Unexpected diversity in northern Europe: trematodes from salmonid fishes in Iceland with two new species of Crepidostomum Braun, 1900

Freshwater trematodes in Iceland and the surrounding north - current advances and questions

Iceland is an isolated, sub-Arctic, oceanic island of volcanic origin in the northern North Atlantic. With a limited faunal diversity and being the most northern point in the distributional range for some species, it is an intriguing model region to study parasite biodiversity and biogeography. Since 2006, there has been a history of intense biodiversity discoveries of freshwater trematodes (Trematoda, Digenea), thanks to the use of integrative taxonomic methods. The majority of digeneans (28 out of 41 known) were characterised with molecular genetic methods and morphological analyses, with some of their life-cycle stages and geographical distribution assessed. A surprising diversity has been discovered, comprising species of the families Allocreadiidae, Cyclocoeliidae, Diplostomidae, Echinostomatidae, Gorgoderidae, Plagiorchiidae, Notocotylidae, Schistosomatidae, and Strigeidae. Many of the recorded species complete their life cycles within Iceland, with three snail species (Ampullaceana balthica, Gyraulus parvus, Physa acuta) known as intermediate hosts. No trematodes endemic for Iceland were found; they appear to be generalists with wide geographical ranges dispersed mainly by migratory birds. Interestingly, fish trematodes recorded in Iceland were found in mainland Europe, indicating that they might be dispersed by anadromous fishes, by human activity, or by migratory birds carrying intermediate hosts. The trematode fauna is mainly Palaearctic, with few species recorded in North America. We highlight the ongoing need for precise species identification via integrative taxonomic methods, which is a baseline for any further ecological studies and adequate epidemiological and conservation measures. Also, there is still a need of obtaining well-preserved vouchers of adults for definite species delimitation.

Description and phylogenetic position of an unusual genus and species of allocreadiid trematode from a characid freshwater fish in South America

The worldwide distributed family Allocreadiidae Looss, 1902 is predominantly parasitic in freshwater fish and occasionally in amphibians and reptiles. It contains 15 genera, of which only Allocreadium, Creptotrema, Wallinia and Creptotrematina have been reported in Argentina. The records of Allocreadium species still require further verification. The discovery of an allocreadiid digenean parasitizing Pseudocorynopoma doriae led us to erect a new genus for which the name Martorellia n. gen is proposed. It is characterized by poorly developed ventrolateral muscular lobes and a transversely elongated ovary, with the most characteristic feature being the presence of vitelline follicles in two fields (one anterior at level of ventral sucker and one posterior to the testis), which are not confluent in the middle. The phylogenetic tree reconstructed with 28S rDNA sequences placed the new genus as the sister taxa of Creptotrematina. Although both genera are separated by a small genetic distance, it is comparable with those separating other allocreadiid genera. The discovery of Martorellia liciae gen et sp. nov. provides further evidence of the great diversity of Allocreadiidae in South America. The analysis of new samples from different South American hosts with an integrative taxonomic approach as the one used here allows a better understanding of the evolutionary history and biogeography of allocreadiids in the Americas.

A New Species of Creptotrematina (Trematoda: Allocreadiidae) from the Red Minor Tetra, Hyphessobrycon eques (Steindachner, 1882) (Characidae) from Brazil, with Comments on the Genetic Divergence of C. Aguirrepequenoi Jiménez-Guzmán, 1973 across a Wide Geographical Range in Middle America

BACKGROUND

Allocreadiids are relatively small digeneans that appear to be restricted to freshwater systems distributed across the world. Allocreadiids are highly diverse in the Americas, particularly in the Neotropical biogeographical region. Their taxonomic history has been rather controversial, with several taxonomic reassessments and the description of new genera and species.

METHODS

We sampled Creptotrematina specimens from a characid collected in the Pardo River, Paranapanema River basin in Brazil, and specimens of C. aguirrepequenoi, from Astyanax spp. in several localities between northern Mexico and Costa Rica. The specimens were studied through integrative approaches using morphological and molecular analyses of the 28S rDNA gene and two different regions of the COI mtDNA gene.

RESULTS

We describe a new species of Creptotrematina which is differentiated from other congeners by the overall body size, but in particular by the size and position of the cirrus-sac, distribution of the vitelline follicles, and extension of uterine loops in the posterior end of body. Phylogenetic analyses of the 28S rDNA and COI mtDNA genes placed the new species in a monophyletic clade together with all other sequenced species of Creptotrematina, and as a sister species of C. batalhensis. Genetic divergences between the new species and other Creptotrematina spp. varied from 1.1 to 1.2% for the 28S rDNA and 12.4-14.3% for the COI mtDNA. Phylogenetic analysis based on COI mtDNA showed the isolates of C. aguirrepequenoi grouped in four monophyletic clades representing populations geographically separated along a wide geographical range spanning between northern Mexico and Costa Rica, with an estimated genetic divergence between 3.9% and 8.9%.

CONCLUSIONS

Our findings based on integrative analyses recognize Creptotrematina saltograndensis n. sp. from a characid collected in the Pardo River, Paranapanema River basin in Brazil and provide validation of the wide geographical distribution of C. aguirrepequenoi across Middle-America parasitizing Astyanax spp.; the genetic divergence of the species through the analysis of two regions of COI mtDNA points towards considering it represent a species complex, although we refrain at the moment on describing a new species, awaiting for further verification using other molecular markers, and obtaining fresh material for a more detailed taxonomic analyses. This study increases the known diversity of allocreadiids and contributes to the understanding of evolutionary relationships, host-parasite relationships, and biogeographic history of the group.

The complete mitochondrial genome of 3 species of allocreadiids (Digenea, Allocreadiidae): characterization and phylogenetic position within the order Plagiorchiida

Trematodes of the family Allocreadiidae are primarily found in the intestines of freshwater fishes around the world. The family includes 15 genera and c. 130 species. The last 2 decades have witnessed an increase in the genetic library of its species. Molecular data have been crucial for species delimitation and species description within Allocreadiidae and for understanding their evolutionary and biogeographical history and classification. Here, the mitogenomes of 3 species of allocreadiids were obtained using high throughput sequencing methods. Mitogenomes were compared with other members of the order Plagiorchiida to determine their molecular composition, gene rearrangement and phylogenetic interrelationships. The complete circular mitogenomes of Allocreadium lobatum, Creptotrematina aguirrepequenoi and Wallinia mexicana were 14 424, 13 769 and 13 924 bp long respectively, comprising 12 protein-coding genes, 22 transfer RNA genes, 2 ribosomal RNA genes and 2 non-coding regions. Gene arrangements were identical to other Xiphidiatan trematodes. Phylogenetic analyses using the mitogenomes revealed Allocreadiidae as a monophyletic group closely related to other members of the suborder Xiphidiata; A. lobatum was yielded as the sister taxon of C. aguirrepequenoi + W. mexicana. Our study increases the complete mitochondrial genome library of trematodes and strengthens our understanding of the phylogenetic relationships and classification of this parasite group.

Rich but morphologically problematic: an integrative approach to taxonomic resolution of the genus Neospirorchis (Trematoda: Schistosomatoidea)

Neospirorchis Price, 1934 is a genus of blood flukes that infect the cardiovascular system, including vessels surrounding the nervous systems of marine turtles. Although the genus comprises just two named species, the available molecular data suggest substantial richness which has not yet been formally described. The lack of description of species of Neospirorchis is probably explained by their small, slender, elongate bodies, which allow them to infect numerous organs and vessels in their hosts, such as the heart and peripheral vessels of nervous system, endocrine organs, thymus, mesenteric vessels, and gastrointestinal submucosa. This morphology and site of infection means that collecting good quality, intact specimens is generally difficult, ultimately hampering the formal description of species. Here we supplement limited morphological samples with multi-locus genetic data to formally describe four new species of Neospirorchis infecting marine turtles from Queensland, Australia and Florida, USA; Neospirorchis goodmanorum n. sp. and Neospirorchis deburonae n. sp. are described from Chelonia mydas, Neospirorchis stacyi n. sp. is described from Caretta caretta, and Neospirorchis chapmanae n. sp. from Ch. mydas and Ca. caretta. The four new species are delineated from each other and the two known species based on the arrangement of the male and female reproductive organs, on the basis of cytochrome c oxidase subunit 1 (cox1), internal transcribed spacer 2 (ITS2), and 28S ribosomal DNA (rDNA) molecular data, site of infection, and host species. Molecular evidence for three further putative, presently undescribable, species is also reported. We propose that this integrated characterisation of species of Neospirorchis, based on careful consideration of host, molecular and key morphological data, offers a valuable solution to the slow rate of descriptions for this important genus. We provide the first known life cycle data for Neospirorchis in Australian waters, from Moreton Bay, Queensland; consistent with reports from the Atlantic, sporocysts were collected from a terebellid polychaete and genetically matched to an unnamed species of Neospirorchis infecting Ch. mydas from Queensland and Florida.

Integrative approach for discovering of the new species within the genus Allocreadium Looss, 1900 (Trematoda: Allocreadiidae) and framing of biogeographical hypotheses for the genus

In July 2012 new Allocreadium species was isolated from Carassius gibelio caught in the Arsenyevka River, Primorsky region, Russia. Analyses on the morphometrics and internal organs' topology revealed that these worms are morphologically closest with A. isoporum but both species are independent according to high genetic distances based on the 28S gene fragment (5.434±0.0073%). Unlike A. isoporum found earlier in Europe, the new species named A. pseudoisoporum sp. nov. has a shorter body length and the vitellarium not reaching the posterior end of the body at some distance and its anterior border is on the level of the ventral sucker. Allocreadium pseudoisoporum sp. nov. differs from seven species previously found in the Russian Far East with the following features: smaller size of the body, suckers' ratio, range values, and topology of internal structures. Newly localities in the Pavlovka River and the Artyomovka River were discovered for A. khankaiensis. Morphological variability of the worms from the Pavlovka River was observed in comparison with A. khankaiensis from the Komissarovka River. Using scanning electron microscope, we examined external surfaces of three species (A. pseudoisoporum sp. nov., A. khankaiensis, A. hemibarbi) and observed structures reminiscent sensory receptors. This study was aimed to describe species diversity of allocreadiids inhabiting the south of Primorsky region, and to clarify phylogenetic relationships between the species from the genus Allocreadium Looss, 1900 using molecular genetic methods. The phylogenetic Bayesian tree based on the 28S gene showed a clear separation of ten Allocreadium species and confirmed the validity of A. pseudoisoporum sp. nov. Allocreadium pseudoisoporum sp. nov. is most similar to A. gotoi (genetic distances - 3.578±0.0051% in 28S, and 18.777±0.0149% in cox1), and represented the earliest divergent lineage in Allocreadium clade on the phylogenetic tree based on the 28S gene, thereby indicating its proximity to the ancestral node. Also, dichotomous keys for 25 Palearctic species of Allocreadium were prepared based on the morphology of the adult worms.

Hemiurid Trematodes (Digenea: Hemiuridae) from Marine Fishes off the Coast of Rio de Janeiro, Brazil, with Novel Molecular Data

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.

Digenean trematodes infecting the frigate tuna Auxis thazard (Scombriformes, Scombridae) off the Rio de Janeiro coast, Brazil, including molecular data

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.

Chelatrematidae n. fam., a new family of digenetic trematodes from the South Western Ghats, India, erected on the basis of morphological and molecular studies

On the basis of the morphological characterization of Chelatrema neilgherriensis Manjula & Janardanan, 2006 recovered from the freshwater fish Barilius gatensis (Valenciennes, 1844) in the Wayanad region of the Western Ghats, the diagnostic features of the genus Chelatrema Gupta & Kumari, 1973 have been modified. Based on the phylogenetic analysis of C. neilgherriensis and comparative morphology studies relative to members of other families of Gorgoderoidea Looss, 1901, this genus is placed in a new family Chelatrematidae n. fam. The studies revealed the molecular and morphological closeness of Chelatrema with Paracreptatrematina limi Amin & Myer, 1982, and the latter is transferred to this new family. Hence the new family Chelatrematidae n. fam. comprises the genera Chelatrema and Paracreptatrematina.

Trematodes from Antarctic teleost fishes off Argentine Islands, West Antarctica: molecular and morphological data

In 2014-2015 and 2019-2021, teleost fishes off Galindez Island (Antarctic Peninsula) were examined for trematodes. Combined morphological and molecular analyses revealed the presence of eight trematode species of four families (Hemiuridae, Lecithasteridae, Opecoelidae, Lepidapedidae) from five fish species. Only adult trematodes were found and all of them are Antarctic endemics with their congeners occurring on other continents. The hemiuroids, Elytrophalloides oatesi (Leiper & Atkinson, 1914), Genolinea bowersi (Leiper & Atkinson, 1914), and Lecithaster macrocotyle Szidat & Graefe, 1967 belong to the most common Antarctic species and together with Lepidapedon garrardi (Leiper & Atkinson, 1914) and Neolebouria georgiensis Gibson, 1976 they were recorded as the least host-specific parasites. The originally sub-Antarctic Neolepidapedon macquariensis Zdzitowiecki, 1993 is a new record for the Antarctic Peninsula and Parachaenichthys charcoti (Vaillant), is a new host record. Neolebouria terranovaensis Zdzitowiecki, Pisano & Vacchi, 1993 is considered a synonym of N. georgiensis because of identical morphology and dimensions. The currently known phylogenetic relationships within the studied families are supported, including the polyphyly of Macvicaria Gibson & Bray, 1982 with the future need to accommodate its Antarctic species in a new genus. The validity of M. georgiana (Kovaleva & Gaevskaja, 1974) and M. magellanica Laskowski, Jezewski & Zdzitowiecki, 2013 needs to be confirmed by further analyses. Genetic sequence data are still scarce from Antarctica, and more studies applying integrative taxonomic approaches and large-scale parasitological examinations of benthic invertebrates are needed to match sequences of larval stages to those of well-characterised adults and to elucidate trematode life-cycles.

Low-level pathogen transmission from wild to farmed salmonids in a flow-through fish farm

While the potential effects of pathogens spread from farmed fish to wild populations have frequently been studied, evidence for the transmission of parasites from wild to farmed fish is scarce. In the present study, we evaluated natural bacterial and parasitic infections in brown trout (Salmo trutta m. fario) collected from the Černá Opava river (Czech Republic) as a potential source of infections for rainbow trout (Oncorhynchus mykiss) reared in a flow-through farm system fed by the same river. The prevalence of bacterial and protozoan infections in farmed fish was comparable, or higher, than for riverine fish. Despite this, none of the infected farmed fish showed any signs of severe diseases. Substantial differences in metazoan parasite infections were observed between wild and farmed fish regarding monogeneans, adult trematodes, nematodes, the myxozoan Tetracapsuloides bryosalmonae found in riverine fish only, and larval eye-fluke trematodes sporadically found in farmed fish. The different distribution of metazoan parasites between brown and rainbow trout most probably reflects the availability of infected intermediate hosts in the two habitats. Despite the river being the main water source for the farm, there was no significant threat of parasite infection to the farmed fish from naturally infected riverine fish.

Three new species of Creptotrema (Trematoda, Allocreadiidae) with an amended diagnosis of the genus and reassignment of Auriculostoma (Allocreadiidae), based on morphological and molecular evidence

Diversity of Creptotrema was investigated using morphological and molecular tools, including data for Creptotrema creptotrema (type-species). Three new species, parasites of Brazilian fishes, are described: Creptotrema conconae n. sp. (type-host, Imparfinis mirini Haseman), Creptotrema schubarti n. sp. (type-host, Characidium schubarti Travassos) and Creptotrema megacetabularis n. sp. (type-host, Auchenipterus osteomystax (Miranda Ribeiro)). The diagnosis of the genus was amended to include new features. The new species differ from each other mainly in terms of body shape, relative sucker size, and testes position. DNA sequences were obtained from Creptotrema spp. from Brazil, including 28S, ITS and COI. Genetic divergences among the new species and C. creptotrema varied from 2.1 to 5.2% (21–49 bp) for 28S, and 6.6 to 16.4% (21–45 bp) for COI. Phylogenetic analysis (28S) placed the newly generated DNA sequences of Creptotrema in a clade (Creptotrema clade sensu stricto) composed of C. creptotrema, the new species described herein, and all species previously described as Auriculostoma, revealing that Auriculostoma is best interpreted as a synonym of Creptotrema based on the principle of priority of zoological nomenclature. Creptotrema funduli, the single sequence of the genus previously available, was not grouped within the Creptotrema clade sensu stricto, suggesting the need for reevaluation of the taxonomic status of this species. Our results showed that Creptotrema represents a monophyletic genus of trematodes widely distributed across the Americas, which currently comprises 19 valid species of parasites of teleosts and anurans.

Diversity of echinostomes (Digenea: Echinostomatidae) in their snail hosts at high latitudes

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.

Systematics of Crepidostomum species from the Russian Far East and northern Japan, with description of a new species and validation of the genus Stephanophiala

Current article touched upon the issue of the complicated taxonomic status of some species from the genus Crepidostomum collected from the freshwater fish in the rivers of Primorsky region, Sakhalin, and Hokkaido Islands. Primary morphological analyses showed affiliation of the worms to the species C. farionis (Müller, 1784) Lühe, 1909; C. metoecus Braun, 1900b; C. chaenogobii Yamaguti and Matsumura, 1942; C. nemachilus Krotov, 1959. We described the new species Crepidostomum achmerovi sp. nov. that is a sibling species of C. nemachilus. Molecular-genetic investigation have shown that C. nemachilus and C. achmerovi sp. nov. are closely related to C. metoecus in both 28S rDNA and cox1 mtDNA markers. Crepidostomum nemachilus forms a separate branch within the C. metoecus clade on the 28S BI tree with strong statistical support and separate clade in relation to C. metoecus clade on the cox1 BI tree. Values of p-distances between Crepidostomum species were at intergeneric level. Crepidostomum metoecus species complex including five species (C. metoecus, C. nemachilus, C. oschmarini, C. brinkmanni, and C. achmerovi sp. nov.) was reconsidered as independent genus Crepidostomum sensu stricto. Minimum Spanning Network showed that C. nemachilus, C. metoecus and C. achmerovi sp. nov. were separated by large number of mutational events and represent independent phyletic lines. An amended diagnosis is provided for the subfamily Crepidostomatinae, the genera Crepidostomum s. str. and Stephanophiala Nicoll, 1909, along with keys to species of both genera.

Conspecific migration and environmental setting determine parasite infracommunities of non-migratory individual fish

Parasite infracommunities tend to be stochastic in nature, although environmental characteristics such as the type of water source in streams and host traits can have an effect on the biotic assemblages and by extension the parasite fauna. We examined the effect of water source and the rate of adult fish migration on the metazoan parasite infracommunities of conspecific juvenile brown trout, Salmo trutta L. among streams flowing into Lake Lucerne (Switzerland). Juvenile (1 to 2-year old) fish harboured higher parasite species richness in groundwater-fed than in surface water-fed streams, whereas the rate of fish migration did not affect infracommunity richness. Heteroxenous species were more common in groundwater-fed streams with high and medium rates of trout migration, whereas infracommunities in surface water-fed streams and streams with low rates of fish migration were dominated by one monoxenous parasite or lacked infections. Similarity in the parasite infracommunity composition of juvenile trout across streams was explained by the interaction between type of water source and adult migration rates. Our conclusions support that similarity in the parasite composition of resident freshwater conspecifics can be predicted by the local environmental settings and host migratory behaviour, whereas parasite richness is mainly influenced by the environmental characteristics.