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. These routine monitorings of bivalve health, however, only survey a few relatively well known parasites, which may leave undetected a range of lesser known opportunistic protozoan agents. Under increased stress, for example due to shifting environmental conditions affecting water quality and nutrient access, these opportunistic parasites may develop pathogenicity 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 catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH), we show that the cells correspond to the H. nelsoni sequence initially isolated from oysters. We also reveal the presence of H. nelsoni in the digestive glands and mantel, and that this parasite might be horizontally transmitted from adult to larvae via infections within gonads, ovaries. This study represents the first report of H. nelsoni in blue mussel.