Shell-boring Polychaetes of Clams


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. and Boccardia spp.

Geographic distribution

Global, although some species probably have limited distributions. The cases in clams were reported from Mexico (Caceres-Martinez et al. 1999) and New Zealand (Read 1975).

Host species

Chione fluctifraga (in Mexico), Chione stutchburyi (in New Zealand) and various species of molluscs living on the surface of the substrate including oysters, 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. Polydora spp. are filter feeders and use the calcareous mollusc shell as a substrate in which to bore a protective shelter. However, its boring activity may reach the inner surface of the mollusc's shell and induce the host to secrete calcite and conchiolin layers forming a blister to isolate the worm. In the black clam, C. fluctifraga, the majority of the burrows were located around the siphon area of both valves and there was a increased intensity of worm infestation with increased clam size. The U-shape channels of the worms resulted in a very porous and brittle clam shell. In heavily infested clams the shell was often broken impinging on the clams ability to close its valves. The worms survived in the shell after the clam was dead.

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 or patches of mud and debris about 1 cm in diameter. Alternately, place shells in a clear glass container of seawater at a temperature ambient to the area where the mollusc was harvested and examine the shell surface for emerging worms under a dissecting microscope.

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

No known methods of prevention or control.


Caceres-Martinez, J., G.D. Tinoco, M.L.U. Bustamante and I.M. Gomez-Humaran. 1999. Relationship between the burrowing worm Polydora sp. and the black clam Chione fluctifraga Showerby. Journal of Shellfish Research 18: 85-89.

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.

Read, G.B. 1975. Systematics and biology of polydroid species (Polychaeta: Spionidae) from Wellington Harbour. Journal of the Royal Society of New Zealand 5: 395-419.

Citation Information

Bower, S.M. (2002): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Shell-boring Polychaetes of Clams.

Date last revised: March 2002
Comments to Susan Bower

Date modified: