck clam (Protothaca staminea). Desser and Bower (1997) speculated that this protozoan in Pacific littleneck clams may be the early developmental stage of a vermiform apicomplexan parasite speculated to be a stage of a heteroxenous coccidian life cycle with a predator of clams serving as the definitive host in which gamogony and sporogony occur. The vermiform stage of the gregarine parasite commonly found in Pacific littleneck clams (between 70 and 100% of the clams infected in some populations) (Desser and Bower 1997) and rarely observed in Manila (Japanese littleneck) clams (Venerupis (=Tapes) philippinarum) (Bower et al. 1992) has not been found to date in geoduck clams. The vermiform stages in the littleneck clams are zoites that have some ultrastructural similarities to merozoites of Aggregata eberthi (class Coccidia, order Eimeriida). Although the cyst (containing numerous closely packed zoites) in littleneck clams can be up to 150 µm in diameter, there was little evidence of a haemocytic response to infection and no associated pathology in the Pacific littleneck clam and Manila clam (Bower 2002).


Bower, S.M. (2002): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Gregarine Parasitism of Clams and Cockles.

Bower, S.M., J. Blackbourn and G.R. Meyer. 1992. Parasite and symbiont fauna of Japanese littlenecks, Tapes philippinarum (Adams and Reeve, 1850), in British Columbia. Journal of Shellfish Research 11: 13-19.

Desser, S.S. and S.M. Bower. 1997. The distribution, prevalence, and morphological features of the cystic stage of an apicomplexan parasite of native littleneck clams (Protothaca staminea) in British Columbia. The Journal of Parasitology 83: 642-646.

Lauckner, G. 1983. Diseases of Mollusca: Bivalvia. In: O. Kinne (ed.). Diseases of Marine Animals. Volume II: Introduction, Bivalvia to Scaphopoda. Biologische Anstalt Helgoland, Hamburg, p. 542-548.

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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