Molecular and morphological screening of Podocotyle spp. (Trematoda: Opecoelidae) sheds light on their diversity in Northwest Pacific and eastern European Arctic

Podocotyle is a genus of marine opecoelid digeneans that parasitize a wide variety of fish as adults. We present the first phylogenetic analysis of several Podocotyle isolates using nuclear 28S rDNA and mitochondrial cox1 DNA regions. New sequences were obtained for Podocotyle specimens from fish caught in the Sea of Okhotsk and the White Sea. Based on morphological and molecular data, eight Podocotyle lineages of species rank were revealed. However, this diversity is poorly formalized within the current taxonomic model of the genus. As a result, we identified Podocotyle cf. angulata, Podocotyle cf. atomon, Podocotyle cf. reflexa, Podocotyle atomon of Sokolov et al., 2019, Podocotyle sp. of Denisova et al., 2023, Podocotyle sp. 1, Podocotyle sp. 2 and Podocotyle sp. 3. We also highlight the unresolved question of the life cycles of representatives of Podocotyle whose intramolluscan stages parasitize the intertidal snails Littorina spp.

Molecular characterisation of three species of Coitocaecum (Digenea: Opecoelidae) infecting Clinus superciliosus (Clinidae) in South Africa, with description of Coitocaecum brayi sp. n

The genus Coitocaecum Nicoll, 1915 is part of the most speciose digenean family, the Opecoelidae Ozaki, 1925, which is found globally in both freshwater and marine fishes. Fifteen opecoelid species have been reported from marine fishes in South Africa, yet only one species of Coitocaecum has been described from this region: Coitocaecum capense Bray, 1987. During an explorative study of the digeneans of the endemic, intertidal fish Clinus superciliosus (Linnaeus) from the Saldanha Bay area, Cape Town harbour, Hermanus, the Tsitsikamma section of the Garden Route National Park and Chintsa East in South Africa, a total of three distinct species of Coitocaecum were identified based on morphological and molecular (28S rDNA, ITS1-5.8S-ITS2 rDNA and COI mtDNA) data: the previously mentioned C. capense, Coitocaecum brayi sp. n. and a third, unnamed species. We provide the first molecular characterisation of species of Coitocaecum from South Africa, accompanied by detailed morphological descriptions. This study illustrates the importance of an integrated taxonomic approach, especially when studying species with similar morphology. These findings further emphasise the lack of information on the true diversity and molecular data for trematodes of marine fishes in South Africa, creating a great capacity for future explorative taxonomic studies and highlighting the use of intertidal areas for conducting such research.

A Review of Sperm Ultrastructural Characters in the Opecoelidae (Digenea) and Their Phylogenetic Implications, with New Data on Peracreadium characis, a Parasite of Diplodus puntazzo in Tunisia

The spermatozoon ultrastructure of Peracreadium characis (Stossich, 1886) (Digenea: Opecoelidae), an intestinal parasite of the sheephead bream Diplodus puntazzo (Walbaum, 1792) (Sparidae), is described by means of transmission electron microscopy (TEM). The mature spermatozoon possesses two axonemes of the 9+'1' trepaxonematan pattern, an anterior electron-dense material, two mitochondria, a nucleus and parallel cortical microtubules distributed in two bundles. The absence of external ornamentation of the plasma membrane and spine-like bodies are the noteworthy characters that distinguish the spermatozoon of P. characis from those of most opecoelids. In fact, only Helicometra fasciata lacks external ornamentation in the spermatozoon. A comparative study with the remaining opecoelids described so far reveals similarities in the ultrastructural organization of their sperm cells. In addition, the current data on sperm ultrastructure in species of the recognized opecoelid subfamilies are compared, namely the Hamacreadiinae, Helicometrinae, Opecoelinae, Opistholebetinae and Plagioporinae.

A Phylogenetic Re-Evaluation of the Stenakrine Opecoelids (Trematoda, Digenea: Opecoeloidea) with Some Taxonomic Novelties

The Opecoeloidea is a large group of xiphidiate digeneans parasitizing marine and freshwater fishes. According to the current taxonomic model, this superfamily contains only one family with numerous subfamilies. This study is devoted to the members of the Stenakrinae. Based on phylogenetic analysis of concatenated sequences of 18S and 28S rRNA genes of stenakrine opecoelids Caudotestis dobrovolski, C. cf. dobrovolski, Hexagrammia zhukovi, Stenakron vetustum, as well as the deep-sea xiphidiate digenean Zdzitowieckitrema incognitum, which so far has had an ambiguous phylogenetic status, we erect a new opecoeloid family, the Zdzitowieckitrematidae fam. nov. The genera Holsworthotrema and Scorpidotrema are removed from the Stenakrinae to the Scorpidotrematinae subfam. nov. within the Opecoelidae. We also remove the Stenakrinae from the Opecoelidae and recognize it as a separate family within the Opecoeloidea. The Stenakridae stat. nov. is a sister taxon to a well-supported Opecoelidae. The Zdzitowieckitrematidae occupies a sister position relative to the stenakrids and the opecoelids taken together. All three families are clearly phylogenetically distinct, however convincing morphological differences are revealed only between the Zdzitowieckitrematidae and the Stenakridae and between the Opecoelidae and the Stenakridae.

Enenterum kyphosi Yamaguti, 1970 and Enenterum petrae n. sp. (Digenea: Enenteridae) from kyphosid fishes (Centrarchiformes: Kyphosidae) collected in marine waters off eastern Australia

Species of the digenean genus Enenterum Linton, 1910 (Lepocreadioidea: Enenteridae) are characterised primarily by their elaborate oral suckers, which are divided into varying numbers of anteriorly directed lobes, and their host-restriction to herbivorous marine fishes of the family Kyphosidae. We describe Enenterum petrae n. sp. from the brassy chub Kyphosus vaigiensis (Quoy & Gaimard) collected off Lizard Island, Great Barrier Reef, Queensland, Australia. Enenterum petrae n. sp. is readily differentiated from congeners by its unique oral sucker morphology, in having a minute pharynx, and the combination of a genital cap and accessory sucker. We also provide the first record of Enenterum kyphosi Yamaguti, 1970 from Australia based on material obtained from the blue sea chub Kyphosus cinerascens (Forsskål) collected off Lizard Island and North Stradbroke Island, Queensland. Morphologically, our specimens of E. kyphosi agree closely with descriptions of this species from Hawaii and South Africa, and despite lack of molecular data from outside of Australian waters, we consider all three reports to represent a single, widespread species. The first ITS2 and COI mtDNA gene sequences for species of Enenterum are provided and molecular phylogenetic analyses of 28S rDNA gene sequences place these species in a strongly-supported clade with the type-species of the genus, Enenterum aureum Linton, 1910. The oral suckers of both E. kyphosi and E. petrae n. sp. can be interpreted as having varying numbers of lobes depending on the particular specimen and how the division between lobes is defined. Scanning electron microscopical images improves understanding of the morphology of the enenterid oral sucker, and permits speculation regarding the evolutionary history leading to its specialisation in this lineage.

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

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

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.

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.

A new genus of Bucephalidae Poche, 1907 (Trematoda: Digenea) for three new species infecting the yellowtail pike, Sphyraena obtusata Cuvier (Sphyraenidae), from Moreton Bay, Queensland, Australia

Three new species of the family Bucephalidae Poche, 1907 (Trematoda: Digenea) are described from the yellowtail pike, Sphyraena obtusata Cuvier (Sphyraenidae), from Moreton Bay, Queensland, Australia. The three species are morphologically consistent with the present broad concept of the genus Bucephalus Baer, 1827, but significant phylogenetic and ecological differences relative to the type-species of Bucephalus require the proposal of a new genus. Aenigmatrema n. g. is proposed for A. undecimtentaculatum n. sp. (type-species), A. inopinatum n. sp. and A. grandiovum n. sp. In addition, based on morphological, ecological and biogeographical similarities, we recombine two existing species of Bucephalus as Aenigmatrema kaku (Yamaguti, 1970) n. comb. and Aenigmatrema sphyraenae (Yamaguti, 1952) n. comb. Although the three species described in this study are extremely morphologically similar, they can be differentiated from each other, and from A. kaku and A. sphyraenae, morphometrically on the basis of egg size, tentacle number and a combination of the caecum and vitelline field lengths. Complete ITS2 rDNA, partial 28S rDNA and partial cox1 mtDNA sequence data were generated for the three new species, which formed a well-supported clade in all 28S phylogenetic analyses. An expanded phylogenetic tree for the subfamily Bucephalinae Poche, 1907 is presented, demonstrating unresolved issues with the morphology-based taxonomy of the subfamily. The three largest genera, Bucephalus, Rhipidocotyle Diesing, 1858 and Prosorhynchoides Dollfus, 1929 remain extensively polyphyletic, indicating the need for significant further systematic revision.

Phylogenetic position of deep-sea opecoelid digenean Tellervotrema beringi (Mamaev, 1965) (Trematoda: Opecoelidae) based on novel genetic data

Adult opecoelid digeneans consistent with Tellervotrema beringi (Mamaev, 1965) were found in the intestine of the deep-water fish Antimora microlepis Bean collected off the northern Kuril Islands (Russia). Earlier, T. beringi was recorded only in macrourid fish. This is the first record of a species of Tellervotrema Gibson & Bray, 1982 from fishes of the family Moridae (Gadiformes). A partial 28S rRNA gene sequence was obtained for this species and was compared with the available sequences on GenBank from other opecoelids. The phylogenetic analysis did not support the current taxonomic hypothesis that the genus Tellervotrema belongs to the subfamily Podocotylinae Dollfus, 1959. Our results demonstrated that species of this genus formed a clade with those of the genera Mesobathylebouria Martin, Huston, Cutmore & Cribb, 2019 and Abyssopedunculus Martin, Huston, Cutmore & Cribb, 2019. The morphological synapomorphies of the Tellervotrema + (Mesobathylebouria + Abyssopedunculus) clade are currently unclear, and this group does not have an adequate subfamilial classification.

High parasite diversity in a neglected host: larval trematodes of Bithynia tentaculata in Central Europe

Bithynids snails are a widespread group of molluscs in European freshwater systems. However, not much information is available on trematode communities from molluscs of this family. Here, we investigate the trematode diversity of Bithynia tentaculata, based on molecular and morphological data. A total of 682 snails from the rivers Lippe and Rhine in North Rhine-Westphalia, Germany, and 121 B. tentaculata from Curonian Lagoon, Lithuania were screened for infections with digeneans. In total, B. tentaculata showed a trematode prevalence of 12.9% and 14%, respectively. The phylogenetic analyses based on 55 novel sequences for 36 isolates demonstrated a high diversity of digeneans. Analyses of the molecular and morphological data revealed a species-rich trematode fauna, comprising 20 species, belonging to ten families. Interestingly, the larval trematode community of B. tentaculata shows little overlap with the well-studied trematode fauna of lymnaeids and planorbids, and some of the detected species (Echinochasmus beleocephalus and E. coaxatus) constitute first records for B. tentaculata in Central Europe. Our study revealed an abundant, diverse and distinct trematode fauna in B. tentaculata, which highlights the need for further research on this so far understudied host-parasite system. Therefore, we might currently be underestimating the ecological roles of several parasite communities of non-pulmonate snail host families in European fresh waters.

Digenean parasites of deep-sea teleosts: A progress report

The developments in the study of digeneans of deep-sea fish in the 21st Century are documented and discussed. Most recent work has been on the bathyal fauna (i.e. 1,000m-2,999 m depth), with virtually nothing on the abyssal fauna (i.e. deeper than 3,000 m). The one study on hydrothermal vent digeneans has indicated that these regions probably harbour a distinctive fauna. The demarcation of the deep-sea fauna is blurred at the poles, where the cold-adapted fauna appears similar to the shallower bathyal fauna. The abyssal fauna, however, appears distinct, possibly due to adaptations to variable or ultra-high pressures. The digenean fauna of bathypelagic fishes is depauperate. Recent phylogenetic studies reinforce the view that the typical deep-sea fauna has radiated in the deep-sea. Encroachment into the deep from shallow water is relatively rare. Overall, the digenean fauna in the deep-sea is distinctly less diverse that the equivalent fauna in shallow waters. A major conclusion is that our understanding of the deep-sea digenean fauna is poor, and that much further work over a much wider area is needed.

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Sampling effort in the deep-sea is poor and restricted to a few areas.

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Deep-sea digenean diversity is distinctly lower than in shallow water.

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Much of the deep-sea digenean fauna appears to have radiated in the deep-sea.

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Encroachment into the very deep sea from shallow waters is rare.

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Adaptation to cold temperature appears to be a major factor in deep-sea digeneans.

A new subfamily for a clade of opecoelids (Trematoda: Digenea) exploiting marine fishes as second-intermediate hosts, with the first report of opecoelid metacercariae from an elasmobranch

Metacercariae of trematodes belonging to the family Opecoelidae were collected from small fishes of the Great Barrier Reef: a blenniid, two gobiids, two labrids, three pomacentrids, a monacanthid, an ostraciid and the epaulette shark, Hemiscyllium ocellatum. Sequences of the second internal transcribed spacer region (ITS2) of ribosomal DNA were generated from these metacercariae in an attempt to match them with adult worms. Three species of Allopodocotyle (Allopodocotyle epinepheli, Allopodocotyle heronensis and an unidentified species), two unidentified species of Hamacreadium and Pacificreadium serrani were detected. Among the Opecoelidae, these species all resolve to a single, phylogenetically and somewhat morphologically distinct clade. Species of this clade are the only known marine opecoelids to exploit fishes as second-intermediate hosts. The clade is proposed to warrant a new subfamily, the Hamacreadiinae subfam. nov. It includes Allopodocotyle, Bentholebouria, Cainocreadium, Choanotrema, Hamacreadium, Pacificreadium, Paraplagioporus, Pedunculacetabulum and Podocotyloides.

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.

Podocotyle atomon (Trematoda: Digenea) impacts reproductive behaviour, survival and physiology in Gammarus zaddachi (Amphipoda)

The Trematoda are a group of phylogenetically diverse metazoan parasites that exhibit complex life cycles that often pass through invertebrate and vertebrate hosts. Some trematodes influence their host's behaviour to benefit transmission. Their parasitic influence may impact host population size by inhibiting an individual's reproductive capacity. We assessed the impact of infection by Podocotyle atomon on the reproductive behaviour and fecundity of its amphipod intermediate host, Gammarus zaddachi, using laboratory and field studies. Parasite prevalence was high in the field, with males more likely to be infected (prevalence in males 64%, in females 39%). Males also suffered a higher parasite burden than females. Infected females were less active, but we found no evidence for a reduction in female reproductive success. Infected females also had comparable pairing success to uninfected females. In males, infection reduced survival and fecundity, with mortality being highest, and sperm numbers lowest, in heavily infected individuals. Trematode parasites are sometimes associated with altered host fecundity, but studies often lack the relevant experimental data to explore the evolution of the trait. We discuss this among information specific to the effect of P. atomon infection in G. zaddachi.