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Shell-boring Polychaetes of Mussels


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

Polydora spp.

Geographic distribution

Global, although some species probably have limited distributions.

Host species

Mytilus edulis and various species of bivalves that live on the surface of the substrate including oysters, scallops and abalone.

Impact on the host

Most infections are innocuous and usually of low intensity with the burrow being limited to the outside margins of the shell. In North American waters, the burrows rarely penetrate through to the inner surface of the shell. However, in European waters, mortalities, a lower condition index, and loss of market quality of blue mussels were caused by Polydora ciliata. The burrows excavated by P. ciliata in blue mussel shells not only cause unsightly blisters containing compacted mud but also weaken the shell. Nacreous blisters produced by blue mussels in response to P. ciliata may result in atrophy and detachment of the adductor muscle and possibly interfere with gamete production when the calcareous ridges occur adjacent to these organs. At two locations in southern Australia, M. edulis had serious shell damage attributed to Polydora.

Diagnostic techniques

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

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. Place the 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.


Kent, R.M.L. 1979. The influence of heavy infestations of Polydora ciliata on the flesh content of Mytilus edulis. Journal of the Marine Biological Association of the United Kingdom 59: 289-297.

Kent, R.M.L. 1981. The Effect of Polydora ciliata on the shell strength of Mytilus edulis. Journal du Conseil International pour l’Exploration de la Mer 39: 252­255.

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.

Pregenzer, C. 1983. Survey of metazoan symbionts of Mytilus edulis (Mollusca: Pelecypoda) in Southern Australia. Australian Journal of Marine and Freshwater Research 34: 387­396.

Citation Information

Bower, S.M., McGladdery, S.E., Price, I.M. (1994): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Shell-boring Polychaetes of Mussels.

Date last revised: Fall 1994
Comments to Susan Bower

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