Family Xyalidae Chitwood, 1951 112. Daptonema dihystera Gagarin & Nguyen, 2005 — {12, 32–34, 61, 62} (5, 0, 23, 18; 8) Population structure and abundance. The species is very numerous at site 33—over 80 juveniles, about 40 females (seven gravid) and 36 males (58 x10 3 /m2). At sites 61 and 62—1 to 3 juvenile specimens, 4 to 9 females, 3 to 4 females with eggs and 3 to 8 males (4–13 x10 3 /m2), at the other sites—1 to 3 females and males (up to 1 x10 3 / m2). Ecology and distribution. Most likely a hydrobiont. Originally described from the rivers and lakes of northern Vietnam (Gagarin & Nguyen 2005a; Nguyen & Nguyen 2005; Gagarin & Nguyen 2008b). Not yet found outside Vietnam. 113. Theristus Bastian, 1865 spp.— {22, 29, 32, 67} (5, 4, 8, 9; 6) Population structure and abundance. In the quantitative samples—single juvenile individual or female, at site 29—five juvenile specimens. Ecology and distribution. Representatives of this genus are mainly marine species. Some species occur in brackish and fresh water, as well as in groundwater and wet soil (Coomans & Eyualem 2006). Remarks. We failed to attribute any of the solitary adult females we found reliably to any described species. Previously, four species of the genus Theristus have been reported from the inland waters of Vietnam: T. longus Platt, 1973, T. orientalis Gagarin & Nguyen, 2005, T. rigidus Gagarin & Nguyen, 2005 and T. minimus Gagarin & Nguyen, 2011. All listed species were found only in brackish waters, except for the second, which was also recovered from freshwater (Canh & Nguyen 2000; Nguyen 2007; Gagarin & Nguyen 2011).

Published as part of Gusakov, Vladimir A. & Gagarin, Vladimir G., 2017, An annotated checklist of the main representatives of meiobenthos from inland water bodies of Central and Southern Vietnam. I. Roundworms (Nematoda), pp. 1-43 in Zootaxa 4300 (1) on page 32, DOI: 10.11646/zootaxa.4300.1.1, http://zenodo.org/record/837117