Diversity and specificity in Diplostomum spp. metacercariae in freshwater fishes revealed by cytochrome c oxidase I and internal transcribed spacer sequences

Helminths of the black-headed gull (Chroicocephalus ridibundus) from breeding colonies in north-central Poland

Among parasites of the digestive tract of the black-headed gull (Chroicocephalus ridibundus) in Poland, the best known are species of digenetic trematodes and cestodes. Nematodes of this bird species are not well known. Black-headed gulls, due to their varied diet, migration, life in a flock, and changes of habitat, can become infected with various species of helminths, and like synanthropic birds, they can spread the dispersal stages of parasites across urban and recreational areas. In the present study, an attempt was made to identify the helminth fauna of C. ridibundus from breeding colonies in north-central Poland. The aim of the study was to describe the taxonomic structure of parasites of the digestive tract of the black-headed gull and determine the quantitative parameters of their occurrence. A total of 43 black-headed gulls were examined post-mortem for gastrointestinal helminths, resulting in the identification of four cestodes (Paricterotaenia porosa, Lateriporus clerci, Anomotaenia micracantha, and Wardium fusum), three trematodes (Diplostomum pseudospathaceum, Plagiorchis laricola, and Apophallus muehlingi), and three nematodes (Eucoleus contortus, Cosmocephalus obvelatus, and Porrocaecum ensicaudatum). Lateriporus clerci (in adult form), C. obvelatus and P. ensicaudatum (in larval form) were recorded for the first time in the black-headed gull in Poland.

Other Schistosomatoidea and Diplostomoidea

Trematodes of the order Diplostomida are well known as serious pathogens of man, and both farm and wild animals; members of the genus Schistosoma (Schistosomatidae) are responsible for human schistosomosis (schistosomiasis) affecting more than 200 million people in tropical and subtropical countries, and infections of mammals and birds by animal schistosomes are of great veterinary importance. The order Diplostomida is also rich in species parasitizing other major taxa of vertebrates. The "Aporocotylidae" sensu lato are pathogenic in fish, "Spirorchiidae" sensu lato in reptiles. All these flukes have two-host life cycles, with asexually reproducing larvae usually in mollusks and occasionally in annelids, and adults usually live in the blood vessels of their vertebrate hosts. Pathology is frequently associated with inflammatory reactions to eggs trapped in various tissues/organs. On the other hand, the representatives of Diplostomidae and Strigeidae have three- or four-host life cycles in which vertebrates often serve not only as definitive but also as intermediate or paratenic hosts. Pathology is usually associated with migration of metacercariae and mesocercariae within the host tissues. The impact of these trematode infections on both farm and wild animals may be significant.

Hidden in the fog: morphological and molecular characterisation of Derogenes varicus sensu stricto (Trematoda, Derogenidae) from Sweden and Norway, and redescription of two poorly known Derogenes species

Derogenes varicus (Müller, 1784) is widely reported as a trematode with exceptionally low host specificity and a wide, bipolar distribution. However, several recent studies have suggested that D. varicus represents a species complex and based on molecular evidence, four genetic lineages (labeled as "DV1-4") have been designated within the D. varicus species complex. This possibility requires improved (ideally molecular) characterisation of specimens from the type-host (Salmo salar) and type-locality (off Denmark). During examination of trematode parasites of fish from Scandinavian and Arctic waters (Sweden and Norway), we found specimens of D. varicus in the stomach of Merlangius merlangus off the coast of Sweden, and in Gadus morhua off the coast of Sweden and Norway; we compared them to D. varicus from the type-host, the Atlantic salmon Salmo salar from Norway, to verify their conspecificity. Newly generated sequences (28S rDNA, ITS2 and cox1) of Scandinavian and Arctic specimens consistent with D. varicus all formed a single clade, DV1. 28S sequences of D. varicus from S. salar from Norway, i.e., close to the Danish type locality, clustered within the DV1 clade along with sequences of D. varicus from various hosts including Limanda limanda, G. morhua and Myoxocephalus scorpius from the White Sea and the Barents Sea (Russia), without any host-related structuring. We thus consider that the lineage DV1 represents D. varicus sensu stricto. Additionally, specimens from M. merlangus had a similar morphology and anatomy to those of D. varicus from L. limanda, G. morhua and M. scorpius from T. Odhner's collection, supporting the presence of a single species in the DV1 lineage designated herein as D. varicus sensu stricto. We redescribe D. varicus sensu stricto, add new morphological characters and provide morphometric data. We infer that D. varicus types DV2-4 all relate to separate species. We also revise type-specimens of Derogenes minor Looss, 1901 from the A. Looss collection in the Swedish Museum of Natural History and provide redescriptions of it and of the type-species of the genus, Derogenes ruber Lühe, 1900. In light of their morphological distinctiveness relative to D. varicus sensu stricto, we reinstate D. parvus Szidat, 1950 and D. fuhrmanni Mola, 1912.

Review of the metazoan parasites of the economically and ecologically important African sharptooth catfish Clarias gariepinus in Africa: Current status and novel records

One of the most widely distributed African freshwater fish is the African sharptooth catfish Clarias gariepinus (Burchell) that is naturally distributed in 8 of the 10 ichthyofaunal regions of this continent. Clarias gariepinus is a highly valued and cheap staple to local communities and an ideal aquaculture species. Consequently, interest in the parasitic communities of C. gariepinus has increased as parasites may accidentally be ingested by humans when eating uncooked fish or can be introduced into culture systems through fish stocks supplied from local rivers which affect yield, growth, and marketability. This review provides an overview of the ∼107 metazoan parasite species known to parasitise C. gariepinus in Africa and their general life cycles, morphology, paratenic and post-cyclic infections, and the biogeography and validity of records are discussed. A brief overview is included on the application of some of these parasites in environmental studies and their link to human health.

No strict host specificity: Brain metacercariae Diplostomum petromyzifluviatilis Müller (Diesing, 1850) are conspecific with Diplostomum sp. Lineage 4 of Blasco-Costa et al. (2014)

Metacercariae of Diplostomum petromyzifluviatilis (Digenea, Diplostomidae) from the brain of European river lamprey Lampetra fluviatilis from the Baltic Sea basin and Arctic lamprey Lethenteron camtschaticum from the White Sea basin were studied with the use of genetic and morphological methods. Phylogenetic analysis based on cox1 marker showed that the parasites of both lamprey species were conspecific with Diplostomum sp. Lineage 4 of Blasco-Costa et al. (2014). The name Diplostomum petromyzifluviatilis Müller (Diesing, 1850) has historical precedence as a species described from the brain of lampreys and should be used in genus nomenclature. There were no morphological qualitative differences between the metacercariae from the two lamprey species but those from L. fluviatilis were larger than those from L. camtschaticum. We expanded the data on the second intermediate hosts and the localization of D. petromyzifluviatilis, showing that its metacercariae occur not only in the brain of lampreys but also in the brain and the retina of three-spined stickleback Gasterosteus aculeatus and the vitreous humour of the perch Perca fluviatilis across the European part of the Palearctic.

Metacercariae in the brain of Erythrinus cf. erythrinus (Characiformes: Erythrinidae) from Iguazú National Park (Argentina): do they belong to Dolichorchis lacombeensis (Digenea, Diplostomidae)?

In Argentina, the family Diplostomidae is composed of eight genera: Austrodiplostomum Szidat & Nani; Diplostomum von Nordmann; Dolichorchis Dubois; Hysteromorpha Lutz; Neodiplostomum Railliet; Posthodiplostomum Dubois; Sphincterodiplostomum Dubois; and Tylodelphys Diesing. During a parasitological survey of fishes from the Iguazú National Park we detected diplostomid metacercariae in the brain of Erythrinus cf. erythrinus. Fish were caught using crab traps, transported alive to the field laboratory, cold-anaesthetized and euthanized by cervical dissection. Some metacercariae were heat-killed in water and fixed in 10% formalin and others were preserved in alcohol 96% for DNA extraction. They were sequenced for the partial segment of the 28S rDNA, internal transcribed spacer (ITS) rDNA and cytochrome c oxidase subunit I (COI) mtDNA genes. Phylogenetic reconstruction was carried out using Bayesian inference and the proportion (p) of absolute nucleotide sites (p-distance) was obtained. In the 28S rDNA tree, the metacercaria sequenced grouped as Dolichorchis sp. The COI mtDNA p-distance between the metacercariae with Dolichorchis lacombeensis was 0.01. There is a small number of ITS sequences for the Diplostomidae family deposited in the GenBank. The oral sucker, ventral sucker, holdfast organ and the distance between oral and ventral suckers are larger in the adult compared with the metacercariae. Additionally, hind-body length and width are larger in the adult due to the development of the genital complex. Further studies using an integrative approach will help confirm the affiliation of other species to the genus Dolichorchis.

Another plea for 'best practice' in molecular approaches to trematode systematics: Diplostomum sp. clade Q identified as Diplostomum baeri Dubois, 1937 in Europe

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.

Assessing the genetic diversity of the metacercariae of Posthodiplostomum minimum (Trematoda: Diplostomidae) in Middle American freshwater fishes: one species or more?

Trematode taxonomy is mainly based on the morphological traits of adults. The identification of metacercariae is challenging because such traits are not developed in larval forms, and they even may show some level of morphological variability. Studies testing the potential correspondence between morphological differences and genetic variation of parasites are still lacking. The metacercariae of Posthodiplostomum minimum are probably the diplostomids more widely distributed in North and Middle American freshwater fish, and their intraspecific morphological variability has been attributed to the effect exerted by the host. Here, we tested the hypothesis whether they represent a single species, or a species complex by assessing the genetic divergence and phylogenetic relationships of metacercariae sampled from several host species in a wide geographical range across Middle America. The internal transcribed spacers (ITS1-5.8S-ITS2), and the mitochondrial COI gene were sequenced for 124 and 55 metacercariae, respectively. Phylogenetic analysis inferred from ITS sequences uncovered six well-supported monophyletic lineages. The six lineages show no correspondence to any Posthodiplostomum species for which sequences are available thus far in GenBank. Lineages exhibit some degree of host specificity; Lineages I, II, IV and V are primarily parasites of cyprinodontiforms of the families Poeciliidae, Goodeidae, Profundulidae and Fundulidae. In poeciliids there are at least four candidate species of Posthodiplostomum, some of them occurring in sympatry; instead, Lineages II and VI are exclusively parasites of cichlids. This study contributes to our understanding of the diversity of larval forms of diplostomids and provides an opportunity to further study their life cycles.

Molecular phylogeny of Diplostomum, Tylodelphys, Austrodiplostomum and Paralaria (Digenea: Diplostomidae) necessitates systematic changes and reveals a history of evolutionary host switching events

The Diplostomidae Poirier, 1886 is a large, globally distributed family of digeneans parasitic in intestines of their definitive hosts. Diplostomum and Tylodelphys spp. are broadly distributed, commonly reported, and the most often sequenced diplostomid genera. The majority of published DNA sequences from these genera originated from larval stages only, which typically cannot be identified to the species level based on morphology alone. We generated partial large ribosomal subunit (28S) rRNA and cytochrome c oxidase subunit 1 (cox1) mtDNA gene sequences from 14 species/species-level lineages of Diplostomum, six species/species-level lineages of Tylodelphys, two species/species-level lineages of Austrodiplostomum, one species previously assigned to Paralaria, two species/species-level lineages of Dolichorchis and one unknown diplostomid. Our DNA sequences of 11 species/species-level lineages of Diplostomum (all identified to species), four species/species-level lineages of Tylodelphys (all identified to species), Austrodiplostomum compactum, Paralaria alarioides and Dolichorchis lacombeensis originated from adult specimens. 28S sequences were used for phylogenetic inference to demonstrate the position of Paralaria alarioides and Dolichorchis spp. within the Diplostomoidea and study the interrelationships of Diplostomum, Tylodelphys and Austrodiplostomum. Our results demonstrate that two diplostomids from the North American river otter (P. alarioides and a likely undescribed taxon) belong within Diplostomum. Further, our results demonstrate the non-monophyly of Tylodelphys due to the position of Austrodiplostomum spp., based on our phylogenetic analyses and morphology. Furthermore, the results of phylogenetic analysis of 28S confirmed the status of Dolichorchis as a separate genus. The phylogenies suggest multiple definitive host-switching events (birds to otters and among major avian groups) and a New World origin of Diplostomum and Tylodelphys spp. Our DNA sequences from adult digeneans revealed identities of 10 previously published lineages of Diplostomum and Tylodelphys, which were previously identified to genus only. The novel DNA data from this work provide opportunities for future comparisons of larval diplostomines collected in ecological studies.

Infection of Diplostomum spp. in invasive round gobies in the St Lawrence River, Canada

The round goby (Neogobius melanostomus) is a successful invader of the Great Lakes-St Lawrence River basin that harbours a number of local parasites. The most common are metacercariae of the genus Diplostomum. Species of Diplostomum are morphologically difficult to distinguish but can be separated using molecular techniques. While a few species have been sequenced from invasive round gobies in this study system, their relative abundance has not been documented. The purpose of this study was to determine the species composition of Diplostomum spp. and their relative abundance in round gobies in the St Lawrence River by sequencing the barcode region of cytochrome c oxidase I. In 2007-2011, Diplostomum huronense (=Diplostomum sp. 1) was the most common, followed in order by Diplostomum indistinctum (=Diplostomum sp. 4) and Diplostomum indistinctum sensu Galazzo, Dayanandan, Marcogliese & McLaughlin (2002). In 2012, the most common species infecting the round goby in the St Lawrence River was D. huronense, followed by D. indistinctum and Diplostomum gavium (=Diplostomum sp. 3). The invasion of the round goby in the St Lawrence River was followed by a decline of Diplostomum spp. in native fishes to low levels, leading to the previously published hypothesis that the presence of the round goby has led to a dilution effect. Herein, it is suggested that despite the low infection levels in the round goby, infections still may lead to spillback, helping to maintain Diplostomum spp. in native fishes, albeit at low levels.

Molecular and morphological characterisation of Diplostomum phoxini (Faust, 1918) with a revised classification and an updated nomenclature of the species-level lineages of Diplostomum (Digenea: Diplostomidae) sequenced worldwide

We characterised morphologically and molecularly Diplostomum phoxini (Faust, 1918) based on cercarial isolates from the snail Ampullaceana balthica (L.) (Gastropoda: Lymnaeidae) and metacercariae from the Eurasian minnow, Phoxinus phoxinus (L.) (Cypriniformes: Leuciscidae), and provided molecular evidence for the identification of the snail intermediate host. Phylogenetic analyses based on the cytochrome c oxidase subunit 1 (cox1) gene depicted 44 molecularly characterised species and genetically distinct lineages of Diplostomum, and resulted in: (i) a re-identification/re-classification of 98 isolates plus D. baeri sampled in North America; (ii) re-definition of the composition of the D. baeri species complex which now includes nine molecularly characterised species/lineages; (iii) re-definition of the composition of the D. mergi species complex which now includes seven molecularly characterised species/lineages; and (iv) an updated nomenclature for the molecularly characterised species-level lineages of Diplostomum.

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.

Morphological and molecular differentiation of Diplostomum spp. metacercariae from brain of minnows (Phoxinus phoxinus L.) in four populations of northern Europe and East Asia

Metacercariae of trematodes from the genus Diplostomum are major helminth pathogens of freshwater fish, infecting the eye or the brain. The taxonomy of the genus Diplostomum is complicated, and has recently been based mainly on the molecular markers. In this study, we report the results of the morphological and molecular genetic analysis of diplostomid metacercaria from the brain of the minnow Phoxinus phoxinus from three populations in Fennoscandia (Northern Europe) and one population in Mongolia (East Asia). We obtained the data on the polymorphism of the partial mitochondrial cox1 gene and ribosomal ITS1-5.8S-ITS2 region of these parasites. РСА-based morphological analysis revealed that the parasites in the Asian and the European groups of Diplostomum sp. were distinctly different. Metacercariae from the brain of Mongolian minnows were much larger than those from the brain of Fennoscandian minnows but had much fewer excretory granules. Considering that the two study regions were separated by a distance of about 4500 km, we also tested the genetic homogeneity of their host, the minnow, using the mitochondrial cytb gene. It was shown that Diplostomum-infected minnows from Mongolia and Fennoscandia represented two previously unknown separate phylogenetic lineages of the genus Phoxinus. Both molecular and morphological analysis demonstrated that the parasites from Fennoscandia belonged the species Diplostomum phoxini, while the parasites from Mongolia belonged to a separate species, Diplostomum sp. MТ.Each of the two studied Diplostomum spp. was associated with a specific, and previously unknown, genealogical lineage of its second intermediate host, P. phoxinus.

Cryptic speciation among Tylodelphys spp.: the major helminth pathogens of fish and amphibians

Larvae of Tylodelphys Diesing, 1950 are major digenean pathogens of fish and amphibians. Tylodelphys spp. may induce mass mortality of fish and increase their susceptibility to predation. Even though Tylodelphys spp. cause substantial damage to aquaculture systems, surprisingly little is known regarding the taxonomy of this commercially important genus with a limited number of visible autapomorphic identification features. The authors obtained the DNA sequences and analyzed the molecular phylogenetics of Tylodelphys spp. adults isolated from bird hosts of Czech origin and provide comparative measurements of the analyzed species. They identified a previously unknown species complex that is subject to cryptic speciation and was previously morphologically identified as Tylodelphys excavata (Rudolphi, 1803) sensu lato. This species complex consists of three morphologically similar but genetically well-separated species. Tylodelphys excavata sensu stricto remains the dominant Tylodelphys isolated from Ciconia ciconia, which also serves as a satellite host of Tylodelphys circibuteonis Odening, 1962, which is the resurrected species for which birds of prey serve as core hosts. The authors describe Tylodelphys nigriciconis sp. n. Heneberg & Sitko as a new species identified in Ciconia nigra. By providing the first sequences of Tylodelphys podicipina Kozicka and Niewiadomska, 1960, they also show that Tylodelphys immer Dubois, 1961 is a junior synonym of T. podicipina. Further research is needed to match the provided molecular data with the DNA of larval Tylodelphys from outbreaks in commercially exploited fish species.

Whole-Genome Diversification Analysis of the Hornbeam Species Reveals Speciation and Adaptation Among Closely Related Species

Speciation is the key evolutionary process for generating biological diversity and has a central place in evolutionary and ecological research. How species diverge and adapt to different habitats is one of the most exciting areas in speciation studies. Here, we sequenced 55 individuals from three closely related species in the genus Carpinus: Carpinus tibetana, Carpinus monbeigiana, and Carpinus mollicoma to understand the strength and direction of gene flow and selection during the speciation process. We found low genetic diversity in C. tibetana, which reflects its extremely small effective population size. The speciation analysis between C. monbeigiana and C. mollicoma revealed that both species diverged ∼1.2 Mya with bidirectional gene flow. A total of 291 highly diverged genes, 223 copy number variants genes, and 269 positive selected genes were recovered from the two species. Genes associated with the diverged and positively selected regions were mainly involved in thermoregulation, plant development, and response to stress, which included adaptations to their habitats. We also found a great population decline and a low genetic divergence of C. tibetana, which suggests that this species is extremely vulnerable. We believe that the current diversification and adaption study and the important genomic resource sequenced herein will facilitate the speciation studies and serve as an important methodological reference for future research.

Morphological and Molecular Characteristics of Clinostomid Metacercariae from Korea and Myanmar

Morphological and molecular characterization of clinostomid metacercariae (CMc) was performed with the specimens collected in fish from Korea and Myanmar. Total 6 batches of clinostomid specimens by the fish species and geographical localities, 5 Korean and 1 Myanmar isolates, were analyzed with morphological (light microscopy and SEM) and molecular methods (the cytochrome c oxidase 1 gene and internal transcribed spacer 1/5.8S rRNA sequence). There were some morphological variations among CMc specimens from Korea. However, some morphometrics, i.e., the size of worm body and each organ, ratio of body length to body width, and morphology of cecal lumens, were considerably different between the specimens from Korea and Myanmar. The surface ultrastructures were somewhat different between the specimens from Korea and Myanmar. The CO1 sequences of 5 Korean specimens ranging 728–736 bp showed 99.6–100% identity with Clinostomum complanatum (GenBank no. KM923964). They also showed 99.9–100% identity with C. complanatum (FJ609420) in the ITS1 sequences ranging 692–698 bp. Meanwhile, the ITS1 sequences of Myanmar specimen showed 99.9% identity with Euclinostomum heterostomum (KY312847). Five sequences from Korean specimens clustered with the C. complanatum genes, but not clustered with Myanmar specimens. Conclusively, it was confirmed that CMc from Korea were morphologically and molecularly identical with C. complanatum and those from Myanmar were E. heterostomum.

Pathology associated with Odhneriotrema incommodum infection in wild-caught American alligators Alligator mississippiensis and assessment of potential first intermediate snail hosts

AIMS

To assess pathological changes associated with natural infections of the trematode Odhneriotrema incommodum in wild-caught American alligators Alligator mississippiensis and assess potential first intermediate hosts.

MATERIALS AND METHODS

Tongues from two wild-caught alligators were obtained from a commercial alligator processor in Port Gibson, Mississippi, USA. Tongues were subjected to gross parasitological examination and routine histological assessment. Eggs were expressed from adult trematodes collected from these tongues into distilled water, where they hatched into infectious miracidia. The snails Planorbella trivolvis, Physa gyrina, and Biomphalaria havanensis were exposed to these miracidia and observed for cercarial emergence for 200 days post-exposure.

RESULTS

Histological assessment of alligator tongues revealed marked hemorrhage, necrosis, presence of bacteria, and inflammation at sites of Odhneriotrema incommodum attachment, differing from previous histological reports from controlled experimental studies. Cercarial emergence was not observed in snails exposed to infectious miracidia.

CONCLUSIONS

Wild-caught alligators infected with Odhneriotrema incommodum exhibit more severe pathology than was previously noted from experimentally infected alligators. This adverse pathology may be associated with microbes present in eutrophic natural habitats that are absent from controlled environments used in experimental exposures. Impacts of this parasite in wild alligator populations are likely underestimated and damage associated with parasite attachment could increase host susceptibility to secondary infections. Given the importance of alligators as game animals and sustained demand for alligator products, further study into the role of O. incommodum on alligator health is warranted. Results of snail exposures to miracidia suggest these snail species are not suitable first intermediate hosts for this trematode and the true first intermediate host of O. incommodum remains unknown.

New record of metacercariae of the North American Posthodiplostomum centrarchi (Digenea, Diplostomidae) in pumpkinseed (Lepomis gibbosus) in Hungary

Two species of the genus Posthodiplostomum (Digenea: Diplostomatidae) (Posthodiplostomum brevicaudatum Nordmann, 1832 and Posthodiplostomum cuticola Nordmann, 1832) are known as parasites of Hungarian native fishes. Metacercariae of P. cuticola are widespread in Europe and cause black spot disease. Several species of Posthodiplostomum were described also from North America but none of them has been isolated in Hungary up to now. Posthodiplostomum centrarchi Hoffman, 1958 has been detected recently in pumpkinseeds (Lepomis gibbosus L., 1758) in several European countries. Posthodiplostomum centrarchi was isolated for the first time in Hungary from pumpkinseeds caught in the Maconka water reservoir in 2015. Thereafter, several natural waters (e.g. the River Danube, Lake Balaton and the Sió channel) were sampled in order to determine its presence and distribution. Only the native species P. cuticola was detected in Lake Balaton on cyprinids but a relatively high infection rate of P. centrarchi was observed in the Sió channel close to the lake. Pathological changes were absent, and metacercariae were mostly attached to the surface of the liver, kidney and heart. The phylogenetic analysis of the ITS and COI sequences of P. centrarchi and P. cuticola clustered into two distinct branches, which was in agreement with the morphological results.

Resolution of the identity of three species of Diplostomum (Digenea: Diplostomidae) parasitising freshwater fishes in South Africa, combining molecular and morphological evidence

Reliable data on the diversity of the genus Diplostomum (Digenea: Diplostomidae) parasitising freshwater fishes in South Africa, as well as in Africa, is almost non-existent. Most of the morphology-based identifications of species within this genus reported from Africa require critical revision. The aim of the present study was to determine the diversity of Diplostomum metacercariae in South African fishes applying molecular and traditional morphological techniques. To achieve this aim, a total of 216 fishes belonging to 21 species collected in the Rivers Phongolo, Riet, Usuthu and Mooi in three provinces of South Africa were examined. Metacercariae of Diplostomum were recovered from the eye lenses of 38 fishes belonging to five species of the families Anguillidae, Cichilidae and Mochokidae, with an overall low prevalence of infection (18%). Metacercariae were subjected to morphological study and molecular sequencing of the partial mithochondrial cox1 and ribosomal 28S rDNA genes as well as of ribosomal ITS1-5.8S-ITS2 region. Morphological and phylogenetic analyses revealed the presence of three species which matched those previously reported from Nigeria, Iraq and China, therefore those from Tilapia sparrmanii and Synodontis zambezensis were named Diplostomum sp.; those from Anguilla labiata, Oreochromis mossambicus and S. zambezensis were named Diplostomum sp. 14; and those from Pseudocrenilabrus philander were named Diplostomum sp. 16. Geographic distribution of several species of Diplostomum appeared to be wider than expected. Morphological description and novel sequence data generated during this study will contribute to the elucidation of the life cycles of Diplostomum sp., Diplostomum sp. 14 and Diplostomum sp. 16 and advance further research of diplostomids in Africa.

A volunteer-populated online database provides evidence for a geographic pattern in symptoms of black spot infections

Infections of parasitic digenean trematode metacercariae may lead to a visually observable syndrome in fish commonly called black spot disease. While black spot has been noted from various locations throughout North America, patterns in prevalence across the continent remain unknown. Funding to investigate continental-wide prevalence of low-mortality parasitic infections represents a barrier to such studies. I utilize iNaturalist.org's photograph database to examine fish for signs of black spot infections across North America. Fish targeted include blacknose dace, creek chub, chubs (Nocomis spp.), and stonerollers (Campostoma spp.). Photos were visually examined for symptomatic black spots indicative of infection by trematode species linked to black spot disease. Regardless of fish species group, symptoms of black spot pathogens were highly prevalent (27.1% of 314 fish) in watersheds of southern Ontario Canada, whereas mean prevalence was comparatively low elsewhere (7.8%). In one instance, a user uploaded a higher number of photos, with a higher percentage exhibiting signs of infection than other users in the watershed. However, it is difficult to tease apart if that user fished in waterbodies with high infection rates, uploaded more photos of symptomatic fishes, or some other explanation for the differences in user-reported fish with symptoms. Beyond this exception, geographic patterns in the frequency of black spot symptoms do not appear to be related to solely the users, suggesting the observed pattern is biological or ecological. While causative explanations remain conjectures, the data reported herein provides evidence that across four groups of fish, signs of black spot infections are more common in southern Ontario than other areas studied in North America. This work also represents an initial and unexpected utility of volunteer-population databases such as iNaturalist. Further data contributions could lead to better understanding of the causative agents to variation in black spot pathogens' occurrences.

The spatial distribution and fecundity of sympatric species ofDiplostomum(subclass Digenea) in single-species and mixed-species infections in the intestine of the Ring-billed Gull (Larus delawarensis)

Interactions between parasite species may influence their distribution and abundance within communities. Experimental single-species infections of Diplostomum spp. in the gut of the definitive host, the Ringed-billed Gull (Larus delawarensis Ord, 1815), were compared with mixed-species infections to explore interactions among parasites. Three species of Diplostomum von Nordmann, 1832 (Digenea), designated as Diplostomum sp. 1, Diplostomum sp. 4, and Diplostomum baeri Dubois, 1937, were examined for intestinal distribution and fecundity in single and mixed infections. In single-species infections, most specimens of Diplostomum sp. 1 and D. baeri were recovered from the mid-region of the intestine, whereas Diplostomum sp. 4 were mainly present in the anterior region. Significant spatial displacement was observed only for D. baeri when Diplostomum sp. 1 was also present. Intensity was directly correlated to the number of occupied intestinal segments, and there was no significant difference in mean linear span for each species between single-species and mixed-species infections. Diplostomum sp. 4 had the highest mean number of eggs per worm in utero in single-species infections. In mixed-species infections, the fecundity of Diplostomum sp. 4 declined dramatically in the presence of Diplostomum sp. 1, whereas fecundity of Diplostomum sp. 1 increased in the presence of D. baeri. These results highlight interspecific interactions that may play a role in population dynamics of Diplostomum spp. and community structure.

Parasitological research in the molecular age

New technological methods, such as rapidly developing molecular approaches, often provide new tools for scientific advances. However, these new tools are often not utilized equally across different research areas, possibly leading to disparities in progress between these areas. Here, we use empirical evidence from the scientific literature to test for potential discrepancies in the use of genetic tools to study parasitic vs non-parasitic organisms across three distinguishable molecular periods, the allozyme, nucleotide and genomics periods. Publications on parasites constitute only a fraction (<5%) of the total research output across all molecular periods and are dominated by medically relevant parasites (especially protists), particularly during the early phase of each period. Our analysis suggests an increasing complexity of topics and research questions being addressed with the development of more sophisticated molecular tools, with the research focus between the periods shifting from predominantly species discovery to broader theory-focused questions. We conclude that both new and older molecular methods offer powerful tools for research on parasites, including their diverse roles in ecosystems and their relevance as human pathogens. While older methods, such as barcoding approaches, will continue to feature in the molecular toolbox of parasitologists for years to come, we encourage parasitologists to be more responsive to new approaches that provide the tools to address broader questions.

Molecular and morphological characterization of the metacercariae of two species of diplostomid trematodes (Platyhelminthes, Digenea) in freshwater fishes of the Batalha River, Brazil

The Diplostomidae include a large group of flatworms with complex life cycles and are frequently found parasitizing the eyes and central nervous system of freshwater fishes. The morphological identification of the metacercariae at species level is not always possible. Thus, molecular tools have become essential to assist in the parasite species determination. This study was aimed at describing two diplostomid metacercariae found in freshwater fish in São Paulo, Brazil, based on morphological characters and in the genetic characterization of COI sequences. Our results showed that the two recognized taxa (Tylodelphys sp. and Diplostomidae gen. sp.) appear to be different from the species already described in South America. Tylodelphys sp. differs morphologically from Tylodelphys xenopi, T. mashonense, T. jenynsiae, and T. scheuringi. The metacercariae of T. clavata and T. conifera are smaller than Tylodelphys sp., while T. podicipina is larger than the metacercariae described here. The phylogenetic analysis of COI sequences yielded Tylodelphys sp. as the sister species of Tylodelphys sp. 4, a species reported from the brain of the eleotrid Gobiomorus maculatus in Oaxaca, Mexico. The metacercariae identified as Diplostomidae gen. sp. are morphologically different from the known diplostomid metacercariae and did not match with other diplostomid sequences available. Diplostomidae gen. sp. is recovered as the sister species of Diplostomum ardeae. Although the morphological evidence and the COI sequences differentiate the metacercariae found, the absence of adult specimens of both species precludes the specific designation. This is one of the first papers that use an integrative taxonomy approach to describe the species diversity of diplostomid trematodes in Brazil.

A fine-scale phylogenetic assessment of digenean trematodes in central Alberta reveals we have yet to uncover their total diversity

Despite over 100 years of digenean trematode parasite species descriptions, from a wide diversity of vertebrate and invertebrate host species, our ability to recognize the diversity of trematode species within a single lake remains an incredible challenge. The most challenging aspect is the identification of species from larval stages derived from intermediate hosts, due to the disjointed data of adult worm morphological descriptions, from which species are named, and links to corresponding molecular identifiers in depauperate databases. Cryptic species also play a significant role in the challenge of linking trematode larvae to adults, species identifications, and estimating diversity. Herein, we utilize a large, longitudinal dataset of snail first-intermediate host infection data from lakes in Alberta, Canada, to infer trematode larval diversity using molecular phylogenetics and snail host associations. From our assessments, we uncover a diversity of 79 larval trematode species among just five snail host species. Only 14 species were identified to a previously described species, while the other 65 species are either cryptic or otherwise unrepresented by mitochondrial genes in GenBank. This study currently represents the largest and most diverse singular molecular survey of trematode larval fauna composed of over one thousand mitochondrial sequences. Surprisingly, rarefaction analyses indicate we have yet to capture the complete diversity of trematodes from our sampling area.

Schistosomatoidea and Diplostomoidea

Trematodes of the order Diplostomida are well known as serious pathogens of man, and both farm and wild animals; members of the genus Schistosoma (Schistosomatidae) are responsible for human schistosomosis affecting more than 200 million people in tropical and subtropical countries, infections of mammals and birds by animal schistosomes are of great veterinary importance. The order Diplostomida is also rich in species parasitizing other major taxa of vertebrates. The Aporocotylidae are pathogenic in fish, Spirorchiidae in reptiles. All these flukes have two-host life cycles, with asexually reproducing larvae usually in molluscs and occasionally in annelids, and adults usually live in the blood vessels of their vertebrate hosts. Pathology is frequently associated with inflammatory reactions to eggs trapped in various tissues/organs. On the other hand, the representatives of Diplostomidae and Strigeidae have three- or four-host life cycles in which vertebrates often serve not only as definitive, but also as intermediate or paratenic hosts. Pathology is usually associated with migration of metacercariae and mesocercariae within the host tissues. The impact of these trematode infections on both farm and wild animals may be significant.

Impairment of retinal function in yellow perch (Perca flavescens) by Diplostomum baeri metacercariae

Histologic studies of fish from Douglas Lake, Cheboygan County, Michigan, USA show that Diplostomum spp. infect the lens of spottail shiners (Notropis hudsonius) and common shiners (Luxilus cornutus). In contrast, infection was confined to the choroidal vasculature of yellow perch (Perca flavescens), and the morphology of the pigment epithelium and retina in regions adjacent to the metacercariae was abnormal. The difference in location of metacercariae within the host suggested that different Diplostomum species may infect shiners and perch in Douglas Lake. Species diversity was investigated by sequencing the barcode region of the cytochrome oxidase I gene of metacercariae. Four species of Diplostomum were identified, all four of which were present in shiner lenses; however, only Diplostomum baeri was present in the perch choroid. To determine whether infection of perch eyes affects the response of the retina to a light stimulus, electroretinograms (ERG) were recorded. The amplitude of the b-wave of the ERG was reduced and the b-wave latency was increased in infected perch, as compared to uninfected eyes, and the flicker-fusion frequency was also reduced. Infection of the yellow perch choroid by Diplostomum baeri, which shows strong host and tissue specificity, has an adverse effect on retinal function, lending support to the hypothesis that parasite-induced impairment of host vision may afford Diplostomum baeri the evolutionary benefit of increasing the likelihood of transmission, via host fish predation, to its definitive avian host.

Histopathological characterisation of retinal lesions associated to Diplostomum species (Platyhelminthes: Trematoda) infection in polymorphic Arctic charr Salvelinus alpinus

The eye represents an immune privileged organ where parasites can escape host reactions. This study provides the first systematic evidence of the pathology associated with Diplostomum sp. infection in the eye retina of fish (i.e. Arctic charr). Histological sections showed that the trematodes caused mechanical disengagement between the retinal pigmentary epithelium and the neurosensory retina, with damaged cones and rods in the outer segment and epithelium reduced to a single layer of pigmentary cells. The metacercariae were “floating” in possibly fluid-filled vesicles together with several round cells, mostly located in the anterio-dorsal and anterio-ventral areas of the eye near the iris. The round cells may indicate internal retinal damage repair mechanisms, without connections to the general immune system. Metacercariae intestines contained pigmented cellular debris indicating that they feed on retinal epithelium. These retinal lesions may have similar vision effects as focal retinal detachment in vertebrates. Diplostomum metacercaria alters fish visual acuity but may in a lesser degree lead to a severe or total visual impairment because of repairing mechanisms. The pathology in the retina seems thereby to be dependent on fish size, age and dose.

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Histological description of the distribution of Diplostomum in the eye of Arctic charr Salvelinus alpinus.

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Metacercaria specifically located in spaces formed between the retinal pigmentary epithelium and the neurosensory retina.

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Lesions display closer similarities with those observed in retinal detachment in other species.

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Changes in the frequency and distribution of the lesions between morphs are suggested.

Exploring the diversity of Diplostomum (Digenea: Diplostomidae) in fishes from the River Danube using mitochondrial DNA barcodes

BACKGROUND

Metacercariae of Diplostomum are important fish pathogens, but reliable data on their diversity in natural fish populations are virtually lacking. This study was conducted to explore the species diversity and host-parasite association patterns of Diplostomum spp. in a large riverine system in Europe, using molecular and morphological data.

METHODS

Twenty-eight species of fish of nine families were sampled in the River Danube at Nyergesújfalu in Hungary in 2012 and Štúrovo in Slovakia in 2015. Isolates of Diplostomum spp. were characterised morphologically and molecularly. Partial sequences of the 'barcode' region of the cytochrome c oxidase subunit 1 (cox1) and complete sequences of the nicotinamide adenine dinucleotide dehydrogenase subunit 3 (nad3) mitochondrial genes were amplified for 76 and 30 isolates, respectively. The partial cox1 sequences were used for molecular identification of the isolates and an assessment of haplotype diversity and possible host-associated structuring of the most prevalent parasite species. New primers were designed for amplification of the mitochondrial nad3 gene.

RESULTS

Only lens-infecting Diplostomum spp. were recovered in 16 fish species of five families. Barcoding of representative isolates provided molecular identification for three species/species-level genetic lineages, D. spathaceum, D. pseudospathaceum and 'D. mergi Lineage 2', and three single isolates potentially representing distinct species. Molecular data helped to elucidate partially the life-cycle of 'D. mergi Lineage 2'. Many of the haplotypes of D. spathaceum (16 in total), D. pseudospathaceum (15 in total) and 'D. mergi Lineage 2' (7 in total) were shared by a number of fish hosts and there was no indication of genetic structuring associated with the second intermediate host. The most frequent Diplostomum spp. exhibited a low host-specificity, predominantly infecting a wide range of cyprinid fishes, but also species of distant fish families such as the Acipenseridae, Lotidae, Percidae and Siluridae. The nad3 gene exhibited distinctly higher levels of interspecific divergence in comparison with the cox1 gene.

CONCLUSIONS

This first exploration of the species diversity and host ranges of Diplostomum spp., in natural fish populations in the River Danube, provided novel molecular, morphological and host-use data which will advance further ecological studies on the distribution and host ranges of these important fish parasites in Europe. Our results also indicate that the nad3 gene is a good candidate marker for multi-gene approaches to systematic estimates within the genus.

Enigmatic decline of a common fish parasite (Diplostomum spp.) in the St. Lawrence River: Evidence for a dilution effect induced by the invasive round goby

As they integrate into recipient food webs, invasive exotic species may influence the population dynamics of native parasites. Here we assess the potential impact of the Eurasian round goby (Neogobius melanostomus) on the abundance of eyeflukes of the genus Diplostomum, which are common parasites in fishes of the St. Lawrence River (Canada). Analyses of data collected over nearly two decades revealed that the infection levels in three native fish [spottail shiner (Notropis hudsonius), golden shiner (Notemigonus crysoleucas) yellow perch (Perca flavescens)] declined sharply throughout the St. Lawrence River after the introduction of the goby. At two sites where data were collected at regular time intervals, declines of Diplostomum spp. in spottail shiners occurred within two years of the goby's first recorded appearance, with prevalence dropping as much as 77-80% between pre-invasion and post-invasion periods. Furthermore, in localities where gobies remained scarce, infection in native species did not change significantly over time. Altogether, these observations suggest that gobies play a role in the eyefluke collapse. The decline in populations of the main definitive host (ring-billed gulls, Larus delawarensis) and changes in hydrology during periods of parasite recruitment were not strongly supported as alternate explanations for this phenomenon. Since other snail-transmitted trematodes with similar life cycles to Diplostomum spp. did not show the same decreasing pattern, we conclude that eyeflukes did not decline as a result of snail depletion due to goby predation. Rather, we suggest that gobies acted as decoys, diluting the infection. As Diplostomum spp. occurred at lower abundance in gobies than in native fish hosts, the replacement of native fish with exotic gobies in the diet of gulls might have played a part in reducing parasite transmission. In contrast to the typically negative impact of invasions, the goby-induced decline of this pathogen may have beneficial effects for native fishes.

Molecular data reveal high diversity ofUvulifer(Trematoda: Diplostomidae) in Middle America, with the description of a new species

Members of the genusUvuliferare distributed worldwide and infect aquatic snails and freshwater fishes as first and second intermediate hosts, respectively, and fish-eating birds (kingfishers) as definitive hosts. Metacercariae ofUvuliferspp. were collected from the fins and skin of 20 species of freshwater fishes in Mexico, Guatemala, Honduras, Nicaragua and Costa Rica, and the adults were recovered from the intestine of kingfishers in four localities of Mexico. The genetic divergence among 76 samples (64 metacercariae and 12 adults) was estimated by sequencing the 28S and 5.8S nuclear genes, as well as the internal transcribed spacers ITS1 and ITS2, and one mitochondrial gene (cox1). Maximum likelihood and Bayesian inference analyses inferred with each dataset showed a high genetic diversity within the genusUvuliferacross Middle America, revealing the existence of four genetic lineages that exhibit some level of host specificity to their second intermediate hosts. The metacercariae of lineage 1 were associated with characids and cyprinids in central and northern Mexico. Metacercariae of lineages 2 and 3 were associated with cichlids distributed widely across Middle America. The lack of adults of these lineages in kingfishers, in lineages 2 and 3, or the fact that just a few adult specimens were recovered, as in lineage 1, prevented a formal description of these species. The metacercariae of lineage 4 were found in poeciliids, across a distribution range comprising Mexico, Guatemala, Honduras and Nicaragua, and the adult was found in the green kingfisher in Mexico. The number of specimens sampled for lineage 4, for both gravid adults and metacercariae, allowed us to describe a new species,Uvulifer spinatusn. sp. We describe the new species herein and we discuss briefly the genetic diversity inUvuliferspp. and the importance of using DNA sequences to properly characterize parasite diversity.

Occurrence and effect of trematode metacercariae in two endangered killifishes from Greece

We report digeneans (Diplostomidae, Crassiphialinae) in the endangered freshwater fishes Valencia letourneuxi and Valencia robertae, endemics of Western Greece. Digenean metacercariae occurred in two forms in the abdominal cavity, excysted and encysted, the latter attached to the gonads, liver and alimentary tract. Parasites were, using morphological and molecular techniques, identified as two representatives of Crassiphialinae, specifically part of the Posthodiplostomum-Ornithodiplostomum clade. The spatial, seasonal, and age class variation in parasite prevalence was examined. Autumn parasite prevalence varied between the six populations sampled (18.2 to 100%). Seasonal prevalence at the two sites sampled quadannually peaked in autumn and reached its lowest value in spring; prevalence increased with size to 100% in young adult fish. We did not find a correlation between prevalence and host sex. Overall parasites' weight averaged 0.64% of the host's, while parasite weight increased with host weight. A comparison of relative condition and hepatosomatic and gonadosomatic indices of infected and metacercariae-free specimens showed that infection did not have a significant effect on host body condition and reproduction. Regarding the parasite's life cycle, planorbid gastropods are proposed as potential first intermediate hosts in view of the host's diet and occurrence data of molluscs in the ecosystem. This is the first record of a diplostomid digenean in valenciid fishes and of representatives of the Posthodiplostomum-Ornithodiplostomum clade in a native Greek freshwater fish. Our findings are discussed in conjunction to fish conservation interventions, since parasites may contribute to the decline of endangered species.

Alligator wrestling: morphological, molecular, and phylogenetic data on Odhneriotrema incommodum (Leidy, 1856) (Digenea: Clinostomidae) from Alligator mississippiensis Daudin, 1801 in Mississippi, USA

Based on specimens collected from harvested American alligator Alligator mississippiensis Daudin, 1801 in Mississippi, USA, novel molecular data for both nuclear ribosomal genes (18S, ITS1-5.8S, ITS2, and 28S) and mitochondrial genes (cytochrome c oxidase subunit 1 and nicotinamide adenine dinucleotide dehydrogenase subunit 1) are provided for Odhneriotrema incommodum (Leidy, 1856), a trematode of the family Clinostomidae Lühe, 1901 infecting A. mississippiensis and the Florida spotted gar Lepisosteus platyrhincus DeKay, 1842. This represents the first sequencing data available for the genus Odhneriotrema and the subfamily Nephrocephalinae Travassos, 1928. Additionally, the results of phylogenetic analyses, additional morphometric data, a photomicrograph, and a line drawing supporting the present identification of O. incommodum are provided. These data will aid in elucidating the life cycle of O. incommodum through molecular identification of larval stages as well as understanding the evolutionary history of Clinostomidae and its subfamilies. Implications for the currently accepted organization of the Clinostomidae are discussed.

Filling the gaps in the classification of the Digenea Carus, 1863: systematic position of the Proterodiplostomidae Dubois, 1936 within the superfamily Diplostomoidea Poirier, 1886, inferred from nuclear and mitochondrial DNA sequences

The Diplostomida Olson, Cribb, Tkach, Bray & Littlewood, 2003 is the less diverse order of the two orders within the subclass Digenea Carus, 1863 and is currently classified into three superfamilies, i.e. Brachylaimoidea Joyeux & Foley, 1930, Diplostomoidea Poirier, 1886, and Schistosomatoidea Stiles & Hassall, 1898. Although the suprageneric-level relationships have been elucidated with the use of molecular markers, the lack of representation of some groups obscure the phylogenetic relationships among families, rendering the classification unstable. Here, we tested the phylogenetic position of the family Proterodiplostomidae Dubois, 1936 based on partial 28S rDNA and complete 18S rDNA sequences for Crocodilicola pseudostoma (Willemoes-Suhm, 1870), a crocodile parasite that has been found as a progenetic metacercaria parasitising the pale catfish Rhamdia guatemalensis (Günther) in Mexico and in other siluruforms in the Neotropics. We augmented the representation of the species, genera and families within the Diplostomida, including mostly representatives of the superfamily Diplostomoidea, and assembled a dataset that contains 49 species for the 28S rRNA gene, and 45 species for the 18S rRNA gene. Additionally, we explored the phylogenetic signal of the mitochondrial gene cox1 in reconstructing the phylogenetic relationships of selected members of the superfamily. Our analyses showed that the family Proterodiplostomidae is the sister taxon to the paraphyletic Diplostomidae Poirier, 1886 and Strigeidae Railliet, 1919, with Cyathocotylidae Mühling, 1898 + Brauninidae Wolf, 1903 as their sister group. Analysis of concatenated 18S + 28S sequences revealed the Liolopidae Odhner, 1912 as the basal group of the superfamily Diplostomoidea, although analyses of independent datasets showed that the position of this family remains uncertain. Analysis based on cox1 unequivocally resolved the Proterodiplostomidae as the sister taxon to the Diplostomidae and Strigeidae, although the Cyathocotylidae was nested in a different clade, along with brachylaimoids and schistosomatoids.

Hook, Line and Infection: A Guide to Culturing Parasites, Establishing Infections and Assessing Immune Responses in the Three-Spined Stickleback

The three-spined stickleback (Gasterosteus aculeatus) is a model organism with an extremely well-characterized ecology, evolutionary history, behavioural repertoire and parasitology that is coupled with published genomic data. These small temperate zone fish therefore provide an ideal experimental system to study common diseases of coldwater fish, including those of aquacultural importance. However, detailed information on the culture of stickleback parasites, the establishment and maintenance of infections and the quantification of host responses is scattered between primary and grey literature resources, some of which is not readily accessible. Our aim is to lay out a framework of techniques based on our experience to inform new and established laboratories about culture techniques and recent advances in the field. Here, essential knowledge on the biology, capture and laboratory maintenance of sticklebacks, and their commonly studied parasites is drawn together, highlighting recent advances in our understanding of the associated immune responses. In compiling this guide on the maintenance of sticklebacks and a range of common, taxonomically diverse parasites in the laboratory, we aim to engage a broader interdisciplinary community to consider this highly tractable model when addressing pressing questions in evolution, infection and aquaculture.

First report on molecular evidence of Tylodelphys cerebralis (Diplostomulum cerebralis) Chakrabarti, 1968 (Digenea: Diplostomidae) from snakehead fish Channa punctata

Freshwater snakehead fish, Channa punctata (Perciformes: Channidae) from Meerut (U.P.), India were screened for infection with metacercaria from the eye (vitreous humor) and brain (cranial cavity) and were analyzed by molecular methods using PCR and sequencing of the complete internal transcribed spacers (ITS1-5.8S-ITS2) of the ribosomal RNA gene. Recovered metacercariae from sites, vitreous humor and cranial cavity were previously diagnosed as Diplostomulum cerebralis Chakrabarti, 1968 on the basis of morphological characteristics. A combination of molecular methods in this study depicts that this is a species of genus Tylodelphys Diesing, 1850 which was misinterpreted as Diplostomulum. Hence, in this study, we validated the status of T. cerebralis (=D. cerebralis) after 49 years from its original description. The present work might contribute to expand our knowledge for identification, biodiversity and taxonomy of diplostomids in Indian fishes.

Host species exploitation and discrimination by animal parasites

Parasite species often show differential fitness on different host species. We developed an equation-based model to explore conditions favouring host species exploitation and discrimination. In our model, diploid infective stages randomly encountered hosts of two species; the parasite's relative fitness in exploiting each host species, and its ability to discriminate between them, was determined by the parasite's genotype at two independent diallelic loci. Relative host species frequency determined allele frequencies at the exploitation locus, whereas differential fitness and combined host density determined frequency of discrimination alleles. The model predicts instances where populations contain mixes of discriminatory and non-discriminatory infective stages. Also, non-discriminatory parasites should evolve when differential fitness is low to moderate and when combined host densities are low, but not so low as to cause parasite extinction. A corollary is that parasite discrimination (and host-specificity) increases with higher combined host densities. Instances in nature where parasites fail to discriminate when differential fitness is extreme could be explained by one host species evolving resistance, following from earlier selection for parasite non-discrimination. Similar results overall were obtained for haploid extensions of the model. Our model emulates multi-host associations and has implications for understanding broadening of host species ranges by parasites.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Parasites as drivers of key processes in aquatic ecosystems: Facts and future directions

Despite the advances in our understanding of the ecological importance of parasites that we have made in recent years, we are still far away from having a complete picture of the ecological implications connected to parasitism. In the present paper we highlight key issues that illustrate (1) important contributions of parasites to biodiversity, (2) their integral role in ecosystems, (3) as well as their ecological effects as keystone species (4) and in biological invasion processes. By using selected examples from aquatic ecosystems we want to provide an insight and generate interest into the topic, and want to show directions for future research in the field of ecological parasitology. This may help to convince more parasitologists and ecologists contributing and advancing our understanding of the complex and fascinating interplay of parasites, hosts and ecosystems.

Life History, Systematics and Evolution of the Diplostomoidea Poirier, 1886: Progress, Promises and Challenges Emerging From Molecular Studies

Members of the Diplostomoidea mature in amniotes and employ vertebrates, annelids and molluscs as second intermediate hosts. Diplostomoid life cycles generally follow a three-host pattern typical of digeneans, but novelties have arisen in some species, including obligate four-host life cycles, vertical transmission, and intracellular parasitism. In this review, we summarize the basic biology of diplostomoids with reference to molecular studies, and present challenges, gaps and areas where molecular data could address long-standing questions. Our analysis of published studies revealed that most molecular surveys find more diplostomoid species than expected, but this tendency is influenced by how much effort goes into examining specimens morphologically and the number of sequenced worms. To date, molecular work has concentrated disproportionately on intraspecific or species-level diversity of larval stages in the Diplostomidae in temperate northern regions. Although the higher taxonomy of the superfamily is recognized to be in need of revision, little molecular work has been conducted at this level. Our phylogenetic analysis indicates several families and subfamilies require reconsideration, and that larval morphotypes are more reflective of evolutionary relationships than definitive hosts. The host associations of adult diplostomoids result from host-switching processes, whereas molecular surveys indicate that larval diplostomoid metacercariae have narrow ranges of second intermediate hosts, consistent with coevolution. Molecular data are often used to link diplostomoid developmental stages, and we provide data from adult Neodiplostomum and Mesoophorodiplostomum that correct earlier misidentifications of their larval stages and propose alternatives to collecting definitive hosts.

Loads of trematodes: discovering hidden diversity of paramphistomoids in Kenyan ruminants

Paramphistomoids are ubiquitous and widespread digeneans that infect a diverse range of definitive hosts, being particularly speciose in ruminants. We collected adult worms from cattle, goats and sheep from slaughterhouses, and cercariae from freshwater snails from ten localities in Central and West Kenya. We sequenced cox1 (690 bp) and internal transcribed region 2 (ITS2) (385 bp) genes from a small piece of 79 different adult worms and stained and mounted the remaining worm bodies for comparisons with available descriptions. We also sequenced cox1 and ITS2 from 41 cercariae/rediae samples collected from four different genera of planorbid snails. Combining morphological observations, host use information, genetic distance values and phylogenetic methods, we delineated 16 distinct clades of paramphistomoids. For four of the 16 clades, sequences from adult worms and cercariae/rediae matched, providing an independent assessment for their life cycles. Much work is yet to be done to resolve fully the relationships among paramphistomoids, but some correspondence between sequence- and anatomically based classifications were noted. Paramphistomoids of domestic ruminants provide one of the most abundant sources of parasitic flatworm biomass, and because of the predilection of several species use Bulinus and Biomphalaria snail hosts, have interesting linkages with the biology of animal and human schistosomes to in Africa.

Integrated spatial health assessment of yellow perch (Perca flavescens) populations from the St. Lawrence River, Quebec, Canada) part A: physiological parameters and pathogen assessment

A multi-disciplinary approach was used to evaluate the health of yellow perch (Perca flavescens) in the St. Lawrence River (Quebec, Canada), which is experiencing a severe population decline in the downstream portion of the river. Physiological parameters, liver alterations, trace metal concentrations, parasite prevalence and abundance, stable isotope composition, and the presence/absence of the viral hemorragic septicemia virus (VHSV) were evaluated in perch collected at six sites along the river: Lake St. François, Lake St. Louis (north and south), Beauregard Island, and Lake St. Pierre (north and south). Trace metal concentrations in surface water were higher in Lake St. Louis and downstream of a major urban wastewater treatment plant discharge, indicating that this effluent was a significant source of Cu, As, Ag, Zn, and Cd. Levels of Pb in surface water exceeded thresholds for the protection of aquatic life in Lake St. Louis and were negatively correlated with body condition index in this lake. In Lake St. Pierre, Cu, Ag, and Cd bioaccumulated significantly in perch liver and lower body condition index and greater liver damage were observed compared to upstream sites. Parasite analyses indicated a higher abundance of metacercariae of the trematodes Apophallus brevis and Diplostomum spp. in Lake St. Louis, and VHSV was not detected in the liver of yellow perch for all studied sites. Overall, results suggested that the global health of yellow perch from Lake St. Pierre is lower compared to upstream studied sites, which could contribute to the documented population collapse at this site.