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Gregarine Parasitism of Oysters


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Parasitism by gregarines, Gymnospore infections of oysters.

Scientific name or taxonomic affiliation

Nematopsis ostrearum, Nematopsis prytherchi, Nematopsis legeri and probably other species of the family Porosporidae.

Geographic distribution

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

Host species

Many species of oysters as well as a wide variety of other marine bivalves including mussels, clams, cockles and scallops.

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 phagocytes 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 intestinal tract of marine arthropods such as crabs.

Diagnostic techniques

Squash Preparations: Microscopic examination (100x) is the most sensitive procedure for detecting the oval oocysts (that have light refractile walls) because of the larger sample size that can be examined in comparison to histology. However, the gymnospores and naked sporozoites may not be detected by this technique.

Histology: 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 low intensity of infection that usually occurs. Oocysts are characterised by a thick hyaline wall and by a densely basophilic "worm-like" sporozoite coiled within. Oval or pyriform thick-walled cysts occur within the gills (primarily N. prytherchi) or in the connective tissue of the mantle and digestive gland (primarily N. ostrearum). Oocysts of N. prytherchi measure 19 x 16 µm while those of N. ostrearum measure 14 x 10 µm. Both species have been observed in the same specimen of Crassostrea virginica.

Methods of control

Spread requires the availability of a suitable crustacean host to complete the sexual stage of the gymnospore life-cycle. At least 4 species of decapod crustaceans are known to host N. ostrearum, the stone crabs (Menippe mercenaria) is the host of N. prytherchi and the crab Eriphia verrucosa is a host 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.

Marteil, L. 1976. La conchyliculture française, 2 partie: biologie de l’huître et de la moule. Chapitre VI: maladies et mortalites. Revue des Travaux de l’Institut des Pêches Maritimes. 40: 285-313.

Winstead, J.T., A.K. Volety and S.G. Tolley. 2004. Parasitic and symbiotic fauna in oysters (Crassostrea virginica) collected from the Caloosahatchee River and estuary in Florida. Journal of Shellfish Research 23: 831-840.

Citation Information

Bower, S.M., McGladdery, S.E. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Gregarine Parasitism of Oysters.

Date last revised: June 2009
Comments to Susan Bower

Date modified: