Aethycteron malleusodes n. sp. (Monopisthocotylea: Dactylogyridae) from the threatened leopard darter, Percina pantherina (Moore & Reeves) (Perciformes: Percidae), in the Cossatot River, Arkansas, USA.

Aethycteron malleusodes n. sp. (Monopisthocotyla: Dactylogyridae) is described from the gills of the federally threatened leopard darter, Percina pantherina (Moore & Reeves) (Perciformes: Percidae), from the Cossatot River (Little River Drainage of the Red River System) in the Ouachita Mountains of Arkansas, USA. Aethycteron malleusodes n. sp. is morphologically most similar to Aethycteron malleus (Mueller (1938) by possessing a ventral bar with a posterior medial protuberance, a relatively straight male copulatory organ (MCO) with a distal spiraling sheath, and tip of the MCO with a pronounced flange. The body size and sclerites are generally smaller in A. malleusodes n. sp. and the spiraling sheath wraps around the MCO 2.5-3 times, compared to 3-4 times in A. malleus. We document the first helminth described from this threatened fish species. This article was registered in the Official Register of Zoological Nomenclature (ZooBank) as urn:lsid:zoobank.org:pub:232173AF-D776-48BD-9A8E 660C1764C006.