Thornenematidae Siddiqi, 1969

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), Zootaxa 4300 (1), pp. 1-43 : 18-19

Family Thornenematidae Siddiqi, 1969

34. Thornenematidae gen. sp.— {70} (0, 0, 0, 9; 1)

Population structure and abundance. One female from an unidentified genus.

35. * Indodorylaimus Ali & Prabha, 1974 (or Sicaguttur Siddiqi, 1970 ?) sp.— {67} (0, 0, 0, 9; 1)

Population structure and abundance. One male.

Ecology and distribution. The representatives of the genus are soil nematodes (edaphobionts), occurring accidentally in water bodies. Both genera include several species reported in Asia (India) and Africa (Coted’Ivoire) ( Andrássy 2009b). Not previously known from Vietnam.

Remarks. The genera Indodorylaimus and Sicaguttur differ from each other only in the structure of female reproductive system and from other genera of the family in the presence of long tails in both males and females ( Andrássy 2009b).

36. * Lagenonema caudatum ( Jairajpuri, Ahmad & Dhanachand, 1979) Andrássy, 1987 — {67, 68} (0, 0, 0,18; 13)

Population structure and abundance. 10 individuals (three juveniles and seven females) in the first sample (4 x10 3 /m2) and three females at the second (~1 x10 3 /m2).

Ecology and distribution. Edaphobiont. Rare species. Registered in soil among plant roots in India and Malaysia (Jairajpuri et al. 1979; Carbonell & Coomans 1986). We failed to find reports on this species from other regions and habitats.

Remarks. Males are unknown ( Carbonell & Coomans 1986).

37. * Thornenema baldum (Thorne, 1939) Andrássy, 1959 — {57, 59, 63, 65, 67–69} (0, 0, 15, 45; 10)

Population structure and abundance. At most sites—1 to 6 juvenile specimens, females, gravid females and also one male (at site 69) (up to 2 x10 3 /m2). At sites 59 and 68—8 to 11 juveniles, 15 to 17 females (including two gravid) and one more male (at site 68) (up to 10 x10 3 /m2).

Ecology and distribution. Considered an edaphobiont but possibly an amphibiont (see Remarks in the discussion of the next species). Very widespread species, possibly cosmopolitan. Has been found in Europe, Asia, Africa, South America, Australia, and island states of the Indian and Pacific oceans ( Carbonell & Coomans 1986; Andrássy 1987; Baniyamuddin & Ahmad 2006). Has not been registered from Vietnam until now. Our material was recovered exclusively from small forest water bodies (brooks and pools), including non-drying out biotopes.

38. * Thornenema mauritianum (Williams, 1959) Baqri & Jairajpuri, 1969 — {22, 55, 59, 63, 64, 66–68} (5, 0, 15, 45; 11)

Population structure and abundance. At sites 55, 59, 67, and 68—11 to 21 juveniles and 7 to 16 females, some gravid (7–13 x10 3 /m2), at the other sites—5 to 11 juveniles and females (2–4 x10 3 /m2).

Ecology and distribution. Considered a soil species but has also been found in rice fields indicating, together with our findings in Vietnamese water bodies, that it is a possible amphibiont. Like T. baldum , it is quite widespread. Known from the same regions, but from more countries, as well as in Central America ( Carbonell & Coomans 1986; Andrássy 1987; Baniyamuddin & Ahmad 2006). Before our present study, not recorded from Vietnam. As with the previously described species, this taxon was found almost exclusively in small forest water bodies of Cát Tiên National Park (Đỗng Nai Province ).

Remarks. Like practically all Thornenematidae , the representatives of the genus Thornenema are classified as soil nematodes ( Andrássy 1987, 2009b; Vinciguerra 2006). However, findings in various water bodies (including those that do not dry out) of significant numbers of juveniles and sexually mature specimens (including gravid females) of both T. baldum and T. mauritianum suggest the ability of populations to develop normally in aquatic environments and, hence, to classify these species as amphibionts. It is worth noting that our studies in this region were carried out during a period of relatively low precipitation. Thus, the accidental introduction into water bodies of relatively large numbers of specimens of these nematodes, of various ages and sexes, from soil due to rainwater runoff is unlikely. For the majority of other species of nematodes considered to be edaphobionts, as opposed to T. baldum and T. mauritianum , we found, as a rule, only solitary specimens in the studied water bodies. This does not allow for reliable judging of their ability to function as amphibionts.