Benthic macroinvertebrates of the watercourses of the South Khangai ( Mongolia )

Based on the original material collected in June 2010, a list of aquatic invertebrates found in the watercourses of the endorheic basins of the South Khangai is given. A total of 129 species, mainly widespread transpalaearctic animals, have been found. In general, the fauna is depleted, but contains a certain number of peculiar endemics.


Introduction
Reophylic invertebrates of Mongolia are studied quite irregularly.Most hydrobiological and hydroentomological studies are traditionally carried out in the rivers of the Selenga Basin on the northern slope of the Khangai and the Khövsgöl Lake region, a lesser attention is paid to the Kerulen Basin in the northeast and to inland basins in the west and south of the country (Zaika, 2012;Maasri, Gelhaus, 2012;Narangarvuu et al., 2015).The latter are of particular interest, since they have been existing in isolation from large river systems for considerable time and thus can exhibit a certain faunistic and ecological originality.The purpose of this work is to provide a preliminary description of the faunal diversity of macroinvertebrates in water bodies of the Southern Khangai.The sources of most of the rivers of the South Khangai are located in zone of the highland tundra or semi-desert, at an altitude of 2.600-3.000m.Steeply falling from the ridge, they flow to the south towards the Gobi Desert, gradually losing water content and flow velocity and significantly mineralizing (including due to the massive grazing of cattle in floodplain meadows).The total length of such rivers does not exceed several hundred kilometers (usually within 300-350); as a result, they form a saline swamp or a small lake in the middle of a stony desert, without further drain.

Materials and methods
During the route expedition in June 2010 we examined middle course of seven rivers and several springs on the slopes of the South Khangai in the area of their intersection with the Ulan-Bator-Altai Highway (the length of the section is about 600 km), at altitudes from 1.600 to 1.900 m.They are the rivers Olgi gol, Taatsyn gol, Shargaljuut gol, Tuyn gol, Naryin gol, Baydrag gol and Tsagaan gol (Fig. 1).This is almost all the watercourses available in the region (except for the sources of the Zavkhan River).The collected material includes 55 samples of macrozoobenthos; the selection was carried out using a standard hydrobiological scoop-net with an area of 0.02 m².

Results and discussion
In the watercourses of the South Khangai we found 129 species of macroinvertebrates belonging to the classes Turbellaria, Oligochaeta, Hirudinea, Gastropoda, Bivalvia, Crustacea and Insecta.The most species-rich group was Insecta presented by 108 species; their dominance is generally typical for the Palearctic (Chertoprud, 2010).
Among amphibiotic insects inhabiting the watercourses of the South Khangai the most numerous were Diptera (64 species), considerably less Ephemeroptera (13 species), beetles (13 species) and caddisflies (11 species); six species of Plecoptera and one Heteroptera were also noted.
Mayflies (Ephemeroptera) are represented by 13 species of six families.Ameletus inopinatus Eaton, 1887 (Ameletidae) was found in the Tuyn gol River only (the vicinity of Bayankhanhorg town).Larvae of Siphlonurus lacustris Eaton, 1870 (Siphlonuridae) were also found here; they inhabited floodplain flows and rhypal zone of the main course.Almost in all the watercourses of the region (except the Olgi gol, where no mayfly was found) Serratella ignita (Poda, 1761) (Ephemerellidae) and Caenis rivulorum Eaton, 1884 (Caenidae) occurred.Cinygmula cava (Ulmer, 1927) and Epeorus (Belovius) pellucidus (Brodsky, 1930) were found on stony whitewaters of large rivers (at least 5-6 meters in width), while Paracinygmula joernensis (Bengtsson, 1909) (Heptageniidae) inhabited plants in relatively calm streams.The watercourses of the South Khangai are characterized by a very specific fauna of swimming rheophilic Baetidae mayflies represented here by at least six species.
The most common were the peculiar larvae of Baetopus sp., which possibly belongs to one of the two endemic species described from the northern regions of Mongolia, Baetopus asiaticus Soldan, 1978or B. montanus Soldan, 1978.Since larva of B. montanus is not described in detail (Waltz, 2002), and in B. asiaticus larval stage is still unknown at all, a reliable identification of the material from the Khangai is impossible at that moment.In any case, the found larvae significantly differ from European B. wartensis Keffermüller, 1960 by elongated and narrow tergalia and a number of traits of the oral appendages.Acentrella charadra Sroka & Arnekleiv, 2010, found in all the rivers, also has a peculiar body structure: it is distinguishable from the type specimens, which were described from south-eastern Kazakhstan, with a long paracercus (12 segments).The same feature is characteristic of the populations that we discovered in regions of Xinjiang (China) adjacent to Mongolia (unpubl.data).This species, endemic for the desert zone of Central Asia, has already been discovered in the rivers of the southern slope of the Khangai (Klyuge, Novikova, 2011).
Previously recorded from north and west Mongolia Baetis vernus Curtis, 1834 and B. feles Kluge, 1980 in the watercourses of the South Khangai were met quite regularly; B. feles choose rather streams and small rivers, while B. vernus prefers large (from 5-6 meters wide) watercourses.Together with the latter insect, transpalaearctic B. buceratus Eaton, 1870 was found quite regularly.It was also known from the Selenga and Kerulen Basin, but absent in faunal reports and catalog of the ephemeropteran fauna of Mongolia (Soldan et al., 2009).