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New morphological and molecular data were generated for trematodes recovered from the intestines of the fish Pseudaspius hakonensis from two locations in the south of the Russian Far East. Morphologically, these trematodes are identical to Pseudozoogonoides ugui (Microphalloidea: Zoogonidae) from Japan. According to results of phylogenetic analysis based on 28S rDNA sequence data, P. ugui was closely related to Zoogonoides viviparus , and P. subaequiporus appears as a sister taxon to these two species. Genetic distance values, calculated based on both 28S rDNA and ITS2 rDNA, between P. ugui and Z. viviparus represents an interspecific differentiation level. Our results have an ambiguous explanation, indicating that the implication of the presence of one or two compact vitellarial aggregations for the differentiation of Zoogonoides and Pseudozoogonoides should be reconsidered or that our results open up the question of the taxonomical status of trematodes previously denoted as Z. viviparus and P. subaequiporus.

Adult trematodes of the genus Metorchis were found in the gallbladders of ducklings that had been experimentally fed freshwater fishes of the genera Rhynchocypris and Rhodeus that were naturally infected by Metorchis metacercariae. Some of the trematodes were identified as Metorchis ussuriensis , whose morphology of developmental stages and molecular data had previously been described in detail. The other trematodes were confirmed as species Metorchis butoridi on the basis of morphological features: subterminal oral sucker, vitelline follicles with interrupted bands, and rosette-shaped testes. An analysis of phylogenetic relationships within Opisthorchiidae using nuclear and mitochondrial markers confirmed that the obtained trematodes were actually from the genus Metorchis . The morphological and molecular features indicated that a number of trematodes found in East Asia and described as Metorchis orientalis belong to M. butoridi . Also, the M. orientalis individuals from Europe are, in fact, representatives of another Metorchis species.

Adult individuals of Echinochasmus pseudobeleocephalus n. sp. were obtained during an experimental study on trematodes' life cycle. An analysis of the morphometric characteristics of the developmental stages and involvement of first intermediate hosts, snails of the genus Boreoelona , in their life cycle, revealed the identity of the obtained trematodes to the European species Echinochasmus beleocephalus previously discovered in the south of the Russian Far East. However, an analysis of molecular data, in particular sequences of the 28S rRNA gene, showed that the Far Eastern trematodes examined do not belong to European E. beleocephalus despite their morphological similarities. An analysis of phylogenetic relationships within the family Echinochasmidae supported the status of E. pseudobeleocephalus n. sp. as an independent species. Our new data confirmed that the individuals attributed to Echinochasmus can be subdivided into two groups on the basis of the number of head-collar spines and the tail length in cercariae on an intergeneric level.

Furcocercariae of the genus Neodiplostomum Railliet, 1919 (Diplostomidae Poirier, 1886) were found in freshwater snails Helicorbis sujfunensis Starobogatov, 1957 (Planorbidae Rafinesque, 1815) collected from three localities in the Russian southern Far East. For the trematodes from each locality, frogs played the role of the second intermediate host, and rats were the definitive host. Chickens were insusceptible to infection. The morphological and molecular data obtained for these trematodes indicated they were representatives of the same species. The experimentally-derived adult individuals were morphometrically similar to the East Asian Neodiplostomum seoulense (Seo, Rim, Lee, 1964), Neodiplostomum oriolinum Oschmarin, 1963, Neodiplostomum leei Chai and Shin, 2002, and Neodiplostomum boryongense Shin et al., 2008. Analysis of available data on the life cycle, developmental stage morphology, and molecular genetic characteristics of East Asian Neodiplostomum revealed a lack of information for objective assessment of the species status of neodiplostomula found in the East Asia region. Based on the considerations above and the data for the cox1 marker, we named the trematode Neodiplostomum cf. seoulense (Seo, Rim, Lee, 1964) sensu Pyo et al. , 2014. In a phylogenetic reconstruction based on nuclear and mitochondrial markers, neodiplostomulas clustered into geographically related groups: South American, North American, European, and East Asian, with the former occupying an external position in the tree, which may indicate South America as a center of Neodiplostomum speciation.

Morphological and genetic data have been obtained for five new East Asian species of the Psilostomatidae. The life cycle of Psilotrema limosum n. sp. was carried out using Parafossarulus manchouricus as the first intermediate host. Compared with the East Asian species of the genus, these worms differ in morphometric characters in both the cercarial and adult stages. Its validity was also confirmed by the 28S rRNA gene data. Data on the life cycle and morphology of developmental stages of Sphaeridiotrema ussuriensis n. sp. and Sphaeridiotrema aziaticus n. sp. were also obtained. Cercariae of these species are found in Parafossarulus and Boreoelona snails, respectively. Sphaeridiotrema ussuriensis n. sp., like Sphaeridiotrema monorchis in China, has one testis, while S. aziaticus n. sp. has two testes. In addition, S. monorchis from Vietnam and Sphaeridiotrema spinoacetabulum from the Russian southern Far East are justified as belonging to the new species named Sphaeridiotrema vietnamensis n. sp. and Sphaeridiotrema pyriforme n. sp., respectively. This proposition is based on the morphology of developmental stages, the list of the first intermediate hosts and the 28S rRNA gene data. Analysis of the phylogenetic relationships within Psilostomatidae revealed three clusters, including taxa with different life cycles strategies. Sphaeridiotrema was also divided into two groups, which combine species according to their intermediate hosts and geographical localisation., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In this study we described two new trematode species, Lecithostaphylus halongi n. sp. (Zoogonidae, Lecithostaphylinae) and Gymnotergestia strongyluri n. sp. (Fellodistomidae, Tergestiinae), on the basis of morphological and molecular data. Adult worms of these two species were collected from, respectively, Hemiramphus spp. (Hemiramphidae) and Strongylura strongylura (Belonidae) caught in the coastal waters of Vietnam. Adult worms of L. halongi n. sp. are morphologically close to Lecithostaphylus gibsoni Cribb, Bray & Barker, 1992 ex Abudefduf whitleyi from Heron Island and Lecithostaphylus depauperati Yamaguti, 1970 ex Hemiramphus depauperatus from Hawaii, but differ from these species in having a larger cirrus sac and a different arrangement of vitelline fields. They also differ from Lecithostaphylus brayi Cabañas-Granillo, Solórzano-García, Mendoza-Garfias & Pérez-Ponce de León, 2020 in the 28S ribosomal DNA (rDNA) sequence data at the interspecific level. Adult worms of G. strongyluri n. sp. ex S. strongylura are morphologically similar to Gymnotergestia chaetodipteri, the only previously known species of this genus, described from Chaetodipterus faber in Jamaica. The new species differs from G. chaetodipteri in body shape, testicular arrangement and the size of the pharynx and eggs. The 28S rDNA-based phylogenetic analysis indicates that G. strongyluri n. sp. is closely related to Tergestia spp., rendering Tergestia paraphyletic. Genetic divergence values between G. strongyluri n. sp. and Tergestia spp. are similar to those among species in the genera Tergestia, Steringophorus and Proctoeces. Our molecular results indicate that G. strongyluri n. sp. and Tergestia spp. may belong the same genus, but additional molecular data are needed for the final conclusion.

New data have been obtained for three representatives of Exorchis; Exorchis convictus sp. n., Exorchis oviformis and Exorchis sp., from fish in the East-Asian region. For the first time, based on combined sequences of the ITS2 rDNA region and the 28S rRNA gene, Exorchis is confirmed to belong Cryptogonimidae. Based on analysis of a mitochondrial marker (cox1), the ‘Japanese’ and ‘Russian’ haplogroups are identified for E. oviformis isolated from Silurus asotus. One specimen of E. oviformis obtained in Japan is identical to the ‘Russian’ haplotype. Haplotype patterns are also observed for metacercariae of Exorchis sp. from Tanakia lanceolata and Carassius sp. fish in Kyushu Island (Japan).

Life cycles, and morphological and molecular data were obtained for Echinostoma chankensis nom. nov., Echinostoma cinetorchis, Echinostoma miyagawai and Isthmiophora hortensis from East Asia. It was established that, based on both life cycle and morphology data, one of the trematodes is identical to the worms designated as Euparyphium amurensis. Genetic data showed that this trematode belongs to Echinostoma. The complex data on biological, morphological and genetic characterizations establish that the distribution of the morphologically similar species, I. hortensis and Isthmiophora melis, in the Old World are limited by the East Asian and European regions, respectively. Data on mature worms of East Asian E. miyagawai revealed morphological and genetic identity with E. miyagawai from Europe. However, E. miyagawai from Europe differs from E. miyagawai from the type locality (East Asia) in terms of reaching maturity and the morphology of cercariae. These data indicate that the European worm, designated E. miyagawai, does not belong to this species. An analysis of the phylogenetic relationships of Echinostomatidae was conducted based on the 28S, ITS2 and nad1 markers. Analysis using the nad1 gene for the known representatives of Echinostomatidae is carried out for the first time, showing that nuclear markers are ineffective separate from mitochondrial ones.

As a result of the experimental infection of rats with metacercariae of Paragonimus heterotremus Chen et Hsia, 1964 from crabs (Potamiscus tannanti) caught in Yen Bai province, Vietnam, it was found that worms migrated into the lungs, to the liver and less frequently to the tissue that lines body cavities of the hosts, where they reached the adult stage, but in the muscles, worms stayed at the larval stage. Studies have shown that for P. heterotremus, rats can simultaneously play the role of the final and paratenic host; herewith, an infection with the trematode of this species can lead to the development of three forms of paragonimiasis: pulmonary, hepatic and muscular. Eggs from the adult worms localised in the liver, unlike eggs from the adult worms localised in the lungs, were not excreted into the external environment, but accumulated inside the organ. Histology and description of changes, which take place on the external surface of organs affected with P. heterotremus, are given in this study. Based on the behavioural characteristics of worms during rat infection and molecular genetic data, we established that worms from Vietnam and India should be assigned to different species of Paragonimus. P. heterotremus distribution is limited to the territory of the Southeast China, Northern Vietnam, Laos and Thailand.

Mature worms of Stephanoprora amurensis sp. nov. were obtained in an experimental study of its life cycle. In the Russian southern Far East, this trematode circulates using freshwater snails Parajuga subtegulata, freshwater fish and birds as the first, second intermediate and final hosts, respectively. Stephanoprora amurensis sp. nov. differs from the well-known representatives of Stephanoprora in a number of morphometric indicators of the developmental stages. The validity of the species was also confirmed by nuclear and mitochondrial DNA markers. In addition, new genetic data were obtained for Echinochasmus suifunensis and Echinochasmus milvi. An analysis of phylogenetic relationships within Echinochasmidae based on the 28S rRNA gene and ITS2 region identified two clusters, one of which combines species of Echinochasmus with 20-22 collar spines and short-tailed cercariae, and the other which includes Stephanoprora spp. and a number of representatives of Echinochasmus with 24 collar spines and long-tailed cercariae. The results of phylogenetic analysis based on ITS2 data show interfamily level of differences between the two clusters and intergeneric differentiation between the three subclusters uniting the species of Stephanoprora and Echinochasmus.

A description and the molecular characterization of two new species in the Haploporidae and Haplosplanchnidae families are provided herein. Parasaccocoelium armatum n. sp. was collected from the intestine of a Mugil cephalus Linnaeus, 1758 from the Primorsky region, Russia, and Pseudohaplosplanchnus catbaensis n. g. n. sp. was collected from Moolgarda seheli (Forsskål, 1775) in the coastal waters of Cat Ba Island, Vietnam. The morphological features of P. armatum n. sp. closely resemble those of Parasaccocoelium polyovum, but these species differ from one another by hermaphroditic sac and vitellaria area length and by maximal egg size. The main difference between P. armatum n. sp. and P. polyovum is the presence of an armed hermaphroditic duct in the new species. Molecular data support the case for inclusion of the studied trematodes in P. armatum n. sp. Worms P. catbaensis n. g. n. sp. from the mullet from Vietnam are morphologically close to Haplosplanchnus (Haplosplosplanchninae). The only difference between P. catbaensis n. g. n. sp. and species of Haplosplanchnus is the presence of few (1-7) large eggs, measuring 135-142 × 92-104 μm, versus numerous small eggs with a maximal size of 75 × 50 μm. Phylogenetic analysis showed that there is a contradiction between the morphological similarity of the worms and their position in the Haplosplanchnidae system, based on the genetic data. Results of this study indicate that P. catbaensis n. g. n. sp. is genetically distant from other representatives of Haplosplanchnus, despite their morphological similarity. According to the molecular data, P. catbaensis n. g. n. sp. is close to Hymenocotta mulli Manter, 1961 (Hymenocottinae). However, these species are considerably different to each other morphologically. Molecular data argue for the possibility of establishing a new subfamily for P. catbaensis n. g. n. sp. However, considering earlier studies of Haplosplanchnidae, we support the view that creating new subfamilies within this family is unreasonable because of the lack of molecular data for most haplosplanchnid species, which are necessary to resolve the problematic systematics and phylogeny of this family.

Adult worms of Erschoviorchis anuiensis sp. n., parasites of the pancreas and liver of birds, were found by feeding the Muscovy ducks Cairina moschata dom. with freshwater fish (Phoxinus percnurus) from the Amur River basin (Russia). The trematodes obtained differ from the only previously known representative of the genus, E. lintoni by the large size of the ventral sucker, testes and ovary, the shape of the ovary (three-lobed vs irregular oval for E. lintoni), and the degree of vitellarium development (well-developed vitellarium with numerous follicles vs weakly developed vitelline fields for E. lintoni). In addition, genetic data were obtained for E. anuiensis sp. n., including nucleotide sequences of the ITS region and the 28S rRNA gene of nuclear DNA, and the mitochondrial сох1 gene. These data show that the genus Erschoviorchis is a sister group to the representatives of the genera Opisthorchis, Clonorchis, and Metorchis. At the same time, it did not cluster with species of Amphimerus, in which E. lintoni has sometimes been placed. The results of the study indicated that E. anuiensis sp. n., as well as E. lintoni, when it occurs in the pancreas, leads to significant associated pathological changes, manifested in an increase in size, changes of structure and tissue density., Competing Interests: Declaration of Competing Interest None., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Two new species of Pseudohaploporinae, Pseudohaploporus pusitestis sp. n. and Parahaploporus elegantus n. g., sp. n., are described from intestines of the Vietnamese mullet fish Moolgarda seheli and Osteomugil cunnesius, respectively. Pseudohaploporus pusitestis sp. n. differs from two known Pseudohaploporus species, P. vietnamensis and P. planiliza, by the absence of a diverticulate hermaphroditic duct and muscular sphincters at the proximal end of the hermaphroditic sac. Metrically, P. pusitestis sp. n. is close to P. vietnamensis and differs from this species and from P. planilizum by lower maximum sizes of most parameters. Parahaploporus elegantus n. g., sp. differs from representatives of Pseudohaploporus by the presence of a single testis and the armament of hermaphroditic duct and is morphologically close to trematodes of the genus Haploporus. However, P. elegantus n. g differs from all known Haploporus species from mugilids of the Indo-West Pacific by the structure of the armament of the hermaphroditic duct and also by size of body, organs and eggs. The validity of designating two new species and a new genus of trematodes is supported by ITS and 28S rDNA sequence data. Phylogenetic reconstructions showed that the new trematodes belong to the Pseudohaploporinae, which formed a well-supported cluster within the monophyletic Haploporidae., Competing Interests: Declaration of Competing Interest None., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The complete cox1 gene sequence was analysed for Metagonimus suifunensis from eight localities in the Russian southern Far East, and the level of variability was compared with that of Clonorchis sinensis from the same territory of Russia. These species belong to the superfamily Opisthorchioidea, have a similar distribution in the Russian southern Far East and share second intermediate and definitive hosts, but are distinguished by their first intermediate hosts belonging to different orders of caenogastropods. The data obtained showed that the nucleotide sequence variability of the M. suifunensis cox1 gene was significantly lower. This fact is considered in connection with a recent bottleneck passage for the M. suifunensis population, in contrast to C. sinensis, which could be due to the features of the Metagonimus life cycle under seasonal freezing temperatures, as well as historical geological and climatic changes in the Russian Far East. These factors could influence the microevolutionary processes and lead to a decrease in the level of variability in the M. suifunensis population. Based on the combination of genetic data and historical geo-processes in the region, the probable route of M. suifunensis expansion from the northern part of its current area in the Amur River basin to the southern territories of the Russian Far East is justified., (Copyright © 2019 Australian Society for Parasitology. Published by Elsevier Ltd. All rights reserved.)

Adult Cryptocotyle lata sp. nov. worms were obtained from experimental studies. In the Russian southern Far East, the life cycle of this parasite is carried out using freshwater snails (Boreoelona ussuriensis), freshwater fish, and birds as the first intermediate, second intermediate, and definitive hosts, respectively. The morphological indices of C. lata sp. nov. are closest to Cryptocotyle concava; however, these two species differ in terms of their sizes of body, oral and ventral suckers, eggs, and the shape of their testes and ovaries. Analysis of the life cycles of the Cryptocotyle representatives suggested that C. concava were at least two cryptic species, one of which circulates using brackish water Hydrobia snails, and the other using freshwater Amnicola snails as the first intermediate hosts. Molecular data (i.e., the 28S gene and the second internal transcribed spacer (ITS2) of rDNA) were used to analyze the phylogenetic relationships of C. lata sp. nov. and other representatives of Opisthorchioidea. The long repeats and secondary structure of the ITS1 region were studied. Representatives of the Opisthorchiidae and several species from Heterophyidae (including the genus Cryptocotyle) were found to have molecular features that suggested that these species belonged to Opisthorchiidae. At the same time, the genetic relatedness of worms, which are united in common clusters on phylogenetic trees, is consistent with the use of the first intermediate hosts from different taxonomic groups in their life cycles; namely, snails of the Truncatelloidea are hosts of trematodes from a cluster with Opisthorchiidae and a number species of the family Heterophyidae, while snails of the Cerithioidea are hosts of worms from a cluster that includes only the Heterophyidae. In addition, the results of genetic studies indicate that Clonorchis sinensis, Metorchis ussuriensis, Metorchis bilis, Metorchis xanthosomus, and Metorchis orientalis should be included in the genus Opisthorchis., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Adult worms that belong to Carassotrema Park 1938 and Elonginurus Lu, 1955 were found in the intestine of Carassius gibelio Bloch, 1782 from the southern Russian Far East and Mugil cephalus Linnaeus, 1758 from northern Vietnam, respectively. Morphometric parameters, geographic location and host species composition of these worms correspond to Carassotrema koreanum Park 1938, which is a known parasite of cyprinid fish in Korea, Japan and China, and Elonginurus mugilus Lu, 1955, first described from M. cephalus in China. The validity of Carassotrema ginezinskajae Kulakova, Ha Ky, 1976, a synonym of C. koreanum, first described from Spinibarbichthys denticulatus Oshima, 1926 in Vietnam, supported the morphometric data. Phylogenetic analysis based on combined ITS2 ribosomal DNA (rDNA) and 28S rRNA indicated that C. koreanum and E. mugilus belong to the subfamily Waretrematinae and are closely related to the genera Skrjabinolecithum and Parasaccocoelium, respectively. Species similarity, revealed through molecular analysis, agreed with the generic diagnoses for Parasaccocoelium and Elonginurus, as well as for Carassotrema and Skrjabinolecithum.

Progressive expansion of the most hazardous human parasitoses caused by trematodes, cestodes and nematodes has been found on the south of the Russian Far East. Decelerating expansion of the trematode Clonorchis sinensis, an agent of clonorchiasis towards the southern Primorye Territory from the Amur River basin, that began 10-15 years ago, was revealed. A prognosis was made on the activation of the natural foci of clonorchiasis and paragonimiasis. Circulation possibilities are discussed of the highly pathogenic trematode Pagonimus heterotremusar in the South Asian regional ecosystems. Our experiments showed that the freshwater gastropods of the Parajuga genus from the Amur River basin and those of Stenothyra genus from Primorsky Territory were resistant to this trematode infection. Nevertheless, this does not exclude the possibility of this parasite penetration into the Far East region via infection of the local gastropods of other genera.

Adult specimens representing Pseudohaploporus vietnamensis n. g., sp. n., Pseudohaploporus planilizum n. g., sp. n., and Pseudohaploporus sp. were collected from the intestines of Osteomugil engeli (Bleeker, 1858), Moolgarda seheli (Forsskål, 1775), and Planiliza subviridis (Valenciennes, 1836) from Vietnam. The new genus Pseudohaploporus n. g. is similar to the genus Haploporus Looss, 1902 in morphology and host-specificity. Species of Pseudohaploporus and Haploporus both possess an elongated body, a long oesophagus, an intestinal bifurcation behind the ventral sucker, a large hermaphroditic bursa, vitellarium consisting of two compact accumulations, concretions in the excretory vesicle, and numerous small eggs that contain miracidia with a pigment eye, and both infect mullet. In contrast to Haploporus, both species of the genus Pseudohaploporus possess two testes. These species differ from each other morphometrically. Pseudohaploporus planilizum sp. n., in comparison with P. vietnamensis sp. n., possess a larger maximum and mean size for the body length, oral and ventral suckers, pharynx, oesophagus length, hermaphrodite bursa, and egg width. Molecular data (ITS and 28S rDNA sequences) support the validity of the new genus and the two new species and indicate the presence of another representative of this genus, Pseudohaploporus sp. Phylogenetic reconstructions show that Pseudohaploporus formed a well-supported clade within a large clade that represents the Haploporidae Nicoll, 1914. Based on morphological and molecular data for P. vietnamensis sp. n. and P. planilizum sp. n., we erected a new subfamily, Pseudohaploporinae subfam. n., which appears to be a sister taxon to Haploporinae Nicoll, 1914. Molecular data show that Pseudohaploporus differ from other haploporids at a level similar to that between other subfamilies., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Data on the life cycles and morphology of the developmental stages of Notocotylus magniovatus, Catatropis vietnamensis n. sp., and Pseudocatatropis dvoryadkini n. sp. were obtained. The Pseudocatatropis genus was restored based on our results. For the studied trematodes, the snails Parajuga spp., Helicorbis sujfunensis (Russia), and Melanoides tuberculata (Vietnam) serve as first intermediate hosts. It has been established that C. vietnamensis n. sp. differs from Catatropis harwoodi and Catatropis pakistanensis in the length of the ridge and metraterm and the location of the anterior papillae. In the life cycle of P. dvoryadkini n. sp., as in Pseudocatatropis joyeuxi, cercariae do not leave the first intermediate host. Both species are very similar in morphometric features, despite the fact that they share no common first intermediate hosts in their life cycles, and the areas of the European and Asian populations of flukes do not overlap. In phylogenetic trees and genetic distances based on the nucleotide sequences of the 28S gene and the ITS2 region of ribosomal DNA, Notocotylus attenuatus, Notocotylus intestinalis, and Notocotylus magniovatus are combined into one systematic group, while C. vietnamensis n. sp. and Catatropis indicus form another group. A third group includes members of different genera: P. dvoryadkini n. sp., and Notocotylus malhamensis, as well as three unclassified Notocotylus spp. The presence in the last group of flukes with three rows of papillae and a median ridge and lateral papillae indicates that these systematic criteria are not determinant in establishing membership of the parasitic worms to one or another genus of Notocotylidae.

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.

Adults of Skrjabinolecithum spinosum n. sp. were discovered in Mugil cephalus from the Gulf of Peter the Great in southern Far-East Russia. Additionally, adults of Unisaccus tonkini n. sp. were found in the intestine of Moolgarda cunnesius and Moolgarda seheli from the coastal waters of Cat Ba Island, Tonkin Bay, northern Vietnam. Skrjabinolecithum spinosum n. sp. possesses a larger body, and ventral and oral sucker size in comparison with Skrjabinolecithum vitellosum, a smaller pharynx size and body length/width rate ratio in comparison to Skrjabinolecithum pyriforme, a smaller body length and prepharynx size in comparison to Skrjabinolecithum lobolecitum and a smaller pharynx length and egg size in comparison to Skrjabinolecithum indicum and S. lobolecitum. The new species also differs from S. indicum, S. lobolecitum and S. vitellosum by the form of the testis, and from the last two species by the presence of a two-branched intestine. The morphometric parameters of S. spinosum n. sp. are similar to those of Skrjabinolecithum spasskii. However, S. spinosum n. sp., unlike S. spasskii, has an armed hermaphroditic duct. Unisaccus tonkini n. sp. is similar to Unisaccus spinosus (Martin, 1973), Unisaccus brisbanensis (Martin, 1973) and Unisaccus overstreeti (Ahmad, 1987) in body size but differs in oral sucker, pharynx and hermaphroditic sac size from U. spinosus, and in ventral sucker and ovary size from U. brisbanensis and U. overstreeti. Bayesian phylogenetic analysis, based on combined data of internal transcribed spacer 2 (ITS2) and partial 28S rRNA gene sequences, confirmed the validity of S. spinosum n. sp. and U. tonkini n. sp. Analysis of interrelationships of the family Haploporidae, including molecular data on new species, showed that the Waretrematinae subfamily is more heterogeneous in comparison with Haploporinae and Forticulcitinae, and includes U. tonkini n. sp.

As a result of experimental studies conducted in the Russian southern Far East, adult worms from the genus Metagonimus were obtained. A comparative analysis of the morphometry of these worms with other Metagonimus representatives showed that they are most similar to M. katsuradai Izumi, 1935 and M. otsurui Shimazu & Urabe, 2002 found in Japan, due to the ratio of suckers and the positions of the testicle, uterus and vitellaria. However, Russian worms differ from species in Japan by other metric characters: they differ from M. otsurui by the maximum size of most organs and from M. katsuradai by body width, pharynx length, and maximum size of testes and ovary. At the same time, they are identical to a trematode from the Russian southern Far East, which was previously identified as M. katsuradai. The validity of this species was also confirmed by genetic data. According to the 28S gene and the internal transcribed spacer 2 (ITS2) region of rDNA, as well as the cytochrome c oxidase I (cox1) gene of mtDNA, the Metagonimus specimens found in Russia differ from published genetic data for other members of this genus. However, both morphological similarity and molecular data showed that M. pusillus sp. nov., M. katsuradai and M. otsurui are most likely cryptic species. Furthermore, additional data based on a mitochondrial marker were provided for M. suifunensis Shumenko, Tatonova & Besprozvannykh, 2017 from Russia.

Specimens of the snails Anisus centrifugops were infected with two types of cercariae relating to the genus Diplodiscus , differing from each other by body and organ sizes were found in a pond in the territory of Vladivostok (Primorsky region, Russia). Further study of their morphology and life cycles allowed us to establish that these flukes belong to Diplodiscus japonicus (Yamaguti, 1936) and D. mehrai Pande, 1937. Morphological and molecular data confirmed the validity of D. japonicus which had previously been synonymized with D. amphichrus Tubangui, 1933 and D. mehrai Pande, 1937., (© 2018 V. V. Besprozvannykh, K. V. Rozhkovan, A. V. Ermolenko, A. V. Izrailskaya, published by Sciendo.)

Adult Aphanurus mugilis Tang, 1981 worms were detected in the intestine of Moolgarda engeli in the shallow waters off Cat Ba Island, Vietnam. Tang (1981) first described this species in Mugil cephalus off China. The worms in Vietnamese mullet were identical to Chinese specimens in a number of morphometric characteristics, with the exception of body and ovary size. In the present study, morphological characteristics, and the first molecular data for A. mugilis are provided. Additionally, molecular phylogenetic analysis of the family Hemiuridae was performed. The results of our molecular phylogenetic study indicate that the presence or absence of an ecsoma was not associated with molecular data for hemiurid subfamilies differentiation. The basal position of Bunocotylinae on the molecular-based phylogenetic tree indicated a primordial nature of ecsoma of hemiurid trematodes. Considerable molecular differentiation of Bunocotylinae from other hemiurids indicated the possibility of the recognition of the family Bunocotylidae Dollfus, 1950. Assuming that Machidatrema chilostoma is considered within the Bunocotylinae, the paraphyly of the Lecithasterinae was supported., (Copyright © 2017 Elsevier B.V. All rights reserved.)

A new species of Skrjabinolecithum (Digenea: Waretrematinae), Skrjabinolecithum pyriforme n. sp., has been found in the intestines of Liza haematocheila and Mugil cephalus from the Primorsky Region, Russia. These worms differ from S. vitellosum and S. lobolecithum by the presence of two caeca, as do S. spasskii, S. indicum and S. bengalensis. These species differ morphologically from S. pyriforme n. sp. by a number of features, including body width, oral sucker, pharynx, eggs and ratio of length and width of the body. The most similar species to S. pyriforme n. sp. is Platydidymus flecterotestis (Zhukov, 1971) with some differences in maximal body length, testis and egg sizes. The results of molecular analysis confirmed that this new species belongs to Skrjabinolecithum on the basis of close relationships with S. spasskii-type species.

Adults of Lasiotocus lizae Liu, 2002 (Monorchiidae) were found in the mullet Liza longimanus (Günther) from Tonkin Bay, near Cat Ba Island, Vietnam. In this region, flukes belonging to the genus Paucivitellosus (Bivesiculudae) were found in Liza subviridis (Valenciennes), Liza engeli (Bleeker) and Valamugil seheli (Forskåi). Results of investigations showed that morphological features of L. lizae (like L. glebulentus Overstreet, 1971) do not meet the criteria of the genus Lasiotocus. In addition, L. lizae is highly differentiated from other species of Lasiotocus from which molecular data were obtained, including L. arrhichostoma Searle, Cutmore et Cribb, 2014 and L. typicum (Nicoll, 1912). Phylogenetic analyses revealed that L. lizae differs considerably from other species of the genus Lasiotocus presented in the GenBank database. We have identified a new species of the genus Paucivitellosus - P. vietnamensis sp. n. - from L. subviridis, which differs from P. fragilis Coil, Reid et Kuntz, 1965 by metrical and molecular (18S rRNA) data, and from P. hanumanthai Mani, 1989 by metric features. Our results also show considerable molecular differentiation between P. vietnamensis sp. n. and Paucivitellosus spp. recovered from L. engeli and V. seheli in Vietnam.

Adult Lecithaster mugilis Yamaguti, 1970 were found in Moolgarda seheli, Valamugil engeli and Liza subviridis in the coastal waters of Cat Ba Island (Halong Bay, Vietnam). Specimens of Lecithaster sudzuhensis n. sp. were found in Mugil cephalus located in an estuary of the Kievka River in the Primorsky region of Russia. Studies have demonstrated that these species share significant morphometric similarities with each other and with specimens of L. helodes Overstreet, 1973 isolated from M. cephalus and Mugil curema from the Mississippi Sound and adjacent waters. These three species differ from one another in the size of the pharynx and ventral sucker and in the ratio of suckers, while they differ from other species in the genus by having a relatively elongated oesophagus. Molecular analysis, using the 18S rRNA and 28S rRNA genes, confirmed the validity of L. mugilis and L. sudzuhensis n. sp. and demonstrated that these species form a shared cluster with L. gibbosus (Rud, 1802).

Human and animal nanophyetiasis is caused by intestinal flukes belonging to the genus Nanophyetus distributed on both North American and Eurasian coasts of Northern Pacific. In spite of the wide geographical distribution and medical and veterinary importance of these flukes, the intra-generic taxonomy of Nanophyetus spp. remains unresolved. The two most widely distributed nominal species, Nanophyetus salmincola and Nanophyetus schikhobalowi, both parasitizing humans and carnivorous mammals, were described from North America and eastern Eurasia, respectively. However, due to their high morphological similarity their interrelationships remained unclear and taxonomic status unstable. In this study, we explored genetic diversity of Nanophyetus spp. from the Southern Russian Far East in comparison with that of samples from North America based on the sequence variation of the nuclear ribosomal gene family (18S, internal transcribed spacers, ITS1-5·8S-ITS2 and 28S). High levels of genetic divergence in each rDNA region (nucleotide substitutions, indels, alterations in the secondary structures of the ITS1 and ITS2 transcripts) as well as results of phylogenetic analysis provided strong support for the status of N. salmincola and N. schikhobalowi as independent species.

Echinostomatoidea is a large, globally distributed and heterogeneous group of hermaphroditic digeneans that parasite, as adults, vertebrate hosts of all classes. Species of this group have received attention from researchers as they can cause diseases in wildlife and humans. Here we describe the biological and molecular phylogenetic characteristics of Stephanoprora chasanensis n. sp. (Digenea: Echinochasmidae). The life cycle of this fluke was experimentally completed by the use of hosts, i.e. Stenothyra recondite Lindholm, 1929 snail (the 1st intermediate), Rhynchocypris percnurus mantschuricus (Berg, 1907) freshwater fish (the 2nd intermediate) and Gallus gallus chicken (the definitive host). In the adult worms, vitelline follicles were distributed anteriorly to the mid-level of the ventral sucker in our specimens whereas they did not reach the level of anterior testis in other species of Stephanoprora previously reported. Phylogenetic analysis based on 28S rDNA revealed that Stephanoprora and Echinochasmus with 20-22 collar spines grouped together in a single cluster. In addition, we showed that Stephanoprora chasanensis n. sp. was closely related to Echinochasmus milvi Yamaguti, 1939. Cercariae of these two echinostomes commonly have a long tail., (Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.)

The trematode from the Russian Southern Far East, earlier defined as M. yokogawai Katsuradai, 1912, was determined as a representative of the new species Metagonimus suifunensis sp. n. The life cycle of this trematode was experimentally studied and morphometric data for the cercariae, metacercariae and adult worm stages were obtained. The molecular data for samples from six Russian localities were received using the ITS1-5.8S-ITS2 region and 28S gene nuclear ribosomal DNA. The results revealed that M. suifunensis sp. n. does not have significant morphometric differences with M. yokogawai, M. takahashii and M. miyatai. However, the new species validity was confirmed by the molecular data. The phylogenetic relationship analysis of Metagonimus representatives showed that M. suifunensis sp. n. is well-differentiated specie with low variability in the whole area., (Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.)

Adults of Haplosplanchnus pachysomus (Eysenhardt, 1829) were found in the intestine of Liza engeli (Bleeker) from the coastal waters of Cat Ba Island, Ha Long Bay, northern Vietnam. Additionally, Provitellotrema crenimugilis Pan, 1984 was discovered in Liza haematocheila (Temminck & Schlegel) from Vostok Bay, Gulf of Peter the Great, southern Far-East Russia. Data concerning morphology, 18S rDNA and 28S rDNA of these worms were obtained.The molecular data confirmed the validity of these species and showed that specimens identified as H. pachysomus are closely related to specimens of H. pachysomus found in Spain, and that P. crenimugilis is closely related to Haplosplanchnus purii. Molecular differentiation of P. crenimugilis and H. purii was 0.92% by combined ribosomal gene sequences that confirmed species validity. Molecular differentiation between P. crenimugilis and H. purii, on the one hand, and H. pachysomus, on the other hand, was much higher, suggesting that the sequence for H. purii in GenBank is for a misidentified species of the genus Provitellotrema Pan, 1984.

In this paper, we review, continent by continent, the trematode fauna of freshwater fishes of the 'Old World', a vast area consisting of the Palaearctic, Ethiopian, Oriental and Australasian zoogeographical regions. Knowledge of this fauna is highly uneven and clearly incomplete for almost all regions, sometimes dramatically so. Although the biggest problem remains the completion of the 'first pass' of alpha taxonomy, there are in addition great problems relating to biogeography and elucidation of life-cycles. For the latter, molecular data, i.e. matching DNA sequences of larval stages and corresponding adults, may represent a powerful tool that should be used in future studies. Another challenging problem represents the existence of cryptic species and, in particular, considerable decrease of experts in taxonomy and life-cycles of trematodes.

Adult Astiotrema odhneri Bhalerao, 1936 sensu Cho & Seo 1977 were found in the intestine of a freshwater turtle, Pelodiscus sinensis (Wiegmann), from the Komissarovka River Basin, Primorsky Region, Russia. It was established that the first intermediate host of this parasite is a snail, Anisus centrifugops, and that the second intermediate hosts include the snails, Helicorbis sujfunensis and A. centrifugops, tadpoles of the frog Rana dybowskii, and the fish Perccottus glenii. The development of A. odhneri includes the formation of sporocyst and xiphidiocercariae, which is typical for species belonging to Plagiorchioidea. Phylogenetic analysis based on 28S rRNA gene sequences showed that A. odhneri, together with Astiotrema monticellii, form a monophyletic clade that was closer to Opisthorchioidea than to any other taxon represented in the tree. However, phylogenetic analysis without outgroup taxon indicated a high degree of differentiation of Astiotrema from both Plagiorchioidea and Opisthorchioidea., (Copyright © 2015. Published by Elsevier Ireland Ltd.)

New data relating to the fauna of Haploporidae (Digenea) in mugilid fish from Primorsky Region of the south of the Russian Far East are presented. In the most recent revision of the family Haploporidae Nicoll, 1914, Parasaccocoelium Zhukov, 1971 was synonymized with Pseudohapladena Yamaguti, 1952 (Overstreet & Curran, 2005). Here, we restore the genus Parasaccocoelium. The species described by Zhukov can be distinguished from the species Pseudohapladena sensu Yamaguti, 1952 using a combination of the following features: (1) diffuse or absent eye-spot pigment; (2) subterminal or terminal oral sucker; (3) present or absent genital atrium; and (4) vitellarium with two large and compact vitelline masses, or vitelline follicles tubular and divided into two groups. For the species of Pseudohapladena, i.e. P. scatophagi, Yamaguti noted that the vitelline follicles occupy almost the entire post-testicular region, but in the present specimens, the vitelline follicles do not occupy the post-testicular region. Based on the diagnosis of Pseudohapladena by Overstreet & Curran (2005), the genus Parasaccocoelium differs in the following features: (1) a single testis that is longitudinal, longitudinally oval, spherical, transversally oval, irregular, bilobed or V-shaped, or a testis that is subspherical to irregular to elongate; (2) the vitellarium in the lateral fields formed from large compact follicles that have an irregular form and extend between the anterior margin of the ventral sucker or pharynx and posterior margin of the testis, and the vitelline follicles do not occupy post-testicular regions; or the vitellarium consists of elongate groups of follicles that usually extend at least one-half of a body length and extend to the posterior end of the body in some and to the posterior margin of the testis in others; (3) the eggs are operculate, unembryonated and developed when laid, or the eggs in some cases are only operculate; and (4) a developed miracidium with or without an eye spot. New data have been added to the description of Parasaccocoelium mugili Zhukov, 1971 based on new material, and two new species of this genus, Pa. haematocheilum n. sp. from Liza haematocheila and Pa. polyovum n. sp. from L. haematocheila and Mugil cephalus, are described using morphological and molecular approaches. We support the taxonomic status of the genus Parasaccocoelium and its inclusion within the sub-family Waretrematinae, and we consider Pseudohapladena lizae Liu et Yang, 2002 to be a junior synonym of Parasaccocoelium mugili Zhukov, 1971.

We examined the phylogeography and the variation of the mitochondrial DNA (mtDNA) cytochrome c oxidase subunit 1 gene (cox1) of the Chinese liver fluke Clonorchis sinensis (Cobbold, 1875) in two geographic localities in the Russian southern Far East and compared them with those from different geographical regions (China, Korea, Japan and Vietnam). The Russian samples differed from those of the other regions in haplotype frequencies, haplotype and nucleotide diversities, and AT/GC ratios. Only 4 of the 18 haplotypes were common to Russian and Chinese samples, and two haplotypes were common to Russia and other regions. The intraspecific genetic distances ranged from 0 to 1.58% for the entire dataset studied and from 0 to 1.25% among the samples from Russia. Phylogenetic trees revealed no significant genealogical clades of samples corresponding to sampling localities and no strong isolation by distance was estimated with Mantel test. Neutrality test analysis suggested a relatively recent population expansion for C. sinensis, whereas goodness-of-fit tests indicated deviation from the strict model of uniform expansion. Therefore, the sequences of the mtDNA cox1 gene provide useful genetic markers for evaluating intraspecific diversity and generating phylogeographic reconstructions for this fish-borne trematode.

Specimens of the nematode genus Rhabdochona Railliet, 1916 (Rhabdochonidae) were collected during helminthological examination of four species of cyprinid fishes in two rivers of the Amur River basin in the Russian Far East (Primorsky Region) in June 2011. Detailed light microscopical (LM) and scanning electron microscopical (SEM) examinations (the latter used for the first time for the reported nematode species) of the available material revealed the presence of three inadequately described nominal species of this genus: R. (Rhabdochona) denudata (Dujardin, 1845) from the spotted steed Hemibarbus maculatus Bleeker (Gobioninae), and R. (Rhabdochona) longispicula Belous in Roytman, 1963 and R. (Globochonoides) coronacauda Belous, 1965 from Culter alburnus Basilewsky (Cultrinae) in the Ilistaya River. Detailed morphological study of these worms, especially SEM examination, made it possible to reveal some previously unreported morphological features (e.g., the presence of sublabia or the character of ventral precloacal ridges) and to confirm other taxonomically important characters such as the shape of deirids, number of anterior prostomal teeth, number and situation of lateral preanal and postanal papillae or the detailed structure of the crown-like formation on the female tail tip in R. coronacauda. Unidentifiable Rhabdochona (Rhabdochona) gravid females were recorded from the humpback Chanodichthys dabryi (Bleeker) (Cultrinae) in the Ilistaya River and from the Amur minnow Rhynchocypris lagowskii (Dybowski) (Leuciscinae) in the Komissarovka River.

Specimens of Asymphylodora perccotti sp. n. (Trematoda: Lissorchidae) were found in the esophagus of the freshwater fish Perccottus glenii (Odobantidae) taken from the Bolshaya Ussurka River Basin (Primorsky Region, Russian Southern Far East). The first intermediate host of this trematode is a gastropod, Parafossarulus manchouricus, and the secondary hosts are the same mollusk and Boreoelona ussuriensis. Specimens of the new species are similar to A. amnicolae identified by Stunkard in 1959, but the mature worms have larger suckers and shorter ceca. The cercariae of these species are distinguished by body, suckers and pharynx size. These organs in A. perccotti sp. n. are more than one-third larger than what is observed in A. amnicolae. In addition, the new species lacks the capacity for progenesis. Finally, the new species is unusual in that it resides in the fish esophagus instead of the intestine, as is common for most Asymphylodora species. Partial ribosomal DNA sequences and phylogenetic reconstruction sequence data indicate that these worms represent a new digenean species., (Copyright © 2011 Elsevier Ireland Ltd. All rights reserved.)

The mammalian liver parasite (Clonorchis sinensis flukes) has spread in the Primorye Territory. The areas of the first intermediate hosts for the parasite mollusks of the genus Parafossarulus have been found to recently become wider south-western predominantly due to lotus acclimatization, resulting in the expansion of a parasite habitation area. The intensity of newly formed foci of clonorchiasis is an order of magnitude greater than that of natural ones and they are a higher hazard since they have formed in the recreation areas used by the population for rest and fishing. The carp is the second intermediate host of trematodes. Patients' medical history data show that European and crucian carps (less frequently skygazer, rudd, gudgeon, minnow) have served as the main sources of human infection with trematodes.

Life cycle and developmental stages of the trematode Echinochasmus spinosus Odhner, 1911 are described. As it was established experimentally, in the conditions of Primorsky Krai circulation of the trematode involves first intermediate host, mollusk Parafossarulus spiridonovi, and the second one, freshwater fishes. Adult worms were reared in chicken.

Opisthorchis felineus, O. viverrini, and Clonorchis sinensis, the trematodes of the family Opisthorchiidae, are important human parasites. Two previous studies (Kang et al. Parasitol Int 57:191-197, 2008; Katokhin et al. Dokl Biochem Biophys 421:214-217, 2008) have provided evidence using ribosomal and mitochondrial sequences that O. viverrini, O. felineus, and C. sinensis are closely related. We developed a novel nuclear marker, Pm-int9, which included the ninth intron of the paramyosin gene and flanking exon sequences. Samples of O. felineus from four localities of West Siberia, C. sinensis from the Russian Far East, and O. viverrini from Thailand were genotyped by Pm-int9. Little variation was detected in exon sequences, however, intron sequences turned out to be more variable than ribosomal internal transcribed spacers. We can conclude that Pm-int9 is valuable for interspecific variation studies. Phylogenetic analysis based on Pm-int9 revealed that O. viverrini and C. sinensis were closer to each other than either of them to O. felineus, supporting the opinion that C. sinensis should be considered the sister species of Opisthorchis spp.

Life cycles of two species of echinostomes Echinochasmus japonicus Tanabe, 1926 and E. beleocephalus (Linstow, 1873) have been studied. In Primorye region the first intermediate hosts E. japonicus are snails of genus Boreoelona, E. beleocephalus develops in Parafossarulus. Second intermediate hosts are freshwater fish and tadpole of frog Rana dubowskii. Encysted cercariae of E. beleocephalus also were found in tissue of first intermediate hosts. Experimental definitive host for both species were Gallus gallus dom. chickens.

Parasite fauna of the snakehead Channa argus warpachowskii was studied in water bodies of Primorsky Krai. Thirty-one parasite species were recorded in this host from the studied area according to original and literary data. Most part of these species are host-specific parasites of snakehead originated from Chinese-Indian region or parasites having a wide host range and able to infest almost all freshwater fishes. Moreover, three brackishwater species of parasites were found in the fishes introduced into the rivers of the southern part of Primorsky Krai. The causes of this invasion are discussed.

As a result of experimental and faunistic investigations it is established that the development of the trematode species Azygia hwangtsiytii Tsin, 1933 in Primorsky Territory is realized in the first intermediate host, snail Cipangopaludina ussuriensis, and in second hosts, fishes Perccottus glehni and Channa argus warpachowskii, which can serve as transit, as well as final hosts. For Azygia robusta Odhner, 1911 terms of the development in the first intermediate host, snail Anisus centrifugus are established.

In contrasts to formerly known data, it is found that two species of the Parasymphilodora trematodes, Parasymphilodora japonica (Yamaguti, 1938) and P. markewitschi (Kulakowskaja, 1947) are actually distributed in the territory of Primorye land. The first intermedial hosts of the former species are snails of the genus Parafossarulus; in the second species, these hosts are snails of Boreoelona. A life cycle and stages of development of both trematodes are described.

The paper presents the results of the rate of infection with the nematode Anisakis simplex in Peter-the-Great gulf fishes that are a source of human invasion. Five of 19 fish types were found to be nematode-free, and the extensiveness and intensity of invasion were observed in Siberian salmons (Oncorhynchus keta) (61% and 2-700 worms, respectively), Alaska pollacks (Theragra chalcogramma) (75.7% and 1-31 worms), and brown rock trouts (Hexagrammos octogrammus) (100% and 1-60 worms). Autopsy showed that the distribution of larvae was ambiguous in different fish species. Data on the survival of Anisakis were also obtained under experimental conditions. The nematodes were ascertained to die at -8 degrees C at the moment of complete fish freezing. Anisakis may be alive in the unfrozen fish for more than 40 days. The death of worms in the salted fish occurs within 3-8 days only when 6-10/1000 concentrations of NaCl salts are achieved at the sites of worms.

The paper presents data on the infection of fishes from the Gulf of Peter the Great with an A. simplex nematode. A total of 308 fishes of 19 species were examined for parasites. Of them 5 species were found to be free from nematodes. The highest infection rates were noted in fishes of the species Oncorhynchus keta, Hexagrammos octogrammus, and Theragra chalcogramma. Differences in the infection rates of fishes of the gulf were explained by the specific features of their vital activity and by the saturation of the environment with an invasive agent. Based on the infection of secondary intermediate hosts with the nematoda, it may be suggested that there are the populations of fishes whose representatives accomplish their life cycle under the conditions of the internal waters of the gulf. The infection rate of these fishes with the parasite is low. The altered situation--an increase in the invasion rate--was associated with the seasonal migration of fishes of these species from the open marine spaces to the cost of the Gulf of Peter the Great. The opposite values of the infection rates of deep-sea (herrings) and bottom-dwelling (bases, greenlings) fishes make it possible to assume that bottom-dwelling invertebrates function as the first intermediate hosts in the life cycle of the nematode.

Description of a life cycle and development stages of a trematode Holostephanus nipponicus Yamaguti, 1939 is given. Experimental studies have shown that in Primorye, the first intermediate host is commonly a snail Parafossarulus spiridonovi, and the second hosts are freshwater fishes Rhodeus sericeus sericeus and Pseudorasbora parva. Adult worms H. nipponicus were reared in a chicken.

Brought description trematodes Psilotrema acutilostris Oschmarin, 1963 received as a result experimental studying a life cycle. Installed that in conditions of Primorye land a role of intermediate host trematodes execute snails of Parafossarulus spiridonovi and P. manjuricus. The most further development trematodes goes with formation adolescaria. Adult worms P. acutilostris were growing in the chicken.