Parvatrema sp. (Digenea: Gymnophallidae) in natural populations of Mytilus galloprovincialis from North Western Adriatic Coast.

Introduction: Parvatrema (Gymnophallidae) is a trematode infecting waterfowl at their adult stage and several species of clams and mussels in larval stages. Mytilus galloprovincialis is a widespread bivalve in the Adriatic Sea, both in natural and farmed populations. Recently, Parvatrema sp. was detected in M. galloprovincialis in a lagoon of Croatia and its presence was associated with the decline of the host population. Methods: After the presence of visible pearls was reported in a natural population of M. galloprovincialis along the Northwestern coast of the Adriatic Sea, samples were collected in seven localities between June 2020-September 2021 to evaluate the etiology of the finding. Parasitological investigations, including morphological and molecular analyses, and histopathology were carried out on infected mussels. Results: Metacercariae of the genus Parvatrema were morphologically identified in 75.3% of the examined specimens (119/158) with an interval of prevalence 0-100% among localities. The intensity of infection ranged from 1 to 3700 parasites/mussel and involved mostly the mantle, with parasites surrounded by a variable numbers of conchiolin layers. The pairwise comparison of the sequenced ITS region, revealed a 96.8% identity of our isolate with Parvatrema duboisi. The phylogenetic analysis demonstrated the independent clustering of the obtained sequences compared to other available Parvatrema species. Conclusions: In addition to be a disease affecting mussels health, the presence of pearls implies a loss in the commercial value of the infected mussels. Several issues, such as identification of the involved parasite species, ecological and epidemiological aspects still need to be further investigated.