Mytilicola orientalis (Red Worm) of Oysters


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Mytilicola disease, 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 (Deslou-Paoli 1981, 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, Steele and Mulcahy 2001). 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

Crassostrea gigas, Ostrea edulis, Ostrea lurida (=conchaphila) as well as a wide range of other marine bivalves including mussels, clams and cockles. In British Columbia, Canada, M. orientalis tends to prefer the mussel as a host and there was 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).

Impact on the host

Although bivalve infestation has been referred to as the disease mytilicoliasis (Elston 1993), the pathological effects of M. orientalis are controversial. Mytilicola orientalis has been reported to cause adverse effects in the condition index (condition factor) of Ostrea lurida and Crassostrea gigas at several locations along the west coast of the United States (Odlaug 1946, Chew et al. 1965, Katkansky et al. 1967, Lauckner 1983). However, Odlaug (1946) noted that there was a greater reduction in condition factor in spawning O. lurida than that caused by the presence of M. orientalis. Other reports indicated that the survival of infested oysters was not affected and there was little evidence of reduction in shell growth (Chew et al. 1965, Katkansky et al. 1967). More recent publications reported minimal impact of M. orientalis on the various species of hosts on the west coast of Canada and in Europe. Deslou-Paoli (1981) detected a difference in the condition index of C. gigas from Marennes-Oléron Basin, France only during the spring and the fall when the oysters were under stress from low food availability or spawning and he suggested that oysters in good condition may reject the parasite. Bernard (1969) and De Grave et al. (1995) did not detect a decrease in the condition index of C. gigas infested with M. orientalis from coastal British Columbia, Canada and Dungarvan Bay, Ireland, respectively. Steele and Mulcahy (1999, 2001) detected no effect on condition, growth, sex, stage of glycogen content of C. gigas in Ireland but did report a correlation with shell burrowing by the polycheate annelid Polydora sp. However, in all these recent publications, the prevalence and intensity of infestation tended to be low. De Grave et al. (1995) noted that although far higher intensities of M. orientalis were recorded in the earlier studies, none quantified the exact relationship between the condition of the oysters and intensity of M. orientalis infestation. De Grave et al. (1995) concluded that at low levels of infestation, M. orientalis does not cause a lowering of the condition index of infested C. gigas. Nevertheless, M. orientalis is considered as a serious pest by some scientists (Holmes and Minchin, 1995). For example, Streftaris and Zenetos (2006) included M. orientalis in their list of 100 worst invasive species into the Mediterranean Sea. The relatively recent inadvertent introduction of M. orientalis into European waters is a concern because it infests native oysters and mussels that are important fisheries resources in Europe (Torchin et al. 2002) and dual infections with Mytilicola intestinalis can occur. This warrants cautious observation for potential synergy with M. intestinalis resulting in disease consequences for bivalve hosts (Stock 1993).

This copepod 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 (Elston 1993). Like the related species M. intestinalis, M. orientalis can cause metaplastic changes in the gut of its host (Lauckner 1983). Specifically, tall columnar epithelium of the stomach near the site of attachment may become cuboidal or squamous and deciliated (Elston 1993). Occasionally the mucosa is completely eroded and an appendage of the copepod may penetrate the underlying connective tissue with concurrent haemocyte infiltration. Beneath the areas of epithelial metaplasia, a fibrosis-like response may occur in the connective tissue suggesting an attempt by the host to protect underlying tissue by encapsulation of the parasite (Sparks 1962, Lauckner 1983). When a single specimen is located in a small section of the intestinal tract or when present in large numbers, M. orientalis may cause distension of the gut wall (described in one case as a pea-size swelling of the rectum). Katkansky and Warner (1968) reported that there was no host response by C. gigas to M. orientalis in the lumen of the intestine but when this copepod occurred in the digestive diverticula, there was marked haemocyte infiltration into the connective tissues surrounding the parasite possibly resulting in absorption of the parasite.

The life cycle of M. orientalis is not known but is probably like that of M. intestinalis (Cheng 1967, Goater and Weber 1996). In California and Oregon, M. orientalis showed continuous reproductive activity (Katkansky et al. 1967, 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). Sparks (1962) reported two reproductive peaks for M. orientalis in Washington State, one in the early spring and the other in late summer and suggested that this parasite is incapable of maintaining itself in the gut of C. gigas for prolonged periods or the life span of the copepod is quite short. In France, Deslou-Paoli (1981) detected M. orientalis all year in the Marennes-Oléron Basin with greater infestation intensity (number of copepods per host) in the spring and a peak infestation in the fall and the oysters first became infested at about six-months of age. 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 (Deslou-Paoli 1981, De Grave et al. 1995, Steele and Mulcahy 2001). Odlaug (1946) found that the greatest number of M. orientalis occurred in the anal portion of the intestinal canal (posterior 10-12 mm of the intestine) of Ostrea lurida. Bernard (1969) reported that C. gigas held out of water for 12 or more hours had a tendency to evacuate M. orientalis and speculated that the thoracic protuberances could then cause lesions of the gut wall.

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. Katkansky et al. (1967) claimed that immersion of oysters without the shells in a solution of nine parts 95% isopropyl alcohol and one part glacial acetic acid for one week facilitated dissection. 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: Examine body cross-sections for large copepods within the lumen of the gut. Copepods may attach by hooked appendages to the gut wall. Focal tissue metaplasia may be present in the gut 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 was a technique used for the detection of all parasitic stages of 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 Mytilicolaspp.

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. orientalis 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 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 Oysters.

Date last revised: January 2010
Comments to Susan Bower

Date modified: