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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.

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.)

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).

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.

Intraspecific genetic differentiation of the trematode Skrjabinolecithum spasskii and its phylogenetic relationships with other species of the family Haploporidae were studied by comparing the nucleotide sequences of a part of the 28S rRNA gene and the ITS1-5.8S-ITS2 rDNA region. Trematodes were isolated from so-iuy mullet Liza haematocheila fishes collected in rivers of Primorye and flathead grey mullet Mugil cephalus fishes collected in water bodies of Vietnam (27 fishes in total). A phylogenetic analysis showed that S. spasskii is close to species of the genus Capitimitta of the subfamily Waretrematinae. By intraspecific variation of rDNA sequences, trematodes were divided into three groups with tree different genotypes, which had fixed nucleotide substitutions. Genotype I was found in trematodes from fishes collected in Primorye. Genotype II was detected in trematodes from M. cephalus fishes collected in the Tonkin Bay, Cat Ba Island, Vietnam. Genotype III was found in five trematodes from L. haematocheila collected in the Kievka River, Primorye. The genetic distances between genotypes I and III from Primorye were 0.4 and 0.65% by 28S and ITS rDNA sequences, respectively. The lowest genetic distances were observed between genotypes II (Vietnam) and III (Primorye), 0.1 and 0.33% by 28S and ITS rDNA sequences, respectively. Possible causes of genetic differentiation of S. spasskii from different geographic locations and different definitive host species are discussed.