A parasitological survey of monogenoids infesting the gills of marine catfish (Ariidae) captured from the Atlantic coastal region of the Amazon Basin was carried out during the 2011-2013 period. The gills of 448 specimens involving twelve ariid species (29 Amphiarius rugispinis (Valenciennes), 52 Aspistor quadriscutis (Valenciennes), 74 Bagre bagre (Linnaeus), 16 Cathorops arenatus (Valenciennes), 13 Cathorops agassizii (Eigenmann & Eigenmann), 17 Cathorops spixii (Agassiz), 3 Cathorops sp., 13 Notarius grandicassis (Valenciennes), 14 Sciades couma (Valenciennes), 64 Sciades herzbergii (Bloch), 48 Sciades parkeri (Traill), 13 Sciades passany (Valenciennes), 92 Sciades proops (Valenciennes) were sampled. No monogenoids were found in Cathorops agassizi, Cathorops arenatus, Cathorops spixii, Cathorops sp. and Sciades parkeri, but the gills of the other sampled species were parasitized by at least one species of monogenoid. We identified four new species of Chauhanellus and one new species of Hamatopeduncularia: Chauhanellus hamatopeduncularoideum n. sp. from Amphiarius rugispinis and Sciades couma; Chauhanellus hypenocleithrum n. sp. from Sciades proops; Chauhanellus susamlimae n. sp. from Sciades herzbergii and Sciades passany; Chauhanellus velum n. sp. from Sciades couma, Sciades herzbergii and Sciades passany; and Hamatopeduncularia cangatae n. sp. from Aspistor quadriscutis and Notarius grandicassis. Four previously described species were reported for the first time parasitizing ariids from Atlantic Amazon: Chauhanellus neotropicalis Domingues & Fehlauer, 2006 from Amphiarius rugispinis, Aspistor quadriscutis, Notarius grandicassis and Sciades passany; Chauhanellus boegeri Domingues & Fehlauer, 2006 from Sciades couma and Sciades herzbergii; Hamatopeduncularia bagre Hargis, 1955 from Bagre bagre; and Neomurraytrematoides proops Zambrano & Añez 1993 from Sciades passany. The monotypic Neomurraytrematoides Zambrano & Añez 1993 was placed in synonymy with Neotetraonchus Bravo-Hollis, 1968 and its type-species, Neomurraytrematoides proops Zambrano & Añez 1993 was transferred to Neotetraonchus as Neotetraonchus proops (Zambrano & Añez 1993) n. comb. The morphology of the haptoral structures supports the transfer of Rhamnocercus seenghali Kumar, 2013 to Chauhanellus as Chauhanellus seenghali (Kumar, 2013) n. comb.

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Due to their high specificity, monogenoids from fish provide an interesting model to study historical associations of hosts and parasites. High agreement between host and parasite phylogeny is often interpreted as evidence of cospeciation. However, cophylogenetic signal may also arise from other, either adaptive or non-adaptive, processes. We applied the recently developed Cophylospace Framework to better understand the evolutionary relationship between monogenoids and marine catfish from the Atlantic coast of South America. The associations between 12 marine catfish and 10 monogenoid species were assessed. Molecular data of host and parasite species were used for phylogenetic reconstruction. We used anchor morphology based on Procrustes coordinates to evaluate whether closely related hosts are associated with morphologically similar parasites. To assess the association between parasite phylogeny and host morphology, we produced a distance matrix based on morphological characters of catfishes. Agreement between phylogenies and between phylogeny and morphology was measured using Procrustes R2 computed with PACo. The parasite phylogeny obtained in this study represents the first complete phylogenetic hypothesis of monogenoids parasitizing ariids from South America. The Cophylospace analysis suggested that phylogenetic and morphological distance of monogenoids contributes similarly to explain the pattern of host–parasite associations, whereas parasite phylogeny is more strongly associated with the morphological traits of the hosts than with host phylogeny. This evidence suggests that cospeciation is not a major force accounting for diversification in the monogenoids studied. Rather host morphological traits seem to be a more important driver, which conforms with evidence from other host‒monogenoid systems.

Two new species of Diaphorocleidus and one new species of Rhinoxenoides n. gen. are described from the gills of Acestrorhynchus falcatus (Bloch) from rivers of north-eastern Pará, Brazil. Diaphorocleidus jaymedeloyolai n. sp. is characterized by a male copulatory organ (MCO) possessing three counterclockwise coils; similar anchors with subtriangular superficial roots; a ventral bar with posteromedial projection; and hooks of pairs 1, 4 and 7 approximately three times longer than hook pair 5. Diaphorocleidus sclerocolpus n. sp. differs from its congeners by a dual-branched accessory piece articulated with the MCO and a sclerotized tubular vagina with a bottle-shaped vestibule. Rhinoxenoides n. gen. is proposed and is characterized by possessing: MCO sclerotized with clockwise coils; an accessory piece articulated to the base of MCO; a sinistroventral vaginal aperture; ventral anchor with conspicuous roots; dorsal anchor with superficial root five times longer than deep root; and absence of dorsal bar. The proposal of Rhinoxenoides n. gen. is also supported by its phylogenetic relationship with Protorhinoxenus prochilodi and species of Rhinoxenus, using 16 morphological characters, which resulted in the following hypothesis of sister-group relationships: Rhinoxenoides n. gen. [Protorhinoxenus (Rhinoxenus curimatae (R. nyttus (R. bulbovaginatus (R. guianensis, R. piranhus, R. euryxenus (R. arietinus, R. anaclaudiae)))))].

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.