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Shell-boring Polychaetes of Oysters


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Shell-boring polychaetes, Mud blister worms.

Scientific name or taxonomic affiliation

Spionid species of Polydora including Polydora websteri, Polydora limicola, Polydora ligni, Polydora variegata, Polydora convexa, Polydora concharum, Polydora hoplura and species of Boccardia including Boccardia (Paraboccardia) knoxi, Boccardia (Paraboccardia) acus, Boccardia atokouica, and Boccardia chilensis.

Geographic distribution

Global, although some species probably have limited distributions.

Host species

Crassostrea virginica, Crassostrea gigas, Ostrea edulis, Saccostrea glomerata (=commercialis) and various species of bivalves living on the surface of the substrate including mussels, scallops and abalone.

Impact on the host

Most infections are innocuous and are usually of low intensity with burrows (containing little or no mud) being confined to the shell. However, on the east and south coasts of the North America, Polydora websteri and Polydora ligni in C. virginica may cause unsightly mud blisters in the shell and yellowish abscesses in the adductor muscle if the burrow comes in contact with the muscle tissue. Mud blisters also occurred in polychaete infestations of C. gigas in southern Brazil; Baja California, Mexico; South Australia and Tasmania, Australia and Marlborough Sounds and Mahurangi Harbour, New Zealand. Prevalence and intensity vary considerably with local conditions. Infection rarely causes mortalities and infected oysters can be marketed. However, mud blisters may interfere with shucking and reduce the commercial value of oysters to be served on the half-shell.

Diagnostic techniques

Gross Observations: Hold clean shell against a bright light and examine the shell matrix for sinuous burrows about 2 mm in diameter or patches of mud and debris up to about 1 cm in diameter.

Figure 1. Infestation of polycheates with mud impaction (arrows) in the shell of a shucked Crassostrea gigas and an additional half shell valve from another C. gigas cultured in the vicinity of Santa Catarina Island, southern Brazil.

Wet Mounts: For specific identification the polychaete must be removed from the shell intact. Break the shell along the burrow using bone shears. Submerge the shell fragments in cool sea water and extract the intact living polychaete from the burrow with fine forceps and needle. Handley (1995) found that infected oyster shells placed in 0.05% phenol in a seawater solution overnight was a more efficient method for extracting Boccardia knoxi than dissecting worms from shell blisters. Place the extracted worm on a piece of plasticine and, using pins positioned along the edges of the body to keep the worm straight, flood with 70% alcohol and store in 50-70% isopropyl alcohol. Note: these procedures are very labourious and time-consuming. For other techniques see Knudsen (1966).

Methods of control

Prevalence and intensity of infection can be reduced by off bottom bivalve culture techniques (at least 0.5 m above the mud substratum) and in intertidal areas. The spread of Polydora websteri along the east coast of Australia (associated mortalities first recorded in 1880) forced Sydney rock oyster producers into an intertidal stick and tray culture system. MacKenzie and Shearer (1961) reported that immersion in a saturated salt solution for 10 to 15 minutes followed by 15 or more minutes of drying in the air or a dip for 1 minute followed by air drying for at least 2 hours killed at least 87% of P. websteri in the shells of living oysters.


Anderson, I.G. 1990. Diseases in Australian invertebrate aquaculture. In: Proceedings, Fifth International Colloquium on Invertebrate Pathology and Microbial Control, Society for Invertebrate Pathology, 20-24 August 1990. Adelaide, Australia, p. 38-48.

Caceres-Martinez, J., P. Macias-Montes de Oca and R. Vasquez-Yeomans. 1998. Polydora sp. infestation and health of the Pacific oyster Crassostrea gigas cultured in Baja California, NW Mexico. Journal of Shellfish Research 17: 259-264.

Handley, S.J. 1995. Spionid polychaetes in Pacific oysters, Crassostrea gigas (Thunberg) from Admiralty Bay, Marlborough Sounds, New Zealand. New Zealand Journal of Marine and Freshwater Research 29: 305-309.

Handley, S.J. 1997. Optimizing subtidal oyster production, Marlborough Sounds, New Zealand: spionid polychaete infestations, water depth and spat stunting. Journal of Shellfish Research 16: 143-150.

Handley, S.J. and P.R. Bergquist. 1997. Spionid polychaete infestations of intertidal Pacific oysters Crassostrea gigas (Thunberg), Mahurangi Harbour, northern New Zealand. Aquaculture 153: 191-205.

Knudsen, J.W. 1966. Biological Techniques - Collecting, Preserving, and Illustrating Plants and Animals. Harper and Row, New York. p. 157-160.

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. 805-817.

MacKenzie, C.L. and L.W. Shearer. 1961. Chemical control of Polydora websteri and other annelids inhabiting oyster shells. Proceedings of the National Shellfisheries Association 50: 105-111.

Nell, J. 2002. The Australian oyster industry. World Aquaculture 33: 8-10.

Sato-Okoshi, W. and K. Okoshi. 1993. Microstructure of scallop and oyster shells infected with boring Polydora. Nippon Suisan Gakkaishi (Bulletin of the Japanese Society of Scientific Fisheries) 59: 1243-1247.

Citation Information

Bower, S.M. (2004): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Shell-boring Polychaetes of Oysters.

Date last revised: June 2004
Comments to Susan Bower

Date modified: