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Gregarine Parasitism of Scallops

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Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Parasitism by gregarines.

Scientific name or taxonomic affiliation

Nematopsis ostrearum, Nematopsis pectinis, Nematopsis duorari, and probably other species of the family Porosporidae.

Geographic distribution

British Columbia, western and eastern United States, and Europe. However, each species may have a confined distribution.

Host species

Patinopecten yessoensis, Chlamys (=Pecten) varia, Argopecten irradians, as well as a wide variety of other marine bivalves including mussels, clams, cockles and oysters.

Impact on the host

The gymnospores and oocysts (gregarine spores containing one or more uninucleate vermiform sporozoites), or naked sporozoites (depending on the species involved) usually occur within a phagocyte that can move within the connective tissue to most organs but are most frequently observed in the gills. Infection is usually associated with a focal, benign inflammatory response, without significant health effects. Multiplication of gregarines is limited in bivalves with the completion of the life cycle occurring in the lumen of the intestinal tract of marine arthropods.

Diagnostic techniques

Squash Preparations

Because of the larger sample size that can be examined in comparison to histology, microscopic examination (100 ×) of gills squashed between glass slides is the most sensitive procedure for detecting the oval oocysts with their light refractile walls. However, the gymnospores and naked sporozoites may not be detected by this technique.


All stages that occur in bivalves can be observed in the connective tissue of various organs but they are most prevalent in the gills. However, the presence of these parasites can be missed easily due to the normally low intensity of infection.

Methods of control

No known methods of prevention or control.


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., McGladdery, S.E., Price, I.M. (1994): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Gregarine Parasitism of Scallops.

Date last revised: Fall 1994
Comments to Susan Bower

Date modified: