The life cycle ofDiploproctodaeum arothroniBray and Nahhas, 1998 (Digenea: Lepocreadiidae), with a comment on the parasitic castration of its molluscan intermediate host

Lepocreadiidae (Trematoda) associated with gelatinous zooplankton (Cnidaria and Ctenophora) and fishes in Australian and Japanese waters

We examined gelatinous zooplankton from off eastern Australia for lepocreadiid trematode metacercariae. From 221 specimens of 17 species of cnidarian medusae and 218 specimens of four species of ctenophores, infections were found in seven cnidarian and two ctenophore species. Metacercariae were distinguished using cox1 mtDNA, ITS2 rDNA and morphology. We identified three species of Prodistomum Linton, 1910 [P. keyam Bray & Cribb, 1996, P. orientale (Layman, 1930), and Prodistomum Type 3], two species of Opechona Looss, 1907 [O. kahawai Bray & Cribb, 2003 and O. cf. olssoni], and Cephalolepidapedon saba Yamaguti, 1970. Two species were found in cnidarians and ctenophores, three only in cnidarians, and one only in a ctenophore. Three Australian fishes were identified as definitive hosts; four species were collected from Scomber australasicus and one each from Arripis trutta and Monodactylus argenteus. Transmission of trematodes to these fishes by ingestion of gelatinous zooplankton is plausible given their mid-water feeding habits, although such predation is rarely reported. Combined morphological and molecular analyses of adult trematodes identified two cox1 types for C. saba, three cox1 types and species of Opechona, and six cox1 types and five species of Prodistomum of which only two are identified to species. All three genera are widely distributed geographically and have unresolved taxonomic issues. Levels of distinction between the recognised species varied dramatically for morphology, the three molecular markers, and host distribution. Phylogenetic analysis of 28S rDNA data extends previous findings that species of Opechona and Prodistomum do not form monophyletic clades.

Evidence that host ecology drives first intermediate host use in the Didymozoidae (Trematoda: Hemiuroidea): an asexual infection in a vermetid (Gastropoda)

The Didymozoidae (Trematoda: Hemiuroidea) is among the most speciose trematode families, known from a wide range of marine teleost fishes. Despite their richness, however, didymozoid life cycles are unusually poorly known; only two first intermediate hosts are known, a marine bivalve (Anadara trapezia) and a pelagic gastropod (Firoloida desmarestia). This study uses multi-locus molecular sequence data to identify a novel first intermediate host for the family, a sessile gastropod of the genus Thylacodes Guettard (Vermetidae). The didymozoid infection is not identified to species but, based on molecular phylogenetic analyses, it is close to Saccularina magnacetabula Louvard et al., 2022, which uses a bivalve as a first intermediate host. The distribution of known first intermediate hosts of didymozoids (a bivalve, a holoplanktonic gastropod and a sessile gastropod that feeds with the use of mucus nets) suggests that first intermediate host use within the Didymozoidae has been opportunistically driven by the trophic ecology of potential mollusc hosts and has involved significant host-switching events.

Gastropod first intermediate hosts for two species of Monorchiidae Odhner, 1911 (Trematoda): I can't believe it's not bivalves!

The trematode superfamily Monorchioidea comprises three families of teleost parasites: the Monorchiidae Odhner, 1911, Lissorchiidae Magath, 1917, and Deropristidae Cable & Hunninen, 1942. All presently known lissorchiid and deropristid life cycles have gastropods as first intermediate hosts, whereas those of monorchiids involve bivalves. Here, we report an unexpected intermediate host for monorchiids; two species of Hurleytrematoides Yamaguti, 1954 use gastropods as first intermediate hosts. Sporocysts and cercariae were found infecting two species of the family Vermetidae, highly specialised sessile gastropods that form calcareous tubes, from two locations off the coast of Queensland, Australia. These intramolluscan infections broadly corresponded morphologically to those of known monorchiids in that the cercariae have a spinous tegument, oral and ventral suckers, a simple tail and distinct eye-spots. Given the simplified morphology of intramolluscan infections, genetic data provided a definitive identification. ITS2 rDNA and cox1 mtDNA sequence data from the gastropod infections were identical to two species of Hurleytrematoides, parasites of butterflyfishes (Chaetodontidae); Hurleytrematoides loi McNamara & Cribb, 2011 from Moreton Bay (south-eastern Queensland) and Heron Island (southern Great Barrier Reef) and Hurleytrematoides morandi McNamara & Cribb, 2011 from Heron Island. Notably, species of Hurleytrematoides are positioned relatively basal in the phylogeny of the Monorchiidae and are a sister lineage to that of species known to infect bivalves. Thus, the most parsimonious evolutionary hypothesis to explain infection of gastropods by these monorchiids is that basal monorchiids (in our analyses, species of Cableia Sogandares-Bernal, 1959, Helicometroides Yamaguti, 1934 and Hurleytrematoides) will all prove to infect gastropods, suggesting a single host switching event into bivalves for more derived monorchiids (17 other genera in our phylogenetic analyses). A less parsimonious hypothesis is that the infection of vermetids will prove to be restricted to species of Hurleytrematoides, as an isolated secondary recolonisation of gastropods from a bivalve-infecting lineage. Regardless of how their use arose, vermetids represent a dramatic host jump relative to the rest of the Monorchiidae, one potentially enabled by their specialised feeding biology.

Lepocreadiidae Odhner, 1905 and Aephnidiogenidae Yamaguti, 1934 (Digenea: Lepocreadioidea) of fishes from Moreton Bay, Queensland, Australia, with the erection of a new family and genus

Digeneans of the lepocreadioid families Lepocreadiidae Odhner, 1905 and Aephnidiogenidae Yamaguti, 1934 from Moreton Bay, off southern Queensland, Australia, are recorded, along with the erection of a new family, Gibsonivermidae. Molecular data were generated for all representatives of these families collected during this study and a phylogram for members of the superfamily was generated based on the partial 28S rDNA dataset, placing these species in context with those previously sequenced. This phylogenetic analysis demonstrates that the monotypic Gibsonivermis Bray, Cribb & Barker, 1997 is isolated from all other lepocreadioids and supports the erection of Gibsonivermidae n. fam., which is defined morphologically, based particularly on the uniquely elongated male terminal genitalia, the distribution of the uterus in the forebody and the presence of a uroproct. Mobahincia teirae n. g., n. sp. is reported from Platax teira (Forsskål) in Moreton Bay and off Heron Island and New Caledonia. Recognition of this new genus is based on molecular results and the combination of caeca abutting the posterior body wall and the lack of an anterior body scoop or flanges. The following lepocreadioid species are reported from Moreton Bay for the first time: Bianium arabicum Sey, 1996 in Lagocephalus lunaris (Bloch & Schneider), Diploproctodaeum cf. monstrosum Bray, Cribb & Justine, 2010 in Arothron hispidus (Linnaeus), Multitestis magnacetabulum Mamaev, 1970 and Neomultitestis aspidogastriformis Bray & Cribb, 2003 in Platax teira and Opechona austrobacillaris Bray & Cribb, 1998 in Pomatomus saltatrix (Linnaeus). Bianium plicitum (Linton, 1928) is reported from Torquigener squamicauda (Ogilby) for the first time. Sequences of newly collected specimens of Austroholorchis sprenti (Gibson, 1987) indicate that the species forms a clade with other members of the Aephnidiogenidae, agreeing with its morphology. The phylogenetic status of all newly sequenced species is discussed.

Survey of trematodes in intertidal snails from Patagonia, Argentina: new larval forms and diversity assessment

Larval trematodes are the main parasites of snails, and they play a crucial role because they usually castrate their snail hosts and can thus alter their population and community dynamics. This study involved a survey of seven gastropod species (Crepipatella dilatata, Fissurella radiosa, Nacella magellanica, Pareuthria fuscata, Siphonaria lessonii, S. lateralis and Trophon geversianus) parasitized by 12 trematode species (one hemiurid, one gymnophallid, two lepocreadiids, two microphallids, one notocotylid, two renicolids, one philophtalmid, one schistosomatid and one zoogonid) from southern Patagonia (47°S, 65°W), Argentina. Only F. radiosa was free of parasites. The study included the description of five new larvae, based on morphological and molecular information, and a comparison of the parasite diversity with that of a northern locality (42°S, 64°W), characterized by a lower mollusc diversity. Species richness and diversity of parasites were higher in the southern site. This suggests a correlation between the level of parasitism and the diversity of molluscs (first intermediate hosts), which is higher at the high-latitude site and seems to attract shorebirds, which disperse the digenean eggs and facilitate the completion of their life cycles. These results support the notion that parasitism is influenced by large-scale factors such as biogeographical patterns, and small-scale factors such as diversity or abundance of intermediate and definitive hosts.

The effect of trematode parthenites on the individual fecundity of Bithynia troscheli (Prosobranchia: Bithyniidae)

We studied the long-term infection of Bithynia troscheli (Paasch, 1842) snails with trematodes and estimated the influence of trematode parthenites on the individual fecundity of female snails from the Kargat River (Chany Lake, Russia). The prevalence of B. troscheli females infected by trematode parthenites varied from 7.12% to 17.35% in different years. Eleven redioid species from 5 families and 9 sporocystoid species from 5 families of trematodes were found during this investigation. Snails' fecundity was analysed in relation to the type of infection (redioid or sporocystoid species). Fecund females of B. troscheli were uninfected or they had pre-patent infections from 5 families of trematodes (Psilostomidae, Notocotylidae, Prosthogonimidae, Lecithodendriidae, and Cyathocotylidae). 89.9% of infected B. troscheli females were infertile. Moreover, 13.57% and 1.29% egg capsules (of infected and uninfected females, respectively) were without embryos (χ² = 323.24, p<0.001). The results of the two-way analysis of variability confirmed that trematode parthenites influenced significantly the individual fecundity of B. troscheli. The age of B. troscheli females alone did not alter the individual fecundity, however age in combination with infection by tremathode parthenites influenced significantly the number of normal egg capsules. We also found that under unfavorable environmental conditions the proportion of fertile females increased by 23.7% among uninfected and by 219% among infected females.

The life cycle of Gyliauchen volubilis Nagaty, 1956 (Digenea: Gyliauchenidae) from the Red Sea

Although nothing is known about gyliauchenid life cycles, molecular phylogenetic studies have placed the Gyliauchenidae Fukui, 1929 close to the Lepocreadiidae Odhner, 1905. The gyliauchenid Gyliauchen volubilis Nagaty, 1956 was found in the intestine of its type-host, Siganus rivulatus, a siganid fish permanently resident in a lagoon within the mangrove swamps on the Egyptian coast of the Gulf of Aqaba. Larval forms of this trematode (mother sporocysts, rediae and cercariae) were found in the gonads and digestive gland of Clypeomorus clypeomorus (Gastropoda: Cerithiidae), a common snail in the same lagoon. So, this life cycle of G. volubilis was elucidated under natural conditions: eggs are directly ingested by the snail; mother sporocysts and rediae reach their maturity 3-6 and 11-13 weeks post-infection; rediae contain 23-29 developing cercariae; fully developed cercariae are gymnocephalus, without penetration glands, emerge from the snail during the night 16-18 weeks post-infection and rapidly encyst on aquatic vegetation (no second intermediate host); encysted metacercariae are not progenetic; 4-day-old metacercariae encysted on filamentous algae fed to S. rivulatus developed into fully mature worms 6-8 weeks post-infection. The cycle was completed in about 26 weeks and followed one of the three known patterns of lepocreadiid life cycles, and except for the gymnocephalus cercariae, the other larval stages are very similar to those of lepocreadiids. Generally, the life cycle of G. volubilis implicitly supports the phylogenetic relationship of Gyliauchenidae and Lepocreadiidae inferred from molecular phylogenetic studies.

Taxonomic approaches to and interpretation of host specificity of trematodes of fishes: lessons from the Great Barrier Reef

The taxonomy of trematodes of Great Barrier Reef (GBR) fishes has been studied in some detail for over 20 years. Understanding of the fauna has been informed iteratively by approaches to sampling, understanding of morphology, the advent of molecular methodology and a feed-back loop from the emergent understanding of host specificity. Here we analyse 658 host-parasite combinations for 290 trematode species, 152 genera and 28 families from GBR fishes. These are reported from 8 orders, 38 families, 117 genera and 243 species of fishes. Of the 290 species, only 4 (1·4%) have been reported from more than one order of fishes and just 23 (7·9%) infect more than one family; 77·9% of species are known from only one genus, and 60% from only one species of fish. Molecular studies have revealed several complexes of cryptic species and others are suspected; we conclude that no euryxenous host distribution should be accepted on the basis of morphology only. The occurrence of individual trematode species in potential hosts is patchy and difficult to predict reliably a priori or explain convincingly a posteriori. These observations point to the need for a vigorous iterative interaction between the accretion of host specificity data and its interpretation.

The phylogeny of the Lepocreadioidea (Platyhelminthes, Digenea) inferred from nuclear and mitochondrial genes: Implications for their systematics and evolution

The phylogenetic relationships of representative species of the superfamily Lepocreadioidea were assessed using partial lsrDNA and nad1 sequences. Forty-two members of the family Lepocreadiidae, six putative members of the Enenteridae, six gyliauchenid species and one Gorgocephalidae, were studied along with 22 species representing 8 families. The Lepocreadioidea is found to be monophyletic, except for the two species of the putative enenterid genus Cadenatella, which are found to be only distantly related to the lepocreadioids. The Lepocreadioidea is formed of five clades in a polytomy, the Gorgocephalidae, a clade containing the Enenteridae and Gyliauchenidae, a small clade of atypical lepocreadiines and the deep-sea lepidapedine lepocreadiids, a small clade consisting of a freshwater form and a group of shallow-water putative lepidapedines and the final clade includes the remaining lepocreadiids. Thus, the generally accepted concept of the Lepocreadiidae is polyphyletic. The Enenteridae (minus Cadenatella) and the Gyliauchenidae are jointly and individually monophyletic, and are sister groups. The nad1 gene on its own places a deep-sea lepocreadiine with the deep-sea lepidapedines, whereas lsrDNA, combined sequences and morphology place this deep-sea lepocreadiine within a group of typical lepocreadiids. It could not be demonstrated that a significant proportion of sites in the nad1 gene evolved under positive selection; this anomalous relationship therefore remains unexplained. Most deep-sea species are in a monophyletic group, a few of which also occur in shallow waters, retaining some characters of the deep-sea clade. Many lepocreadioid species infect herbivorous fish, and it may be that the recently discovered life-cycle involving a bivalve first intermediate host and metacercariae encysted on vegetation is a common life-cycle pattern. The host relationships show no indication of co-speciation, although the host-spectrums exhibited are not random, with related worms tending to utilize related hosts. There are, however, many exceptions. Morphology is found to be of limited value in indicating higher level relationships. For example, even with the benefit of hindsight the gyliauchenids show little morphological similarity to their sister group, the Enenteridae.

Trematodes from Red Sea fishes: On the validity of Helicometra marmoratae Nagaty et Abdel-Aal, 1962 and the description of H. aegyptense sp. nov. (Opecoelidae Ozaki, 1925)

Specimens of the fishes Siganus spinus L. (Siganidae) and Sargocentron spiniferum Forsskål (Holocentridae) were caught in the Red Sea off the coast of Sharm El-Sheikh, South Sinai, Egypt. Twenty-three (46%) and 12 (20%) of these fishes, respectively, were found to harbour intestinal trematodes. Siganus spinus was parasitized by Helicometra marmoratae Nagaty et Abdel-Aal, 1962 (Opecoelidae Ozaki, 1925) and S. spiniferum by Helicometra aegyptense sp. nov. (Opecoelidae Ozaki, 1925). Based on a large number of specimens collected from its type host and locality, H. marmoratae is broadly redescribed for the first time, and its type specimen is re-examined. The validity of this species is discussed and considered a new synonym of H. fasciata (Rudolphi, 1819) Odhner, 1902 (type species of the genus). Helicometra aegyptense sp. nov. is similar to H. equilata (Manter, 1933) Siddiqi et Cable, 1960, H. nasae Nagaty et Abdel-Aal, 1962, H. pteroisi (Gupta, 1956) Fischthal et Kuntz, 1965 and H. interrupta Hassanine, 2005 in having a short forebody and a long cirrus sac extending posterior to the ventral sucker, but differs significantly from them or unique in having a distinctly elongate pharynx, a larger sucker ratio and vitelline follicles grouped in small clusters arranged in two lateral rows on each side of the body. The valid species of Helicometra are listed in four principal groups.