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Cercariae, the free-living larval stages of trematodes, have adopted an amazing variety of transmission strategies. One of them is prey-mimetism, i.e. cercariae mimicking prey to attract motile hosts to be eaten. In a period between 2002 and 2019, we examined small planorbid snails, Bathyomphalus contortus, Gyraulus parvus and Planorbis planorbis from lakes in Finland and Iceland and from the Curonian Lagoon in Lithuania. Cercariae with conspicuously enlarged tails and unusual swimming behaviour, likely mimicking invertebrate prey, were detected and studied by the use of morphological and molecular (cox1, ITS1-5.8S-ITS2 and 28S rDNA) methods. Cercariae of two species belonging to the genus Apatemon (Strigeidae) were recognised. We consider Apatemon sp. 5 ex P. planorbis from the Curonian Lagoon identical to Cercaria globocaudata U. Szidat, 1940. Cercariae ex G. parvus from Iceland and ex B. contortus from Finland were conspecific, and we named them Apatemon sp. 6; these cercariae could not be associated with any known species. For the first time, we verified that cercariae of the Bulbocauda group belong to the genus Apatemon. We provide a mini-review on records of furcocercariae of the family Strigeidae with enlarged tails reported in freshwaters of the northern hemisphere and reveal that it is not only Apatemon but also Australapatemon and most likely Strigea which belong to the Bulbocauda group, rendering it a purely ecological assemblage. Understanding which invertebrate swimming behaviour these cercariae are mimicking will enhance our knowledge of the processes behind trematode transmission and will help to assess evolutionary pathways of host-finding strategies in trematodes., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Parasites might represent a helpful tool in understanding the historical dispersion and phylogeography of their hosts. In order to reveal whether the migration routes and diversification of hosts can be traceable in the genetic structures of their parasites, we investigated the diversity of paramphistomoid trematodes of Pelophylax frogs in 2 geographically distant European regions. Water frogs belonging to the genus Pelophylax represent a striking example of a species with a high variety of ecological adaptations and a rich evolutionary history. The parasites were collected from 2 Balkan endemic species, P. epeiroticus and P. kurtmuelleri , and 2 species in Slovakia, P. esculentus and P. ridibundus . While in Slovakia, Pelophylax frogs harboured 2 species, the diplodiscid Diplodiscus subclavatus and the cladorchiid Opisthodiscus diplodiscoides , only the former was recorded in the south-western Balkans. Remarkably high genetic diversity (16 unique mitochondrial cox 1 haplotypes, recognized among 60 novel sequences) was observed in D. subclavatus , and subsequent phylogenetic analyses revealed a strong population-genetic structure associated with geographical distribution. We also evidenced the existence of 2 divergent D. subclavatus cox 1 haplogroups in the south-western Balkans, which might be associated with the historical diversification of endemic water frogs in the regional glacial microrefugia.

DNA sequence data became an integral part of species characterization and identification. Still, specimens associated with a particular DNA sequence must be identified by the use of traditional morphology-based analysis and correct linking of sequence and identification must be ensured. Only a small part of DNA sequences of the genus Diplostomum (Diplostomidae) is based on adult isolates which are essential for accurate identification. In this study, we provide species identification with an aid of morphological and molecular (cox1, ITS-5.8S-ITS2 and 28S) characterization of adults of Diplostomum baeri Dubois, 1937 from naturally infected Larus canus Linnaeus in Karelia, Russia. Furthermore, we reveal that the DNA sequences of our isolates of D. baeri are identical with those of the lineage Diplostomum sp. clade Q , while other sequences labelled as the ‘D. baeri’ complex do not represent lineages of D. baeri. Our new material of cercariae from Radix balthica (Linnaeus) in Ireland is also linked to Diplostomum sp. clade Q. We reveal that D. baeri is widely distributed in Europe; as first intermediate hosts lymnaeid snails (Radix auricularia (Linnaeus), R. balthica) are used; metacercariae occur in eye lens of cyprinid fishes. In light of the convoluted taxonomy of D. baeri and other Diplostomum spp., we extend the recommendations of Blasco-Costa et al. (2016, Systematic Parasitology 93, 295–306) for the ‘best practice’ in molecular approaches to trematode systematics. The current study is another step in elucidating the species spectrum of Diplostomum based on integrative taxonomy with well-described morphology of adults linked to sequences.

Although some parasitological efforts have focused on the frigate tuna Auxis thazard (Lacepède) (Scombriformes, Scombridae) in Brazil, its digenean fauna remains poorly known. Combining morphological and molecular methods, we investigated the diversity of digenean trematodes of A. thazard collected from the coastal waters off the state of Rio de Janeiro, Brazil in 2021. Six species belonging to four families were recorded: the bucephalid Rhipidocotyle cf. angusticolle Chandler, 1941, the didymozoid Didymocystis sp. 6 sensu Louvard et al. (2022), the fellodistomid Tergestia sp., and three hemiurids, Dinurus euthynni Yamaguti, 1934, Lecithochirium floridense (Manter, 1934), and L. synodi Manter, 1931. The current study brings the total number of digenean trematode species parasitising A. thazard in Brazil up to eight, with hemiuroid trematodes being most diverse. Auxis thazard is a new host record for L. floridense, L. synodi and potentially for R. angusticolle. The geographic distribution of several species found in our study appeared to be wider than previously known. Our study is the first to apply a DNA-based approach to digenean diversity in marine fishes in Brazil and we believe that both morphological descriptions and molecular sequence data provided in our study will aid future research assessing the diversity of digenean trematodes of A. thazard and other marine fishes in Brazil., (© C. Pantoja et al., published by EDP Sciences, 2022.)

A new species of Crassicutis Manter, 1936 (Digenea: Megaperidae) is described from the intestine of Satanoperca jurupari (Cichlidae) in the Amazon River basin, Brazil. The genus Crassicutis currently contains eight species. Crassicutis manteri n. sp. is morphologically very similar to Crassicutis cichlasomae Manter, 1936, a parasite of cichlids reported from Mexico, the Antilles, and Central and South America. Molecular data revealed that C. cichlasomae represents a species complex in Middle American cichlids. The new species can be readily distinguished from C. cichlasomae sensu lato, and the other congeners, by a combination of morphological traits such as a narrow, elongate mouth opening (versus spherical in other species), the tandem position of testes (symmetrical or oblique in most congeners), narrow body widening towards its posterior end (versus widely oval, leaf-like in other species), and short intestinal caeca ending close to the posterior end of the posterior testis (versus reaching more posteriorly in other species). Six novel sequences of 28S rDNA, ITS1, and cox1 were generated for two isolates of the new species. Sequences of the 28S rRNA gene were used to corroborate that Crassicutis is sister taxa of Homalometron Stafford, 1904. Mitochondrial DNA corroborated the distinction of the new species with previously sequenced congeners in Middle American cichlids; the interspecific divergence between the new species and the genetic lineages of C. cichlasomae was very high, varying between 23.7 and 27.2%. Biogeographical implications of our findings are briefly discussed including questionable validity of records of C. cichlasomae from South America.

The biodiversity of freshwater ecosystems globally still leaves much to be discovered, not least in the trematode parasite fauna they support. Echinostome trematode parasites have complex, multiple-host life-cycles, often involving migratory bird definitive hosts, thus leading to widespread distributions. Here, we examined the echinostome diversity in freshwater ecosystems at high latitude locations in Iceland, Finland, Ireland and Alaska (USA). We report 14 echinostome species identified morphologically and molecularly from analyses of nad1 and 28S rDNA sequence data. We found echinostomes parasitising snails of 11 species from the families Lymnaeidae, Planorbidae, Physidae and Valvatidae. The number of echinostome species in different hosts did not vary greatly and ranged from one to three species. Of these 14 trematode species, we discovered four species (Echinoparyphium sp. 1, Echinoparyphium sp. 2, Neopetasiger sp. 5, and Echinostomatidae gen. sp.) as novel in Europe; we provide descriptions for the newly recorded species and those not previously associated with DNA sequences. Two species from Iceland (Neopetasiger islandicus and Echinoparyphium sp. 2) were recorded in both Iceland and North America. All species found in Ireland are new records for this country. Via an integrative taxonomic approach taken, both morphological and molecular data are provided for comparison with future studies to elucidate many of the unknown parasite life cycles and transmission routes. Our reports of species distributions spanning Europe and North America highlight the need for parasite biodiversity assessments across large geographical areas., (© C. Pantoja et al., published by EDP Sciences, 2021.)

The first molecular assessment of phylogenetic relationships of cladorchiid digeneans (superfamily Paramphistomoidea Fischoeder, 1901) from freshwater fishes based on 28S rDNA, ITS2 and cox1 sequences reveals the subfamilies Dadayiinae Fukui, 1929 and Kalitrematinae Travassos, 1933 as non-monophyletic, whereas Dadaytrema Travassos, 1931 represented by three species is monophyletic. Fourteen species of cladorchiids were found in characiform, perciform and siluriform fishes in the Neotropical Region (Brazil and Peru), with numerous new host and geographical records. The first scanning electron micrographs of seven species are presented. Two new species of dadayiine and one new species of kalitrematine paramphistomes are described. Microrchis macrovarium sp. n. from Pimelodella cristata (Müller et Troschel) (type host), Tetranematichthys quadrifilis (Kner) and Pterodoras granulosus (Valenciennes) in Brazil and Peru differs from all three congeners in that the testes are directly tandem, not oblique, and that the ovary, between the caecal ends, is widely separated from the testes. Pronamphistoma philippei sp. n. from Heros sp. in Brazil is distinguished from the type and only species, Pronamphistoma cichlasomae Thatcher, 1992, by the absence of the anterior collar-like expansion present in the type species, the presence of extramural rather than intramural pharyngeal sacs, and the unusual development of the dorsal and ventral exterior circular muscle fibre series in the acetabulum. Pseudocladorchis romani sp. n. from P. granulosus (type host), Brachyplatystoma vaillantii (Valenciennes), Calophysus macropterus (Lichtenstein), Megalodoras uranoscopus (Eigenmann et Eigenmann) and Oxydoras niger (Valenciennes) in Brazil and Peru, is most similar to Pseudocladorchis nephrodorchis Daday, 1907 but differs in the shape of the testes (irregular, versus reniform in the latter species) and the size of the ovary (as large as, or larger than, the testes in the new species). The generic diagnosis of Pronamphistoma Thatcher, 1992 is amended. Dadaytremoides parauchenipteri Lunaschi, 1989 is transferred to Doradamphistoma Thatcher, 1979 as D. parauchenipteri (Lunaschi, 1989) comb. n. based on morphological and molecular evidence.

The freshwater fish digenean Pseudosellacotyla lutzi ( Freitas, 1941 ) Yamaguti, 1954 has had an unsettled taxonomic history, and has at various times been classified as a member of Nanophyetidae, Heterophyidae, Microphallidae, Faustulidae, and Cryptogonimidae. Nine individual specimens of the trahira, Hoplias malabaricus (Bloch, 1794), were sampled in the Paraná River basin, Paraná State, Brazil; 22 specimens of P. lutzi were collected. One specimen of P. lutzi was used to obtain a sequence of the domains D1-D3 of the 28S rRNA gene, and to perform a phylogenetic analysis to assess their position and classification within Plagiorchiida. The resulting tree unequivocally shows that the species, along with acanthostomines, belong to the Cryptogonimidae, corroborating recent findings based on the morphology of the cercariae, and in the characteristics of the life cycle. In addition, the study of the ultrastructure of the tegumental spines through scanning electron microscopy allowed us to characterize them as pectinate spines possessing 3 to 8 digitiform projections at their distal end and extending from the anterior to the posterior extremity of the body. This study also provides the first molecular data for a cryptogonimid from South America.

Two new genera and three new species of paramphistomoid digeneans are described in the family Cladorchiidae Fischoeder, 1901 from doradid, heptapterid, pimelodid and pseudopimelodid fishes in the Amazon River in Peru. Goeldamphistomum amazonum n. g., n. sp. (type-species) from Goeldiella eques (Müller & Troschel) (type-host) and Tenellus trimaculatus (Boulenger), and Goeldamphistomum peruanum n. g., n. sp. from Duopalatinus peruanus Eigenmann & Allen (type-host), Calophysus macropterus (Lichtenstein) and Microglanis sp. are placed in the Dadayiinae Fukui, 1929. Both species have an accessory acetabular sucker, which distinguishes the genus from all taxa previously reported from South American freshwater fishes. They differ from each other primarily in that G. amazonum has a prebifurcal genital pore and oblique, separated testes, the levels of which rarely overlap longitudinally, whereas G. peruanum has a postbifurcal genital pore and testes directly to obliquely tandem. Iquitostrema papillatum n. g., n. sp. (Kalitrematinae Travassos, 1933) from the intestine of Hassar orestis (Steindachner) differs from other members of the subfamily in the combination of a massive acetabulum with a papillate luminal surface and symmetrical testes which overlie the caeca close to the caecal arch. These are the first records of paramphistomes from the five host species studied here.

Cercariae of Plagiorchis spp. are frequently reported in European freshwater snails, but their true diversity is difficult to estimate due to subtle differences in morphology. We molecularly characterized 67 isolates of Plagiorchis cercariae collected from four lymnaeid snail hosts, Ampullaceana balthica, Ampullaceana lagotis, Radix auricularia and Lymnaea stagnalis in freshwater ecosystems in the Czech Republic and Poland. Based on mitochondrial cox1 and nuclear 28S sequences, ten species or species-level lineages were identified, including the first molecular evidence of P. vespertilionis from snail hosts and two species-level lineages reported for the first time. Previously undescribed species and species-level lineages are characterized morphometrically. We confirm the overlapping spatial distribution of Plagiorchis spp. in their snail hosts from Central Europe with those from Western and sub-Arctic Europe. Our results increase the known diversity of Plagiorchis spp. in Europe to 25 species/lineages in snails, but further research is needed to establish links between life cycle stages and to assess the host specificity of these parasites. [ABSTRACT FROM AUTHOR]

Despite recent advances in the molecular knowledge of amphistome trematodes, most genera known from fish remain to be genetically characterized. This is the case for Kalitrema, a genus of the speciose family Cladorchiidae and the type of Kalitrematinae. The type and only species of this genus, Kalitrema kalitrema Travassos, 1933, was originally proposed based on two specimens found in an armored suckermouth catfish from Brazil, and its phylogenetic position has not been evaluated. In this study, paramphistomes found in Hypostomus alatus (2/9; 22.2%) and Hypostomus francisci (4/143; 2.8%) from the Paraobepa River (São Francisco River basin), Minas Gerais, Brazil, between December 2019 and November 2021, were subjected to morphological study. The parasites were identified in low intensity of infection [1.2 (1–2)] and redescribed as K. kalitrema. This species exhibits unique features such as a linguiform body with a circular ridge near the anterior end and a deep, median notch present at the posterior extremity of the body, apparently dividing the body into two lobes. A subset of specimens was further subject to phylogenetic analyses based on the most densely sampled markers, the nuclear ribosomal RNA (28S and ITS2) and mitochondrial cox1, which revealed the inclusion of K. kalitrema in a Neotropical clade of fish paramphistomes. The most comprehensive phylogenetic tree, based on the 28S dataset, confirmed K. kalitrema as an independent, early diverging lineage among Neotropical fish cladorchiids. However, the monophyly of Kalitrematinae was not sustained, given that species of the other kalitrematine genera Pseudocladorchis and Iquitostrema included in the phylogenetic analysis fell in a distinct clade with other fish cladorchiids. As a result, we propose here a narrower concept for Kalitrematinae sensu stricto, accommodating only Kalitrema (type genus) until a more natural subfamilial or familial classification is provided. [ABSTRACT FROM AUTHOR]

Simple Summary: Brazil, with its remarkably diverse marine habitats, harbour one of the world's richest fish diversities. Consequently, the diversity of their trematode parasites is also expected to be extremely rich. However, our current knowledge on this group of animals is incomplete and there are many unknown trematode species that await discovery and genetic characterisation. The Hemiuridae (Digenea) is the second most speciose trematode family in marine fishes from Brazil; however, to date, it remains understudied. We examined forty-three specimens of nine fish species belonging to eight families (Carangidae, Clupeidae, Haemulidae, Muraenidae, Percophidae, Pinguipedidae, Trichiuridae, and Triglidae) collected from the coastal zone off Rio de Janeiro, Brazil and found hemiurid trematodes in the stomach of 14 fishes. Using morphological and molecular analyses, we identified eight species from four genera of the family Hemiuridae. One of these species is reported in Brazil for the first time, four are reported from new fish hosts, and four were genetically characterised for the first time. Our novel data contributes to the knowledge on marine biodiversity in Brazil and will further contribute to the classification of the family Hemiuridae. Brazil is a tropical country with remarkably diverse marine habitats that harbour a rich diversity of fish. Only a small portion of this fish diversity has been investigated for parasites, and thus the diversity of their trematode parasites remains unexplored. Moreover, only 5 out of 184 known digenean trematode species of marine fish in Brazil have been genetically characterised. The Hemiuridae Looss, 1899 is the second most speciose trematode family in marine fishes from Brazil but, in many ways, it remains a neglected group. Forty-three trematode specimens from nine fish species were collected from the coastal zone off Rio de Janeiro, Brazil. Trematodes were found in the stomach of 14 specimens of 9 fish species belonging to 8 families (Carangidae, Clupeidae, Haemulidae, Muraenidae, Percophidae, Pinguipedidae, Trichiuridae, and Triglidae). Trematode specimens were studied using morphological and molecular genetic analyses. A total of eight hemiurid species from four genera, Ectenurus, Lecithochirium, Myosaccium, and Parahemiurus were identified. This paper reports on new host records for four species of hemiurids, adds a new record on the geographical distribution for one species, and provides the first DNA sequence data supplemented with the detailed description of morphology for five species. Phylogenetic analyses supported that the subfamily classifications of the Hemiuridae—based entirely on morphological characters—needs to be reconsidered, taking into account a wider range of information sources. [ABSTRACT FROM AUTHOR]

Parasites are important components of ecosystems, thus identifying patterns of their diversity is crucial for studies investigating ecosystem functioning, food web structure, trophic interactions and epidemiological parameters. Via an integrative taxonomic approach, we studied the diversity and distribution of trematodes of the genus Plagiorchis Lühe, 1899 in snails in the sub‐Arctic freshwater ecosystems of Iceland and for comparative purposes in West and North Europe (Ireland and Finland, respectively), supplemented by data from Alaska, USA, all regions with remarkably few data on trematode diversity. Molecular genetic and morphological analyses of 68 samples of cercarial isolates of Plagiorchis collected from 3,069 snails belonging to six species revealed a total of 16 species/species‐level lineages of Plagiorchis with five found in Iceland, six in Finland, nine in Ireland and five in Alaska. Two species‐level lineages found in Iceland, Finland and Ireland were reported for the first time; for all species/species‐level lineages identified by DNA sequences we provide the first morphological characterization. Our analysis of the diversity of Plagiorchis and their snail host spectrum in Europe provides evidence for Radix balthica and Lymnaea stagnalis as primary first intermediate hosts. By comparing DNA sequence data for Plagiorchis spp. from this and previous studies, we demonstrate no overlapping spatial distribution for members of the genus in our material between the continents of Europe and North America. We infer that the spatial distribution of Plagiorchis spp. is constrained mainly by migratory birds' ranges. Finally, in Europe, further research is needed to evaluate whether the diversity of Plagiorchis spp. in R. balthica is unique to high latitude ecosystems. [ABSTRACT FROM AUTHOR]