Oyster Trematode Diseases


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Gymnophallid metacercariae of oysters, Metacercaria infection of oysters, Oyster trematode disease, bucephalosis.

Scientific name or taxonomic affiliation

  1. Gymnophalloides tokiensis and possibly other metacercariae of the family Gymnophallidae.
  2. Bucephalus haimeanus, Bucephalus cucullus, Bucephalus spp. and possibly other metacercariae of the family Bucephalidae.
  3. Metacercariae of Proctoedes sp. probably P. maculatus.

Geographic distribution

  1. Metacercariae of Gymnophalloides tokiensis known only from Japan, but metacercariae of a Gymnophallid that appears to be a different species occurs in British Columbia and similar unidentified metacercariae have been observed in Atlantic Canada (S. McGladdery, personal communication).
  2. Widely distributed in temperate and tropical marine waters but not yet reported from Canada.
  3. Widely distributed in temperate and tropical marine waters including the United States (Gulf of Mexico), but not yet reported from Canada.

Host species

  1. Crassostrea gigas, Crassostrea virginica (digenean metacercariae also have been reported from a wide variety of other bivalves including mussels, clams and cockles).
  2. Crassostrea virginica, Crassostrea madrasensis, Ostrea edulis, Ostrea belcheri, Tiostrea chilensis (=Ostrea lutaria), Pinctada martensi, and Pinctada fuscata. Other species in the family Bucephalidae have been reported in mussels.
  3. Crassostrea virginica. Metacercariae also occur in various other Mollusca, Polychaeta and Echinoidea. Adults occur in tropical mollusc-eating fish (labrids and sparids) and temperate Gastropoda, Lamellibranchiata and Polychaeta. Adults and sporocysts occur in mussels.

Impact on the host

  1. In some areas of Japan, prevalences approach 100% and mean intensities of 402 metacercaria per oyster were reported with between 235 and 1,026 metacercaria found in 2-year-old oysters. Heavily infected oysters have a higher water content, reduced levels of glycogen, protein and fat, poor growth, general debilitation, and reduced survival. The Gymnophallidae in Canada occur at low prevalence and intensity and are thought to be innocuous. Neither the primary nor the definitive host are known for Gymnophallidae of oysters.
  2. Sporocysts, often dichotomously branching, and cercariae infiltrate most body organs and have been reported to cause castration, hermaphroditism and protandry in C. virginica, to have serious effects on the formation of pearls in P. martensi, and to significantly reduce resistance to environmental stress. In the Gulf of Mexico, the prevalence of infections tends to be low (less than 3%), except for some locations with low salinities. The correlation of bucephalosis with low salinities may be due to the presumed definitive and second intermediate hosts (the true gar Lepisosteus sp. and millet Mugil cephalus) preference for lower salinity waters (Winstead et al. 2004).
  3. Few unencysted metacercariae occurred in the gonoducts and gonadal follicles at low prevalences (less than 1%) and caused no pathology. Infected oysters were found in close association with hooked mussels (Ischadium recurvum) infected with sporocysts, cercarie and progenetic (sexually mature, ovigerous digenetic trematodes occurring in invertebrates) stages of Proctoeces sp. which were thought to be the source of infection for the oysters (Winstead et al. 2004).

Diagnostic techniques

Squash Preparations: Metacercariae and some cercariae and sporocysts can be observed in thin slices (about 3 mm thick) of the oyster body pressed between two plexiglass plates and examined under a dissecting microscope (50x). Metacercariae of G. tokiensis seem to occur predominantly at the anterior end of the oysters especially in and around the labial palps.

Histology: Metacercariae, cercariae and sporocysts can be observed in cross sections within the vesicular connective tissues. In heavy infections of Bucephalus sp, in C. virginica, the sporocysts within the vesicular connective tissue grow to displace normal tissue. In the gonad, the parasite disrupts tissue integrity and causes gamete degeneration, probably by metabolic disruption. The end result is almost total replacement of oyster tissue by sporocysts which probably leads to the death of the oyster. Haemocyte response by the oyster is generally minimal except when sporocysts become degenerate or die.

Methods of control

a) In heavily infected areas of Japan, repeated raking of oyster beds seemed to reduce the intensity of infection. Infections were heaviest in oysters from muddy beds with prevalence and intensity of infection decreasing in oysters closer to the water surface and away from the shade (indicating that the cercariae may be negative phototactic).
b) No known methods of prevention or control.


Ching, H.L. 1972. A redescription of Gymnophalloides tokiensis Fujita, 1925 (Trematoda: Gymnophallidae). Canadian Journal of Zoology 50: 1299-1302.

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. 728-729.

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

Sakaguchi, S. 1964. Studies on a trematode parasitic of the pearl oyster II: its effects on the pearl oyster as the intermediate host. Bulletin of the National Pearl Research Laboratory 9: 1161-1169. (In Japanese, with English Summary).

Sakaguchi, S. 1968. Studies on the life history of the trematode parasitic in the pearl oyster, Pinctada fucata, and on the hinderance for pearl culture. Bulletin of the National Pearl Research Laboratory 13: 1635-1688. (In Japanese, with English Summary).

Sparks, A.K. 1985. Synopsis of invertebrate pathology exclusive of insects. Elsevier Science Publishers B.V., Amsterdam, p. 337-342.

Tripp, M.R. 1973. Hermaphroditism in Bucephalus-infected oysters. Journal of Invertebrate Pathology 21: 321-322.

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. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Oyster Trematode Diseases.

Date last revised: June 2009
Comments to Susan Bower

Date modified: