Mytilicola orientalis (Red Worm) of Mussels


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Mytilicola parasitism, Red worm.

Scientific name or taxonomic affiliation

Mytilicola orientalis (Copepoda, family Mytilicolidae) [not a worm] (Mori 1935). In 1938, it was erroneously redescribed as Mytilicola ostreae (Lauckner 1983).

Geographic distribution

Mytilicola orientalis was originally described from oysters (Crassostrea gigas) and mussels (Mytilus crassitesta) in Japan (Mori 1935). This parasite is believed to have been introduced from Japan to the west coast of the United States with seed oysters as early as the 1930s and is now widely spread along the west coast of North America (including Canada's west coast). It was also introduced into France in the 1970s with imported C. gigas (Lauckner 1983, Grizel 1985). Following the implementation of EC Council Directive 91/67/EEC, the free movement of trade in shellfish commenced in January 1993 resulting in the range extension of M. orientalis from France to Ireland (Minchin et al. 1993, Minchin 1996). It was also recorded for the first time from the North Sea area (Netherlands) in 1993 (Stock 1993) and now also occurs in the Mediterranean Sea (Streftaris and Zenetos 2006).

Local (regional) distribution at least throughout the Northeastern Pacific, and probably elsewhere, seems restricted to sheltered muddy estuaries, where bivalves near the low tide mark seem to be most heavily infested. Goater and Weber (1996) attributed this distribution to factors that restrict colonization by the free-swimming larvae suggesting that wave action, tidal currents, salinity and/or substratum conditions may play a role.

Host species

Mytilus crassitesta (from Inland Sea of Japan), Mytilus trossulus, Mytilus galloprovincialis, Mytilus californianus, Mytilus edulis and a wide range of other bivalves including oysters, clams and cockles. In British Columbia, Canada, M. orientalis tends to prefer the mussel as a host and there is a high correlation between areas where this parasite occurs and where oyster seed from Japan was planted (Quayle 1988). In various locations in San Francisco and Humboldt bays, California and in Puget Sound, Washington, USA, the prevalence of infestation in mussels (36.9% to 73.6% infested) was considerably greater than in O. lurida (0% to 9.6% infested) (Odlaug 1946, Bradley and Siebert 1978). The prevalence of infection was also high (65%) in the California mussel M. californianus (Chew et. al. 1964).

Impact on the host

The pathological effects of M. orientalis are controversial. Recent publications reported minimal impact of M. orientalis on the various species of hosts on the west coast of Canada and in Europe. Nevertheless, M. orientalis is considered as a serious pest by some scientists (Holmes and Minchin 1995, Streftaris and Zenetos, 2006). Mytilicola orientalis can alter the morphology of the epithelial lining of the gut. Like the related species Mytilicola intestinalis, M. orientalis attaches to the gut wall with the distal segments of the second antennae which has two spine-like setae and terminates in a curved claw and can cause metaplastic changes in the gut. A fibrosis-like response may occur in the connective tissue beneath the areas of epithelial metaplasia, suggesting an attempt by the host to protect underlying tissue by encapsulation of the parasite (Lauckner, 1983). However, pathology is believed to be negligible or minimal in most cases. The relatively recent inadvertent introduction of M. orientalis into European waters is a concern because it infests native mussels and oysters that are important fisheries resources in Europe (Torchin et al. 2002) and dual infections with M. intestinalis can occur. This introduction warrants cautious observation for potential synergy with M. intestinalis resulting in disease consequences for bivalve hosts (Stock 1993). Streftaris and Zenetos (2006) included M. orientalis in their list of 100 worst invasive species into the Mediterranean Sea.

The life cycle of Mytilicola orientalis is not known but is probably like that of M. intestinalis (Cheng 1967, Goater and Weber 1996). In California, M. orientalis showed continuous reproductive activity (Bradley and Siebert 1978) while in British Columbia, there was a single reproductive period from June to late August and larval stages were in the water column for a short period and did not travel far (Bernard 1969). Goater and Weber (1996) found this parasite in the intestine and rectum but not the stomach of Mytilus trossulus. The intensity of infestation tends to be relatively low with usually less than 10 M. orientalis per host and larger hosts accommodating the most copepods (Goater and Weber, 1996). Juvenile mussels (less than 10 mm in length) are rarely infected.

Diagnostic techniques

Gross Observations: Tease open the stomach and intestine of fresh whole bivalves to reveal reddish coloured elongate copepods. To aid detection, the dissected intestinal tract can be compressed between two glass plates prior to microscopic examination. 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. It has five thoracic segments each with paired posterolateral triangular protuberances (processes) followed by a genital segment and then a narrower abdomen with incomplete segmentation. The female is about 6 to 12 mm in length, 1.3 mm in greatest width and can have paired elongate ovisacs (about 7 mm in length each containing about 200 eggs) attached to the genital segment and possibly extending beyond the posterior end of the abdomen. The male is smaller than the female with a total length of about 2 to 5 mm and greatest width of about 0.5 mm (Grizel, 1985). The head of M. orientalis carries a median red eye spot, the first pair of antennae has four segments and the second has two. 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 three species of Mytilicola can be differentiated by external morphological characteristics. The adult female of M. orientalis is 10-12 mm long with a pair of slender processes extending from the posterior corners of the head, whereas the adult female of M. intestinalis is about 8 mm long with smoothly rounded posterior corners of the head. In the adults of both sexes, the second antenna has two segments in M. orientalis and three segments in M. intestinalis. The posterolateral thoracic protuberances are more prominent in M. orientalis, except for the first pair which is absent in male M. orientalis. Both M. orientalis and M. intestinalis can be differentiated from Mytilicola porrecta (an intestinal parasite of various commercial molluscs in southeastern USA) which is shorter (female about 5 mm long) and has four segments on the second antenna and no mandibles. 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: Presence of large copepods in 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 M. intestinalis 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. Bivalves from areas known to be affected (currently or historically) should not be moved to areas with no record of Mytilicola spp.

The risk of introduction lies in transplanting infested bivalves from one location to another. Currently, the greatest risk of introduction is associated with the bivalve aquaculture industry during transplantation and transportation of seed and farmed stocks. The reality of this risk has been exemplified by the introduction of M. orientalis from Japan to the west coast of North America in the 1930s and then to France in the 1970s and subsequently Ireland in 1993 through the transplanting of infested Crassostrea gigas (Steele and Mulcahy, 2001). The critical number of M. orientialis required to establish a population may depend on local conditions. Enclosed inlets with poor to moderate tidal flushing are more likely to develop local populations (Holmes and Minchin, 1995). The risk of introduction can be significantly reduced by the implementation of regulations that prohibit the movement of infested bivalves.


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Citation Information

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

Date last revised: January 2010
Comments to Susan Bower

Date modified: