An updated look at the uneven distribution of cryptic diversity among parasitic helminths

Challenges in the recognition of trematode species: Consideration of hypotheses in an inexact science

The description and delineation of trematode species is a major ongoing task. Across the field there has been, and currently still is, great variation in the standard of this work and in the sophistication of the proposal of taxonomic hypotheses. Although most species are relatively unambiguously distinct from their congeners, many are either morphologically very similar, including the major and rapidly growing component of cryptic species, or are highly variable morphologically despite little to no molecular variation for standard DNA markers. Here we review challenges in species delineation in the context provided to us by the historical literature, and the use of morphological, geographical, host, and molecular data. We observe that there are potential challenges associated with all these information sources. As a result, we encourage careful proposal of taxonomic hypotheses with consideration for underlying species concepts and frank acknowledgement of weaknesses or conflict in the data. It seems clear that there is no single source of data that provides a wholly reliable answer to our taxonomic challenges but that nuanced consideration of information from multiple sources (the 'integrated approach') provides the best possibility of developing hypotheses that will stand the test of time.

Far from home: Morphological, ultrastructural, and molecular characterization of Thometrema patagonicum (Szidat, 1956) (Hemiuroidea: Derogenidae) from an unexpected host and locality, with taxonomic implications

While most higher taxa in the Hemiuroidea exhibit remarkable diversity and thrive worldwide in marine ecosystems, a few groups have successfully radiated into freshwater environments. Among them, species of Thometrema are well known from freshwater and brackish fishes in the New World. In this study, specimens of Thometrema from a single specimen of the endangered pimelodid catfish Steindachneridion scriptum in the Upper Paraná River basin, State of São Paulo, Brazil, were subjected to morphological, ultrastructural (first scanning electron microscopy data for the genus) and molecular analyses. Newly obtained sequences of the partial large subunit nuclear ribosomal RNA (28S rDNA) and partial mitochondrial cytochrome c oxidase subunit I (COI mtDNA) provided strong evidence supporting the identification of the specimens as T. patagonicum, a trematode previously known only from endemic percichthyid fishes (Percichthyidae) in the Argentinian Patagonia. This unexpected finding, along with critical morphological evaluation of new and deposited material, led us to propose T. overstreeti and T. bonariense as junior synonyms of T. patagonicum. Moreover, we reassessed certain morphological features, revealed new potential distinguishing characters (papillae) and raised doubts about the taxonomic value of commonly used features for species differentiation (e.g., vitellarium masses). To overcome taxonomic problems in Thometrema due to the paucity of morphological features to clearly set species boundaries, new descriptions could leverage integrative approaches, as employed here, to establish robust species hypotheses and ultimately unravel the actual species composition in the Americas.

Validating a web application's use of genetic distance to determine helminth species boundaries and aid in identification

BACKGROUND

Parasitic helminths exhibit significant diversity, complicating both morphological and molecular species identification. Moreover, no helminth-specific tool is currently available to aid in species identification of helminths using molecular data. To address this, we developed and validated a straightforward, user-friendly application named Applying Taxonomic Boundaries for Species Identification of Helminths (ABIapp) using R and the Shiny framework. Serving as a preliminary step in species identification, ABIapp is designed to assist in visualizing taxonomic boundaries for nematodes, trematodes, and cestodes. ABIapp employs a database of genetic distance cut-offs determined by the K-means algorithm to establish taxonomic boundaries for ten genetic markers. Validation of ABIapp was performed both in silico and with actual specimens to determine its classification accuracy. The in silico validation involved 591 genetic distances sourced from 117 publications, while the validation with actual specimens utilized ten specimens. ABIapp's accuracy was also compared with other online platforms to ensure its robustness to assist in helminth identification.

RESULTS

ABIapp achieved an overall classification accuracy of 76% for in silico validation and 75% for actual specimens. Additionally, compared to other platforms, the classification accuracy of ABIapp was superior, proving its effectiveness to determine helminth taxonomic boundaries. With its user-friendly interface, minimal data input requirements, and precise classification capabilities, ABIapp offers multiple benefits for helminth researchers and can aid in identification.

CONCLUSIONS

Built on a helminth-specific database, ABIapp serves as a pioneering tool for helminth researchers, offering an invaluable resource for determining species boundaries and aiding in species identification of helminths. The availability of ABIapp to the community of helminth researchers may further enhance research in the field of helminthology. To enhance ABIapp's accuracy and utility, the database will be updated annually.

Twenty thousand fishes under the seas: Insights into the collection and storage of trematodes from the examination of 20,000 fishes in the tropical Indo west-Pacific

The techniques employed to collect and store trematodes vary between research groups, and although these differences are sometimes necessitated by distinctions in the hosts examined, they are more commonly an artefact of instruction. As a general rule, we tend to follow what we were taught rather than explore new techniques. A major reason for this is that there are few technique papers in the published literature. Inspired by a collaborative workshop at the Trematodes 2024 symposium, we outline our techniques and processes for collecting adult trematodes from fishes and discuss the improvements we have made over 40 years of dissections of 20,000+ individual marine fishes. We present these techniques for two reasons: first, to encourage unified methods across the globe, with an aim to produce optimally comparable specimens across temporal periods, across geographic localities, and between research groups; and second, as a resource for inexperienced researchers. We stress the importance of understanding differences in host biology and the expected trematode fauna, which ultimately enables organised and productive dissections. We outline our dissection method for each key organ separately, discuss handling, fixation, and storage methods to generate the most uniform and comparable samples, and explore ethical considerations, issues of accurate host identification, and the importance and potential of clear record keeping.

Life cycle of the seabird digenean Gymnophallus minor (Gymnophallidae) in the Arctic

Gymnophallidae is one of the digenean families featuring bivalves as first intermediate hosts. However, the exact bivalve host species remain unknown for most members of this family. Gymnophallids have been one of the targets in our continuous efforts to reveal the diversity of digeneans in the higher north. Here, we focus on Gymnophallus minor, which we found in eiders from various locations in the Arctic and sub-Arctic. Sexual adults (maritae) of G. minor can be easily identified because they have a distinctive character: the roughly equal size of the pharynx and the ventral sucker. We also matched them, using DNA markers, with the intramolluscan stages (sporocysts, cercariae, and metacercariae) from the bivalve Liocyma fluctuosa collected on Spitsbergen. Taken together, we compile the first data on the life cycle of G. minor and discuss them in the context of other gymnophallids.

Complex species structure of Lecithaster salmonis (Digenea: Lecithasteridae), a fish parasite in the Arctic and Pacific Northwest

Studying complexes of cryptic or pseudocryptic species opens new horizons for the understanding of speciation processes, an important yet vague issue for the digeneans. We investigated a hemiuroidean trematode Lecithaster salmonis across a wide geographic range including the northern European seas (White, Barents, and Pechora), East Siberian Sea, and the Pacific Northwest (Sea of Okhotsk and Sea of Japan). The goals were to explore the genetic diversity within L. salmonis through mitochondrial (cox1 and nad5 genes) and ribosomal (ITS1, ITS2, 28S rDNA) marker sequences, to study morphometry of maritae, and to revise the life cycle data. Mitochondrial markers showed that L. salmonis is likely divided into six lineages (referred to as operational taxonomic units, OTUs), which often occur in sympatry, sometimes in a single host specimen. Variation in rDNA was not consistent with that in the mitochondrial markers. Morphometric analysis of maritae was performed for four out of six OTUs; it showed that some OTUs had significant differences from the others, but some did not. The effect of host species on the morphometric characteristics cannot be excluded. Intramolluscan stages were identified for two OTUs; they differed clearly by the structure of cercariae and also by the species of the first intermediate host. The case of L. salmonis is instructive in how different criteria for species delimitation can contradict each other. We regard this as a sign of recent or ongoing speciation and suggest using the name Lecithaster cf. salmonis. The most promising criteria to differentiate genetic lineages within L. cf. salmonis are first intermediate hosts and morphological characteristics of the cercariae: shape of the delivery tube and caudal cyst, and length of the filamentous appendage.

Intestinal helminths of bats in the States of Nayarit and Veracruz, Mexico, with redescription of Bidigiticauda vivipara Chitwood

Although surveys on bat parasites in the Americas have been conducted, information on helminths remains limited. In Mexican states such as Nayarit and Veracruz only 1 and 8 helminth species have been reported in bats, respectively. In this study, we provide new helminthological records for bats using morphological techniques. Also, the 28S rRNA gene of specimens from several helminth taxa was successfully amplified and sequenced. To confirm the identification at the generic level, and in some cases at the specific level, and the genealogical relationships of the parasites, we performed the phylogenetic analyses using the new 28S rRNA sequences. From March to May 2022, 16 bats of 10 species are captured and examined for helminths. Three bat species of two families, two Vespertillionidae (Bauerus dubiaquercus and Rhogeessa parvula) and one Phyllostomidae (Glossophaga mutica), are parasitized by helminths. Seven helminth taxa are morphologically identified: the trematodes Urotrema scabridum and Anenterotrema cf. hastati, the cestodes Vampirolepis macroti, Vampirolepis sp. (1), and Vampirolepis sp. (2), and the nematodes Bidigiticauda vivipara and Capillariidae gen. sp. The first helminthological records for R. parvula and B. dubiaquercus and the 28S rRNA gene data of B. vivipara, V. macroti, and Vampirolepis spp. are provided. The findings of the present study increase the number of helminth taxa recorded in Mexican bats from 78 to 79, as well as the number of bat species with helminthological records from 35 to 37.

Nomenclatural stability and the longevity of helminth species names

Although most Latin binomial names of species are valid, many are eventually unaccepted when they are found to be synonyms of previously described species, or superseded by a new combination when the species they denote are moved to a different genus. What proportion of parasite species names become unaccepted over time, and how long does it take for incorrect names to become unaccepted? Here, we address these questions using a dataset comprising thousands of species names of parasitic helminths from four higher taxa (Acanthocephala, Nematoda, Cestoda, and Trematoda). Overall, among species names proposed in the past two-and-a-half centuries, nearly one-third have since been unaccepted, the most common reason being that they have been superseded by a new combination. A greater proportion of older names (proposed pre-1950) have since been unaccepted compared to names proposed more recently, however most taxonomic acts leading to species names being unaccepted (through either synonymy or reclassification) occurred in the past few decades. Overall, the average longevity of helminth species names that are currently unaccepted was 29 years; although many remained in use for over 100 years, about 50% of the total were invalidated within 20 years of first being proposed. The patterns observed were roughly the same for all four higher helminth taxa considered here. Our results provide a quantitative illustration of the self-correcting nature of parasite taxonomy, and can also help to calibrate future estimates of total parasite biodiversity.

Gaps in parasitological research in the molecular era

We live in the age of molecular biology and '-omics', and molecular methods have opened up unimagined possibilities for biological research, including parasitology. However, too one-sided a focus on new approaches can lead to major gaps as less 'cool' topics are neglected. Selected areas of research are briefly discussed to highlight the gaps caused by the current excessive focus on molecular and '-omics' methods. It is crucial to combine both 'classical' and modern methods without neglecting the complexity of the interactions of parasites with their hosts and the environment (One Health concept), which is even more urgent in today's rapidly changing world. Parasitologists should be more involved in field studies and multidisciplinary assessment of parasites.

NEW RECORDS OF HELMINTHS OF THE JAGUAR IN MEXICO, WITH AN UPDATED LIST OF SPECIES IN THE AMERICAS

An inventory of parasites infecting the jaguar (Panthera onca) across its distribution range is relevant for the conservation of this threatened big cat. In this study, we report the occurrence of helminths in a jaguar from Mexico using morphological techniques (cleared and stained mounts and scanning electron microscopy) and partial sequences of the 28S ribosomal RNA (28S rRNA) gene and the cytochrome c oxidase 1 mitochondrial (COI) gene. We also provide an updated list of helminth species reported in jaguars in the Americas. Three helminth taxa are identified in the jaguar examined from Mexico: Toxocara cati, Physaloptera sp., and Taenia sp. The new 28S rRNA sequences of To. cati, Physaloptera sp., and Taenia sp. and the COI sequence of Taenia sp. corroborate the identity of the helminths isolated from this host. One hundred and twenty-nine records of helminths parasitizing jaguars from 49 studies up to May 2023 were identified in the Americas. In most of these studies (73.6%), helminths were identified using coproparasitological techniques. Sixteen helminths (7 nematodes, 5 cestodes, 3 acanthocephalans, and 1 trematode) were identified at the species level in free-ranging and captive jaguars. The study demonstrates the value of an integrative taxonomy approach to increase the accuracy of parasite identification in wildlife, especially when helminth specimens are scarce or poorly fixed.

Molecular species delimitation of marine trematodes over wide geographical ranges: Schikhobalotrema spp. (Digenea: Haplosplanchnidae) in needlefishes (Belonidae) from the Pacific Ocean and Gulf of Mexico

Geographical distribution plays a major role in our understanding of marine biodiversity. Some marine fish trematodes have been shown to have highly restricted geographical distributions, while some are known to occur over very wide ranges; however, very few of these wide distributions have been demonstrated genetically. Here, we analyse species of the genus Schikhobalotrema (Haplosplanchnidae) parasitizing beloniforms from the tropical west Pacific, the eastern Pacific and the Gulf of Mexico (GoM). We test the boundaries of these trematodes by integrating molecular and morphological data, host association, habitat of the hosts and geographical distribution, following a recently proposed and standardized delineation method for the recognition of marine trematode species. Based on the new collections, Schikhobalotrema huffmani is here synonymized with the type-species of the genus, Schikhobalotrema acutum; Sch. acutum is now considered to be widely distributed, from the GoM to the western Pacific. Additionally, we describe a new species, Schikhobalotrema minutum n. sp., from Strongylura notata and Strongylura marina (Belonidae) from La Carbonera coastal lagoon, northern Yucatán, GoM. We briefly discuss the role of host association and historical biogeography of the hosts as drivers of species diversification of Schikhobalotrema infecting beloniforms.

Selected Wildlife Trematodes

The trematodes are a species-rich group of parasites, with some estimates suggesting that there are more than 24,000 species. However, the complexities associated with their taxonomic status and nomenclature can hinder explorations of the biology of wildlife trematodes, including fundamental aspects such as host use, life cycle variation, pathology, and disease. In this chapter, we review work on selected trematodes of amphibians, birds, mammals, and their snail intermediate hosts, with the goal of providing a tool kit on how to study trematodes of wildlife. We provide a brief introduction to each group of wildlife trematodes, followed by some examples of the challenges each group of trematodes has relative to the goal of their identification and understanding of the biology and interactions these organisms have with their wildlife hosts.

Morphological constraint obscures richness: a mitochondrial exploration of cryptic richness in Transversotrema (Trematoda: Transversotrematidae)

Species of Transversotrema Witenberg, 1944 (Transversotrematidae) occupy a unique ecological niche for the Trematoda, living externally under the scales of their teleost hosts. Previous studies of the genus have been impeded partly by limited variation in ribosomal DNA sequence data between closely related species and partly by a lack of morphometrically informative characters. Here, we assess richness of the tropical Indo-west Pacific species through parallel phylogenetic and morphometric analyses, generating cytochrome c oxidase subunit 1 mitochondrial sequence data and morphometric data for hologenophore specimens from Australia, French Polynesia, Japan and Palau. These analyses demonstrate that molecular data provide the only reliable basis for species identification; host distribution, and to a lesser extent morphology, are useful for identifying just a few species of Transversotrema. We infer that a combination of morphological simplicity and infection site constraint has led to the group displaying exceptionally low morphological diversification. Phylogenetic analyses of the mitochondrial data broadly support previous systematic interpretations based on ribosomal data, but also demonstrate the presence of several morphologically and ecologically cryptic species. Ten new species are described, eight from the Great Barrier Reef, Australia (Transversotrema chrysallis n. sp., Transversotrema daphnidis n. sp., Transversotrema enceladi n. sp., Transversotrema hyperionis n. sp., Transversotrema iapeti n. sp., Transversotrema rheae n. sp., Transversotrema tethyos n. sp., and Transversotrema titanis n. sp.) and two from off Japan (Transversotrema methones n. sp. and Transversotrema panos n. sp.). There are now 26 Transversotrema species known from Australian marine fishes, making it the richest trematode genus for the fauna.

How we study cryptic species and their biological implications: A case study from marine shelled gastropods

Methodological and biological considerations are intertwined when studying cryptic species. A potentially large component of modern biodiversity, the frequency of cryptic species among taxonomic groups is not well documented. The term "cryptic species" is imprecisely used in scientific literature, causing ambiguity when interpreting their evolutionary and ecological significance. This study reviews how cryptic species have been defined, discussing implications for taxonomy and biology, and explores these implications with a case study based on recently published literature on extant shelled marine gastropods. Reviewed gastropods were recorded by species. Records of cryptic gastropods were presented by authors with variable levels of confidence but were difficult to disentangle from inherent biases in the study effort. These complexities notwithstanding, most gastropod species discussed were not cryptic. To the degree that this review's sample represents extinct taxa, the results suggest that a high proportion of shelled marine gastropod species are identifiable for study in the fossil record. Much additional work is needed to provide a more adequate understanding of the relative frequency of cryptic species in shelled marine gastropods, which should start with more explicit definitions and targeted case studies.

Let's restart formally naming 'larval' trematodes

Arguably the most unique biological features of trematode parasites involve their clonal parthenitae and cercariae. These life stages are biologically fascinating, medically and scientifically important, and often studied for years, lacking knowledge of their corresponding sexual adult stages. But sexual adults are the focus of trematode species-level taxonomy, partially explaining the relative neglect of documenting the diversity of parthenitae and cercariae and why researchers who do describe them give them only provisional names. Provisional names are unregulated, unstable, often ambiguous, and, I argue, often unnecessary. I suggest that we recommence formally naming parthenitae and cercariae using an improved naming scheme. The scheme should permit us to reap the benefits of formal nomenclature and thereby enhance research involving these diverse and important parasites.

Evidence for two morphologically cryptic species of Hysterolecitha Linton, 1910 (Trematoda: Lecithasteridae) infecting overlapping host ranges in Moreton Bay, Australia

Integration of morphological and molecular approaches to species delineation has become an essential part of digenean trematode taxonomy, particularly when delimiting cryptic species. Here, we use an integrated approach to distinguish and describe two morphologically cryptic species of Hysterolecitha Linton, 1910 (Trematoda: Lecithasteridae) from fishes of Moreton Bay, Queensland, Australia. Morphological analyses of Hysterolecitha specimens from six fish species demonstrated a complete overlap in morphometric data with no reliable differences in their gross morphological characters that suggested the presence of more than one species. Distinctions in ITS2 rDNA and cox1 mtDNA sequence data for corresponding specimens suggested the presence of two forms. A principal component analysis on an imputed dataset showed clear separation between the two forms. These two forms are partially separated on the basis of their host's identity. Therefore, we describe two morphologically cryptic species: Hysterolecitha melae n. sp. from three species of Abudefduf Forsskål and one species of Parma Günther (Pomacentridae), with the Bengal sergeant, Abudefduf bengalensis (Bloch), as the type-host; and Hysterolecitha phisoni n. sp. from species of Pomacentridae (including A. bengalensis), Pomatomidae and Siganidae, with the black rabbitfish, Siganus fuscescens (Houttuyn), as the type-host.

A new cryptic species of Echinostoma (Trematoda: Echinostomatidae) closely related to Echinostoma paraensei found in Brazil

Echinostoma paraensei, described in Brazil at the end of the 1960s and used as a biological model for a range of studies, belongs to the ‘revolutum’ complex of Echinostoma comprising species with 37 collar spines. However, molecular data are available only for a few isolates maintained under laboratory conditions, with molecular prospecting based on specimens originating from naturally infected hosts virtually lacking. The present study describes Echinostoma maldonadoi Valadão, Alves & Pinto n. sp., a species cryptically related to E. paraensei found in Brazil. Larval stages (cercariae, metacercariae and rediae) of the new species were found in the physid snail Stenophysa marmorata in the State of Minas Gerais, Brazil, the same geographical area where E. paraensei was originally described. Adult parasites obtained experimentally in Meriones unguiculatus were used for morphological (optical microscopy) and molecular [28S, internal transcribed spacer (ITS), nad1 and cox1] characterization. The morphology of larval and adult parasites (most notable the small-sized dorsal spines in the head collar), associated with low (0–0.1%) molecular divergence for 28S gene or ITS region, and only moderate divergence for the mitochondrial cox1 gene (3.83%), might suggest that the newly collected specimens should be assigned to E. paraensei. However, higher genetic divergence (6.16–6.39%) was found in the mitochondrial nad1, revealing that it is a genetically distinct, cryptic lineage. In the most informative phylogenetic reconstruction, based on nad1, E. maldonadoi n. sp. exhibited a strongly supported sister relationship with E. paraensei, which may indicate a very recent speciation event giving rise to these 2 species.

Mussel parasite richness and risk of extinction

Parasite conservation is important for the maintenance of ecosystem diversity and function. Conserving parasites relies first on understanding parasite biodiversity and second on estimating the extinction risk to that biodiversity. Although steps have been taken independently in both these areas, previous studies have overwhelmingly focused on helminths in vertebrate hosts over broad scales, providing low resolution and excluding a large proportion of possible host and parasite diversity. We estimated both total obligate parasite richness and parasite extinction risk in freshwater mussels (Unionidae and Margaritiferidae) from Europe and the United States to provide a case study for considering parasite conservation in a severely understudied system. We used currently reported host-parasite relationships to extrapolate parasite diversity to all possible mussel hosts and then used the threat levels of those hosts to estimate the extinction risk for both described and undescribed parasites. An estimated 67% of parasite richness in freshwater mussels is undescribed and over 80% of the most host-specific groups (digenean trematodes and ciliates) are undescribed. We estimated that 21% of this total parasite fauna is at immediate risk of extinction, corresponding to 60 unique species, many of which will likely go extinct before being described. Given the important roles parasites play in community structure and function and the strong ecosystem engineering capacities of freshwater mussels, such extinctions are likely to severely affect freshwater ecosystems. Our detailed study of mussel parasites provides compelling evidence for the hidden conservation threat to parasites through extinction cascades and shows parasites are deserving of immediate attention.

Fish-parasite interaction networks reveal latitudinal and taxonomic trends in the structure of host-parasite associations

In recent years, treating host–parasite associations as bipartite interaction networks has proven a powerful tool to identify structural patterns and their likely causes in communities of fish and their parasites. Network analysis allows for both community-level properties to be computed and investigated, and species-level roles to be determined. Here, using data from 31 host–parasite interaction networks from local fish communities around the world, we test for latitudinal trends at whole-network level, and taxonomic patterns at individual parasite species level. We found that while controlling for network size (number of species per network), network modularity, or the tendency for the network to be subdivided into groups of species that interact mostly with each other, decreased with increasing latitude. This suggests that tropical fish–parasite networks may be more stable than those from temperate regions in the event of community perturbations, such as species extinction. At the species level, after accounting for the effect of host specificity, we observed no difference in the centrality of parasite species within networks between parasites with different transmission modes. However, species in some taxa, namely branchiurans, acanthocephalans and larval trematodes, generally had higher centrality values than other parasite taxa. Because species with a central position often serve as module connectors, these 3 taxa may play a key role in whole-network cohesion. Our results highlight the usefulness of network analysis to reveal the aspects of fish–parasite community interactions that would otherwise remain hidden and advance our understanding of their evolution.

Essential Oils as Novel Anthelmintic Drug Candidates

Helminths, with an estimated 1.5 billion annual global infections, are one of the major health challenges worldwide. The current strategy of the World Health Organization to prevent helminth infection includes increasing hygienic awareness, providing better sanitation and preventative anthelmintic drug therapy in vulnerable populations. Nowadays, anthelmintic drugs are used heavily in livestock, both in case of infection and as a preventative measure. However, this has led to the development of resistance against several of the most common drugs, such as levamisole, ivermectin and thiabendazole. As many as 70% of the livestock in developed countries now has helminths that are drug resistant, and multiple resistance is common. Because of this, novel anthelmintics are urgently needed to help combat large-scale production losses. Prior to this review, no comprehensive review of the anthelmintic effects of essential oils and their components existed. Multiple review articles have been published on the uses of a single plant and its extracts that only briefly touch upon their anthelmintic activity. This review aims to provide a detailed overview of essential oils and their components as anthelmintic treatment against a wider variety of helminths.

Autochthonous transmission of the Indomalayan parasite, Transversotrema patialense, in the Caribbean: Molecular, morphological, and experimental evidence

The Asian freshwater snail Melanoides tuberculata has been established since the 1960s in the Americas, where it transmits cercariae of a small number of digenetic trematode species from its native range. In 2021-2022, 24 M. tuberculata were discovered shedding transversotrematid cercariae in Puerto Rico, where parasites of this snail have not been previously studied. Adult transversotrematids (in some cases, gravid) were found on field-caught fish and on fish exposed to shedding snails, including on fish species native to Puerto Rico. Adults and cercariae were identified as Transversotrema patialense (Soparkar, 1924), a species native to the Indomalayan region. Morphological identification was supported with 28S rDNA sequences closely matching that from unidentified transversotrematid cercariae in Thailand. The absence of T. patialense in snails collected prior to 2021, increasing prevalence of infection in snails collected thereafter, and lack of variation in parasite DNA sequences (28S, internal transcribed spacer 2, cytochrome c oxidase I) from three isolates are consistent with a recently introduced and possibly expanding parasite population. Transversotrema patialense has been recorded outside its native range before, but most studies (including a prior record in the Americas) reported the parasite from captive hosts from commercial sources such as pet shops. The present results thus provide the first demonstration of natural transmission of T. patialense in the Americas. Phylogenetic analysis of 28S but not of ITS2 show the transversotrematid genus Transversotrema Witenberg, 1944 is paraphyletic, with Crusziella Cribb, Bray and Barker1992 nested within it.

What lies behind the curtain: Cryptic diversity in helminth parasites of human and veterinary importance

Parasite cryptic species are morphologically indistinguishable but genetically distinct organisms, leading to taxa with unclear species boundaries. Speciation mechanisms such as cospeciation, host colonization, taxon pulse, and oscillation may lead to the emergence of cryptic species, influencing host-parasite interactions, parasite ecology, distribution, and biodiversity. The study of cryptic species diversity in helminth parasites of human and veterinary importance has gained relevance, since their distribution may affect clinical and epidemiological features such as pathogenicity, virulence, drug resistance and susceptibility, mortality, and morbidity, ultimately affecting patient management, course, and outcome of treatment. At the same time, the need for recognition of cryptic species diversity has implied a transition from morphological to molecular diagnostic methods, which are becoming more available and accessible in parasitology. Here, we discuss the general approaches for cryptic species delineation and summarize some examples found in nematodes, trematodes and cestodes of medical and veterinary importance, along with the clinical implications of their taxonomic status. Lastly, we highlight the need for the correct interpretation of molecular information, and the correct use of definitions when reporting or describing new cryptic species in parasitology, since molecular and morphological data should be integrated whenever possible.

A world of taxonomic pain: cryptic species, inexplicable host-specificity, and host-induced morphological variation among species of Bivesicula Yamaguti, 1934 (Trematoda: Bivesiculidae) from Indo-Pacific Holocentridae, Muraenidae and Serranidae

The taxonomy of species of Bivesicula Yamaguti, 1934 is analysed for samples from holocentrid, muraenid and serranid fishes from Japan, Ningaloo Reef (Western Australia), the Great Barrier Reef (Queensland), New Caledonia and French Polynesia. Analysis of three genetic markers (cox1 mtDNA, ITS2 and 28S rDNA) identifies three strongly supported clades of species and suggests that Bivesicula as presently recognized is not monophyletic. On the basis of combined morphological, molecular and biological data, 10 species are distinguished of which five are proposed as new. Bivesicula Clade 1 comprises seven species of which three are effectively morphologically cryptic relative to each other; all seven infect serranids and four also infect holocentrids. Bivesicula Clade 2 comprises three species of which two are effectively morphologically cryptic relative to each other; all three infect serranids and one also infects a muraenid. Bivesicula Clade 3 comprises two known species from apogonids and a pomacentrid, and forms a clade with species of Paucivitellosus Coil, Reid & Kuntz, 1965 to the exclusion of other Bivesicula species. Taxonomy in this genus is made challenging by the combination of low resolving power of ribosomal markers, the existence of regional cox1 mtDNA populations, exceptional and unpredictable host-specificity and geographical distribution, and significant host-induced morphological variation.

New collections of blood flukes (Aporocotylidae) from fishes of the tropical Indo-west Pacific, including a new genus, two new species and molecular evidence that Elaphrobates chaetodontis (Yamaguti, 1970) is widespread in the region

We report new collections of the Aporocotylidae from Australia, French Polynesia, and Japan. A new species of Cardicola Short, 1953 is described from Scomberomorus commerson (Lacépède) (Scombridae), off Lizard Island. Cardicola nolani n. sp. can be distinguished from its congeners based on the position of the oötype, the position of the male genital pore, and the absence of an oral sucker. A new species is described from Abalistes stellatus (Anonymous) (Balistidae), also from off Lizard Island. Phylogenetically the new species forms a strongly-supported clade with Cardicola yuelao Yong, Cutmore & Cribb, 2018, which also infects balistids. These two species are distinct from all other aporocotylids in the combination of exceptionally short anterior and long posterior caeca, a lanceolate body, a single testis, an entirely post-ovarian uterus and the position of the oötype; a new genus, Balistidicola, is proposed for them. Balistidicola corneri n. sp. and B. yuelao (Yong, Cutmore & Cribb, 2018) n. comb. are essentially morphologically cryptic, only distinguishable by the form of the spination (B. corneri has five spines per row and B. yuelao has six). Elaphrobates chaetodontis (Yamaguti, 1970) is reported from 21 species of butterflyfishes (Chaetodontidae) from nine locations in tropical Indo-west Pacific; cox1 sequence data demonstrate extensive geographical structuring in this species. Braya jexi Nolan & Cribb, 2006, Elaphrobates milleri (Nolan & Cribb, 2006), and P. corventum Overstreet & Køie, 1989 are each re-reported from their type-hosts, and Pearsonellum pygmaeus Nolan & Cribb, 2004 and Balistidicola yuelao are each reported from a new host.

The potential use of mitochondrial ribosomal genes (12S and 16S) in DNA barcoding and phylogenetic analysis of trematodes

Background

Genetic markers like the nuclear ribosomal RNA (rRNA) genes, internal transcribed spacer regions, mitochondrial protein-coding genes, and genomes have been utilized for molecular identification of parasitic trematodes. However, challenges such as the design of broadly applicable primers for the vast number of species within Digenea and the genetic markers’ ability to provide sufficient species-level resolution limited their utility. This study presented novel and broadly applicable primers using the mitochondrial 12S and 16S rRNA genes for Digenea and aimed to show their suitability as alternative genetic markers for molecular identification of orders Plagiorchiida, Echinostomida, and Strigeida.

Results

Our results revealed that the mitochondrial 12S and 16S rRNA genes are suitable for trematode molecular identification, with sufficient resolution to discriminate closely related species and achieve accurate species identification through phylogenetic placements. Moreover, the robustness of our newly designed primers to amplify medically important parasitic trematodes encompassing three orders was demonstrated through successful amplification. The convenience and applicability of the newly designed primers and adequate genetic variation of the mitochondrial rRNA genes can be useful as complementary markers for trematode molecular-based studies.

Conclusions

We demonstrated that the mitochondrial rRNA genes could be alternative genetic markers robust for trematode molecular identification and potentially helpful for DNA barcoding where our primers can be widely applied across the major Digenea orders. Furthermore, the potential of the mitochondrial rRNA genes for molecular systematics can be explored, enhancing their appeal for trematode molecular-based studies. The novelty of utilizing the mitochondrial rRNA genes and the designed primers in this study can potentially open avenues for species identification, discovery, and systematics in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08302-4.

The biodiversity of marine trematodes: then, now and in the future

Trematodes are the richest class of platyhelminths in the marine environment, infecting all classes of marine vertebrates as sexual adults and many phyla of marine invertebrates as part of their life cycles. Despite the cryptic nature of their existence (almost all marine trematodes are internal parasites), they have been the focus of study for almost 250 years, with the first species described in 1774. Here we review progress in the study of the "biodiversity" of these parasites, contrasting the progress made in the last 50 years (post-1971) to that in the almost 200 years before it (pre-1972). We consider an understanding of biodiversity to require knowledge of the species present in the system, an understanding of their evolutionary relationships (which informs higher classification), and, specifically for trematodes, an understanding of their complex life cycles. The fauna is now large, comprising well over 5,000 species. Although species description continues, we see evidence of a slow-down in all aspects of discovery. There has been only one completely new family identified since 1984 and the proposal of new genera is in decline as is the description of new species, especially for those of tetrapods. However, the extent to which this slow-down reflects an approach to the richness asymptote is made uncertain by changes in the field; reduced effort and difficulty of study may be important components of the effect. Regardless of how close we are to a complete description of the fauna, we infer that the outline is well-understood although the details are not. Adoption of molecular methodologies over the last 40 years have complemented morphometric analyses to facilitate objective recognition of species; however, despite these objective data, there is still inconsistency between authors on species delimitation. Molecular methodologies have also completely revolutionised inference of relationships at all levels, from within genera to between orders, and underpinned elucidation of novel life cycles. We expect the next 50 years to produce further dividends from technological innovations. The backdrop to the field will be global environmental concerns and the growing problem of funding for basic biodiversity studies.

A paradigm for the recognition of cryptic trematode species in tropical Indo-west Pacific fishes: the problematic genus Preptetos (Trematoda: Lepocreadiidae)

Molecular data have transformed approaches to trematode taxonomy by providing objective evidence for the delineation of species. However, although the data are objective, the interpretation of these data regarding species boundaries is subjective, especially when different markers conflict. Conserved markers can lead to an underestimation of richness and those used for finer species delineation have the capacity to inflate species recognition, perhaps unrealistically. Here we examine molecular and morphological evidence for species recognition in an especially confusing system, the lepocreadiid genus Preptetos Pritchard, 1960 in acanthuriform fishes of the tropical Indo-west Pacific. We consider species boundaries within this genus based on combined data (ITS2 and 28S rDNA; cox1 mtDNA and morphometrics) for substantial new collections. Delineation of species using only morphological data suggest fewer species than analysis of the sequence data; the latter suggests the presence of potential cryptic species and analysis of different markers suggests the presence of differing numbers of species. We conclude that an integrative interpretation creates the most satisfying taxonomic hypothesis. In the light of the new data, we have chosen and propose a model of trematode species recognition that demands reciprocal monophyly in the most discriminating available molecular marker plus distinction in morphology or host distribution. By invoking these criteria, we distinguish eight species in our new tropical Indo-west Pacific collections. Six of these are new (Preptetos allocaballeroi n. sp., Preptetos paracaballeroi n. sp., Preptetos pearsoni n. sp., Preptetos prudhoei n. sp., Preptetos quandamooka n. sp. and Preptetos zebravaranus n. sp.) and we continue to recognise Preptetos cannoni Barker, Bray & Cribb, 1993 and Preptetos laguncula Bray and Cribb, 1996. Notably; two of the new species, P. allocaballeroi n. sp. and P. paracaballeroi n. sp., are morphologically cryptic relative to each other. Our criteria lead us to recognise, as species, populations with unvarying morphology and similar host relationships but which may have a complex population structure over their range. In our view, this paradigm has the capacity to render tractable the interpretation of the species status of the huge trematode fauna of the tropical Indo-west Pacific.

Two new species of threadlike blood flukes (Aporocotylidae), with a molecular revision of the genera Ankistromeces Nolan & Cribb, 2004 and Phthinomita Nolan & Cribb, 2006

Ankistromeces Nolan & Cribb, 2004 and Phthinomita Nolan & Cribb, 2006 are sister genera of threadlike blood flukes (Trematoda: Aporocotylidae) infecting teleost fishes of the tropical Indo-west Pacific. Here, we report new collections of these genera from Australia, Indonesia, and Japan. A new species of Ankistromeces, Ankistromeces kawamurai n. sp., is described from Siganus spinus (Linnaeus) off Okinawa, Japan, and a new species of Phthinomita, Phthinomita abdita n. sp., from Choerodon cephalotes (Castelnau), in Moreton Bay, Australia; the new species are morphologically cryptic within their respective genera and are delineated by molecular and ecological data. Ankistromeces olsoni Nolan & Cribb, 2006 is reported from Siganus fuscescens (Houttuyn) off Heron Island (southern Great Barrier Reef), Lizard Island (northern Great Barrier Reef), and Okinawa and Wakayama Prefectures, Japan and from Siganus spinus (Linnaeus) from off Bali, Indonesia. Ankistromeces mariae Nolan & Cribb, 2004 is re-reported from the type-host, Meuschenia freycineti (Quoy & Gaimard), from a new location, Gypsy Bay, Tasmania. Phthinomita poulini Nolan & Cribb, 2006 is re-reported from its type-locality, Lizard Island, from a range of mullids, including five new host species, and its range is extended to include Moreton Bay. Phthinomita symplocos Nolan & Cribb, 2006 is reported from Bali and P. hallae Nolan & Cribb, 2006, P. jonesi Nolan & Cribb, 2006, P. littlewoodi Nolan & Cribb, 2006, and P. munozae Nolan & Cribb, 2006 are each re-reported from their type-host and type-localities. New cox1 mtDNA data were generated for all known species of these two genera from new and archival material. Analyses of these data enabled an evaluation of all known Phthinomita species; P. robertsthomsoni Nolan & Cribb, 2006 is synonymised with P. adlardi Nolan & Cribb, 2006, and P. brooksi Nolan & Cribb, 2006 is synonymised with P. sasali Nolan & Cribb, 2006. We highlight the failure of ITS2 data to delineate closely related aporocotylid species. In contrast, cox1 sequence data are proving reliable and effective in this context and we recommend their incorporation in future studies of blood fluke taxonomy.

The rise of ecological parasitology: twelve landmark advances that changed its history

In the five decades since the first publication of the International Journal for Parasitology, ecological parasitology has grown from modest beginnings to become a modern discipline with a strong theoretical foundation, a diverse toolkit, and a multidisciplinary approach. In this review, I highlight 12 advances in the field that have spurred its growth over the past 50 years. Where relevant, I identify pivotal contributions that have altered the course of research, as well as the influence of developments in other fields such as mainstream ecology and molecular biology. The 12 key advances discussed are in areas including parasite population dynamics and community assembly, the regulation of host population abundance and food web structure, parasites as agents of natural selection, the impacts of biodiversity and anthropogenic changes on host-parasite interactions, the biogeography of parasite diversity, and the evolutionary genetics of parasites. I conclude by identifying some challenges and opportunities lying ahead, which need to be met for the future growth of ecological research on host-parasite interactions.

Morphological and molecular characterization of Quinqueserialis (Digenea: Notocotylidae) species diversity in North America

Estimates of trematode diversity are inaccurate due to unrecognized cryptic species and phenotypic plasticity within species. Integrative taxonomy (genetics, morphology and host use) increases the clarity of species delineation and improves knowledge of parasite biology. In this study, we used this approach to resolve taxonomic issues and test hypotheses of cryptic species in a genus of trematode, Quinqueserialis. Specimens from throughout North America were field collected from hosts and obtained from museums. We found three morphologically distinct groups and successfully sequenced specimens from two of these groups. DNA sequencing at the 28S and CO1 gene regions revealed that two of the three groups were genetically distinct. One genetic group included two morphological clusters demonstrating host-induced phenotypic plasticity within Quinqueserialis quinqueserialis. The other unique genetic group is a novel species, Quinqueserialis kinsellai n. sp., which is described herein. Our study illustrates the importance of integrating multiple sources of evidence when investigating trematode diversity to account for the influence of cryptic species or phenotypic plasticity. However, further sampling is needed to understand Quinqueserialis spp. diversity as some species have no genetic information associated with them.

Gorgocephalidae (Digenea: Lepocreadioidea) in the Indo-West Pacific: new species, life-cycle data and perspectives on species delineation over geographic range

The digenetic trematode family Gorgocephalidae comprises just a few species, and the literature devoted to the lineage consists of only a handful of reports. With one exception, all reports have been based on material collected in the Indo-West Pacific, an expansive marine ecoregion stretching from the east coast of Africa to Easter Island, Hawaii and French Polynesia. We collected adult and intramolluscan gorgocephalids from kyphosid fishes and littorinid gastropods from several Australian localities, and from South Africa and French Polynesia. Specimens of Gorgocephalus kyphosi and G. yaaji were collected from, or near, their type-localities, providing new morphological and molecular (COI, ITS2 and 28S) data needed for a revised understanding of species boundaries in the family. Two new species are recognized: Gorgocephalus euryaleae sp. nov. and Gorgocephalus graboides sp. nov. New definitive host records are provided for described species and three new intermediate hosts are identified. These new records are all associated with Kyphosus fishes and littorinid gastropods, reaffirming the restriction of gorgocephalids to these hosts. Most significantly, we provide evidence that G. yaaji is distributed from South Africa to French Polynesia, spanning the breadth of the Indo-West Pacific. Our findings have significant relevance regarding digenean species delineation over geographic range.

What would it take to describe the global diversity of parasites?

How many parasites are there on Earth? Here, we use helminth parasites to highlight how little is known about parasite diversity, and how insufficient our current approach will be to describe the full scope of life on Earth. Using the largest database of host-parasite associations and one of the world's largest parasite collections, we estimate a global total of roughly 100 000-350 000 species of helminth endoparasites of vertebrates, of which 85-95% are unknown to science. The parasites of amphibians and reptiles remain the most poorly described, but the majority of undescribed species are probably parasites of birds and bony fish. Missing species are disproportionately likely to be smaller parasites of smaller hosts in undersampled countries. At current rates, it would take centuries to comprehensively sample, collect and name vertebrate helminths. While some have suggested that macroecology can work around existing data limitations, we argue that patterns described from a small, biased sample of diversity aren't necessarily reliable, especially as host-parasite networks are increasingly altered by global change. In the spirit of moonshots like the Human Genome Project and the Global Virome Project, we consider the idea of a Global Parasite Project: a global effort to transform parasitology and inventory parasite diversity at an unprecedented pace.

First confirmed record of Trichobilharzia franki Müller & Kimmig, 1994, from Radix auricularia (Linnaeus, 1758) for Austria

Avian schistosomes are of medical and veterinary importance as they are responsible for the annually occurring cercarial dermatitis outbreaks. For Austria, so far, only Trichobilharzia szidati Neuhaus 1952 was confirmed on species level as causative agent of cercarial dermatitis. Here we present the first record of Trichobilharzia franki Müller & Kimmig 1994 in Austria. The species was detected during a survey of digenean trematodes in Upper Austrian water bodies. Furthermore, we provide DNA barcodes of T. franki as well as measurements of several parasite individuals to indicate the intraspecific diversity. We also recommend the usage of an alternative primer pair, since the “standard COI primer pair” previously used for Schistosomatidae amplified an aberrant fragment in the sequence of T. franki. Overall, our study shows how limited our knowledge about occurrence and distribution of avian schistosomes in Austria is and how important it is to acquire such a knowledge to estimate ecological and epidemiological risks in the future.

Pinworms of the red howler monkey (Alouatta seniculus) in Colombia: Gathering the pieces of the pinworm-primate puzzle

Pinworms of primates are believed to be highly host specific parasites, forming co-evolutionary associations with their hosts. In order to assess the strength and reach of such evolutionary links, we need to have a broad understanding of the pinworm diversity associated with primates. Here, we employed an integrative taxonomic approach to assess pinworm diversity in red howler monkeys in Colombia. Molecular and morphological evidence validate the presence of at least four different species of Trypanoxyuris occurring in red howler monkeys: T. minutus, a widely distributed species, and three new species, T. seunimiii n. sp., T. kemuimae n. sp. and T. kotudoi n. sp. The mitochondrial COI gene and the 28S ribosomal gene were used for phylogenetic assessments through Bayesian inference. The three new species were morphologically distinct and formed reciprocally monophyletic lineages. Further molecular lineage subdivision in T. minutus and T. kotudoi n. sp. without morphological correspondence, suggests the potential scenario for the existence of cryptic species. Phylogenetic relationships imply that the different species of Trypanoxyuris occurring in each howler monkey species were acquired through independent colonization events. On-going efforts to uncover pinworm diversity will allow us to test the degree of host specificity and the co-phylogenetic hypothesis, as well as to further unravel the primate-pinworm evolutionary history puzzle.

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Three new species of pinworms from red howler monkeys are described.

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Phylogenetic analysis suggests the potential existence of cryptic species.

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Our results agree with pinworms of primates as highly host specific parasites.

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Trypanoxyuris species occurring in each howler species are not closely related.

Morphological comparison of genetically differentiated Polymorphus cf. minutus types

In the present study, we analyzed the morphology of three genetic types of the bird-infecting acanthocephalan Polymorphus cf. minutus (PspT1, PspT2, PspT3), mainly based on the cystacanth-stage obtained from amphipods (Gammarus fossarum, Gammarus pulex, Gammarus roeselii, Echinogammarus spp.). Males and females were pooled as there was no considerable difference between the sexes concerning the hook measurements. Additionally, we conducted a laboratory infection of one domestic duck for each Polymorphus type, to compare their performance and localization in this host species, and to obtain adult specimens for morphological comparison. The recovery rate from the ducks 4 weeks after infection was 16% for PspT1, 23.8% for PspT2, and 25% for PspT3. The adult worms were gravid, and the females contained mature eggs. Hook size did not differ considerably between cystacanths and adults of the respective type. The three Polymorphus types could be distinguished based on the cystacanth stage by a linear discriminant analysis that included hook measurements, proboscis length, proboscis width, and number of longitudinal hook rows and hooks per row. Furthermore, PspT3 was more different from PspT1 and PspT2 than the latter types from each other. Mainly the number of longitudinal hook rows differed in PspT3 from the existing descriptions of P. minutus (mainly 14 vs. mainly 16 rows). Potentially, PspT3 could be a non-indigenous parasite that was introduced with G. roeselii and that adapted to use the indigenous G. pulex as a host, while PspT2 might have been introduced to central Europe together with Echinogammarus spp.

An identity crisis in the Indo-Pacific: molecular exploration of the genus Koseiria (Digenea: Enenteridae)

We explore the growing issue of cryptic speciation in the Digenea through study of museum material and newly collected specimens consistent with the enenterid genus Koseiria from five species of the Kyphosidae and Chaetodontoplus meredithi Kuiter (Pomacanthidae) collected in the Indo-Pacific. We use an integrated approach, employing traditional morphometrics, principal components analysis (PCA), and molecular data (ITS2 and 28S rDNA). Our results support recombination of Koseiria allanwilliamsi Bray & Cribb, 2002 as Proenenterum allanwilliamsi (Bray & Cribb, 2002) n. comb. and transfer of Koseiria huxleyi Bray & Cribb, 2001 to a new genus as Enenterageitus huxleyi (Bray & Cribb, 2002) n. comb. Molecular data indicate the presence of four further species consistent with Koseiria, one from Western Australia (sequence data only) and three from eastern Australia. All three eastern Australian species are morphologically consistent with Koseiria xishaensis Gu & Shen, 1983, but distinct from all other previously described species. Although K. xishaensis has been reported from Australia, we conclude that the similarity of the present forms to the original description of K. xishaensis means records of this species from Japan, Palau and Australia are unreliable. Because the eastern Australian forms cannot be reliably ascribed to K. xishaensis, we describe Koseiria argalea n. sp., Koseiria laiphopharophora n. sp., and Koseiria pyknophora n. sp., following application of PCAs and iterative refinement of species concepts and type series. These analyses did not allow convincing identification hypotheses for all specimens examined. In this genus, both morphological and molecular data, together with reliable host identifications, are essential for species recognition, and thus we refrain from attempting to name samples lacking molecular data. The issues presented by these taxa encapsulate those of trematodes in the region as a whole. Many records require dramatically improved supporting data, leading to substantial uncertainly in the identification of this fauna.

Acanthocephalan infection patterns in amphipods: a reappraisal in the light of recently discovered host cryptic diversity

Amphipods are model species in studies of pervasive biological patterns such as sexual selection, size assortative pairing and parasite infection patterns. Cryptic diversity (i.e. morphologically identical but genetically divergent lineages) has recently been detected in several species. Potential effects of such hidden diversity on biological patterns remain unclear, but potentially significant, and beg the question of whether we have missed part of the picture by involuntarily overlooking the occurrence and effects of cryptic diversity on biological patterns documented by previous studies. Here we tested for potential effects of cryptic diversity on parasite infection patterns in amphipod populations and discuss the implications of our results in the context of previously documented host-parasite infection patterns, especially amphipod-acanthocephalan associations. We assessed infection levels (prevalence and abundance) of 3 acanthocephalan species (Pomphorhynchus laevis, P. tereticollis and Polymorphus minutus) among cryptic lineages of the Gammarus pulex/G. fossarum species complex and G. roeseli from sampling sites where they occur in sympatry. We also evaluated potential differences in parasite-induced mortality among host molecular operational taxonomic units (MOTUs)-parasite species combinations. Acanthocephalan prevalence, abundance and parasite-induced mortality varied widely among cryptic MOTUs and parasite species; infection patterns were more variable among MOTUs than sampling sites. Overall, cryptic diversity in amphipods strongly influenced apparent infection levels and parasite-induced mortality. Future research on species with cryptic diversity should account for potential effects on documented biological patterns. Results from previous studies may also need to be reassessed in light of cryptic diversity and its pervasive effects.

Spirorchiidiasis in marine turtles: the current state of knowledge

Blood flukes of the family Spirorchiidae are important disease agents in marine turtles. The family is near cosmopolitan in distribution. Twenty-nine marine species across 10 genera are currently recognized, but taxonomic problems remain and it is likely that more species will be discovered. Spirorchiids infect the circulatory system, where they and their eggs cause a range of inflammatory lesions. Infection is sometimes implicated in the death of the turtle. In some regions, prevalence in stranded turtles is close to 100%. Knowledge of life cycles, important for control and epidemiological studies, has proven elusive until recently, when the first intermediate host identifications were made. Recent molecular studies of eggs and adult worms indicate that a considerable level of intrageneric and intraspecific diversity exists. The characterization of this diversity is likely to be of importance in exploring parasite taxonomy and ecology, unravelling life cycles, identifying the differential pathogenicity of genotypes and species, and developing antemortem diagnostic tools, all of which are major priorities for future spirorchiid research. Diagnosis to date has been reliant on copromicroscopy or necropsy, which both have significant limitations. The current lack of reliable antemortem diagnostic options is a roadblock to determining the true prevalence and epidemiology of spirorchiidiasis and the development of effective treatment regimes.

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.

Taxonomic and geographic bias in the genetic study of helminth parasites

The use of genetic information is now fundamental in parasite taxonomy and systematics, for resolving parasite phylogenies, discovering cryptic species, and elucidating patterns of gene flow among parasite populations. The accumulation of available gene sequences per geographical area or per parasite taxonomic group is likely proportional to species richness, but not without some biases. Certain areas and certain taxonomic groups receive more research effort than others, possibly causing a deficit in the relative number of parasite species being characterized genetically in some areas or taxonomic groups. Here, we use data on the number of parasite records per country or helminth family from the London Natural History Museum host-parasite database, and matching data on the number of gene sequences available from the National Center for Biotechnology Information (NCBI) GenBank database, to determine how available gene sequences scale with species richness across countries or parasitic helminth families. Our quantitative analysis identified countries/regions of the world and helminth families that have received the most effort in genetic research. More importantly, it allowed us to generate lists (based on residuals from the statistical model) of the 20 countries/regions and the 20 helminth families with the largest deficit in available gene sequences relative to their helminth species richness. We propose these lists as useful guides toward future allocation of effort to maximise advances in parasite biodiscovery, systematics and population structure.

A closer look at the morphological and molecular diversity of Neoechinorhynchus (Acanthocephala) in Middle American cichlids (Osteichthyes: Cichlidae), with the description of a new species from Costa Rica

Neoechinorhynchus is one of the most speciose genera of acanthocephalans, with approximately 116 described species. A recent study, aimed at establishing the genetic diversity of Neoechinorhynchus in Middle American freshwater fishes, validated nine species molecularly and morphologically and revealed the existence of 10 putative candidate species. Neoechinorhynchus golvani, a parasite commonly found in cichlids throughout Middle America with an allegedly large intraspecific morphological variability, was found to represent a species complex; species delimitation methods uncovered three additional genetic lineages. Here, we re-analyse the morphological and molecular data for N. golvani species complex infecting cichlids in that geographical area. A multivariate analysis of variance (MANOVA) was conducted particularly for the length of apical, middle and posterior hooks of the species/lineages of Neoechinorhynchus in cichlids, revealing morphological variation in the length of apical hooks for Lineage 8, although no morphological distinction was observed for Lineages 9 and 10. A new concatenated phylogenetic analysis of one mitochondrial and two ribosomal DNA genes was used to further corroborate the species delimitation among lineages; Neoechinorhynchus Lineage 8 was found to be morphologically and genetically distinct from its sister taxa, N. golvani and other two undescribed genetic lineages, and is formally described as a new species. Neoechinorhynchus costarricense n. sp. is described from the intestines of eight species of cichlids in Costa Rica. The new species is distinguished from the other species/lineages of Neoechinorhynchus in cichlids mainly by the size of the apical hooks of the proboscis.

Diversity and phylogenetic relationships of European species of Crepidostomum Braun, 1900 (Trematoda: Allocreadiidae) based on rDNA, with special reference to Crepidostomum oschmarini Zhokhov & Pugacheva, 1998

BACKGROUND

Within the genus Crepidostomum Braun, 1900, identification of species and taxonomic decisions made only on the basis of adult morphology have resulted in great problems associated with evaluating actual diversity and validity of species. Life-cycle data, while equal in importance to adult characters, are scarce, controversial or incomplete for most Crepidostomum spp. In this study, rDNA sequences generated from adult and larval Crepidostomum spp. and some other allocreadiid species were analysed to reveal the diversity and phylogenetic relationships of the species and their host range. Detailed morphological description based on light microscopy, SEM tegumental surface topography and genetic data are provided for the poorly known trematode C. oschmarini Zhokhov & Pugacheva, 1998 found in the intestine of two teleost fish species, Barbatula barbatula (L.) and Cottus gobio L.

RESULTS

We characterized 27 isolates of adult and larval parasites. Based on newly obtained 28S and ITS1-5.8S-ITS2 rDNA sequences, new intermediate and final hosts were ascertained, and life-cycles clarified for some allocreadiids. New knowledge on the diversity and phylogenetic relationships of European Crepidostomum spp. was gained. The validity of C. oschmarini was verified based on comparative sequence analysis. Ophthalmoxiphidiocercariae of C. oschmarini were recorded in sphaeriid bivalves Pisidium (Euglesa) casertanum (Poli). Additionally, morphological differences between gravid specimens of C. oschmarini and other related species were observed.

CONCLUSIONS

Species of the Allocreadiidae parasitizing fishes in Europe are distributed among two monophyletic genera, Allocreadium and Bunodera, and two paraphyletic Crepidostomum clades. A complex of Crepidostomum metoecus (syn. C. nemachilus), C. oschmarini and Crepidostomum sp. 2 clustered in one clade, and a complex of C. farionis, Crepidostomum sp. 1 and, probably, C. wikgreni in the other. Molecular data indicated that C. oschmarini and Crepidostomum sp. 2 presumably have a wide geographical distribution in Europe. The new data provided evidence that Crepidostomum is a more diverse genus than can be judged from morphological data and host switching in this genus may occur independently of fish-host phylogeny.

Best practice guidelines for studies of parasite community ecology

In recent decades, parasite community ecology has produced hundreds of studies on an ever-growing number of host species, and developed into an active sub-discipline of parasitology. However, this growth has been characterized by a lack of standards in the practices used by researchers, with many common approaches being flawed, unjustified or misleading. Here, in the hope of promoting advances in the study of parasite community ecology, I identify some of the most common errors or weaknesses in past studies, and propose ten simple rules for best practice in the field. They cover issues including, among others, taxonomic resolution, proper and justifiable analytical methods, higher-level replication, controlling for sampling effort or species richness, accounting for spatial distances, using experimental approaches, and placing raw data in the public domain. While knowledge of parasite communities has expanded in breadth, with more and more host species being studied, true progress has been very limited with respect to our understanding of fundamental general processes shaping these communities. It is hoped that the guidelines presented here can direct researchers away from the entrenched use of certain approaches flawed in design, analysis or interpretation, by offering a more rigorous and standardized set of practices, and, hopefully, a way forward.

A hyper-diverse genus of acanthocephalans revealed by tree-based and non-tree-based species delimitation methods: Ten cryptic species of Neoechinorhynchus in Middle American freshwater fishes

The genus Neoechinorhynchus represents a hyper-diverse group of acanthocephalans, parasites of fresh and brackish water fish and freshwater turtles, with approximately 116 species described worldwide. Forty-nine species have been recorded in the Americas, nine of them in Middle America. Even though species delimitation methods using DNA sequences have been rarely used for parasitic helminths, the genetic library for species of Neoechinorhynchus has grown in the past few years, enhancing the possibility of using these methods for inferring evolutionary relationships and for establishing more robust species boundaries. In this study, we used non-tree-based and tree-based methods through a coalescent approach to explore the species limits of specimens of Neoechinorhynchus collected in 57 localities across Middle America. We sequenced a large number of individuals to build a comprehensive dataset for three genes: the mitochondrial cytochrome c oxidase subunit I (352 individuals), the internal transcribed spacers (330 individuals), and the D2 + D3 domains of the large subunit (278 individuals). Several species delimitation methods were implemented, i.e., Automatic Barcode Gap Discovery (ABGD), General Mixed Yule-Coalescent Model (GMYC), Bayesian species delimitation (BPP) and species tree (∗BEAST). Additionally, we conducted a detailed morphological study of the diagnostic traits associated with the proboscis of 184 males and 169 females. Overall, our analyses allowed us to validate nine nominal species of Neoechinorhynchus and to identify 10 additional genetic lineages herein regarded as candidate species. This unexpected genetic diversity and the lack of reliable morphological traits show that the genus Neoechinorhynchus includes a group of cryptic species, at least in Middle America.

Systematics and diversification of Anindobothrium Marques, Brooks & Lasso, 2001 (Eucestoda: Rhinebothriidea)

Tapeworms of the genus Anindobothrium Marques, Brooks & Lasso, 2001 are found in both marine and Neotropical freshwater stingrays of the family Potamotrygonidae. The patterns of host association within the genus support the most recent hypothesis about the history of diversification of potamotrygonids, which suggests that the ancestor of freshwater lineages of the Potamotrygonidae colonized South American river systems through marine incursion events. Despite the relevance of the genus Anindobothrium to understand the history of colonization and diversification of potamotrygonids, no additional efforts were done to better investigate the phylogenetic relationship of this taxon with other lineages of cestodes since its erection. This study is a result of recent collecting efforts to sample members of the genus in marine and freshwater potamotrygonids that enabled the most extensive documentation of the fauna of Anindobothrium parasitizing species of Styracura de Carvalho, Loboda & da Silva, Potamotrygon schroederi Fernández-Yépez, P. orbignyi (Castelnau) and P. yepezi Castex & Castello from six different countries, representing the eastern Pacific Ocean, Caribbean Sea, and river basins in South America (Rio Negro, Orinoco, and Maracaibo). The newly collected material provided additional specimens for morphological studies and molecular samples for subsequent phylogenetic analyses that allowed us to address the phylogenetic position of Anindobothrium and provide molecular and morphological evidence to recognize two additional species for the genus. The taxonomic actions that followed our analyses included the proposition of a new family, Anindobothriidae fam. n., to accommodate the genus Anindobothrium in the order Rhinebothriidea Healy, Caira, Jensen, Webster & Littlewood, 2009 and the description of two new species-one from the eastern Pacific Ocean, A. carrioni sp. n., and the other from the Caribbean Sea, A. inexpectatum sp. n. In addition, we also present a redescription of the type species of the genus, A. anacolum (Brooks, 1977) Marques, Brooks & Lasso, 2001, and of A. lisae Marques, Brooks & Lasso, 2001. Finally, we discuss the paleogeographical events mostly linked with the diversification of the genus and the protocols adopted to uncover cryptic diversity in Anindobothrium.

Molecular and morphological evidence for nine species in North American Australapatemon (Sudarikov, 1959): a phylogeny expansion with description of the zygocercous Australapatemon mclaughlini n. sp

Zygocercous (aggregating) cercarial larvae were recently discovered emerging from a physid snail during a molecular survey of cercariae from molluscs in lakes in central Alberta, Canada. This manuscript delves into the characterization of these cercariae through morphological and molecular techniques and provides the first genetic information for a zygocercous larval trematode. Analyses of cytochrome c oxidase I of mitochondrial DNA and two partial regions of nuclear ribosomal DNA sequences revealed the zygocercous cercariae to belong to the genus Australapatemon Sudarikov, 1959. Further analyses of sequences of Australapatemon burti (Miller, 1923), from cercariae and adults collected from across North America, indicate a complex of nine genetically-distinct lineages within this species, a surprising level of diversity. The zygocercous cercariae, along with adult worms collected from ducks in Manitoba, Canada, and from Mexico, represent one of these lineages, and are herein described as Australapatemon mclaughlini n. sp. Seven lineages cannot yet be identified, but one is tentatively identified as Australapatemon burti.