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The complete life cycle and developmental stages of the fluke, Metorchis ussuriensis sp. nov. (Trematoda, Platyhelminthes), are herein described. The results of the present experiments showed that, for flukes from the Primorsky Region in the Russian southern Far East, the first intermediate hosts are the snails Parafossarulus spiridonovi and Boreoelona ussuriensis, and the second intermediate hosts are freshwater fish, tadpoles, and snails. The definitive host in this experiment was Anas platyrhynchos dom. Morphometric parameters of M. ussuriensis sp. nov. demonstrate similarities with Metorchis taiwanensis, but the two species differ in the sizes of their bodies, sizes of suckers of adult worms, and sizes of cercariae, as well as respective positions of the finfold in cercariae. Phylogenetic reconstructions and genetic distances using the cox1 gene sequences support the conclusion that M. ussuriensis sp. nov. is well distinguished from all other species of the genus Metorchis, while sequences of internal transcribed spacers (ITS1 and ITS2) failed to separate M. ussuriensis sp. nov., Metorchis bilis, and Metorchis xanthosomus. In addition, we sequenced 1,402 bp of the 28SrRNA gene of M. ussuriensis sp. nov. being the first 28S sequences in the genus Metorchis. Comparison to other trematodes suggests that 28SrRNA could proof suitable for the differentiation of trematode species.

SummaryNew cercariae similar to those of Paracoenogonimus (Cyathocotylidae, Prohemistomatinae), have been found in gastropods Amuropaludina and Cipangopaludina from reservoirs of the Primorsky Region (southern Far East of Russia). Two species of freshwater fishes (Pseudorasbora parva and Perccottus glenii) were experimentally-infected second intermediate hosts. Adults reared in chicken, Gallus gallus dom. They belong to a new species, Paracoenogonimus ussuriensis sp. n. They differ from those of P. szidati (Anderson, 1914) and Mesostephanus appendiculatus (Ciurea, 1916) (=P. skworzowi (Petrov, 1950) sensu Sudarikov 1961) by the size of the body, pharynx and ovary and, further, from the latter by the size of the suckers and testes. Adults differ from P. ovatus Katsurada, 1914 by the smaller dimensions of the oral sucker, pharynx and testes, and also by the absence of anterior interval between vitelline follicles.

Four representatives of the genus Lecithaster and one representative of the genus Hysterolecithoides were found during investigation of the trematode fauna of fish species in Vietnamese, Japanese and eastern coastal waters of the Russian Far East. Based on morphometric data, adult trematodes from Vietnamese Strongylura strongylura and Russian Acanthogobius flavimanus were identified as Lecithaster confusus, trematodes from Vietnamese Hemirhamphus marginatus as L. sayori and from osmerid fishes as L. salmonis. Further, a single specimen of Lecithaster sp. and representatives of Hysterolecithoides epinepheli were found in Vietnamese Siganus fuscescens. Morphological and molecular data, including 18S ribosomal DNA (rDNA) V4 fragment, 28S rDNA D1-D3 fragment, internal transcribed spacers (ITS) and a mitochondrial COI gene fragment were analysed for Lecithaster spp. The results revealed that L. sayori and L. salmonis are not synonyms of L. stellatus and L. gibbosus, respectively, but that Hysterolecithoides frontilatus and H. guangdongensis are junior synonyms of H. epinepheli. The 28S-rDNA-based phylogenetic tree of Hemiuroidea showed a distinct position for the genus Lecithaster with internal differentiation into three subclades, including L. confusus, L. sayori and Lecithaster sp. within the first subclade, L. mugilis and L. sudzuhensis within the second subclade and L. salmonis and L. gibbosus within the third subclade. Bayesian phylogenetic reconstructions of Hemiuroidea showed four clades for members of Hemiuridae and Lecithasteridae. The first clade consisted of Hemiuridae representatives and the second clade represented the genus Lecithaster. The third clade included genera Aponurus and Lecithophyllum (Lecithasteridae) and the fourth clade combined members of lecithasterid Quadrifoliovariinae and Hysterolecithinae and hemiurid Opisthadeninae and Bunocotylidae with high statistical support.

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