Asaccotrema vietnamiense n. gen.; n. sp. (Trematoda: Monorchioidea), a new aberrant representative of lissorchiid trematodes from the sidestripe rasbora, Rasbora paviana Tirant (Actinopterygii: Cyprinidae), Vietnam

: In this paper we erect Asaccotrema n. gen. of Lissorchiidae Magath, 1917 (Trematoda, Monorchioidea) and provide a description of Asaccotrema vietnamiense n. sp., that infects freshwater fish in Vietnam. The morphology of Asaccotrema n. gen. is significantly different from that of other lissorchiid genera due to the absence of the cirrus-sac, and position of the genital pore sublaterally, near the middle level of the esophagus. The male terminal genitalia are characterized by the presence of the seminal vesicle, which is surrounded by prostate gland-cells, a tubular pars prostatica and a long unarmed ejaculatory duct. The terminal portion of the cluster of prostate gland-cells, which surrounds pars prostatica and the distal end of seminal vesicle, is covered with a thin-walled open-ended membrane, and the other constituents of the male terminal genitalia are naked. Asaccotrema vietnamiense n. sp. specimens are found as progenetic metacercariae on the liver of the sidestripe rasbora, Rasbora paviana Tirant (Actinopterygii: Cyprinidae). A partial sequence was obtained for the 28S rRNA gene (rDNA) of A. vietnamiense n. sp. The 28S rDNA-based phylogenetic analyses support the integration of A. vietnamiense n. sp. into the Lissorchiidae clade, which also includes Lissorchis kritskyi Barnhart Powell, 1979 and Asymphylodora perccotti Besprozvanykh, Ermolenko Atopkin, 2012. This is the second record of a lissorchiid species in fish in Vietnam.