<i>Hirsutonosema embarrassi</i> n. gen. n. sp. (Phylum Microsporidia) in the Ovary of Mucket (<i>Actinonaias ligamentina</i>), Plain Pocketbook (<i>Lampsilis cardium</i>), and Fatmucket (<i>Lampsilis siliquoidea</i>) (Unionidae) from the Embarrass River, Wisconsin, USA

Abstract

During an epidemiological survey following a mortality event of freshwater mussels in 2018 in the Embarrass River, Wisconsin, USA, we identified a novel microsporidian parasite in the ovaries of mucket (Actinonaias ligamentina), plain pocketbook (Lampsilis cardium), and fatmucket (Lampsilis siliquoidea) (Unionidae). Histopathology showed round-to-oval microsporidian spores in the cytoplasm of oocytes in 60% (3/5) of female mucket, 100% (4/4) of female plain pocketbook, and 50% (1/2) of female fatmucket. Using transmission electron microscopy, we found that mature spores were round-to-oval and measured 4.13 ± 0.64 µm (3.14–5.31) long by 2.88 ± 0.37 µm (2.36–3.68) wide. The spores had a thin electron-dense exospore with a spiky “hairy” coat, a thick electron lucent endospore, diplokaryotic nuclei, a polar vacuole, and 27–28 polar filaments arranged in 1–3 rows. Sequencing of the small subunit rRNA produced a 1356 bp sequence most similar to that of Pseudonosema cristatellae (92%), and phylogenetic analysis grouped it within the freshwater Neopereziida. Genetic, morphological, and ultrastructural characteristics did not closely match those of other Pseudonosema spp., and a new genus and species, Hirsutonosema embarrassi n. gen. n. sp., were designated. Additional studies could evaluate host susceptibility, distribution, seasonality, transmission, and lethal or sub-lethal effects of this parasite on freshwater mussels.

Keywords

• <i>Actinonaias ligamentina</i>
• freshwater mussel
• <i>Hirsutonosema embarrassi</i> n. gen. n. sp.
• histopathology
• <i>Lampsilis cardium</i>
• <i>Lampsilis siliquoidea</i>