Parasites and mortality in the endangered fluted kidneyshell Ptychobranchus subtentus from the Clinch River,...

The fluted kidneyshell Ptychobranchus subtentus is an endangered freshwater mussel that, as of 2012, has experienced nearly 70% decline from its historical range in the Cumberland and Tennessee River drainages of the southeastern USA. Understanding the factors contributing to morbidity and mortality is essential for the long-term conservation of the species and the development of effective recovery plans. In this study, we incorporated morphological, histological, and molecular techniques to identify and describe parasites and associated lesions in 18 P. subtentus collected from the Clinch River, USA, in 2018 and 2021-2023. We identified several parasites, including metacercariae of Homalometron sp. (Homalometridae; 6/7 case mussels, 7/11 controls), sporocysts of putative Phyllodistomum sp. (Gorgoderidae; 4/7 case mussels, 2/11 controls), adult Aspidogastridae, and gonadotropic microsporidia (2/7 case mussels, 6/11 controls). Notably, this is the first report of P. subtentus as the first intermediate host for Phyllodistomum sp. and second intermediate host for Homalometron sp. Additionally, we report the first documentation of Unionicola sp. mites and gonadotropic microsporidia in P. subtentus. Our findings contribute to the understanding of parasites in mussel populations and emphasize the need for further research on the role of parasitism in mussel mortality, especially in endangered species.