You have a Mytilicola intestinalis, Steuer, 1902 – Red worm disease?
Synonyms: None known.
Common names: The condition caused by this parasitic copepod is known as "red worm disease"; Le copepod rouge (FR).
This is a parasitic copepod living in the intestine of bivalves, in particular mussels, but also oysters. It has a characteristic red color, which makes it very conspicuous inside the host. Adults have elongate ("worm-like") bodies with very short appendages. Cephalic appendages, including mouthparts have been described and illustrated (Hockley, 1951). Females are larger than males, 9 mm against 4.5 mm (Gee & Davey, 1986). The paired external egg-sacs of the females are also red. Adult copepods are found in the posterior part of the intestine of the mussel whereas the copepodite stages are found in the stomach digestive gland, and anterior part of the intestine (Gresty, 1992).
Mytilicola intestinalis, male and ovigerous female (from Hockley 1951)
Similar species: Mytilicola orientalis, 1935 (syn: Mytilicola ostreae, Wilson, 1938), NON Myicola ostreae (Hoshina & Sugiura, 1953) (=Mytilicola ostreae Hoshina & Suguira, 1953). The latter species parasitizes gills rather than intestine of bivalves. Both species are found in European countries, often associated with the introduced Pacific oyster, Crassostrea gigas (Thunberg, 1793). More species of similar parasitic copepods from mussels and oysters have been described from the Pacific Ocean.
Native area: Most likely the Mediterranean where it was first described from Mytilus galloprovincialis Lamarck, 1818 in the Adriatic Sea. In 1914 it was found in Mytilus edulis Linnaeus, 1758 in the Mediterranean (Bolster, 1954).
Introduced area: In 1937 a single specimen was found in Mytilus edulis at Portsmouth in southern England, but it was not until 1951 that this was published, and 1947 is usually listed as the first date (Campbell, 1970). In 1938 it was found in the German Wadden Sea (Bolster, 1954). M. intestinalis was first found in the Limfjord in 1964 (Theisen, 1964, 1966) and only in 1994 were the first infected mussels found in the Danish Wadden Sea (Theisen, pers comm.). The first record from the French Atlantic coast was from 1949 (Goulletquer et al., 2002). In the Netherlands the first observations were also from 1949 (Wolff, 2005), and in Belgium from 1950 (Kerckhof et al., 2007). In Ireland the first record was from 1948 (Minchin, 2007). It has not yet been found in Norway or Sweden (Hopkins, 2001; Främmande arter, alert list). Nor has it been found in Poland, Lithuania, Latvia, Estonia or Finland.
There is an old record of this species from the Malacca Strait, Indian Ocean (Wickstead, 1960), but there are no records of the species since then, so this record must be considered dubious.
Vector: Probably infected mussels transported as fouling on ships' hulls or infected mussels transferred for aquaculture (Hockley, 1951; Theisen, 1966; Korringa, 1968). Secondary dispersal through free swimming larvae seems unlikely as the actively swimming stages last only a few days (Hockley, 1951).
Most of the information available is on prevalence and effects of the parasite on the host animals, and very little information is available about the ecology of the parasite. Mytilicola intestinalis in Mytilus galloprovincialis seems fairly rare in the Aegean Sea, infecting about 11% of the mussels with 1-4 individuals per mussel (Rayyan et al., 2004). However, these were mussels cultured on hanging ropes, which are less prone to infection by this parasite (Theisen, 1987). It also seems rare in M. galloprovincialis from Italy whereas almost 60% of the M. galloprovincialis imported to Italy from Spain were infected (Trotti et al., 1998), and even higher infection rates were recorded from cultured mussels from the Spanish Atlantic coast (Fuentes et al., 1995).
The effect on Mytilus edulis depends on the number of parasites, but even the presence of one copepod may cause visible effects (Korringa, 1968), but see also below for impacts. The highest number of parasites are found in large sized mussels. This has been attributed to higher filtration rates rather than age (Williams, 1967; Paul, 1983), though this has also been contested (Davey, 1989). In most places relatively low numbers (1-10 individuals) of parasites are found (Grainger, 1951; Williams, 1969; Theisen, 1987). Mussels in beds on the bottom are more heavily infected than mussels on vertical surfaces away from the bottom (Korringa, 1968). The smallest mussel infected was about 10 mm long (Williams, 1967). Fewer parasitic copepods are found in mussels infected by the shell-boring polychaete Polydora ciliata (Johnston, 1838) (Williams, 1968). Adult females have almost the same diameter as the intestine of the host, but a groove formed by appendages on the dorsal side permits the flow of food to pass, the copepod only picking up enough for its own nutrition (Hockley, 1951). What constitutes the diet of M. intestinalis has been discussed, but it seems certain that it does not include mussel tissues (Davey, 1989; Gresty, 1992). If an adult parasite is expelled it is not able to move enough to find a new host (Grainger, 1951).