Common, generally accepted names of the organism or disease agent Mytilicola parasitism, Red worm. Recently, this copepod was introduced into France where dual infections with Mytilicola intestinalis can occur. It may have also become established in other European locations such as Ireland due to transfers of Crassostrea gigas spat from France.
Pathology is believed to be negligable or minimal in most cases.
However, M. orientalis can alter the morphology of the epithelial lining of the gut, and when present in numbers it produces a pea-size swelling of the rectum and has been accused of causing a loss of condition. Tease open the stomach and intestine of fresh whole bivalves to reveal reddish coloured elongate copepods. The various 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 .