First report of Hexamita nelsoni in blue mussel (Mytilus edulis): morphology, phylogeny, and host-parasite interaction.

Bivalve diseases caused by protozoan parasitic infection are monitored by coordinated surveillance programs across European member countries. However, these routine monitoring of bivalve health only survey a few relatively well-known parasites, which may leave undetected a range of less known opportunistic protozoan agents. Under increased stress, notably due to shifting environmental conditions affecting water quality and nutrient access, these opportunistic parasites may turn into pathogens impacting reproduction or growth and therefore decreasing the success and future development of aquaculture or wild population sustainability. During routine histopathological surveillance of blue mussels in Sweden, we noticed cryptic lesions in the adductor muscle in high prevalence over a period ranging from 2017 to 2022. These lesions suggested cell infiltration and degenerative changes in the muscle tissues. In this study, we report on the isolation, identification, and culture of the protistan cells likely responsible for at least some of these lesions. Using histology and cytology, molecular phylogeny, and catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH), we show that the cells correspond to an Hexamita nelsoni sequence initially isolated from oysters. We also reveal the presence of H. nelsoni in the digestive glands and mantle, and that this parasite might be vertically transmitted from adult to larvae via infections within gonads and ovaries. This study represents the first report of H. nelsoni in blue mussel.