Lissorchiidae Magath, 1917

Sokolov, Sergey G. & Gordeev, Ilya I., 2019, 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, Zootaxa 4674 (4), pp. 451-462 : 456

Family Lissorchiidae Magath, 1917

Emended diagnosis (based on Shimazu [1992] and Bray [2008] with changes). Body small to medium sized, elongate-oval to fusiform. Tegument usually spinous. Oral sucker subterminal. Ventral sucker equatorial or more anterior. Prepharynx short. Pharynx well developed. Esophagus short. Intestinal bifurcation usually in forebody. Ceca short to long, blind. Testes single or double, in posterior half of hindbody; if two, tandem, oblique or symmetrical. Cirrus-sac well developed and containing bipartite or unipartite seminal vesicle, variable pars prostatica and ejaculatory duct connecting with armed or unarmed cirrus, or absent. If cirrus-sac absent, only terminal portion of cluster of prostate gland-cells, which surrounds pars prostatica and distal end of seminal vesicle, covered with thin-walled open-ended membrane, and other constituents of male terminal genitalia are naked. Genital atrium small to apparently absent. Genital pore lateral or ventrally sublateral in forebody or at level of ventral sucker. Ovary entire or lobed often trilobed, median or submedian, pretesticular. Canalicular seminal receptacle absent or small. Laurer’s canal present. Uterine seminal receptacle present. Uterus long, coiled; uterine field extending throughout most of hindbody, encircling gonads, often with distinct post-testicular zone. Eggs numerous, small, usually operculate, embryonated. Vitellarium follicular, in small lateral fields, occasionally confluent dorsally. Excretory vesicle tubular, I-shaped, pore terminal or subterminal. In freshwater teleosts; Holarctic and Oriental Realms. Type genus Lissorchis Magath, 1917 .

Among the species of trematodes exhibiting progenesis there are species with either obligate or facultative precocious life cycles ( Lefebvre & Poulin 2005; Kasl et al. 2018). In the obligate precocious life cycle an expected definitive host is absent, and the second intermediate host plays this role instead. In the facultative precocious life cycle, the parasite becomes sexually mature while still encysted in the second intermediate host; however, encysted adults can still be trophically transmitted to a definitive host ( Lefebvre & Poulin 2005; Kasl et al. 2018). At the moment it is difficult to determine which of these two types of life cycle A. vietnamiense n. sp. exhibits. This species is only the second lissorchiid where metacercariae are known to encyst in fish. Fish have also been recorded as the second intermediate host of Asymphylodora innominata ( Faust, 1924) ( Yamaguti 1938; Shimazu 2016). As for the rest of the lissorchiids with a known life cycle, the role of their second intermediate hosts is played by invertebrates, namely gastropods, aquatic oligochaetes, chironomid larvae and planarians ( Magath 1917; Wallace 1941; Smith 1968; Schell 1973; Macy & English, 1975; Lambert 1976; Besprozvannykh 2005; van den Broek & de Jong 1979; Besprozvannykh et al. 2012; Shimazu 2016).

This is the second record of a representative of the family Lissorchiidae in fish in Vietnam. The first record was made by Ha & Duc (2005) (compare this with Arthur & Te 2006), who found Asymphylodora japonica Yamaguti, 1938 and Asymphylodora sp. in cyprinid fish in northern Vietnam. In total, reports on lissorchiid trematodes in Mainland Southeast Asia are few; moreover, as a rule, the authors have not been able to identify the parasites to the species level ( Scholz 1991; Hla Bu & Seng 1997; Sey et al. 2004; Ha & Duc 2005).