Dorylaimidae de Man, 1876

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 : 19-23

Family Dorylaimidae de Man, 1876

39. Dorylaimidae gen. spp.— {3, 15, 53} (5, 7, 0, 0; 4)

Population structure and abundance. In sample 53—about 20 juveniles (6 x10 3 /m2), in the other samples—a juvenile specimen and two females from undetermined genera.

40. * Amphidorylaimus cf. flagellicauda Monteiro, 1970 — {55, 57, 59, 69, 70} (0, 0, 23, 18; 7)

Population structure and abundance. 1 to 4 individuals (juveniles, females or gravid females) at the each site (up to 1 x10 3 /m2).

Ecology and distribution. Edaphobiont. Occurs in forest soil around tree roots. Presumably ingress into water bodies is accidental. Until now, only reported in Brazil and India ( Ahmad & Jairajpuri 1982; Baniyamuddin & Ahmad 2006).

Remarks. Judging by the main morphometric features, adult females found in Vietnam are close to the specimens from a population of A. flagellicauda described from India ( Ahmad & Jairajpuri 1982; Baniyamuddin & Ahmad 2006). However, to reliably identify the species, males (absent from our samples) are needed since the females of the closely related species, A. infecundus (Cobb in Thorne & Swanger, 1936) Andrássy, 1960 have similar characteristics. The males of these two species differ in number of supplements and their position ( Ahmad & Jairajpuri 1982; Ahmad & Araki 2003). The females of A. flagellicauda from India differ slightly from the type population from Brazil (they have a slightly longer body, prerectum, odontostyle, more posterior vulva position, etc.). It is assumed that these differences reflect geographic variability ( Baniyamuddin & Ahmad 2006). It is also worth noting that recently two more species close to A. flagellicauda were described from India on the basis female specimens (males were not found): A. jamali Siddiqi & Shah, 2015 and A. longistylus Siddiqi & Shah, 2015 ( Siddiqi & Shah 2015). The authors compare their new species with specimens of the type population of A. flagellicauda from Brazil, but not with the specimens of A. flagellicauda from India noted above ( Ahmad & Jairajpuri 1982; Baniyamuddin & Ahmad 2006). At the same time, examining all available data indicates that the majority of characteristics of the new Indian species and of populations described in other papers as A. flagellicauda , are overlapping ( Ahmad & Jairajpuri 1982; Baniyamuddin & Ahmad 2006; Siddiqi & Shah 2015). There are thus two possibilities: (1) in all cases the same morphologically variable species ( A. flagellicauda ?) is mentioned; (2) different Indian populations (and similar specimens from Vietnam), previously identified as A. flagellicauda , are in reality separate new species. Certainly, to solve this problem, a revision of the genus is needed and, in particular, additional analysis of Indian populations of all species noted above.

41. ** Crocodorylaimus minor Gagarin & Gusakov, 2013 — {3, 25, 27, 31–33} (5, 11, 15, 0; 8)

Population structure and abundance. An abundant species, reaching relatively high numbers at some localities. The maximum was found at site 25—more than 200 juveniles, 17 females (including some with eggs) and 23 males (90 x10 3 /m2). At sites 27 and 31—34 to 51 juveniles, 8 to 22 females and 8 to 16 males (19–33 x10 3 / m2), at the rest of the sites—some specimens of various age and sex (up to 3 x10 3 /m2).

Ecology and distribution. Probably a hydrobiont. New species ( Gagarin & Gusakov 2013d). To date this species is known only from the lakes and irrigation ditches of Cam Ranh Peninsula (Khánh Hòa Province) and one of the reservoirs in Đǻk Lǻk Province.

42. ** Dorylaimus proximus Gusakov & Gagarin, 2015 — {6, 13, 14} (5, 7, 0, 0; 4) Population structure and abundance. At site 6—six juvenile specimens, 15 females, two gravid females and 13 males (13 x10 3 /m2), at the other sites—2–3 females and/or males (up to 1 x10 3 /m2). Ecology and distribution. Hydrobiont. New species, recently found only in two large water bodies (Gusakov & Gagarin 2015a).

43. Dorylaimus stagnalis Dujardin, 1845 — {15, 16, 52–54} (0, 18, 0, 0; 7)

Population structure and abundance. A relatively abundant species at some biotopes. The greatest quantity was recorded at site 54—about 170 juveniles, six females (one gravid) and seven males (67 x10 3 /m2). At site 15 and 16 11–38 juveniles, 3 to 7 females and single males were found (7–16 x10 3 /m2), at the other sites—a single juvenile, female and/or male (up to 1 x10 3 /m2).

Ecology and distribution. Hydrobiont. Prefers aquatic biotopes but sometimes occurs in wet soil. One of the most common species of the genus. Registered on all continents except for Australia and Antarctica. Repeatedly recorded in Vietnam ( Nguyen 2007; Andrássy 2009b; Gagarin & Nguyen 2012).

44. Dorylaimus Dujardin, 1845 (or Ischiodorylaimus Andrássy, 1969 ?) spp.— {55–57, 63, 65–68, 70} (0, 0, 23, 55; 13)

Population structure and abundance. One to eight juveniles at each site (up to 3 x10 3 /m2). Remarks. The taxon contains juvenile specimens for which reliable identification to either of the noted closely-related genera is only possible with the presence of adult specimens in the same samples.

45. * Ischiodorylaimus minimus Khan, Jairajpuri & Ahmad, 1989 — {58} (0, 0, 8, 0; 1)

Population structure and abundance. One female and one male.

Ecology and distribution. Hydrobiont, but possibly amphibiont as well (see Remarks). Rare species. Described from boggy soil in India ( Khan et al. 1989). Our finding is apparently the second locality known for this species.

Remarks. The representatives of the genus Ischiodorylaimus are considered aquatic nematodes ( Vinciguerra 2006; Andrássy 2009b). Because I. minimus is rare, it is difficult to assess its ecological preferences. Occurrence in boggy soil (type habitat) and in a small, possibly ephemeral, water body (where it was found in Vietnam) is more characteristic for amphibiont nematodes.

46. ** Ischiodorylaimus paracognatus Gagarin & Gusakov, 2016 — {58} (0, 0, 8, 0; 1)

Population structure and abundance. Numerous (60) juveniles, five females and two males (25 x10 3 /m2). Ecology and distribution. Like the preceding species, possibly an amphibiont. Found in the same water body, as well. This is the only known habitat to date ( Gagarin & Gusakov 2016).

47. ** Laimydorus gigas Gagarin & Gusakov, 2014 — {51, 52} (0, 7, 0, 0; 3)

Population structure and abundance. Eight specimens (three juveniles, five females and one male) at site 51 (3 x10 3 /m2) and single juvenile and female at the second locality.

Ecology and distribution. Amphibiont or hydrobiont. Found in small, forest (possibly ephemeral) lake. New species ( Gagarin & Gusakov 2014b). Not yet found outside type locality.

48. Laimydorus oxurus Gagarin & Nguyen, 2005 — {16, 53–55, 63–65, 67} (0, 11, 8, 36; 11)

Population structure and abundance. At site 53—10 juvenile specimens, 18 females and 13 males (15 x10 3 / m2), at most other sites—4 to 14 individuals of varying stages and sexes (1–5 x10 3 /m2), at site 65—alone female.

Ecology and distribution. Most likely hydrobiont. Dwells predominantly in aquatic environments, although may be found in wet soil near water bodies ( Nguyen 2007). This species was described from a northern Vietnam river ( Gagarin & Nguyen 2005b). As observed in further studies, including the present paper, relatively widespread in various water bodies of that country ( Nguyen 2007; Gagarin & Nguyen 2008a, b). Has not been recorded outside Vietnam yet.

49. ** Laimydorus propinquus Gagarin & Gusakov, 2016 — {38, 40, 42, 43, 47, 49} (0, 21, 0, 0; 8) Population structure and abundance. At site 49—six juveniles, four females (including two gravid) and one male (more than 3 x10 3 /m2), at the other sites—1 to 7 juveniles (up to 3 x10 3 /m2).

Ecology and distribution. Hydrobiont. New species ( Gagarin & Gusakov 2016). All noted habitats belong to one relatively large floodplain lake. Has not been recorded from other water bodies.

50. * Laimydorus siddiqii Baqri & Jana, 1982 — {5, 6, 12, 15, 16, 21, 50–54, 56, 58, 63, 64} (21, 25, 15, 18; 21)

Population structure and abundance. An abundant species in many habitats. At site 56—over 100 juvenile specimens, 43 females and 11 males (58 x10 3 /m2), at sites 16, 52–54, and 58—8 to 37 juveniles, 1 to 16 females (including gravid individuals) and 1 to 8 males (11–21 x10 3 /m2), at 15, 50, 51 and 64—10–20 specimens of different age and sex (3–6 x10 3 /m2) and at the other sites—1 to 6 various specimens (up to 2 x10 3 /m2).

Ecology and distribution. Most likely an amphibiont. Until recently known only in India, where it was recorded from rice fields ( Baqri & Jana 1982; Ahmad & Ahmad 2002; Vinciguerra 2006). In Vietnam, found both in large and small water bodies (presumably, some of the latter are ephemeral). One of the most common and relatively numerous nematodes of the studied regions.

Remarks. L. siddiqii is a morphologically variable species. Many features of the populations described in India vary within a wide range; in some features specimens from different locations differ noticeably ( Ahmad & Ahmad 2002). The majority of adult females and males found in Vietnam are most similar to the Indian population from Hyderabad ( Ahmad & Ahmad 2002). However, even among these specimens from various localities noticeable variations may also be noted. In our opinion, this inter-population variability of L. siddiqii is worthy of special study. It is possible that the present taxon is actually a set of morphologically similar (or cryptic) species.

51. ** Laimydorus tenuis Gagarin & Gusakov, 2014 — {50, 51, 53, 54} (0, 14, 0, 0; 6)

Population structure and abundance. At site 51—one juvenile, two females and two males (2 x10 3 /m2), at the remaining biotopes—1 to 2 females or females with eggs and 16 to 30 juvenile individuals (7–11 x10 3 /m2). Ecology and distribution. Amphibiont or hydrobiont. New species ( Gagarin & Gusakov 2014b). To date found only in small, possibly ephemeral, forest and floodplain lakes.

52. Laimydorus Siddiqi, 1969 (or Prodorylaimus Andrássy, 1959 ?) spp.— {1, 3, 7, 16, 17, 32, 33, 50, 56, 57, 60, 62, 63, 66–68} (16, 7, 46, 45; 23)

Population structure and abundance. At the majority of samples—single or a few juveniles, females and, rarer, gravid females and males (up to 1 x10 3 /m2), at sites 57, 68 and 66—7 to 15 juveniles (3–6 x10 3 /m2).

Remarks. The taxon unites immature specimens and, as rule, solitary males and females, that are impossible to attribute unequivocally to any known species of the genus Laimydorus (not less than 4–5 various potential species). It is possible that, in some samples noted above, juveniles of the closely-related genus Prodorylaimus may be present. One species of the latter genus was found in the studied areas (see below).

53. * Mesodorylaimus aegypticus ( Andrássy, 1958) Andrássy, 1959 — {65, 66} (0, 0, 0, 18; 3)

Population structure and abundance. Single female in sample 65 and four females (including two gravid) at site 66 (more than 1 x10 3 /m2).

Ecology and distribution. Like the majority of species of this genus, likely, an amphibiont ( Gagarin 2001; Vinciguerra 2006; Andrássy 2009b). Initially found in the freshwater pond ( Andrássy 1988). In Vietnam, found in a small forest brook. Precise data on the type of biotopes (aquatic or terrestrial) in other known localities lacking. Rare , but judged on occurrence in various parts of the world, relatively widespread species. Found in several European countries, in Central Asia ( Kyrgyzstan), Egypt and South Africa , on the islands of the southern Indian Ocean ( Kerguelen Archipelago ) ( Andrássy 1988; Vinciguerra 2006). Not reported from Southeast Asia before the present study.

Remarks. The females found in Vietnam correspond almost exactly to the description by Andrássy (1958). Males are as yet unknown.

54. ** Mesodorylaimus dolychurus Gagarin & Gusakov, 2012 — {25} (0, 4, 0, 0; 1) Population structure and abundance. Numerous (about 50) juvenile specimens, 24 females and 18 males (33 x10 3 /m2).

Ecology and distribution. Hydrobiont or amphibiont. New species (Gagarin & Gusakov 2012). To date only found in one habitat: the boggy shore of small lake.

55. Mesodorylaimus lutosus Gagarin & Nguyen, 2005 — {15, 16} (0, 7, 0, 0; 3) Population structure and abundance. 1 to 2 juvenile specimens, 3 to 7 females and 1 to 10 males at the each locality (2–6 x10 3 /m2).

Ecology and distribution. Likely hydrobiont. To date found only in relatively large permanent water bodies: in rivers of northern Vietnam ( Gagarin & Nguyen 2005b; Gagarin & Nguyen 2008a, 2012). Not yet found outside Vietnam.

56. Mesodorylaimus mesonyctius (Kreis, 1939) Andrássy, 1959 — {33, 50, 52, 57, 59, 62–70} (0, 7, 23, 82; 20)

Population structure and abundance. Rather numerous at site 50—36 juveniles, 31 females (including one gravid) and 17 males (31 x10 3 /m2). At sites 33, 59, 63, and 67–69—7 to 15 juveniles, females and males (3–6x10 3./ m2), at the remaining biotopes—1 to 4 individuals of various stages and sexes (up to 1 x10 3 /m2).

Ecology and distribution. Amphibiont. Dwells in mosses, wet soil and water bodies (predominantly small). Likely cosmopolitan. To date is registered on all continents except for Australia and Antarctica ( Gagarin 1992; Vinciguerra 2006; Andrássy 2009b). Frequently collected in Vietnam ( Nguyen 2007). One of the most common species of nematodes in the small forest water bodies and brooks of Cát Tiên National Park (Đỗng Nai Province ) that we studied.

57. * Mesodorylaimus cf. spicatus Tauheed & Ahmad, 2010 — {3} (5, 0, 0, 0; 1)

Population structure and abundance. One juvenile, two females and one male (more than 1 x10 3 /m2).

Ecology and distribution. Possibly an amphibiont. The species was recently described from soil in India ( Tauheed & Ahmad 2010). Found in Vietnam on a reservoir shore. This is probably the second known locality of this species.

Remarks. General body structure and practically all morphometrics of the Vietnamese specimens similar to descriptions of M. spicatus . At the same time, there are still some doubts concerning our identification since, owing to poor position on the mount, we failed to determine vulva orientation. In M. spicatus it is transverse ( Tauheed & Ahmad 2010), while in M. recurvus Andrássy, 1964 (most similar in size and main morphometric features), the vulva is longitudinal ( Andrássy 1964). It is worth noting that the latter species has been recorded to date only in Europe and males have not yet been described ( Andrássy 2009b). Thus the specimens from Vietnam most likely belong to M. spicatus .

58. ** Mesodorylaimus vietnamicus Gagarin & Gusakov, 2014 — {53} (0, 4, 0, 0; 1) Population structure and abundance. Large numbers of juveniles (115 individuals), females (172) and males (147) (161 x10 3 /m2).

Ecology and distribution. Most likely an amphibiont. New species recorded in massive amounts in the only known habitat to date: a small, possibly ephemeral forest lake ( Gagarin & Gusakov 2014a).

59. Mesodorylaimus Andrássy, 1959 spp.— {29, 63} (0, 4, 0, 9; 3)

Population structure and abundance. One male and one female with significant differences in body size, odontostyle length, etc.; these likely represent two undetermined species.

60. * Prodorylaimus depressus Loof, 1973 — {46, 56} (0, 4, 8, 0; 3)

Population structure and abundance. At site 46—one female, three gravid females and three juveniles (a qualitative sample), at site 56—one gravid female.

Ecology and distribution. Possibly a hydrobiont. Rare species. Found and described in various small water bodies and bogs in Surinam (South America) ( Loof 1973) and, according to available data ( Loof 1996; Vinciguerra 2006), has not been noted anywhere else. It is likely that Vietnam is the second country where this species has been observed.

Remarks. The main morphological characteristics of the females collected in Vietnam do not differ significantly from the original species description. Males are unknown ( Loof 1973, 1996).