Mytilicola intestinalis (Red Worm Disease) of Clams and Cockles
Category 1 (Not Reported in Canada)
Common, generally accepted names of the organism or disease agent
Mytilicola disease, Red worm disease.
Scientific name or taxonomic affiliation
Mytilicola intestinalis (Copepoda, family Mytilicolidae) [not a worm] (Steuer 1902, 1905).
Mytilicola intestinalis appears to be confined to European waters including coastal areas of the Adriatic Sea, Mediterranean Sea and North Sea (specifically, from Italy to Denmark including the British Isles and Ireland but not in the Baltic Sea). To date, it has been reported beyond the Mediterranean Sea and the west coast of Europe on only one occasion; in the unusual circumstance of being found in a plankton sample in the Indian Ocean-Malacca Strait area 67 nautical miles from the nearest shore and depth of 951 meters (Wickstead 1960). Although various reasons for finding an intestinal parasite of European bivalves at this location in the water column were discussed (including the possibility of infected bivalves attached to the hull of the ship), Wickstead (1960) could not explain the phenomenon.
Tapes(=Paphia) decussatus (=decussata), Venerupis (=Paphia) pullastra, Macoma baltica and Cerastoderma (=Cardium) edule and a wide range of other marine bivalves including oysters and mussels and in the laboratory, Crepidula fornicata.
Impact on the host
Pathogenicity to clams and cockles has not been reported. However, this parasitic copepod has been accused of causing disease and mortalities in mussels but there is marked controversy with respect to its significance for oysters.
Gross Observations: Tease open the stomach and intestine of fresh whole bivalves to reveal reddish coloured elongate copepods. Because of the relatively elongate morphology and small limbs of this parasitic copepod, it looks like a worm to the unaided eye, thus the common name of red worm. Hockley (1951) occasionally found active specimens with no colour. The body of adult M. intestinalis has thoracic segments with paired processes and the segmentation of the abdomen is incomplete. The male becomes sexually mature at about 2.8 mm in length and can grow to a maximum length of 4.5 mm. The female becomes sexually mature at about 4.6 mm in length, reaches a maximum length of about 9.0 mm. Paired egg sacs attached to the genital segment (located posterior to the thorax) of the female can extending beyond the posterior end of the abdomen. The head of M. intestinalis carries a median red eye spot, the first pair of antennae has four segments and the second has three. The second antennae are modified as a pair of stout hooks that are used as anchors for resisting expulsion from the host. There is an overall reduction in the length and complexity of the appendages in comparison to free living copepods. The loss of complexity is greatest in the mouth parts where the mandibles are entirely lacking and maxillulae, maxillae and maxilipeds are extremely simplified (Hockley, 1951). Juvenile stages of M. intestinalis (Copepodite II through V) and sexually immature preadult stages, all less than about 2.5 mm in length, also inhabit the intestinal tract of its host (Gee and Davey 1986).
The three species of Mytilicola can be differentiated by external morphological characteristics. Specifically, the caudal ramus of M. intestinalis is elongated (237 µm) and widely divergent, the caudal ramus of M. orientalis is also elongated (233 µm) but not widely divergent, and the caudal ramus of M. porrecta is short (96 µm) and not divergent. In adults of both sexes, the second antenna has three segments (podomeres) in M. intestinalis, two segments in M. orientalis and four segments in M. porrecta. The posterolateral thoracic protuberances are more prominent in M. orientalis, except for the first pair which is absent in male M. orientalis. The adult male of M. porrecta has reduced posterolateral thoracic protuberances that are almost indiscernible. The claw of the maxilliped of male M. porrecta is short, stout and strongly hooked in comparison to the elongated and not strongly hooked maxilliped claw of male M. intestinalis and M. orientalis. Also, female M. intestinalis (4.6 to 9.0 mm in length) tend to be shorter than female M. orientalis (10 to 12 mm in length) and longer than female M. porrecta (about 5 mm in length).
Histology: Examine body cross sections for large copepods within the lumen of the gut. Copepods may attach by hooked appendages to the intestine wall. Focal tissue metaplasia may be present in the intestinal epithelium.
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 for diagnostic identity of the parasite.
Methods of control
No known methods of prevention or control.
Carballal, M.J., D. Iglesias, J. Santamarina, B. Ferro-Soto and A. Villalba. 2001. Parasites and pathologic conditions of the cockle Cerastoderma edule populations of the coast of Galicia (NW Spain). Journal of Invertebrate Pathology 78: 87-97.
Cheng, T.C. 1967. Marine molluscs as hosts for symbioses with a review of known parasites of commercially important species. In: F.S. Russell (ed.). Advances in Marine Biology. Volume 5. Academic Press Inc., London, p. 286-296.
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.
Gee, J.M. and J.T. Davey. 1986. Stages in the life cycle of Mytilicola intestinalis Steuer, a copepod parasite of Mytilus edulis (L.), and the effect of temperature on their rates of development. Journal du Conseil International pour l'Exploration de la Mer 42: 254-264.
Hepper, B.T. 1953. Artificial infection of various molluscs with Mytilicola intestinalis, Steuer. Nature 172(4371): 250.
Hockley, A.R. 1951. On the biology of Mytilicola intestinalis (Steuer). Journal of the Marine Biological Association of the United Kingdom 30: 223-232.
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. 817-829.
Steuer, A. 1902. Mytilicola intestinalis n. gen. n. sp. aus dem Darme von Mytilus galloprovincialis Lam. Zoologischer Anzeiger 25: 635-637.
Steuer, A. 1905. Mytilicola intestinalis n. gen. n. sp. Arbeiten aus dem Zoologischen Instituten der Universität Wien und der Zoologischen Station in Triest 15: 1-46.
Wickstead, J. 1960. A new record of Mytilicola intestinalis Steuer, a parasitic copepod of mussels. Nature 185 (4708): 258.
Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Mytilicola intestinalis (Red Worm Disease) of Clams and Cockles.
Date last revised: December 2009
Comments to Susan Bower
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