A new species of Myxobolus (Cnidaria: Myxosporea: Myxobolidae) from the gills of the green sunfish, Lepomis cyanellus (Perciformes: Centrarchidae), from the Ouachita River Drainage of Western Arkansas.

Forty-two species of myxozoans, including 31 species of Myxobolus Bütschli, 1882, have been described from centrarchid fishes. One species, the green sunfish (Lepomis cyanellus), has been reported to host at least three species of Myxobolus. Between March 2023 and June 2024, 42 L. cyanellus were collected from watersheds in Montgomery and Polk counties, Arkansas, and their gills, gallbladders, urinary bladders, fins, integument, other major organs, and musculature were examined for myxozoans. Two (5%) L. cyanellus from Polk County were found to harbor a new species of Myxobolus infecting the gill lamellae. A qualitative and quantitative morphological description was based on formalin-fixed preserved plasmodia and myxospores. Plasmodia of Myxobolus polkensis n. sp. are 133 µm long × 123 µm wide, and myxospores are 19.8 µm long × 6.5 µm wide, with two narrowly pyriform unequal polar capsules, one usually longer (9.2 µm) than the other (8.5 µm). Molecular data consisted of a 2,025 base pair sequence of the partial small subunit rRNA gene (SSU). Variably sized, polysporic plasmodia were randomly distributed throughout the gill lamellae in intralamellar locations. Large plasmodia displaced adjacent lamellae inducing mild epithelial proliferation but minimal inflammatory changes. Phylogenetic analysis revealed that M. polkensis n. sp. is a member of a clade of myxozoan species that predominately infect centrarchids from North America. This is the fourth report of a Myxobolus from L. cyanellus but the first report of a species infecting the gill lamellae. This article was registered in the Official Register of Zoological Nomenclature (ZooBank) as urn:lsid:zoobank.org:pub:E9BA08E0-0A7E-487B-80AE-C5C723DBB661.