Proposal of Rotundianchoratus dactyliriformis gen. nov. sp. nov. and two new species of Urocleidoides (Monopisthocotyla, Dactylogyridae) parasitizing the gills of Hemiodus unimaculatus (Characiformes, Hemiodontidae)

Abstract Rotundianchoratus gen. nov. is proposed to accommodate a new species of dactylogyrid monopisthocolylan parasitizing the gills of Hemiodus unimaculatus (Characiformes: Hemiodontidae) from the Tocantins–Araguaia River basin, Brazil. Rotundianchoratus gen. nov. is mainly characterized by the presence of a well-developed protuberance at the middle shaft of the ventral anchor, a feature not previously reported for other genera within Dactylogyridae. The new genus also presents a tubular male copulatory organ (MCO) with one to two counterclockwise rings, an accessory piece articulated by a copulatory ligament, and a sclerotized vagina. This genus can be distinguished from other genera by the combination of the morphology of the copulatory complex and the unique structure of the anchors, particularly the conspicuous mid-shaft protuberance on the ventral anchor. Additionally, two new species of Urocleidoides Mizelle & Price, 1964 are described from the same host species. Both species are characterized by the presence of a copulatory ligament, a sclerotized vagina, and a distinct hook configuration, with pairs 6 and 7 larger than the remaining pairs. The new species differ from their congeners by features of the copulatory complex and structures of the haptor. This study represents the first record of monopisthocolylans infecting H. unimaculatus and expands the knowledge of dactylogyrid diversity in Neotropical freshwater systems. The findings also reinforce the morphological variability within Urocleidoides and highlight the importance of detailed morphological analyses for the recognition of taxonomic diversity within Dactylogyridae.