Cestode Parasitism of Scallops


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Cestode parasitism of scallops, Larval cestode infection.

Scientific name or taxonomic affiliation

Representatives of three orders and eight genera including Lecanicepalidea, Polypocephalus sp. and Tylocephalum sp.; Trypanorhyncha, Eutetrarhynchus sp. and Parachristianella sp.; and Tetraphyllidea, Rhinebothrium sp., Anthobothrium sp., Acanthobothrium sp. and "Scolex pleuronectis".

Geographic distribution

Northern part of the Gulf of Mexico and southern coastal areas of Japan.

Host species

Larvae (metacestodes - a term used for any larval form between the egg and adult cestode) reported from Argopecten irradians concentricus as well as from other pelecypods including oysters (e.g., Crassostrea virginica) and clams (e.g., Mercenaria mercenaria) and from molluscivorous gastropods (Cake 1976). Tylocephalum sp. also reported from Chlamys nobilis in Japan (Lauckner 1983).

Impact on the host

The scallops and other pelecypods appeared to serve as primary intermediate hosts while molluscivorous gastropods as well as other marine organisms including crustaceans and fish appeared to serve as secondary intermediate or paratenic hosts. All species were thought to have elasmobranch definitive (final) host (especially demersal elasmobranchs) where the cestodes became sexually mature. Pelecypods became infected by ingesting eggs, gravid proglottids or free-swimming coracidia released from the intestinal tract of the definitive host.

A mean infection intensity of 18.5 ± 5.5 (range of 1 to 75) metacestodes of Polypocephalus sp. per scallop was detected in 76% of A. concentricus from four Florida bays adjacent to the Apalachicola River delta (Cake 1979). The majority of the metacestodes (plerocercoids) were encysted singly in thin transparent sacs in the mesenteries and the remainder occurred in the stomach wall or digestive gland. In heavy infections small groups of four to eight individually encysted metacestodes occurred together in the mesenteries.

Diagnostic techniques

Squash Preparations: Squash the dissected organs, especially the gills, labial palps, stomach, intestine and digestive gland, of scallops between glass plates and examine with the aid of a stereo-zoom, dissection microscope. Metacestodes can be removed from the tissues intact by permitting the visceral tissue to putrefy in a Petri dish containing filtered seawater. In order to examine the morphology of the metacestodes, they should be killed in an expanded or relaxed condition achieved by emersion in tepid tap water or hot (about 50°C) AFA (80 parts 70% ethanol, 10 parts formaldehyde and 10 parts glacial acetic acid). Preserve in 70% ethanol and 5% glycerine. Stain larvae with Ehrlich's acid haematoxylin and mount in Permount using standard helminthological techniques.

Culture: Incubating viable metacestodes in a glucose-enriched artificial elasmobranch saline medium (250 mM NaCl, 44 mM KCl, 5.1 mM MgCl2 and 10 mM tris (hydroxymethyl) amino methane-maleate buffer at pH 7.2) for up to 150 hours may reveal additional features of the cestodes. Larval features may be modification or lost and some rudimentary adult features, such as bothridal hooks, may develop.

Methods of control

No known methods of prevention or control. However, elasmobranchs infected with the adult stage of these cestodes must occur in the vicinity of the scallops in order to initiate the infection in scallops.


Cake, E.W. 1976. A key to larval cestodes of shallow-water, benthic mollusks of the northern Gulf of Mexico. Proceedings of the Helminthological Society of Washington 43: 160-171.

Cake, E.W. 1979. Polycephalus sp. (Cestoda; Lecanicephalidae): a description of tentaculo-plerocercoids from bay scallops of the northeastern Gulf of Mexico. Proceedings of the Helminthological Society of Washington 46: 165-170.

Lauckner, G. 1983. Diseases of Mollusca: Bivalvia. In: Kinne, O. (ed.) Diseases of marine animals. Volume II: Introduction, Bivalvia to Scaphopoda, Vol. 2. Biologische Anstalt Helgoland, Hamburg. pp. 477-961 (section on Cestoda, pg. 762-784).

Citation Information

Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Cestode Parasitism of Scallops.

Date last revised: July 2009
Comments to Susan Bower

Date modified: