Four new species, assigned to the proposed new genus Tocantinsiella n. gen., were described from the gills of Caenotropus labyrinthicus collected in the Tocantins and Gurupi river basins, northern Brazil: Tocantinsiella maranhensis n. sp., Tocantinsiella brassicae n. sp., Tocantinsiella vesiculosa n. sp., and Tocantinsiella flabellata n. sp. The new species are characterized mainly by the presence of a sclerotized, robust, and sinistral vaginal vestibule, as well as a copulatory ligament connecting the male copulatory organ (MCO) from the accessory piece, which represent morphological and diagnostic characteristics for the proposed new genus. Tocantinsiella n. gen. is morphologically similar to Heterothecium due to the presence of a sclerotized, sinistral vaginal vestibule, differing due to the presence of a copulatory ligament and a non-articulated MCO. Philocorydoras also shares similarities related to the morphology of the copulatory complex. However, species of this genus are recorded only in Siluriformes fishes. This study provides a significant expansion of the diversity of helminth fauna in fishes of the family Chilodontidae by describing a new genus and four new species of Monopisthocotyla in the host studied.

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.

Authorship of the nominal family Gyrodactylidae has been inconsistently cited for more than a century, being generally attributed either to van Beneden & Hesse (1863) or to Cobbold (1864). We evaluated the competing attributions under Article 11.7.2 of the International Code of Zoological Nomenclature. This provision allows a pre-1900 family-group name published in non-latinized form to retain its original authorship if it was later latinized and has been generally accepted as valid and as dating from the first publication in vernacular form. Van Beneden & Hesse (1863) originally established the taxon under the French vernacular Gyrodactylidés. Shortly thereafter, Cobbold (1864) published the latinized form Gyrodactylidae. Review of the historical literature demonstrated persistent attribution to van Beneden & Hesse (1863) across a broad range of taxonomic traditions and geographic regions, even while attribution to Cobbold (1864) has also continued. The conditions of Article 11.7.2 are satisfied, and the nominal family of the family-group should be cited as Gyrodactylidae Van Beneden & Hesse, 1863. This clarification resolves a long-standing nomenclatural inconsistency that persists in both the primary literature and major taxonomic databases.

Parasitological studies on Cyphocharax spp. from different regions of South America, have recorded different species of Urocleidoides Mizelle & Price, 1964 parasitizing their gills. To date, no parasites have been documented for Cyphocharax spilurus (Günther) from the Peruvian Amazonia. Thus, the main objective of the present study was to survey the monopisthocotylan parasites from the gills of C. spilurus collected in the Peruvian Amazonia. Twenty specimens of C. spilurus were collected from the Nanay River, Loreto, Peru. Parasites were removed from the gills using dissection needles and cleared in Hoyer's medium for the study of sclerotized structures. Two new species were described: Urocleidoides nanayensis n. sp. characterized by the male copulatory organ consisting in an elongated sclerotized tube forming a loop-like reservoir at its base, and an articulated accessory piece bearing a multi-pronged terminal apparatus with hook-like projections. The vaginal sclerite is can-opener-shaped, and the haptor presents unequal ventral and dorsal anchors with poorly developed roots; hooks are arranged in seven pairs: pairs I and V share a similar shape with elongated filaments, while pairs II-IV and VI-VII are composed of two subunits with straight shanks and short filaments. Urocleidoides loretoensis n. sp. is characterized by the shape of the MCO, which consists in a knife-blade-shaped with a cone-shaped base and a smoothly arched, needle-tipped sclerotized tube, and an articulated accessory piece bearing a stippled central rod with a curved branch and fan-like membranous sheath. The vaginal sclerite is irregular, with a ventrally directed process, arched lamina, and recurved distal end. The haptor presents unequal ventral and dorsal anchors with poorly developed roots; the ventral bar is straight and robust while the dorsal bar is slender with a small mid-region projection. Hook pairs I and V are morphologically distinct from the remaining pairs (II-IV and VI-VII), which share an erect thumb, slightly curved shank, and short filament. The discovery of two new species of Urocleidoides brings the number of valid species in the genus to 67 and constitutes the first parasitological record for C. spilurus; it also represents the first species of the genus described in a curimatid host from the Peruvian Amazon.

Studies concentrating on description, identification, and analysis of monopisthocotyls are required to further our understanding of this group of parasites connected to Amazonian fish. Based on the above, the main objective of this research was to identify new species of monopisthocotyls parasitizing the gills of Acestrorhynchus falcirostris (Cuvier) collected in the Itaya River, Loreto-Peru. Twenty (20) A. falcitrostris were collected in the Itaya River (3°47'20.70″S 73°15'40.90″W), in Loreto-Peru in October 2025. The collected fish were transported in boxes with ice to the Laboratory of Parasitology of the "Instituto de Investigaciones de la Amazonía Peruana" (IIAP), in Iquitos, Loreto-Peru for parasitological analyses. Four new species were discovered and described: Characithecium peruense n. sp., C. cachorroi n. sp., C. iquitense n. sp., and C. loretoense n. sp. The new species are differentiated from their congeners, mainly by the morphology of the copulatory complex. In C. peruense n. sp. the male copulatory organ (MCO) is a J-shaped tube, the accessory piece is articulated to the base of the MCO and comprises two subunits, being the second unit hook-shaped. In C. cachorroi n. sp. the accessory piece consists of an elongated proximal unit, with a gentle curvature in its middle portion, whose upper end forms a loop that passes behind the distal end of the cirrus; and an equally elongated distal unit, which forms a loop at its anterior end and has an expansion in its middle region that projects posteriorly. In C. iquitense n. sp. the MCO consists of an elongated sclerotized tube forming two and a half counterclockwise rings before reaching the base, which is articulated to the accessory piece, presenting two units, the first unit articulated to the base of the MCO, "L" shaped, with a rounded termination, the second unit is complex, featuring a stippled (dotted) subunit, formed by two asymmetrical arms. In C. loretoense n. sp. the MCO is tubular, elongated, whose sinuous path describes four and a half clockwise rings; the base of the cirrus articulates with the accessory piece, that is presented as robust plate-like structure with a broad bulbous basal region that tapers to an acutely pointed, down-curved distal tip. The results of the present investigation represent the first species of the Characithecium described from a host collected in the Peruvian Amazon, increasing the knowledge of the biodiversity of monopisthocotyls reported for Peru and the Neotropical region.

Isopods of the suborder Epicaridea Latreille, 1825 are obligatory parasites of other crustaceans, using them as intermediate and definitive hosts during their life cycles. All species of the family Cryptoniscidae Kossmann, 1880 are endoparasitic or mesoparasitic, and can be found as direct parasites of decapods or as hyperparasites of rhizocephalan barnacles infesting decapod hosts. Females usually lose all body segmentation, lacking antennae, pereopods, oostegites and pleopods, and their bodies have two portions, an anterior portion that is embedded in the host cuticle and a posterior one that is external to the host. Males, on the other hand, are neotenous, and retain the morphology of the cryptoniscus larval stage. The genus Zeuxokoma Grygier, 1993 comprises six valid species, all of which are direct parasites of caridean shrimps in the Atlantic and Pacific Oceans. Here we describe a new species of Zeuxokoma parasitizing the snapping-shrimp host Synalpheus apioceros Coutière from Brazil. The new species is morphologically most similar to Z. setosa (Nierstrasz & Brender à Brandis, 1930), but can be distinguished by the position of the shield and the insertion of the trunk. We also comment on the morphology of the last embryonic stage, provide a map detailing the distribution of all currently valid species of Zeuxokoma, and construct a key based on the females to assist in the identification of species. Finally, we include a table compiling the morphological characters, definitive hosts, and known distribution of the females of each species.

Feather mites (Acariformes: Astigmata) are commonly regarded as harmless ectosymbionts of birds. In some cases, however, they may cause pathological conditions in their hosts. A new species of feather mite, Fainalges irritans sp. nov., is described from green-and-red macaws, Ara chloropterus Gray (Psittaciformes: Psittacidae) in Brazil, where it is associated with skin irritation and feather loss. An updated key to males and females of the genus Fainalges Gaud & Berla, 1963 is provided, and previous records of feather mites causing injuries in birds are briefly discussed.

A new coccidian species, Isospora melanotidis n. sp. from the blue mockingbird Melanotis caerulescens (Swainson), from Morelia, Michoacán State, Mexico. It is described and characterized molecularly. Sporulated oöcyst of the new species are subspherical, 25‒29 × 21‒25 (26.9 × 23.1) µm, (n = 30), with a length/width (L/W) ratio 1.0‒1.1 (1.04). Wall bi-layered 0.6‒1.1 (0.8) thick, clear outer and dark interior. One polar granule present, but micropyle and oöcyst residuum are absent, Sporocysts are ovoidal, measuring on average 14‒22 × 10‒13 (16.6 × 10‒13) µm, with a L/W ratio of 1.5; Stieda, sub-Stieda and para-Stieda bodies are present. A sporocyst residuum is present as a diffuse mass of granules. The 18S rRNA gene loci segment of Isospora sp., used for phylogenetic analysis by means of a maximum likelihood (ML) tree, revealed that Isospora sp., from M. caerulescens is distinct from Isospora sp., from Bewick's Wren Thryomanes bewickii, with node support and bootstrap length of 91.5%. To date, this is the third report on Isospora Schneider 1881 (Apicomplexa: Eimeriidae), which infects Passeriformes of the Mimidae family of the Americas.

The platyhelminth Microcotyle pomatomi Goto, 1899 was originally described from the gills of the bluefish, Pomatomus saltatrix (Linnaeus) collected in Newport, Rhode Island (USA). The original description was brief and lacked detailed morphological, anatomical, and molecular information. As a result, the species has long been the subject of several taxonomic studies, with historical records often attributed to different nominal taxa due to insufficient diagnostic features. In the present study, specimens of M. pomatomi were collected from the gills of Pomatomus saltatrix (Linnaeus) off the Algerian coast in the western Mediterranean, representing the first molecular characterization of this species in this region. Parasites were carefully examined, measured, and illustrated to provide a comprehensive morphological redescription. In parallel, molecular characterization was conducted using a partial fragment of the mitochondrial cox1 gene, and host identity was confirmed through analysis of gill tissue. The integration of morphological and molecular data allowed for a more precise identification and improved understanding of the species. The obtained cox1 sequence clearly supports the validity of M. pomatomi and distinguishes it from closely related congeners. Furthermore, our findings confirm the synonymy of several previously described species. Overall, this study clarifies the taxonomic status and phylogenetic position of M. pomatomi within the Microcotylidae.

The tapeworm genus Mariauxiella (Onchoproteocephalidea, Proteocephalidae) was recently revised and its species composition reappraised; however, its actual species diversity remains incompletely understood. In the present study, an integrative taxonomic approach combining morphological, ultrastructural, and molecular phylogenetic analyses was applied to proteocephalid tapeworms collected from the driftwood catfish Ageneiosus militaris (Siluriformes: Auchenipteridae) in the Upper Paraná River basin, Brazil. The evidence obtained supports the proposal of Mariauxiella saltogrande n. sp., the third species assigned to the genus. The new species can be differentiated from the most similar species, Mariauxiella brevifilis, by its body size, proglottis width, scolex shape, and the arrangement of the osmoregulatory canals in the proglottides. Phylogenetic analyses based on the partial large subunit nuclear ribosomal RNA (RNA28S) and the complete mitochondrial cytochrome c oxidase subunit I (MT-CO1) sequence data strongly supported the monophyly of Mariauxiella and recovered Mariauxiella saltogrande n. sp. as sister to M. brevifilis. The discovery of this new species expands the known diversity of Mariauxiella and provides further evidence for the usefulness of integrative taxonomy in resolving species boundaries and evolutionary relationships among Neotropical proteocephalids.

As part of a project to discover the diversity of fish parasites from Acapulco Bay on the coast of Guerrero in the Mexican Pacific, four species of the Monopisthocotyla (including two new species) and a species of the Polyopisthocotyla are described and compared with congeneric species: Octouncuhaptor sp. from the gills of the blue bobo, Polydactylus approximans (Lay & Bennett, 1839); Pseudempleurosoma sp. (Dactylogyridae) from the oesophagus of the yellow bobo Polydactylus opercularis (Gill); Acanthocercodes bullardiodes n. sp. and Acanthocercodes pacificus n. sp. (Diplectanidae) (all of the Monopisthocotyla) from the gills of P. opercularis (type host for both) and P. approximans; and Microcotyloides impudicus Caballero, Bravo & Grocott, 1955 (Polyopisthocotyla: Microcotylidae) from the gills of P. approximans. Octouncuhaptor sp. is mainly distinguished from its unique congeneric species, Octouncuhaptor eugerrei Mendoza-Franco, Roche & Torchin, 2008, on Eugerres brasilianus (Cuvier) (Gerreidae) from Panama by the morphology of the copulatory complex. Octouncuhaptor sp. represents a new species, but a specific name is not proposed for a single specimen in which the position of the structures of the internal soft organs (i.e., prostatic reservoir and seminal vesicle) are not visible, and those hard parts of the anatomy are not well defined to permit a complete description. The new diplectanids are distinguished from their congeners by the morphology and the size of the sclerotized parts of the haptor and peduncular spines. Measurements, supplementary observations, and new illustrations for Pseudempleurosoma sp. and M. impudicus are provided. The occurrence of interoceanic species of Acanthocercodes and Microcotyloides from the coasts of the Gulf of Mexico, South Atlantic Ocean (Brazil), and Mexican Pacific are briefly discussed.

Monopisthocotyls represent one of the most diverse groups of ectoparasites associated with Neotropical freshwater fishes; however, their diversity in the Peruvian Amazon remains poorly known. The present study aimed to describe new species of monopisthocotyls parasitizing the gills of Metynnis luna Cope collected in the Nanay River, Loreto, Peru. Twenty specimens of M. luna were examined, from which five new species of monopisthocotyls belonging to the family Dactylogyridae were recovered: four species of the genus Anacanthorus Mizelle & Price, 1965 and one species of Notozothecium Boeger & Kritsky, 1988. Specimens were fixed in 96% ethanol, cleared in Hoyer's medium, and analyzed using phase-contrast microscopy. Taxonomic descriptions were primarily based on the sclerotized structures of the male copulatory organ and the haptor. The results allowed description of Anacanthorus nanayensis n. sp., characterized by a J-shaped tubular cirrus, a short accessory piece, and hooks with an expanded pinhead-like base; A. nanayoides n. sp., distinguished by a simple accessory piece reaching approximately half the length of the cirrus and hooks composed of two subunits; A. ramulus n. sp., characterized by a branch-shaped accessory piece with a conspicuous median projection, slightly undulated cirrus margins, and relatively robust hooks; A. iquitensis n. sp., distinguished by a thick accessory piece with a conspicuous median projection and a length corresponding to approximately three-quarters of the cirrus length; and Notozothecium iquitense n. sp., characterized by an accessory piece basally articulated to the cirrus, composed of two subunits, and dorsal anchors with short, rounded deep roots. These findings expand the current knowledge of dactylogyrid diversity associated with Amazonian serrasalmids and highlight the high, yet still undocumented, parasitic diversity occurring in fishes from the Peruvian Amazonia. Furthermore, this study represents the first record of species of Anacanthorus and Notozothecium parasitizing M. luna.

The type species of the aporocotylid blood flukes of the genus Psettarium Goto & Ozaki, 1930, P. japonicum (Goto & Ozaki, 1929) from Takifugu pardalis (Temminck & Schlegel) and Takifugu snyderi (Abe), is redescribed. In addition, three new and two unidentified species of the genus are described: Psettarium gotoi n. sp. from T. pardalis and Takifugu flavipterus Matsuura, Psettarium ozakii n. sp. from T. flavipterus, Psettarium shimazui n. sp. from Takifugu alboplumbeus (Richardson), Psettarium sp. 1 from T. pardalis and Psettarium sp. 2 from Takifugu porphyreus (Temminck & Schlegel). Each of the three new species can be variously differentiated from the congeners by the combination of the ovary position, testis distribution, the number of tegumental spine rows, the length of oesophagus, anterior caecum, metraterm and the confluent part of the vas deferens. The definition of the genus Psettarium has been emended. Psettarium pseudupenei (Lebedev & Parukhin, 1972) and Psettarium rachycentri (Lebedev & Paruchin, 1972) are treated as incertae sedis, as a conical projection at the male terminalia is lacking and the uterus turns clockwise in the former species, and, in the latter species, the male terminalia is posterior to oötype and the posterior caeca have thornlike projections. Psettarium now comprises 19 species including two Psettarium spp. from T. pardalis and T. porphyreus, nine of which infect Takifugu spp. Phylogenetic analyses of 28S rDNA and ITS2 rDNA sequences from five Psettarium species parasitizing Takifugu spp. revealed a monophyletic group. Each species formed a distinct clade based on 28S rDNA, with P. gotoi n. sp., P. sinense, and P. shimazui n. sp. closely related; this relationship was also supported by cox1 analysis.

Aglaiogyrodactylus Kritsky, Vianna & Boeger, 2007 comprises ectoparasitic metazoans recorded mainly on the body surface of loricariid fishes. Although the genus currently includes only eight species, its actual diversity remains largely underestimated. In this study, three new species of Aglaiogyrodactylus collected from specimens of Hisonotus spp. in Brazil are described, and a new locality record of Aglaiogyrodactylus pedunculatus Kritsky, Vianna & Boeger, 2007 is reported. The descriptions are based on diagnostic morphological characteristics, with the male copulatory organ as the main distinguishing structure. Aglaiogyrodactylus matildeae n. sp. presents an accessory piece composed of two branches: a sinistral spatulated branch and a dextral tubular branch. Aglaiogyrodactylus nonnae n. sp. possesses an accessory piece divided into three branches: two sinistral spatulated branches that serve as a guide for the male copulatory organ and a bifurcated dextral branch derived from an annular structure. Aglaiogyrodactylus marinae n. sp. presents an accessory piece formed by three branches, one spatulated sinistral branch with a semicircular end, and dextral branches with distal ends having inconspicuous digitiform projections. These descriptions expand the taxonomic knowledge of Aglaiogyrodactylus and document the occurrence of new lineages associated with Hisonotus spp. in southern Brazil.

Species of Micropleura Linstow, 1906 (Dracunculoidea: Nematoda) are common parasites of crocodilians worldwide. Studies related to this genus remain scarce, particularly regarding genetic characterizations, due to the difficulty in obtaining the parasite specimens. Although the morphology of M. vazi Travassos, 1933, a parasite of Caiman spp. in the Neotropics, has been previously evaluated, some morphological inconsistencies persist, and certain features are insufficiently described. Moreover, there is no genetic data for the species. In this regard, we provide the first genetic characterization of M. vazi along with detailed morphological analysis using light and scanning electron microscopy (SEM). We report for the first time in M. vazi the presence of phasmids and of oesophageal gland nuclei, in addition to provide a clearer view of the cephalic region and the accurate position of the vulva. Genetic characterization of the species was conducted using partial sequences of the 18S and 28S rDNA, and the phylogenetic reconstructions using these first indicated the monophyly of Micropleura and of most families within Dracunculoidea. Moreover, Micropleuridae appears to be the sister group to Dracunculidae, and Philonematidae should be considered valid as it is not close to Philometridae. Furthermore, Anguillicolidae should be allocated in another superfamily, Anguillicoloidea, which is basal relative to Dracunculoidea and far from it. The absence of males in the current sampling may suggest a seasonal occurrence of these parasites, as has been reported for other dracunculoid nematodes.