The Pseudoplagioporinae, a new subfamily in the Opecoelidae Ozaki, 1925 (Trematoda) for a small clade parasitizing mainly lethrinid fishes, with three new species

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.

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.

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.

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.