Sirolpidium zoophthorum (Larval Mycosis) of Oysters


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Larval mycosis.

Scientific name or taxonomic affiliation

Sirolpidium zoophthorum.

Geographic distribution

Hatchery-related mortalities recorded from along eastern U.S. Also occurs in wild populations, but not yet associated with observed mortalities.

Host species

Crassostrea virginica; (also affects Mercenaria mercenaria).

Impact on the host

Affects larvae ranging in size from early veliger larvae to post-metamorphic juveniles measuring up to 400 µm in diameter. Fungus spreads throughout the soft-tissues causing them to disintegrate. The sporangia produce tubes which protrude outside the shell and release motile biflagellate zoospores that transmit the disease. In heavily infected larval cultures over 90% of larvae can be killed within two days. Zoospores of S. zoophthorum can germinate and grow on nutrient agar in the absence of bivalve larvae. Disease probably related to poor husbandry.

Diagnostic techniques

Wet Mounts: Larvae contain looped, sparsely branched mycelia, with constrictions at intervals between segments which may be swollen and frequently lobed. The fungus can be detected in living larvae placed in a neutral red, seawater solution. The fungus takes on a deeper red colour than the oyster tissue facilitating its differentiation.

Methods of control

No known treatment. Batches containing infected individuals should be destroyed in an approved manner; disinfect all containers and equipment in contact with the infected stock. The paucity of reports of this disease suggests that it may be limited in its occurrence by water treatment, husbandry and sanitary procedures that are typically used in production-scale hatcheries today.


Davis, H.C. and V.L. Loosanoff. 1955. A fungus disease in bivalve larvae. Proceedings of the National Shellfisheries Association 45: 151-156.

Davis, H.C., V.L. Loosanoff, W.H. Weston and C. Martin. 1954. A fungus disease in clam and oyster larvae. Science 120: 36-38.

Elston, R.A. 1999. Health management, development and histology of seed oysters. Chapter 13. Fungal infections of oyster larvae. pp. 81-82. World Aquaculture Society, Baton Rouge, Louisiana, USA. 110 pp.

Johnson, T.W. and F.K. Sparrow. 1961. Fungi in Oceans and Estuaries. Cramer, Weinheim, p 347-356.

Vishniac, H.S. 1955. The morphology and nutrition of a new species of Sirolpidium. Mycologia 47: 633-645.

Citation Information

Bower, S.M. (2001): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Sirolpidium zoophthorum (Larval Mycosis) of Oysters.

Date last revised: April 2001
Comments to Susan Bower

Date modified: