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Gregarine Parasitism of Mussels

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Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Parasitism by gregarines, gymnospore infections of mussels.

Scientific name or taxonomic affiliation

Nematopsis schneideri, Nematopsis legeri, Nematopsis spp., Porospora galloprovincialis, Porospora gigantea, and probably other species of the family Porosporidae.

Geographic distribution

Probably ubiquitous, although each species may have a confined distribution.

Host species

Mytilus edulis, Mytilus trossulus, Mytilus galloprovincialis, and Geukensia demissa, as well as a wide variety of other marine bivalves (including oyster, clams, cockles and scallops), gastropods and chitons.

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. Some infections are thought to result in the production of pearls or calcareous excrescencies on the inner surface of the valves of Mytilus edulis. Multiplication of gregarines is limited in bivalves. The life cycle is completed 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 x) of gill squashes 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. Oocysts of N. schneideri measure 12 x 8 µm while those of P. galloprovincialis measure 14.5 x 8.5 µm. The oocysts of both species are characterised by a thick hyaline wall and by a densely basophilic "worm-like" sporozoite coiled within.

Methods of control

Spread requires the availability of a suitable crustacean host to complete the sexual stage of the gymnospore life-cycle and therefore, is limited by the distribution of the crab host (Eriphia spinifrons for P. galloprovincialis, Carcinus maenas and Macropipus depurator for N. schneideri and Eriphia verrucosa for N. legeri). There are no known methods of prevention or control, apart from avoidance of the crustacean hosts.


Belofastova, I.P. 1996. Gregarines of the genus Nematopsis (Eugregarinida, Porosporidae) - parasites of the Black Sea invertebrates. Parazitologiya 30: 159-173. (Russian, English summary).

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 (2001): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Gregarine Parasitism of Mussels.

Date last revised: June 2001
Comments to Susan Bower

Date modified: