Alfjorden, A. | Search bioRxiv

bioRxiv2024

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.

pathology↗

bioRxiv2023

Identification of a new gregarine parasite in mass mortality events of freshwater pearl mussels (Margaritifera margaritifera) in Sweden

Freshwater bivalves play key ecological roles in lakes and rivers, largely contributing to healthy ecosystems. The freshwater pearl mussel, Margaritifera margaritifera, is found in Europe and on the East coast of North America. Once common in oxygenated streams, M. margaritifera is rapidly declining, and consequently assessed as a threatened species worldwide. Deterioration of water quality has been considered the main factors for the mass mortality events affecting this species. Yet, the role of parasitic infections has not been investigated. Here, we report the discovery of three novel protist lineages found in Swedish populations of M. margaritifera belonging to one of the terrestrial groups of gregarines (Eugregarinorida, Apicomplexa). These lineages are closely related-but clearly separated- from the tadpole parasite Nematopsis temporariae. In one lineage, which is specifically associated with mortality events of M. margaritifera, we found using microscopy and in situ hybridization cysts in the gills and other organs of diseased individual, containing single vermiform zoites. This represents the first report of a parasitic infection in M. margaritifera that may be linked to the decline of this mussel species. We propose a tentative life cycle with distribution of different developmental forms and potential exit from the host into the environment.

microbiology↗