First detection of Marteilia pararefringens in Mytilus trossulus

Marteilia pararefringens is a protistan parasite that mainly infects the digestive gland of mussels Mytilus spp. Despite observations of marteiliosis in mussels since the 1970s, the reported host species has generally not been verified by molecular methods. The 3 closely related mussels Mytilus edulis, M. galloprovincialis, and M. trossulus cannot be distinguished morphologically and hybridise in regions where they overlap. Norway is the only country where both M. pararefringens and M. trossulus are known to occur. Here, we report the first detection of marteiliosis, caused by M. pararefringens, in M. trossulus and an M. edulis-M. trossulus hybrid in a heliothermic oyster pond-a poll-in Hardangerfjorden, Norway. The observed infections were severe, containing early, intermediate, and advanced (sporulating) stages present in the digestive epithelium. There was no host reaction associated with the infections. This finding confirms that all known Mytilus species present in Europe are susceptible to M. pararefringens.