Taxonomy and Distribution of the Genus Sellaphora Mereschowsky (Bacillariophyceae: Sellaphoraceae) in Aquatic Ecosystems of Mongolia

During our investigation, 28 species belonging to the genus Sellaphora Mereschowsky were found in different types of water bodies and streams in Mongolia. Of these, 14 species have already been described and/or shown earlier in the flora of Mongolia, and we found 14 species for the first time. One new taxonomic combination has been proposed: Sellaphora glomus (J.R. Carter and A.E. Bailey-Watts) Glushchenko and Kulikovskiy comb. nov. One species is proposed for description as new to science: Sellaphora dorofeyukae Glushchenko and Kulikovskiy sp. nov. All species were illustrated with original light micrographs, and a number of species were also illustrated with scanning electronic photographs. The distribution of the identified species within Mongolia and the world is discussed.

The few revisions of the genus Sellaphora were prepared by different authors [13][14][15]; however, these revisions had no floristic orientation and concerned the study of taxonomic histories of different species.Diatoms from different ecosystems in Mongolia have been studied for more than 100 years [16].According to the latest data in the literature, at least 30 species of Sellaphora are known from Mongolia [17], which we list in Table 1, of which 13 were first described directly from aquatic ecosystems or their bottom sediments in this country [18].This is a rather small number of species for such a vast floristically interesting territory.Thus, there is a need for a new revision of the genus Sellaphora from the aquatic ecosystems of Mongolia.
A complex relief and climatic conditions determine the size and variety of Mongolian aquatic ecosystems.Mongolian water resources are limited and unevenly common.The lakes and the rivers of Mongolia belong to the water collection pools of the Arctic and Pacific Oceans, as well as to the Central Asian basin [17].The aim of this publication is investigation of the diversity, morphology, taxonomy, and distribution of Sellaphora species from Mongolia with the description of a new species.

Results
Of the 30 species previously known in the flora of Mongolia, 16 have been described from this region (see Table 1).
During this investigation, 28 species belonging to the genus Sellaphora were found.Of them: • Fourteen species-previously known in the flora of Mongolia according to literary data (including 11 described directly from the country's aquatic ecosystems);

•
Thirteen species-known from other regions and shown for the first time in the flora of Mongolia; • One species-new to science.Remarks.The specimens in our samples were 17.8-35.0µm long, 7.9-9.3µm wide, and had 18-22 striae in 10 µm.
This species was found on slides no.02605 and 03151.Remarks.The specimens in our samples were   predominantly shorter.The number of areolae in lacunae ranges from 2 to 10.Additionally, in the central part of the valve, there are small single rounded areolae without visible lacunae.Closer to the ends of the valves, the lacunar device is replaced by the usual small rounded areolae, especially closer to the valve margins.The valve margins are smooth and devoid of areolae (Figure 7A,D).Two distinct longitudinal depressions are present on either side of the raphe-sternum.Encompassing the complex of raphe-sternum, flaps of the conopeum and parallel apical depressions are straight (not undulating in parts), ca. 3 µm broad (Figure 7A-C).
The density of areolae in lacunae is 50 in 10 µm in the central part of the valve, up to 70 in 10 µm in lacunae located near the central area.The density of areolae at the valve ends is 40-45 in 10 µm.The distal raphe end is bent to the valve margin, located parallel and fairly close to one of the stria (Figure 7D).SEM, internal view (Figure 8A-F).The raphe is straight, located on a slightly raised sternum (Figure 8A-E).The central raphe ends are bent to one side.Distal raphe ends terminate in narrow helictoglossae.Helictoglossae are located near the apical pits (Figure 8F).The polar bar is well expressed (Figure 8A,E).Several rows of short striae are located on the polar bar (Figure 8E).Uniseriate striae consist of areolae covered with rounded or rectangular hymenes (Figure 8D,E).

Species Previously Described or/and Shown in Mongolia
Sellaphora pseudobacillum has previously been shown in the plankton and benthos of lakes Khövsgöl and Hangal, as well as for the benthos of the Changa and Tuul rivers [17].The valve margins are smooth and devoid of areolae (Figure 7A,D).Two distinct longitudinal depressions are present on either side of the raphe-sternum.Encompassing the complex of raphe-sternum, flaps of the conopeum and parallel apical depressions are straight (not undulating in parts), ca. 3 µm broad (Figure 7A-C).
The density of areolae in lacunae is 50 in 10 µm in the central part of the valve, up to 70 in 10 µm in lacunae located near the central area.The density of areolae at the valve ends is 40-45 in 10 µm.The distal raphe end is bent to the valve margin, located parallel and fairly close to one of the stria (Figure 7D).SEM, internal view (Figure 8A-F).The raphe is straight, located on a slightly raised sternum (Figure 8A-E).The central raphe ends are bent to one side.Distal raphe ends terminate in narrow helictoglossae.Helictoglossae are located near the apical pits (Figure 8F).The polar bar is well expressed (Figure 8A,E).Several rows of short striae are located on the polar bar (Figure 8E).Uniseriate striae consist of areolae covered with rounded or rectangular hymenes (Figure 8D,E).

Species Previously Described or/and Shown in Mongolia
Sellaphora pseudobacillum has previously been shown in the plankton and benthos of lakes Khövsgöl and Hangal, as well as for the benthos of the Changa and Tuul rivers [17].It was also found by us in Lake Khövsgöl.The species has a Holarctic distribution, apparently often identified as S. bacillum s.l.On the whole, our finding of S. pseudobacillum fits into the type description of the species.
Sellaphora mongolocollegarum was previously shown in the benthos of the Ynegtiin and Shurguitm Jargalant rivers and Lake Hangal [17].We have shown new localities of this species from lakes Bayan, Khövsgöl (and the nameless reservoir adjacent to it separated by a sandy spit), Holbo, Ögii, as well as in the Suman River.In general, our finding of S. mongolocollegarum fits into the morphology of the original description of this species.
Sellaphora perobesa was previously shown only in the benthos of Lake Hangal [17].We found it in Lake Bayan and in an unnamed reservoir separated by a sandy spit from Lake Khövsgöl.Our material differs from the typical material, with a somewhat higher density of striae in 10 µm (18-21 in the type material versus 20-22 in our material).The species is known only from Mongolia.
Sellaphora ellipticolanceolata was previously recorded in the sediments of lakes Hangal and Buyr, as well as in the benthos and epiphyton of small ponds of Buyr Lake catchment area Hangal [17].We illustrate new localities of this species from lakes Bayan, Terkhiin Tsagaan, as well as the rivers Suman and Terkhiin.We found valves smaller than indicated in the original description (length from 16.9 µm, width from 6.6 µm in our sample versus 21 µm in length and 7 µm in width for the type material).The species is known from Mongolia, but some specimens of Sellaphora from Bear Island can be interpreted as S. ellipticolanceolata ([19], Figure 7, pp.10-12).Thus, the distribution of the species requires clarification.
Sellaphora permutata was found in the benthos of the Herlen River [17].We show new locations at lakes Bayan, Ögii, Zavagitn.Our material fits into the original description of this species.The species is known from Mongolia.
Sellaphora auldreekie was previously shown in the sediments of Great Lakes Depression (sine loco) lakes [17].We demonstrate new findings of this species outside this basin from lakes Khövsgöl and Ögii.Our studied population differs from the type material by a somewhat larger width of valves (6.6-8.0 µm in the original description versus 7.9-9.3µm in our material).The species was described from Great Britain [13] but, apparently, is more widely distributed and can be identified as S. pupula s.l.
Sellaphora mutata was previously found in the plankton of the Hag and Buyr lakes, as well as in the plankton of the Urd Tamir River [17].We have shown new localities of this species in the lakes Terkhiin Tsagaan and Zavagitn Nur as well as in an unnamed reservoir separated from Lake Khövsgöl by a sandy spit.The species has a Holarctic distribution.
Sellaphora boltziana was found in the benthos of Lake Hoh.The new location was an unnamed reservoir separated by a sandy spit from Lake Khövsgöl.Our species differs from the type material by a somewhat smaller width of valves (12-13 µm in the original description versus 11.2-11.8µm in our material).
Sellaphora khangalis was found in the benthos of Lake Hangal and Lake Hoh [17].We also found this species in Lake Khövsgöl and in an unnamed river.Our material is characterized by a smaller width of valves (9.5-10.0µm for the type material versus 7.3-9.0for our material).The species is known only from Mongolia.
Sellaphora pseudopupula was found in the Nuur Sphagnum bog [16,20].It was shown by us in a nameless reservoir separated by a sandy spit from Lake Khövsgöl, a nameless lake and a nameless stream next to it, Lake Zavagitn Nur, and the Terkhiin River.Our material differs from the European material in a slightly smaller valve width (5.5-5.8 µm in our species versus 6-7 µm in the European population) [21].This is a Holarctic species.
Sellaphora perlaevissima was previously shown in the benthos of the Barun Burh, Bayan, Jargalant, and Tsenher rivers [17].It is illustrated by us in Lake Davaa and an unnamed reservoir separated by a sandy spit from Lake Khövsgöl, as well as from the Suman River.Our material is distinguished by a slightly higher stria frequency in 10 µm (15-16 for our material versus 13-15 for the type material).The species is known only from Mongolia.
Sellaphora kusberi was previously found in the benthos of the Tuul, Ynegtiin, Herlen rivers and Lake Hangal [17].Our findings of this species are shown in lakes Davaa and Ögii, an unnamed reservoir separated by a sandy spit from Lake Khövsgöl, and two unnamed rivers.Our material as a whole is distinguished by a somewhat higher striae frequency in 10 µm (22-23 in our material versus 21-22 in the type material of this species) and also by a smaller width of valves (5.6-6.3 µm in our material versus 6.6 µm in the type material).
Sellaphora intermissa was previously shown in the benthos of the Ayagan Byrd, Bayan, Zuun Burh, and Jargalant Ynegtiin rivers [17].We also found it in Lake Bayan and additionally in Khövsgöl Lake, Heeguer Bay, an unnamed lake near Khövsgöl Lake separated by a sandbar, Zavagitn Nur Lake, and in an unnamed river.The specimens illustrated by us generally have shorter valves (16.3-26.1 µm), while the type material is represented by longer valves (23-30 µm).In this regard, the width of the valves of our specimens is, on the whole, smaller than that of the type material (3.8-4.6 µm for our specimens versus 4.5-5.0 for the type material).The striae frequency in our material is somewhat higher than that for the type material (24-27 µm for our material versus 23-25 µm for the type material).The species is known only from Mongolia.
Sellaphora seminulum was previously shown in the sediments of Lake Davaa [17].We illustrate this species from lakes Bayan and Khövsgöl.Our material, on the whole, fits into the accepted concept of this species.This is a Holarctic species.

First Findings for Mongolian Flora
Sellaphora obesa has been described from the UK [13].This species was found by us in Mongolia and shown in an unnamed river flowing out of the Davaa Lake (periphyton) Khövsgöl Lake (benthos), an unnamed lake near Khövsgöl Lake separated by a sandbar (periphyton), and the Jargalant River (periphyton).Our material is morphologically close to the type material of this species.This species is known from the Holarctic [3].
Sellaphora lanceolata has been described from the UK [13].This species was noted by us from the benthos of Khövsgöl Lake and sand from Terkhiin Tsagaan Lake.In general, our material is distinguished by a slightly smaller valve width, 6.6-7.8 µm, compared to the type material, 7.1-8.1 µm [13].This species is a Holarctic species [3].
Sellaphora pelagonica was described from a dystrophic pond from North Macedonia [22].We show this species from the benthos of different parts of Khövsgöl Lake, from the bottom sediment of an unnamed lake near Khövsgöl Lake separated by a sandbar, and from the periphyton of the Terkhiin River.The morphology of our findings is close to the two populations of species illustrated by Kochoska et al. [22].Sellaphora bisexualis has been described from the UK [23].We illustrate this view from Mongolia from the sediment from Lake Davaa, and also from an unnamed lake near Khövsgöl Lake separated by a sandbar.In general, our finding is distinguished by a slightly higher striae density (20-23 µm in our finding versus 18.5-21.5 in the type material of S. bisexualis).
Sellaphora medioconvexa is known from Europe [24][25][26].We show this species from the benthos and bottom sediments of Davaa Lake, as well as from the periphyton of an unnamed river flowing out of the Davaa Lake.
Sellaphora digna has been described from Germany [15,27,28].We illustrate this species from the sediment and sand of Davaa Lake and from an unnamed lake near Khövsgöl Lake separated by a sandbar.
Sellaphora disjuncta has been described from Germany [29].The species is also known from North America [30] and from the European part of Russia [31].From Mongolia, we illustrate this species from the periphyton of an unnamed river flowing out of the Davaa Lake, from the sediments of an unnamed lake near Khövsgöl Lake separated by a sandbar, from the phytoplankton of an unnamed lake near the valley of the river flowing into Khövsgöl Lake, as well as from the benthos of the unnamed river.On the basis of striae density values, our findings are closest to North American material of this species (24-25 in 10 µm in our findings versus 23-28 in 10 µm in the North American material of the species), while the material from Lake Glubokoe (Russia) has a lower striae density (21-23 in 10 µm).
Sellaphora atomoides was described as Navicula atomoides from Antwerp, Belgium [32].In Mongolia, we show this species in the unnamed lake separated from Khövsgöl Lake by a sandbar (sediments) and the benthos of Lake Terkhiin Tsagaan.It is a rather heterogeneous taxon that is poor in morphological features [31].It belongs to a widespread species [3].
Sellaphora pseudoventralis has been described as Navicula pseudoventralis from Germany, Großer Madebrökensee [33,34].We found this species in the bottom sediment and the benthos of Lake Davaa.The species is a widespread, probably cosmopolitan species [35].In general, our finding corresponds to another population of this species from Lake Glubokoe and European material [21,31,35].
Sellaphora cosmopolitana was originally described as Navicula arvensis var.major Lange-Bertalot in Krammer and Lange-Bertalot [36].We found this species in the benthos of a puddle with mosses next to the Davaa Lake.The species is known from Ecuador [37] and Europe [36].
Sellaphora stroemii was described as Navicula stroemii from Norway [38].In Mongolia, we show this species from the Jargalant River periphyton.In general, our finding morphologically corresponds to the European population of this species shown by Cantonati et al. [21].
Sellaphora multiconfusa has been described as Naviculadicta multiconfusa from James Lake, Canada [39].We have shown this species in the benthos and bottom sediments of Davaa Lake, as well as from the bottom sediments of an unnamed lake near Khövsgöl Lake separated by a sandbar.The species is a Holarctic species known from Canada and Europe [39].Our findings are morphologically close to the type material of this species [39].
Sellaphora vekhovii has been described as Naviculadicta vekhovii from the Yugorskiy Shar Peninsula, Siberia [40].We illustrate this species from the bottom sediments of Lake Davaa, from the periphyton of an unnamed river flowing out of the Davaa Lake, from the benthos of a puddle with moss next to the Davaa Lake, from the periphyton of an unnamed lake near the Terkhiin Tsagaan Lake, and also from a number of nameless rivers.The species is distributed in the European part of Russia, the far north of Western Siberia, and the Baikal region [41].Our findings are morphologically close to the type material of this species [40].
Sellaphora glomus comb.nov.has been described from Scotland as Navicula glomus [42].It was found by us in the bottom sediments of an unnamed lake near Khövsgöl Lake separated by a sandbar.The species is a Holarctic species known from the Shetland Islands, Alps, Yamal Peninsula, and Russia [25,43].Our findings are morphologically consistent with the diagnosis given by Krammer and Lange-Bertalot [25].
Sellaphora dorofeyukae sp.nov., according to electron microscopic examination, belongs to a morphologically interesting group of species known from Asia and the tropics of the New World.These species are characterized by significant external massive silicification of valves, which is unknown for described groups of Sellaphora sensu Mann et al. [14].These species are known from both ancient Lake Baikal (S. amicula Kulikovskiy, Metzeltin and Lange-Bertalot, S. psedamicula Kulikovskiy, Metzeltin and Lange-Bertalot, S. ovalis Kulikovskiy, Metzeltin and Lange-Bertalot) and tropical streams in South America (S.lambda (Cleve) Metzeltin and Lange-Bertalot, S. renata Metzeltin and Lange-Bertalot, etc.).A brief description of the new species and morphologically related species of Sellaphora is given by us in Table 2.A separate mention should be made for the genus Okhapkinia Glushchenko, Kulikovskiy and Kociolek in Kulikovskiy et al., described by us from Southeast Asia and attributed to representatives of the family Sellaphoraceae.A representative of the genus has, in addition to the characteristic generic characters, massive silicification of the valves from the outer surface [4].
The thickness of this silica layer is different; for example, in Okhapkinia and tropical Sellaphora, it is developed to a greater extent than in Baikal Sellaphora.A thick silicified layer from the outer surface of the previously mentioned Sellaphora species has wide openings (hereinafter referred to as "elliptical lacunae" after Metzeltin and Lange-Bertalot) [46] and has a shape from rounded to elongated, at the bottom of which there are separate areolae, from one to several [4], which, according to Metzeltin and Lange-Bertalot [46], together form a system of holes of the first and second order.In the case of Okhapkinia, the areolae are crater-shaped and separated by ridge-like thickenings, which was not noted for Sellaphora [4,47].In general, the presence of species and even genera (for example, Altana, Placoneis) with heavily silicified valves is typical of tropical regions and ancient lakes [8,47].
Sellaphora dorofeyukae sp.nov.has the greatest morphological similarity with the Baikal species S. amicula and S. pseudoamicula (see Table 2).Sellaphora dorofeyukae sp.nov.have a common raphe-sternum structure (the presence of a conopeum and two parallel apical depressions), a similar width of the central and axial areas, and a similar width of valves (16.6-18.4µm in S. dorofeyukae sp.nov.versus 17.3-21.3µm in S. amicula) as well as a similar shape and localization of lacunae on the outer surface of the valves (see Table 2).The number of areolae in each of the lacunae is also similar (from 2 to 10 areolae in S. dorofeyukae sp.nov.versus 2-11 areoles in S. amicula).The species differ in valve shape: In S. dorofeyukae sp.nov., the shape is linear with parallel margins and widely rounded ends.In S. amicula, the shape ranges from linear to linear-elliptical with slightly convex margins and widely rounded valve ends (Table 2).Striae density in S. dorofeyukae sp.nov. is 16-19 in 10 µm, and in S. amicula striae density is 20-22 in 10 µm (Table 2).The density of areolae also varies between species.In Kulikovskiy et al. [8], when describing the species the density of areolae was taken to indicate first-order areolae (according to Metzeltin and Lange-Bertalot) [46], not separately taking into account the areolae located in lacunae (second-order areolae, according to Metzeltin and Lange-Bertalot) [46].This indicator is also similar to ours (15-25 areolae in 10 microns).Our observations show that the density of areolae lying in lacunae (second-order areolae (according to Metzeltin and Lange-Bertalot) [46] in the central region of the valve in S. amicula is approximately 38 in 10 µm [8] (Pl.98, Figures 1 and 2).In S. dorofeyukae sp.nov., the density of second-order areolae is more variable in different parts of the valve: 50 areolae in 10 µm in the central part of the valve, up to 70 areolae in 10 µm near the central area, and at the ends of the valve, there are 40-45 areolae in 10 µm (Table 2).
Sellaphora krsticii is described from St. Naum springs, Lake Ohrid [44], close to S. dorofeyukae sp.nov.Both species have a conopeum, distinct lacunae, and similar valve widths (13.5-20.0µm in Sellaphora krsticii versus 16.6-18.4µm in S. dorofeyukae sp.nov.).At the same time, the species differ in the shape of the valves (linear to linear-lanceolate with obtusely rounded, unattenuated ends, versus linear with parallel edges and widely rounded ends in S. dorofeyukae sp.nov.).The ultrastructure of the species is also radically different: Sellaphora krsticii lacks a siliceous layer pierced by holes, at the bottom of which areolae lie (areolae of the first and second orders, according to Metzeltin and Lange-Bertalot).The areolae density in S. krsticii is 40-42 in 10 µm, while in S. dorofeyukae sp.nov. the second-order areolae density is 15-25 in 10 µm (Table 2).In S. krsticii, the only elongated areolae are located near the central area, but they do not have a lacuna structure (Table 2).
Sellaphora lambda is similar in its ultrastructure to S. dorofeyukae sp.nov.Both species have similar features such as the structure of the raphe-sternum and the presence of firstand second-order areolae.The density of areolae is close between the species, 16-19 in S. dorofeyukae sp.nov.versus 18-20 at the ends and 13-16 striae in the central part in S. lambda.The number of first-order areolae (visible in the LM) in S. lambda is 14-15 in 10 µm; in S. dorofeyukae sp.nov.these data are more variable, 15-25 in 10 µm.This range is due to the alternation of lacunae and individual areolae, which is clearly seen in SEM (Figure 7).The number of second-order areolae (located in lacunae) is also similar in the species: 2-9 areolae in S. lambda versus 2-10 areolae in S. dorofeyukae sp.nov.At the same time, the outer silica layer in S. lambda is more pronounced than in S. dorofeyukae sp.nov.The shapes of the valves of the species also differ; S. lambda has linear valves with slightly concave margins and widely rounded ends, while S. dorofeyukae sp.nov.has linear valves with parallel edges and widely rounded ends (Table 2).The density of the areolae of the basal layer (second-order areolae) is different between species; in S. lambda, these data can be given only for an unknown part of the valve [46] (p.402, Taf.84, Figure 3) and are 40-50 in 10 µm (Table 2).In S. dorofeyukae sp.nov., the density of second-order areolae is more variable in different parts of the valve: 50 areolae in 10 µm in the central part of the valve, up to 70 areolae in 10 µm near the central area, and 40-45 areolae in 10 µm at the ends of the valve (Table 2).
Thus, the number of Sellaphora known from the aquatic ecosystems of Mongolia is 45 species.Of course, even this number of species will not reflect the complete composition of the genus in Mongolia.Additional data on the distribution of species already known in the Mongolian flora, including those known only from the type locality, have been obtained.
Further research is needed to add new localities of both previously known species for the country and to identify new species.It is necessary to conduct molecular genetic studies that will allow a greater degree of comparison between European and Asian populations of Sellaphora species.

Sampling
Samples from Mongolia were collected by M.S. Kulikovskiy in 2015.Physical and chemical water parameters were measured with a Hanna Combo (HI 98129) multiparameter probe (Hanna Instruments Ltd., Inc., New York, NY, USA).Samples had been collected from different parts of the bay and from different substrates.A list of slides and their characteristics is given in Table 3.

Preparation of Slides and Microscope Investigation
Samples have been processed by means of a standard procedure involving treatment with 10% HCl and concentrated hydrogen peroxide.After treatment with HCl, the sample was washed with deionized water.Permanent diatom preparations were mounted in Naphrax ® (Brunel Microscopes Ltd., Chippenham, UK; refractive index = 1.73) [47].Light microscopic (LM) observations were performed by means of a Zeiss Axio Scope A1 (Carl Zeiss Microscopy GmbH, Göttingen, Germany) microscope equipped with an oil immersion objective (×100/n.a.1.4,Nomarski differential interference contrast, DIC) and a Zeiss Axio Cam ERc 5s camera (Carl Zeiss NTS Ltd., Oberkochen, Germany).The valve ultrastructure was examined by means of a JSM-6510LV 6510LV scanning electron microscope (JEOL Ltd., Tokyo, Japan) operated at 10 kV and 11 mm working distance.For scanning electron microscopy (SEM), parts of the suspensions were fixed on aluminum stubs after air drying.The stubs were sputter-coated with 50 nm of gold in an Eiko IB 3 (Eiko Engineering Co., Ltd., Hitachinaka, Japan).
Samples and slides are deposited in the collection of Maxim Kulikovskiy at the herbarium of the Institute of Plant Physiology Russian Academy of Sciences, Moscow, Russia.

Table 2 .
Comparison of morphological features of Sellaphora dorofeyukae sp.nov.and related species.
* counted from published data; ** given the meaning of ordinary areolae; n.d.-no data.

Table 3 .
List of the collected samples and their characteristics.