Analysis of Oribatid Fauna of the East European Tundra with First Reported Data of Subpolar Urals

: This study presents data on the oribatid mite fauna of the Subpolar Urals for the ﬁrst time. Observations were made in the Lembekoyu River valley and 35 species of oribatid mites from 24 genera and 21 families were found. The analysis of taxonomic diversity and distribution of East European tundra oribatid mite species is presented based on available literature and the author’s own research ﬁndings. The taxonomic list includes 163 species from 81 genera and 45 families. Ceratozetidae (15 species), Crotoniidae (14 species), Oppiidae (12 species), Suctobelbidae (12 species), Damaeidae (9 species), Brachychthoniidae (8 species), Phthiracaridae (5 species), Humerobatidae (5 species), Achipteriidae (5 species), Punctoribatidae (5 species), and Galumnidae (5 species) are the leading families, comprising more than 58% of all species. The zoogeographical structure of the fauna is dominated by widely distributed Holarctic, cosmopolitan, and semi-cosmopolitan species. The share of Palaearctic species is 23%. The speciﬁcity of the fauna of East European tundra manifests itself in the small group of Arctic species, both in the mainland tundra and on the Arctic islands. A complex of arctic-boreal species, widely distributed in the Eurasian sector of the Arctic, is distinguished.

Attention is being paid to the study of taxonomic diversity of invertebrates, including oribatid mites, in anthropogenic altered soils in the Arctic and Sub-Arctic conditions and the application of this group of animals in bioindication [13,[21][22][23]. Inventory of modern biodiversity and study of geographic trends of soil fauna diversity is important for biodiagnostics of natural communities under conditions of anthropogenic impact and forecasting changes in these communities in the long-term. In addition, the study of modern biodiversity is important for assessing the changes that take place in response to global warming.
In the European North-East, studies on oribatid mite fauna have been conducted both in the mainland tundra (plain and mountain tundra) and on the Arctic islands. The first data on Vaigach Island oribatid mites were presented in the form of a monograph by Koch [24], who processed the data Synonyms follow Subías [43]. Synonyms of species were given when the author of a publication mentioned the species using a different name. For example, the species Eupelops plicatus (Koch, 1835) (=Pelops auritus Koch, 1839) was noted in Bolshezemelskaya tundra [30] and was named Eupelops auritus Koch, 1839. In some cases, a synonym was given if the authors of several publications cited synonyms.

Subpolar Urals
This is the first report of oribatid mite fauna of the Subpolar Urals. In total, 35 species, 24 genera, and 21 families of oribatid mites were found ( Table 2, Appendix B). The largest number of species was recorded in the families Crotoniidae, Oppiidae and Suctobelbidae. In the studied five plant communities, 11 to 18 species were found. Heminothrus (H.) longisetosus, Nanhermannia (N.) sellnicki, Tectocepheus velatus and Melanozetes sellnicki were common in different types of communities. The species Malaconothrus (M.) monodactylus, Heminothrus (P.) peltifer, Hypochthonius rufulus, Atropacarus (A.) striculus and Neoribates (N.) aurantiacus were recorded only in the floodplain of the stream. The first two species are known to prefer high humidity habitats.

Characteristic Families Typical of Tundra Zone Species
It is possible to distinguish species characteristics of different families for the East European tundra. From Crotoniidae, Heminothrus (Platynothrus) punctatus and Camisia (C.) horrida are consistently found in the local tundra fauna. Both species are often found in the Eurasian sector of the Arctic [28]. Camisia (C.) biverrucata, C. (C.) invenusta, and C. (Ensicamisia) lapponica, are associated with high latitude. Hermannia reticulata and H. scabra Hermanniidae and Ceratoppia bipilis and C. sphaerica Ceratoppiidae are also characteristic of high latitudes. In the tundra zone, the holarctic species Nothrus borussicus Nothridae was often observed.
From Carabodidae, the most common species in the tundra are the circumpolar Carabodes labyrinthicus and the Palearctic C. subarcticus and C. marginatus. For high latitudes, there are Holarctic species Oppiella (M.) neerlandica, O. (O.) splendens, and Moritzoppia unicarinata unicarinata Oppiidae, and Suctobelbella acutidens acutidens Suctobelbidae. From Ceratozetidae, Edwardzetes edwardsi and Ceratozetella sellnicki are common at high latitudes. Diapterobates notatus and Svalbardia paludicola Humerobatidae are widely distributed in the Eurasian sector of the Arctic; they are found on the Arctic islands and archipelagos [11][12][13]17]. The first of these species has been noted as the most abundant in various plant communities in Svalbard [11][12][13].
This In conclusion, all these species are typical of the high latitudes of Eurasia.

Types of Longitudinal Distribution
In this section, the Holarctic, Palearctic, cosmopolitan and semi-cosmopolitan species of oribatid fauna are distinguished by their longitudinal distribution. Holarctic species predominate the fauna ( Figure 3). For comparison, in Spitsbergen, Holarctic species make up 50% of all species [7,11]. It has previously been noted that the European sector of the Arctic is characterised by an increase in the proportion of Holarctic oribatid species in the latitudinal gradient, from the taiga to mainland tundra to the Arctic islands and archipelagos [40]. Thus, in the taiga zone of the European North-East, the share of Holarctic species (41.5%) is the least [40].
The share of Palearctic species (34 species) in the local fauna was much smaller (23.4%) than that of Holarctic species. For comparison, in the taiga zone of the European North-East, Palearctic species make up 37.5% of the faunal list [40]. The highest share of Palearctic species is observed in the fauna of the Polar Urals and Bolshezemelskaya tundra (Figure 3).
The composition of the Palearctic species was specific to each local fauna. In total, five Palearctic species from Damaeidae were recorded, four of which were found only in the Polar Urals. Only one species, Damaeus (E.) bituberculatus, was distributed in three local faunas: in the Bolshezemelskaya tundra, the Polar Urals and the Northern Urals. This species is widely distributed in the taiga zone of the European North [44].

BZ tundra P Urals SP Urals N Urals total
holarctic semicosmopolitan cosmopolitan palaearctic Only a few Palearctic species were common to several local faunas. In addition to D. (E.) bituberculatus mentioned above, the species Carabodes (C.) marginatus was recorded in four mainland faunas: Polar Urals, Subpolar Urals, Yugor Peninsula, and Bolshezemelskaya tundra. The generalisate Palearctic species for two faunas: the Yugor Peninsula and the Vaigach Island was Minunthozetes (M.) pseudofusiger.
The share of cosmopolitan and semi-cosmopolitan species in the total fauna structure is 14.5%. In local faunas, they account for 13.9% to 18.2%. This is much higher than their share in the taiga zone, where they account for 9.5% of the total species [40].
It can thus be concluded that the findings of previous research [40] regarding the decrease in the proportion of Palearctic species and the increase in the proportion of Holarctic, cosmopolitan, and semi-cosmopolitan species in the oribatid fauna along the latitudinal gradient (from the taiga to mainland tundra to the Arctic islands and archipelagos) holds true.

Arctic Species
In the composition of the oribatid fauna of the East European tundra, species with arctic, arctic-boreal, temperate, and polyzonal types of latitudinal distribution have been documented. For the East European tundra, as well as for the mainland tundra of the European part of Russia as a whole, only a single Arctic oribatid species is known until now -S. paludicola. In addition to the Eastern European sector, S. paludicola was found in the Kola Peninsula [50]. In the Arctic archipelagos of the Palearctic region, it was found in Spitsbergen [7,11] and Novaya Zemlya [1,11]; in northern Siberia, in Yamal and Taimyr [34,45]; and in the northern Far East, in Chukotka [48]. The species has a circumpolar distribution and is found in Greenland [49], Alaska, and Yukon [47]. The data obtained confirm earlier Diversity 2020, 12, 235 8 of 19 conclusion about the small number of Arctic species in the Eastern European sector of the Arctic, both in the island and mainland regions [28,40].
The oribatid species Sphaerozetes arcticus Hammer, 1952, noted in the Polar Urals [35], should be referred to as an arctic-boreal species. In the European part of Russia, this species is rarely found. In addition to the Polar Urals, the species was recorded in the taiga zone, in the Arkhangelsk region [44]. In northern Siberia, it is more widespread, with its distribution covering the arctic-boreal zone, but most findings are in the tundra zone [45]. The species is found in Chukotka. Thus, it can be concluded that S. arcticus is characteristic of high latitudes. Subías [43] indicates a boreal distribution of this species.
For comparison, two Arctic species, S. paludicola and Oribatella (O.) arctica arctica, have been registered in the fauna of Novaya Zemlya. Oribatella (O.) arctica arctica, was observed in northern Siberia [34] and Chukotka [45], in addition to Spitsbergen. Therefore, it cannot be ruled out that this species may be found in the East European tundra in the future.

Arctic-Boreal Species
The tundra oribatid fauna includes a complex of arctic-boreal species occupying the Arctic islands and archipelagos, the continental part of the tundra zone, and the taiga zone of Eurasia. The base of this complex is represented by species that constitute the majority of tundra fauna, both in the European sector of the Arctic (mainland part and island part: Vaigach Island, Novaya Zemlya, and Spitsbergen), and in Siberia and the Far East, and, accordingly, contribute to the higher percentage of tundra fauna similarity in Eurasia. These species are H. punctatus, C. sphaerica, H. reticulata, and D. notatus, with a circumpolar distribution. On Vaigach Island, this list further includes Ameronothrus lineatus, A. nigrofemoratus, and Oromurcia lucens. Species A. lineatus in the European sector of the Arctic is also distributed in Spitsbergen and Kola tundra. Like A. nigrofemoratus, it is also a boreal species, while Oromurcia lucens (L. Koch, 1879), also found on Spitsbergen, is a boreal-alpine species, according to Subías [43].
In the biocoenoses of the Pai-Khoi Ridge (Yugor peninsula), the complex of arctic-boreal species is complemented by Moritzoppia unicarinata clavigera, Pyroppia lanceolata, and Banksinoma setosa. For the first of these species, Subías [43] indicates a boreal-alpine distribution. The species B. setosa, in addition to the Yugor peninsula, is noted in the North Urals. It is mainly located in the Siberian and Far Eastern sectors. The share of arctic-boreal species in different local faunas is not high, making up 7.7% to 18.6% of the total fauna. In the overall structure of the fauna, their contribution is 7.4%.
Some arctic-boreal species are also present in the Palaearctic mountain ranges, such as D. notatus in the Altai [46], C. sphaerica in the Tien Shan [46], and M. unicarinata clavigera and P. lanceolata in the Caucasus [41]. Arctic-boreal species H. punctatus, D. notatus, Hermannia scabra, H. reticulata, A. lineatus, and C. sphaerica are also found in the Western European tundra sector (Kola Peninsula) [50]. It can be thus concluded that the arctic-boreal species which are widely distributed along the longitudinal gradient of the European sector of the Arctic are all common. No arctic-boreal species specific to the Eastern European sector have been identified. In the European North-East, the complex of arctic-boreal species also includes M. unicarinata clavigera, P. lanceolata, B. setosa, Sphaerozetes arcticus, and Peloribates pilosus.

Temperate and Polyzonal Species
The largest number of species recorded in the East European tundra have a temperate or polyzonal type of latitudinal distribution. Temperate species typical of East European tundra are Liochthonius Cosmopolitan species T. velatus and O. nova and semi-cosmopolitan species Q. quadricarinata and S. laevigatus, which in the latitudinal aspect are also distributed polyzonal [52], are common in the tundra. These polyzonal and temperate species that are common in the tundra zone, as well as the Arctic and arctic-boreal species, mentioned above, we call 'species of northern complex'.

'Southern' Elements within the Oribatid Fauna
Attention is also drawn to species which are mainly found in the lower latitudes. These species can be called 'conditionally southern'. For example, Hydrozetes thienemanni, with a temperate type of distribution, was found only at Yugor Peninsula in the European sector of the tundra zone [28]. In the European part of Russia, it is distributed in taiga and coniferous/broad-leaved forests [41,44,52]. In the tundra zone of Eurasia, H. thienemanni was previously found only in Chukotka by Grishina [34].
Holarctic species Malaconothrus (Trimalaconothrus) tardus, according to Subías [43], is absent in the northern Palaearctic region. It is found in the Polar Urals [35]. In the European part of Russia, it is found in the northernmost regions. This species was observed mainly in broad-leaved forests and steppe zone [41,44].
Palaearctic species Eueremaeus oblongus silvestris found in the Polar Urals [37] is not characteristic of the tundra zone. In the European part of Russia, it is mainly found in the zone of broad-leaved and coniferous/broad-leaved forests [41,52]. In Siberia, the species was observed in taiga forests and the Altai [45].

Summary
This publication presents a generalised taxonomic list of oribatid mites of East European tundra, based on available literature and new data. The checklist of East European tundra oribatid mites includes 163 species, 81 genera, and 45 families. This study presents data on the oribatid mite fauna of the Subpolar Urals for the first time. To date, 35 species, 24 genera, and 21 families of oribatids have been registered from this region.
The leading families in the fauna structure are Crotoniidae, Ceratozetidae, Oppiidae, Suctobelbidae, Damaeidae, Brachychthoniidae, Phthiracaridae, Humerobatidae, Achipteriidae, Punctoribatidae, and Galumnidae. The greatest number of species is Holarctic. Circumpolar distributed species make 12.3% of the total. The share of Palaearctic species is low (23.4%), which distinguishes the fauna of the tundra zone from the taiga zone.
The specificity of the oribatid fauna of East European tundra manifests itself in the small group of Arctic species, both in the mainland tundra and on the Arctic islands. In the majority of local fauna, there is only one Arctic species, Svalbardia paludicola. Sphaerozetes arcticus, noted in the Polar Urals, that has been classified as an arctic-boreal species. It could also be called 'conditionally arctic', as it sometimes penetrates into the taiga zone.
The fauna of the East European tundra is characterised by a complex of arctic-boreal species, based on circumpolar species common with the Western European sector of the Arctic, as well as with the Siberian and Far Eastern sectors, such as Heminothrus punctatus, Ceratoppia sphaerica, Hermannia reticulata, and Diapterobates notatus. In different local faunas, this complex is complemented by species Ameronothrus lineatus, A. nigrofemoratus, Banksinoma setosa, Pyroppia lanceolata, Moritzoppia unicarinata clavigera, Peloribates pilosus, and Oromurcia lucens.
The largest number of species in the East European tundra is polyzonal. No species specific to the East European sector of the tundra zone was identified.
Acknowledgments: The author thanks Leonid N. Rybin from the Institute of Biology, Komi Science Center, Ural Branch of RAS, Syktyvkar (Russia) for map.

Conflicts of Interest:
The author declares no conflict of interest.