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Oncoceratium n. gen. is proposed to accommodate Oncoceratium amphidactylum n. gen. n. sp. (Monogenoidea, Gyrodactylidae). Although the proposed genus is monotypic, the differences presented in the diagnosis clearly establish it as distinct from species from other genera, such as Gyrodactylus von Nordmann, 1832 and especially those that occur exclusively in the Neotropical region. The new genus is diagnosed by the presence of a bulbous male copulatory organ, with one row of spinelets of similar sizes and shapes, while the only spine usually found in the gyrodactylids is absent or not differentiated; a haptor with anchors presenting points folded outwards, and superficial and deep roots continuous and indistinguishable; the absence of a deep bar; and hooks in two bilateral clusters of eight hooks. Oncoceratium amphidactylum n. gen. n. sp. is described from the tamboatá fish, Hoplosternum littorale (Hancock) (Callichthyidae), and is characterized by the presence of a pair of horn-shaped anchors with the point turned outwards, a superficial bar shield and absent deep bar, hooks disposed in bilateral lobes, and a MCO with no spine or with a row with large spinelets visible in place of the spine. In addition to the morphological features, distance analysis and Bayesian inference, based on 5.8s and partial ITS2, support placing the new species in a new genus, and not including it in a cluster of species of Gyrodactylus and near to Gyrodactyloides bychowskii Albova, 1948, Ieredactylus rivuli Schelkle et al., 2011 and Laminiscus gussevi (Bychowsky & Polyansky, 1953) Pálsson & Beverely-Burton, 1983., (© 2023. The Author(s), under exclusive licence to Springer Nature B.V.)

In this study, a new dactylogyrid species is described from the gills of two ariid fish species of the Brazilian coast, Genidens barbus and Genidens genidens , by combining morphological characters with partial 18S rDNA sequences. The new species can be distinguished from its congeners by the following morphological characteristics: a male copulatory organ (MCO), a coiled tubular shaft of around one counterclockwise ring, base with a sclerotized cap; MCO with tapered distal region; a T-shaped accessory piece; a vagina, a cup-shaped vaginal vestibule, slightly sclerotized; and a pharynx comprising a muscular, glandular bulb. Supplementary taxonomic data for Chauhanellus velum from Sciades couma (type-host) are also presented and new 18S rDNA sequences of Chauhanellus spp. from other ariid fish from South America are provided. Phylogenetic analyses based on partial 18S rDNA gene sequences placed Chauhanellus riograndinensis n. sp. and Chauhanellus velum as two early divergent lineages within Chauhanellus from South America. Finally, a way to test the monophyly of Chauhanellus and Hamatopeduncularia is also discussed, which may be useful for future studies.

Gyrodactylidae and Dactylogyridae (Monogenoidea) are described or reported from three species of Characidium Reinhardt (Crenuchidae), small species of Characiformes, from streams located in southern Brazilian states. Gyrodactylus carolinae sp. n. (Gyrodactylidae) is described from the body surface of Characidium lanei Travassos (type host), C. pterostictum Gomez, and Characidium sp. from streams in the states of Paraná and São Paulo. This new species closely resembles species of Gyrodactylus von Nordmann, 1832 described from species of Poeciliidae, from which it differs by the morphology of the hooks and nucleotide sequences of ITS1-5.8S-ITS2 rDNA. Gyrodactylus inesperatus sp. n. is described from the body surface of Characidium sp. from a stream in the State of São Paulo. The latter new species is characterised by lacking a shield on the superficial bar and by the morphology of the hooks, both unique characteristics for Neotropical species of Gyrodactylus. Marumbius gen. n. (Dactylogyridae) is proposed to accommodate two species, M. dorsivaginatus sp. n. from the gills of Characidium pterostictum (type host) and C. lanei, and M. amplexus sp. n. from the gills of C. lanei (all from the state of Paraná). Both species are characterised by having dorsal vagina, hook pairs 2-4, 6 and 7 composed by two subunits, hook pairs 1 and 5 lacking proximal subunit, and by the length of proximal subunits (when present) varying among hook pairs, completely or partially overlapping gonads, and male copulatory organ (MCO) represented by an incomplete coil of a sclerotized tube articulated to the accessory piece by a copulatory ligament. Cacatuocotyle paranaensis Boeger, Domingues et Kritsky, 1997 is reported from C. lanei at low prevalence in the Rio Marumbi (state of Paraná). The Monogenoidea that parasitize species of Characidium are members of several independent lineages, some of distant evolutionary relationships, suggesting a complex origin for this parasitic fauna.