Rhabdolaimidae Chitwood, 1951

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 : 12

Family Rhabdolaimidae Chitwood, 1951

4. Rhabdolaimus terrestris de Man, 1880 — {25} (0, 4, 0, 0; 1)

Population structure and abundance. One juvenile individual and two females (~1 x10 3 /m2).

Ecology and distribution. Amphibiont. Occurs in various aquatic and terrestrial biotopes. Presumably cosmopolitan. Widely distributed in Europe, Asia and Africa; noted in some countries of North and South America and Oceania ( Andrássy 2005). Observed in diverse aquatic and soil biotopes in northern Vietnam ( Gagarin & Nguyen 2005a; Nguyen & Nguyen 2005; Nguyen 2007; Gagarin & Nguyen 2008a, 2012).

5. * Udonchus tenuicaudatus Cobb, 1913 — {2, 20, 31, 58, 62} (11, 4, 8, 9; 7)

Population structure and abundance. From 1 to 6 females (including those with eggs) and sporadic juveniles at the majority of sites (up to 4 x10 3 /m2), with 27 specimens (two juveniles, 15 females and 10 gravid females) at site 2 (10 x10 3 /m2).

Ecology and distribution. Amphibiont. Until recently considered a hydrobiont as it was noted only from water bodies of several European and African countries, and in some countries of Central ( Jamaica) and South ( Colombia) America and Asia (Sumatra) ( Andrássy 2005). However, a 2012 paper ( Tahseen et al. 2012) reported U. tenuicaudatus populations from the soils of various regions of northern India. Until the beginning of our studies on meiobenthos, this genus had not been recorded in Vietnam (Gusakov et al. 2014).

Remarks. Males are unknown ( Tahseen et al. 2012). In our earlier paper (Gusakov et al. 2014), we described this taxon under the name U. cf. merhatibebi . This was because we could not conclusively assign any of our initial specimens to U. merhatibebi Eyualem & Coomans, 1996 , described from Ethiopia ( Eyualem & Coomans 1996), and were unaware of the morphologically similar U. tenuicaudatus . Further analysis of Vietnamese samples has shown that a majority of the adult specimens found (as assessed on the basis of a set of features) conform to descriptions of U. tenuicaudatus ( Eyualem & Coomans 1996; Andrássy 2005; Tahseen et al. 2012). At the same time, even within a single habitat we found specimens displaying in various combinations the diagnostic features of both species (shapes of the lip region, cardia and associated gland cells, and of the tail tip, amphid position, etc.). Without considering the whole range of variation in these features, some individuals of these populations could be identified as U. merhatibebi . A similar mixing of the features of these two species was noted in the description of soil populations of U. tenuicaudatus from India ( Tahseen et al. 2012), where this phenomenon is discussed and illustrated in detail. Those authors suggest that U. tenuicaudatus is a widespread and morphologically variable species depending on geographical location and that the few diagnostic features that distinguish it from the presumably close species U. merhatibebi are unreliable. Our study on populations of U. tenuicaudatus from Vietnamese inland waters has led us to a similar opinion.