Mytilicola porrecta (Red Worm) of Mussels


Category 3 (Host Not in Canada)

Common, generally accepted names of the organism or disease agent

Mytilicola parasitism, Red worm.

Scientific name or taxonomic affiliation

Mytilicola porrecta (Copepoda, family Mytilicolidae) [not a worm].

Geographic distribution

Gulf of Mexico.

Host species

Geukensia (=Modiolus) demissa (ribbed mussel), Ischadium (=Mytilus, =Brachidontis) recurvum (hooked mussel), may also occur in some species of clams (for example Mercenaria mercenaria) and oysters.

Impact on the host

Nothing is known about the relationship between this parasite and its pelecypod hosts.

Diagnostic techniques

Gross Observations: Tease open the stomach and intestine of fresh whole bivalves to reveal reddish to brownish orange coloured elongate copepods. Mytilicola porrecta can be differentiated from M. orientalis and M. intestinalis by the smaller size of the adult female (about 5 mm long compared to lengths over 8 mm in the other two species) and by the presence of four segments on the second antenna in the adults of both sexes. The other two species have three or fewer segments on the second antenna. The adult male of M. porrecta has reduced posterolateral thoracic protuberances that are almost indiscernible. The claw of the maxilliped is short, stout and strongly hooked in comparison to the elongated and not strongly hooked maxilliped claw of male M. orientalis and M. intestinalis.

Histology: Examine body cross sections for large copepods within the lumen of the gut.

Digestion: Chemical disruption of tissues will expose copepods for easy quantification. Specifically, pepsin digestion of the flesh that was removed from the shells of bivalves followed by filtration of the disintegrated tissues through sieves (348 µm and 124 µm pore size) and examination of the residues for Mytilicola under a binocular microscope is a technique used for the detection of all parasitic stages including egg sacs and early infective stages (0.45 µm long) intact (Dare, 1982). This process is recommended for large scale surveys rather than diagnostics.

Methods of control

No known methods of prevention or control.


Cheng, T.C. 1967. Marine molluscs as hosts for symbioses with a review of known parasites of commercially important species. In: F.S. Russel (ed.) Advances in Marine Biology. Volume 5. Academic Press Inc., London, p. 293-296.

Dare, P.J. 1982. The susceptibility of seed oysters of Ostrea edulis L. and Crassostrea gigas Thunberg to natural infestation by the copepod Mytilicola intestinalis Steuer. Aquaculture 26: 201-211.

Humes, A.G. 1954. Mytilicola porrecta n. sp. (Copepoda: Cyclopoida) from the intestine of marine pelecypods. The Journal of Parasitology 40: 186-194.

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. 827-829.

Citation Information

Bower, S.M. (2010): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Mytilicola porrecta (Red Worm) of Mussels.

Date last revised: January 2010
Comments to Susan Bower

Date modified: