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During studies on parasites of fishes from Moa River, state of Acre, five species of Dactylogyridae were identified parasitizing Brycon cephalus (Günther, 1869): Anacanthorus elegans Kritsky, Thatcher & Kayton, 1979, Anacanthorus kruidenieri Kritsky, Thatcher & Kayton, 1979, Anacanthorus spiralocirrus Kritsky, Thatcher & Kayton, 1979, Annulotrematoides bryconi Cuglianna, Cordeiro & Luque, 2003, and Tereancistrum kerri Kritsky, Thatcher & Kayton, 1980. New morphological and biogeographic data are provided. Records of these parasites in congeneric hosts reinforce the host specificity of these species, restricted to species of Bryconidae.

Opecoeloides Odhner, 1928, is represented by 19 valid species found in marine fish, of which five have been reported in Brazil. Specimens of Opecoeloides pedicathedrae Travassos, Freitas & Bührnheim, 1966, were collected from the intestine of smooth weakfish Cynoscion leiarchus, a new host record, from off the coast of Rio de Janeiro, Brazil. They were examined using light and confocal laser microscopy. New partial sequences of 18S and 28S rDNA genes of O. pedicathedrae were obtained. Bayesian inference analysis on the partial 28S rDNA dataset resulted in a phylogram in which O. pedicathedrae formed a well-supported clade with Opecoeloides fimbriatus and Opecoeloides furcatus. The K2p distance between O. pedicathedrae and O. fimbriatus was 0.34%, with 3 divergent nucleotides; and between O. pedicathedrae and O. furcatus was 4.18%, with 38 divergent nucleotides. A Bayesian-inference phylogenetic tree based on the 18S rDNA recovered two main clades with five subfamilies. A clade of Opecoelinae showed that O. pedicathedrae was closer to Pseudopecoeloides tenuis; the K2p distance between these species was 2.14%, with 28 divergent nucleotides. The new nucleotide sequences presented inclusion of a phylogenetic analysis that can help to clarify the understanding of this complex taxon.

Five new species of Cosmetocleithrum were described parasitizing the gill filaments of neotropical doradid fishes. Cosmetocleithrum undulatum n. sp., Cosmetocleithrum brachylecis n. sp. and Cosmetocleithrum ludovicense n. sp. are described from Platydoras brachylecis from a market-place of São Luís, State of Maranhão, Brazil. Cosmetocleithrum sacciforme n. sp. and Cosmetocleithrum basicomplexum n. sp. are described from Oxydoras niger from Juruá River, State of Acre, Brazil. Cosmetocleithrum undulatum and Cosmetocleithrum brachylecis resemble Cosmetocleithrum falsunilatum Feronato, Razzolini, Morey & Boeger, 2022 mainly by the unique male copulatory organ (MCO) morphology but differ from these and all congeneric species mainly by the morphology of the MCO, accessory piece and hooks pairs. Cosmetocleithrum ludovicense is closer to Cosmetocleithrum confusus Kritsky, Thatcher & Boeger, 1986 and to Cosmetocleithrum akuanduba Soares, Santos Neto & Domingues, 2018 but differs from those mainly by the morphology of the accessory piece. Cosmetocleithrum sacciforme differs from all congeneric species mainly by the morphology of the accessory piece formed by a single plate of saccular appearance. Cosmetocleithrum basicomplexum also shares morphological characters with Cosmetocleithrum gigas Morey, Cachique & Babilonia, 2019 considering the size of the body and shape of the anchors, but differs mainly in the morphology of the bars and hooks. Besides the new species, new data are presented for Cosmetocleithrum leandroi Soares, Neto & Domingues, 2018 , C. akuanduba and C. confusus regarding morphological characteristics and biogeography. Cinq nouvelles espèces de Cosmetocleithrum sont décrites, parasitant les filaments branchiaux de poissons Doradidae néotropicaux. Cosmetocleithrum undulatum n. sp., Cosmetocleithrum brachylecis n. sp. et Cosmetocleithrum ludovicense n. sp. sont décrits de Platydoras brachylecis provenant d’un marché de São Luís, État du Maranhão, Brésil. Cosmetocleithrum sacciforme n. sp. et Cosmetocleithrum basicomplexum n. sp. sont décrits d’ Oxydoras niger de la rivière Juruá, État d’Acre, Brésil. Cosmetocleithrum undulatum et Cosmetocleithrum brachylecis ressemblent à Cosmetocleithrum falsunilatum Feronato, Razzolini, Morey & Boeger, 2022 principalement par la morphologie unique de l’organe copulateur mâle (OCM), mais diffèrent de ces espèces et de toutes les espèces congénères principalement par la morphologie de l’OCM, de la pièce accessoire et des paires de crochets. Cosmetocleithrum ludovicense est proche de Cosmetocleithrum confusus Kritsky, Thatcher & Boeger, 1986 et de Cosmetocleithrum akuanduba Soares, Santos Neto & Domingues, 2018 mais en diffère principalement par la morphologie de la pièce accessoire. Cosmetocleithrum sacciforme se distingue de toutes les espèces congénères principalement par la morphologie de la pièce accessoire formée d’une seule plaque d’aspect sacculaire. Cosmetocleithrum basicomplexum partage également des caractères morphologiques avec Cosmetocleithrum gigas Morey, Cachique & Babilonia, 2019 compte tenu de la taille du corps et de la forme des anchors, mais en diffère principalement par la morphologie des barres et des crochets. Outre les nouvelles espèces, de nouvelles données sont présentées pour Cosmetocleithrum leandroi Soares, Neto & Domingues, 2018, C. akuanduba et C. confusus concernant les caractéristiques morphologiques et la biogéographie.

Hypophthalmus marginatus is among the most commercialized fish from the Amazon region, with fish fillets exported to Southeastern Brazil and other countries. In the present study, the H. marginatus analyzed were parasitized by third-stage larvae of Hysterothylacium sp. and Anisakis sp. These nematodes are being reported for the first time in H. marginatus collected from the Tocantins River, representing a new host and geographical records, and expanding knowledge of the hosts of these nematodes in the Neotropical Region. A brief description with original measurements is presented.

Several studies have demonstrated parasitism by monogenoids in characiform fish in the Neotropics. During studies on the helminth fauna of curimatids from the Tocantins River, specimens of Psectrogaster amazonica Eigenmann & Eigenmann, 1889 were examined and species of Curvianchoratus Hanek, Molnar & Fernando, 1974 were found. Species of the genus are characterized mainly by the complex shape of haptoral anchors with a modified dorsal anchor, composed by two subunits, dorsal-median and dorsal. To date, two species of Curvianchoratus are known to parasitize curimatid fishes: the type species Curvianchoratus hexacleidus Hanek, Molnar & Fernando, 1974 and Curvianchoratus singularis (Suriano, 1980). During examination of specimens of P. amazonica collected in the Tocantins River, Embiral, Imperatriz, Maranhão State, Brazil, two new species of Curvianchoratus were found and are described herein. Curvianchoratus psectrogasteri sp. nov. and Curvianchoratus dominguesi sp. nov. are characterized by possessing the male copulatory organ formed by a long cirrus and a claw-shaped accessory piece, connected to the base of the male copulatory organ by a ligament. The new species differs from the two known congeneric species mainly by the morphology of the dorsal-median and dorsal subunits of the dorsal anchor. Curvianchoratus psectrogasteri sp. nov. also differs from other species of the genus by the absence of the ventral bar and Curvianchoratus dominguesi sp. nov. by the size and shape of the ventral bar. An amendment to the diagnosis of Curvianchoratus is provided to accommodate the new species. The present study increases the number of Curvianchoratus species to four and extends the occurrence of the genus to the Tocantins-Araguaia Basin.

Pseudorhabdosynochus jeanloui n. sp. (Monogenoidea, Diplectanidae) is described from specimens collected from the gills of the Pacific creolefish, Paranthias colonus (Perciformes, Serranidae) from a fish market in Chorrillos, Lima, Peru. The new species is differentiated from other members of the genus by the structure of its sclerotized vagina, which has two spherical chambers of similar diameter. This is the first Pseudorhabdosynochus species described from the Pacific coast of America, the third species of the genus reported from South America and the first described from a member of Paranthias. Pseudorhabdosynochus jeanloui n. sp. (Monogenoidea, Diplectanidae) est décrit de spécimens collectés sur les branchies de la badèche du Pacifique, Paranthias colonus (Perciformes, Serranidae) d’un marché aux poissons de Chorrillos à Lima au Pérou. La nouvelle espèce se différencie des autres membres du genre par la structure de son vagin sclérifié, qui a deux chambres sphériques de diamètre similaire. Ceci est la première espèce de Pseudorhabdosynochus décrits de la côte Pacifique de l’Amérique, la troisième espèce du genre rapportée d’Amérique du Sud, et la première décrite d’une espèce de Paranthias.

A new and two previously described species of Ameloblastella Kritsky, Mendoza-Franco & Scholz, 2000 were found in the gills of two pimelodid catfishes from the Tocantins River, Maranhão State, Brazil: Ameloblastella priman. sp. from Pimelodina flavipinnis Steindachner, and A. edentensis Mendoza-Franco, Mendoza-Palmero & Scholz, 2016 and A. peruensis Mendoza-Franco, Mendoza-Palmero & Scholz, 2016 from Hypophthalmus marginatus Valenciennes. The new species differs from its congeners by having an accessory piece tubular, covered by a delicate membranous cap at 2/3 of the length, by the shape of the ventral and dorsal anchors, in which the ventral anchor is represented by a curved shaft in a position of 80° degrees, with delicate ornamentations located at the superficial and deep root areas, and the dorsal anchor with a long, well-developed and convex superficial root and short deep root, also presenting delicate ornamentations at the superficial and deep root areas. The present study provides new biogeographical data on the two previously described species of Ameloblastella and improves the knowledge on the species of Monogenoidea from the Neotropical Region, increasing to 14 the number of known species of Ameloblastella for this region.

Mymarothecium boegeri sp. n. (Monogenea: Dactylogyridae) is described from the gills of Colossoma macropomum Cuvier (Characidae), collected from the aquaria of the \"Centro de Pesquisas em Aquicultura, Departamento Nacional de Obras Contra as Secas (DNOCS)\", Pentecoste City, State of Ceará, Brazil. Mymarothecium viatorum Boeger, Piasecki et Sobecka, 2002 is reported from the type host, Piaractus brachypomus (Cuvier) (Characidae) and from a new host, P. mesopotamicus (Holmberg) (Characidae), confirming the occurrence of M. viatorum in the Neotropical Region. The new species differs from the congeneric species in the structure of male copulatory complex; it is more closely related to M. viatorum by the presence of a posteromedial projection on ventral bar.

Monogenoids are ectoparasites that have a simple one-host lifecycle, high species diversity, and a relatively high host specificity. During studies on the helminth fauna of fishes from the Jurua River, in Acre State, Brazil, a new species of the monotypic genus Unibarra Suriano & Incorvaia, 1995 was found parasitizing Oxydoras niger Valenciennes, 1821. Unibarra juruaensis n. sp. is allocated in the genus based on the presence of a single haptoral bar, marginal hooks similar in shape and size, gonads partially overlapping, and a conspicuous filament which connects the base of the male copulatory organ with the accessory piece. The new species differs from the only species of the genus by the smaller size of the body and of the structures, by the morphology of copulatory complex, with an accessory piece thinner than that of U. paranoplatensis Suriano & Incorvaia, 1995 and by the presence of two eyespots. The type species, U. paranoplatensis , is referred in a new host, Pimelodus blochii Valenciennes, 1840, with new morphological data. A table of measurements of the new species and previous and the present reports of U. paranoplatensis is presented.

Opecoeloides Odhner, 1928, is represented by 19 valid species found in marine fish, of which five have been reported in Brazil. Specimens of Opecoeloides pedicathedrae Travassos, Freitas & Bührnheim, 1966, were collected from the intestine of smooth weakfish Cynoscion leiarchus, a new host record, from off the coast of Rio de Janeiro, Brazil. They were examined using light and confocal laser microscopy. New partial sequences of 18S and 28S rDNA genes of O. pedicathedrae were obtained. Bayesian inference analysis on the partial 28S rDNA dataset resulted in a phylogram in which O. pedicathedrae formed a well-supported clade with Opecoeloides fimbriatus and Opecoeloides furcatus. The K2p distance between O. pedicathedrae and O. fimbriatus was 0.34%, with 3 divergent nucleotides; and between O. pedicathedrae and O. furcatus was 4.18%, with 38 divergent nucleotides. A Bayesian-inference phylogenetic tree based on the 18S rDNA recovered two main clades with five subfamilies. A clade of Opecoelinae showed that O. pedicathedrae was closer to Pseudopecoeloides tenuis; the K2p distance between these species was 2.14%, with 28 divergent nucleotides. The new nucleotide sequences presented inclusion of a phylogenetic analysis that can help to clarify the understanding of this complex taxon.