Systematic infection of the foot of the mediterranean mussel (Mytilus galloprovincialis) by a trematode parasite (Digenea: Monorchiidae) in the farms of Tyrrhenum and Adriatic Sea (Italy)

Phases of development of a trematode parasite were observed in the histological samples at foot level of the Mediterranean mussel M. galloprovincialis during routine studies between 2008 and 2021 along Campania coast. Macroscopically, affected animals may display nodular areas and discolouration at foot level. Histopathology revealed the presence of cercaria and metacercariae present in variable number, from 1 to 6 per foot, variously distributed either at the level of the bixogenic glands, the collagen gland or the adhesive plate, or in the muscular portion, with a prevalence in the affected areas ranging from 3.3 % to 20 %. The invasion appeared to originate at the level of the pedal blood vessels located along the structure of the foot or in the terminal part, up to reaching the various muscle or glandular sites. Molecular assay of the rRNA 18S showed an identity of 96.1 % with a trematode belonging to the Monorchidae familiy named Opistomonorcheides delicatus , commonly infecting Carangidae fishes in Pacific Ocean. This is the first morphological and molecular description of this genus affecting the foot of the mediterranean mussels in Italian waters. The presence of this parasite could affect the mussel thread strength to the farm ropes and help the predation of the succeeding host. • The presence of a trematode parasite affecting mussel food is evaluated from 2008 to 2021 in mussel farms of Campania region. • Parasite presence affects the byssus gland of mussels. • Molecular diagnostic reports an identity of 96 % with the Opistomonorcheides genus. • Effect of the mussel byssus strength need to be considered.