A.G.D.C. Feitosa

Anacanthorus is one of the most diverse genera of monopisthocotyl parasites associated with Neotropical Characiformes, particularly serrasalmid fishes. This study aimed to inventory the monopisthocotyl parasites of serrasalmid fish from the Lower São Francisco River, northeastern Brazil. Twenty-nine specimens of Pygocentrus nattereri were collected, and their parasites were examined using standard morphological methods. Our results allowed the description of Anacanthorus penedensis n. sp., supported by morphological and molecular data, and the assessment of its phylogenetic relationships within the Dactylogyridae using partial 28S rDNA sequences. Additionally, previously described monopisthocotyls are reported, including Anacanthorus reginae , Amphithecium falcatum , and Enallothecium aegidatum , whereas Rhinoxenus sp. was identified to the genus level and another dactylogyrid remained unidentified. Anacanthorus penedensis n. sp. can be distinguished from its congeners by having an accessory piece comprising two distinct expansions: a submedial expansion extending to the distal region and a conspicuous laminar subterminal expansion originating from one of the margins of the shaft, with a hooked acute tip, as well as by the relative lengths of the male copulatory organ and the accessory piece. Phylogenetic analyses of 28S rDNA sequences, based on maximum likelihood and Bayesian inference, placed the new species within the genus Anacanthorus and supported the monophyly of the group based on the dataset used in the present study. These findings reinforce the need for future integrative taxonomic studies that combine complementary lines of evidence for species delimitation and suggest the diversity of Anacanthorus remains underestimated.