The genus Diploneis Ehrenberg ex Cleve (Bacillariophyta) from Lake Hövsgöl, Mongolia

ELENA JOVANOVSKA; ZLATKO LEVKOV; MARK B. EDLUND

Lake Hövsgöl is an ancient lake situated in north-central Mongolia at the southern end of the Baikal Rift Zone. Throughout its limnological history, Lake Hövsgöl has developed an extraordinary diversity in different groups of organisms, such as fish and invertebrates. However, the one group that presumably holds the largest taxonomic diversity in the lake—the diatoms—has been largely overlooked. This study presents a detailed taxonomic overview of the genus Diploneis from Lake Hövsgöl as well as a few localities from the neighboring Arkhangai province. A total of 25 taxa were identified, five of which are new and potentially endemic. Alongside the endemic species, this study confirms the presence of several taxa recently described from other Mongolian lakes (e.g., Diploneis linearielliptica, Diploneis aff. heteromorphiforma and Diploneis cf. eximia), presumably peculiar for this mid-Asian region. A relatively high number of widespread species were identified in Lake Hövsgöl, commonly reported from Europe and elsewhere. Interestingly, a population of Diploneis praeclara that is morphologically closely related to populations from Neogene fossil deposits in Romania was also observed in Lake Hövsgöl. Detailed LM and SEM observations are provided, together with species descriptions and comparisons to morphologically related species. Greater knowledge of the genus Diploneis opens and guides the way towards better and more comprehensive approaches of uncovering biological diversity and biogeographical patterns on the Eurasian continent and among the ancient lakes.

Phytotaxa 217 (3): 201–248 www.mapress.com/phytotaxa/ Copyright © 2015 Magnolia Press ISSN 1179-3155 (print edition) Article PHYTOTAXA ISSN 1179-3163 (online edition) http://dx.doi.org/10.11646/phytotaxa.217.3.1 The genus Diploneis Ehrenberg ex Cleve (Bacillariophyta) from Lake Hövsgöl, Mongolia ELENA JOVANOVSKA1*, ZLATKO LEVKOV2 & MARK B. EDLUND3 1 Department of Animal Ecology and Systematics, Justus-Liebig University, Heinrich-Buff-Ring 26-32, 35392 Giessen, Germany Institute of Biology, Faculty of Natural Sciences, Arhimedova 3, 1000 Skopje, R. of Macedonia 3 St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, Minnesota 55047, USA *Corresponding author: E-mail: jovanovska.eci@gmail.com 2 Abstract Lake Hövsgöl is an ancient lake situated in north-central Mongolia at the southern end of the Baikal Rift Zone. Throughout its limnological history, Lake Hövsgöl has developed an extraordinary diversity in different groups of organisms, such as fish and invertebrates. However, the one group that presumably holds the largest taxonomic diversity in the lake—the diatoms—has been largely overlooked. This study presents a detailed taxonomic overview of the genus Diploneis from Lake Hövsgöl as well as a few localities from the neighboring Arkhangai province. A total of 25 taxa were identified, five of which are new and potentially endemic. Alongside the endemic species, this study confirms the presence of several taxa recently described from other Mongolian lakes (e.g., Diploneis linearielliptica, Diploneis aff. heteromorphiforma and Diploneis cf. eximia), presumably peculiar for this mid-Asian region. A relatively high number of widespread species were identified in Lake Hövsgöl, commonly reported from Europe and elsewhere. Interestingly, a population of Diploneis praeclara that is morphologically closely related to populations from Neogene fossil deposits in Romania was also observed in Lake Hövsgöl. Detailed LM and SEM observations are provided, together with species descriptions and comparisons to morphologically related species. Greater knowledge of the genus Diploneis opens and guides the way towards better and more comprehensive approaches of uncovering biological diversity and biogeographical patterns on the Eurasian continent and among the ancient lakes. Key words: ancient lakes, Diploneis, diversity, Lake Hövsgöl, taxonomy, new species Introduction Lake Hövsgöl is an ancient graben lake located in north-central Mongolia at the southern end of the Baikal Rift Zone (Goulden et al. 2006). With an estimated age of approximately 2 million years, Lake Hövsgöl is considered to be one of the oldest lakes in the world (Goulden et al. 2006). Throughout its history Lake Hövsgöl has provided a stable habitat for continuous processes of speciation; and at the same time served as an evolutionary refugia preserving relict species lineages (Goulden et al. 2006). Such a combination of processes has resulted in interesting and unique morphological diversity in some groups of organisms (e.g. crane flies, diatoms), but not nearly the level of diversity and endemism as seen in nearby Lake Baikal (Kozhov 1963). While other ancient lakes such as lakes Ohrid, Baikal and Tanganyika were subjects of intense scientific studies that resulted in improved species databases of the respective lakes, Lake Hövsgöl remains relatively understudied (e.g. Kozhov 1963). In the last century most of the studies in Lake Hövsgöl were focused on fish and invertebrates (Kozhova et al. 1994). Even though some of these aquatic organisms were intensely studied from both taxonomic and morphological standpoints, our knowledge of the presumably most diverse taxa in the lake—the diatoms—is largely restricted to a few studies (Østrup 1908; Edlund et al. 2006a,b; Levkov 2009; Pavlov et al. 2013). The first work on diatoms revealed 32 species and two unknowns for the lake (Dorogostaïsky 1904). Later, Ostenfeld (1907) reported 21 planktonic diatoms, while Østrup (1908) identified 179 species, including 16 new for the lake (Edlund et al. 2006b). Patterns of diatom diversity in Lake Hövsgöl include endemics, cryptic species groups, low Accepted by David Williams: 18 May 2015; published: 26 Jun. 2015 201 planktonic diversity and species flocks within benthic taxa (Edlund et al. 2003, 2006a, b; Edlund & Soninkhishig 2009; Levkov 2009). The most recent survey on the diatom flora comes from Edlund et al. (2006b) and gives a checklist of 373 diatom taxa belonging to 67 genera, including only seven widespread taxa from the genus Diploneis (Ehrenberg) Cleve (1894: 76). Diploneis is a marine biraphid pennate genus with only a small number of freshwater species, most of which are globally widespread (Round et al. 1990). Diploneis is characterized with a heavily silicified silica cell wall, with valves containing complex chambers through which the cell communicates with the environment. Each valve is divided into two parts: i) marginal part composed of alveolate striae and ii) longitudinal canal (Hustedt 1935, 1937; Round et al. 1990; Jovanovska et al. 2013a). The external opening of the chambers are morphologically diverse and vary among species (Hustedt 1935, 1937; Jovanovska et al. 2013a). They can open via one or multiple rows of areolae that are generally covered with cribrate occlusions [e.g. Diploneis elliptica (Kützing) Cleve (1894: 92), Diploneis parma Cleve (1891: pl. 2, fig. 10), Diploneis krammeri Lange-Bertalot & Reichardt (2000: pl. 4, figs 1–10, 12)]. In some species the external opening of alveoli are covered with vola [e.g. Diploneis oculata (Brébisson in Desmazières) Cleve (1894: 92), Diploneis peterseni Hustedt (1937: 676, fig. 1068 f–h)], or can open externally via narrow transapically elongate slits as in Diploneis alpina Meister (1912: pl. 14, fig. 1) and Diploneis budayana (Pantocsek) Hustedt (1937: 700, fig. 1081B) (Jovanovska et al. 2013a, b). The alveolate external openings (i.e. vela) obscure observations of the areolae structure. For instance, in some species (e.g., Diploneis stoermeri sp. nov.) the cribrate occlusions can become complex in structure towards the valve margins. The complexity is a result of an enlargement of the cribrum due to the tendency of the pores to scatter over the transapical costa by narrowing the boundaries between them (Idei & Kobayasi 1986a). Usually, towards the valve margins, a thin hyaline areas split the complex cribrum into two to three smaller occlusions, which do not necessarily corresponds with the number of areolae. On the other side, in some species they are distinguishable and alternately positioned, corresponding with the number of areolae (e.g. D. parma, Diploneis paraparma sp. nov.). In this context, it is rather challenging to observe the number of areolae only from the valve exterior, and therefore, an internal view is preferable when observing the alveolus structure. From the interior, the alveoli are covered with thin perforate silica layers (Idei & Kobayasi 1989a). They open to the interior either by a single continuous opening, as in D. krammeri and D. elliptica, or by a single opening interrupted with thick silica bars protruding from the transapical ribs [e.g. D. alpina, D. budayana, Diploneis transylvanica Jovanovska, Buczkó, Nakov & Levkov (2014: figs 1–21, 23, 26)]. In contrast, the longitudinal canal opens only to the exterior by one or multiple rows of areolae covered with cribrate or volate occlusions similar to the alveolate striae. Internally, the longitudinal canal is closed with a thick silica plate and only communicates with the interior through the alveoli (Idei & Kobayasi 1988, 1989a). According to Droop (1998), some species have pores on the internal side of the longitudinal canal; the given examples [e.g. Diploneis bomboides (Schmidt) Cleve (1894: 88) and Diploneis subcincta (Schmidt) Cleve (1894: 86)] are marine taxa. The longitudinal canals are integral to the structure of the valve as in Neidium Pfitzer (1871: 39), Muelleria (Frenguelli) Frenguelli (1945:172), Scoliopleura Grunow (1860: 554), Scoliotropis Cleve (1894: 72) and Progonia Schrader (1969: 58) (Droop 1998). Some species have thickly silicified canals [e.g., Diploneis hovsgolensis sp. nov., Diploneis ostracodarum (Pantocsek) Jovanovska, Nakov & Levkov (2013a: 246, figs 43–59). D. alpina], while others have a finer and less silicified canals [e.g. Diploneis linearielliptica Metzeltin, Lange-Bertalot & Nergui (2009: pl. 107, fig. 6–9), Diploneis fontanella Lange-Bertalot (2004: pl. 74, fig. 1–15; pl. 75, fig. 4), Diploneis boldtiana Cleve (1891: 43, pl. 2, fig. 12 )]. Wide morphological variation is also observed in the proximal and distal raphe endings. The external proximal raphe ends can be bent (e.g. D. elliptica, D. krammeri) or simple (e.g. D. oculata, D. peterseni). Meanwhile the external distal raphe endings can be hooked forming a question mark shape, as in D. alpina, D. budayana, Diploneis parabudayana Jovanovska, Nakov & Levkov (2013a: 239, figs 1–13), Diploneis mauleri (Brun) Cleve (1894: 98); bent into short terminal fissures (e.g. D. parma, D. ostracodarum); or simple, as in D. oculata and D. peterseni. Hustedt (1935) used only two morphologically distinct characters, the longitudinal canal and the alveolus structure, for creating a key of 10 divisions and 2 subdivisions within Diploneis. After a few decades, Droop (1998) suggested a separation of the D. sejuncta group (the tenth subdivision of Hustedt’s key) into a separate genus alongside Diploneis. In his paper, Droop argues that additional data, such as studies on culture conditions, valve ultrastructure, plastids and pyrenoid morphology will refine Hustedt’s proposed key (Droop 1998). In order to overcome these questions, Droop’s suggestions should be taken into consideration and further include some molecular data as additional support in unraveling the evolutionary relationship among the proposed species groups within the genus Diploneis (Hustedt 1935). However, detailed morphological data are similarly needed in concert with molecular analyses in order to use the full and informative range of data. Coupled analyses may help explain the high morphological variation of the genus Diploneis, and may support a potential separation into different genera based on monophyletic groups within Diploneis sensu lato. Further studies should include intensive morphological, 202 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. physiological, biogeographical, and molecular analyses to give greater insight into the extraordinary ultra-structural complexity and diversity of the genus. Diploneis is one of the remaining diatom genera represented in both marine and freshwater ecosystems, but is characterized with a considerably lower number of freshwater species than marine taxa (Hustedt 1937). In most freshwater regional floras the number of Diploneis species is usually dominated by cosmopolitan species (Krammer & Lange-Bertalot 1986, Siver et al. 2005). In contrast, Diploneis species richness in ancient lakes like Ohrid (Macedonia) and Baikal (Russia) is reported to be higher, characterized with a mixture of: i) widespread species; ii) new forms, potentially endemic; and iii) relict species previously reported from fossil deposits (Skvortzow & Meyer 1928, Jurilj 1954, Levkov & Williams 2012, Jovanovska et al. 2013a). Recent observations have significantly increased the number of Diploneis species in ancient lakes, and recent modifications in the diatom species concept have further supported discovery of many new taxa (Jovanovska et al. 2013a, Kulikovskiy et al. in prep). However, previous observations of Lake Hövsgöl (Edlund et al. 2006b) revealed only seven Diploneis species, most of which are widespread [D. elliptica, Diploneis elliptica var. ladogensis Cleve (1891: pl. 2, fig. 9), D. oculata, Diploneis ovalis (Hilse in Rabenhorst) Cleve (1891: pl. 2, fig. 13), D. parma, Diploneis puella (Schumann) Cleve (1894: 92), and Diploneis smithii var. pumila (Grunow) Hustedt (1937: 650, fig. 1052d, e)]. So far, there are no data on endemic and/or relict Diploneis species in the lake. This could be the result of scarce data on the extant diatom flora or those found in fossil deposits in areas surrounding Lake Hövsgöl. However, in recent observations Metzeltin et al. (2009) described six Diploneis species from east-central Mongolian lakes, which are presumably peculiar for the region [Diploneis curiosa Metzeltin, Lange-Bertalot & Nergui (2009: pl. 262, fig. 2), Diploneis spectabilis Metzeltin, Lange-Bertalot & Nergui (2009: pl. 106, fig. 1–7), Diploneis eximia Metzeltin, Lange-Bertalot & Nergui (2009: pl. 259, 260 & 261, fig. 1–3), D. linearielliptica, Diploneis mongolica Metzeltin, Lange-Bertalot & Nergui (2009: pl. 107, fig. 1–4) and Diploneis heteromorphiforma Metzeltin, Lange-Bertalot & Nergui (2009: pl. 108, fig. 1–16)]. Based on previous observations and available literature, the diversity of the genus Diploneis in Lake Hövsgöl appears to be underestimated and poorly understood. This study attempts to review and evaluate the diversity of the genus Diploneis in ancient Lake Hövsgöl and contributes toward determining the number of potentially endemic, relict and widespread species in the lake. Such increased attention would undoubtedly benefit our knowledge on Lake Hövsgöl and ancient lakes, mostly by clarifying diversity within the genus Diploneis and by recognizing any potential endemic and/or relict species in the lake. Material and methods Lake Hövsgöl is an ancient tectonic lake, situated in northwest Mongolia. It is located in the southern end of the Baikal Rift Zone about 200 km southwest of its sister Lake Baikal. Lake Hövsgöl drains via the Egiin gol (river), which is a tributary to the Selenge River, the largest inflowing river to Lake Baikal. Lake Hövsgöl is 135 km long, with a maximum water depth of 262 m and with a total volume of 480.7 km³, making it the largest freshwater lake in Mongolia by volume, and among the 20 largest lakes in the world (Fedotov et al. 2004, Goulden et al. 2006, Prokopenko & Bonvento 2009). Lake Hövsgöl is located at 1645 m a. s. l., is primarily surrounded by mountains, covers a surface area of 2760 km2, and drains a watershed area encompassing 4920 km2 (Goulden et al. 2006). Even though the lake represents an important biodiversity hotspot, its origin and age of formation remain uncertain. The current hypothesis proposes that the basin began to form in early Cenozoic, following volcanic activities in surrounding regions (Goulden et al. 2006, Krivonogov 2006). TABLE 1. Collection data for Mongolian diatom samples examined in this study. Accession numbers given for slides deposited at the California Academy of Sciences (CAS) Diatom Herbarium; n.a. = data not recorded or not applicable. Sample ID Collecting date Province Water body Coordinates Depth (m) Sample type CAS slide # n.a. mosses 917052 M52A 9 June 1996 Hövsgöl Lake Hövsgöl 50°40’44.53”N M63A 12 June 1996 Hövsgöl Lake Hövsgöl M68A 11 June 1996 Hövsgöl Lake Hövsgöl M77A 12 June 1996 Hövsgöl Lake Hövsgöl 100°14’36.57”E 50°26’N 100°10’E 50°26’N 100°10’E 50°26’N 100°10’E 5.0 epipelon 917063 n.a. Ulothrix/epilithic 917068 1.0 sediments 917077 ...continue on next page DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 203 TABLE 1. (Continued) Sample ID Collecting date Province Water body M121A 12 June 1996 M129A 13 June 1996 Hövsgöl Hövsgöl Coordinates Depth (m) Sample type CAS slide # Lake Hövsgöl 50°26’N n.a. periphyton 918021 Lake Hövsgöl 100°10’E 50°26’N n.a. epipelon 918029 n.a. periphyton 918066 n.a. peat sediments 918067 40.0 sediments 919047 40.0 sediments 919048 n.a. epipsammic 919062 4.0 Chara 919072 4.0 midge tubes 919073 4.0 sediments 919074 5.0 epilithic 919076 5.0 marl 919080 5.0 marly material 919081 6.0 sediments 919084 30.0 sediments 919087 30.0 sediments 919088 30.0 sediments 919089 30.0 sediments 919090 20.0 sediments 919091 n.a. marl 920029 n.a. sediments 920030 4.0 sediments 920031 n.a. sediments 920051 M166A 3 July 1998 Arkhangai Taruugiin Gol M167A 3 July 1998 Arkhangai Taruugiin Gol M247A 18 July 1998 Hövsgöl Lake Hövsgöl M248A 18 July 1998 Hövsgöl Lake Hövsgöl M262A 19 July 1998 Hövsgöl Lake Hövsgöl M272A 19 July 1998 Hövsgöl Lake Hövsgöl M273A 19 July 1998 Hövsgöl Lake Hövsgöl M274A 19 July 1998 Hövsgöl Lake Hövsgöl M276A 19 July 1998 Hövsgöl Lake Hövsgöl M280A 19 July 1998 Hövsgöl Lake Hövsgöl M281A 19 July 1998 Hövsgöl Lake Hövsgöl M284A 19 July 1998 Hövsgöl Lake Hövsgöl M287A 19 July 1998 Hövsgöl Lake Hövsgöl M288A 19 July 1998 Hövsgöl Lake Hövsgöl M289A 19 July 1998 Hövsgöl Lake Hövsgöl M290A 19 July 1998 Hövsgöl Lake Hövsgöl M291A 20 July 1998 Hövsgöl M329A 20 July 1998 Hövsgöl Hodon gol Bulan Lake Hövsgöl M330A 20 July 1998 Hövsgöl Lake Hövsgöl M331A 20 July 1998 Hövsgöl Lake Hövsgöl M351A 23 July 1998 Hövsgöl Lake Hövsgöl 100°10’E 47°26.171’N 101°22.971’E 47°26.171’N 101°22.971’E 50°55.056’N 100°37.912’E 50°55.056’N 100°37.912’E 51°2.793’N 100°43.438’E 51°27.268’N 100°43.11’E 51°27.268’N 100°43.11’E 51°27.268’N 100°43.11’E 51°30.407’N 100°39.239’E 51°30.407’N 100°39.239’E 51°30.407’N 100°39.239’E 51°32.423’N 100°29.58’E 51°32.421’N 100°29.634’E 51°32.421’N 100°29.634’E 51°32.421’N 100°29.634’E 51°32.421’N 100°29.634’E 50°20.607’N 100°17.865’E 50°58.035’N 100°30.3’E 50°58.035’N 100°30.3’E 50°56.071’N 100°15.385’E 50°25.676’N 100°9.088’E Material used in this study was collected from numerous Lake Hövsgöl stations at several depths (0.1–40 m) during field expeditions in June 1996 and July 1998 (Table 1). Epilithic, epipsammic, epiphytic, and sediment samples were preserved in the field in either 10% formaldehyde solution or air-dried. Two additional collections made 350 km south of Lake Hövsgöl in Arkhangai province were also used for floristic comparison. Microscope slides and subsamples of all material have been deposited in diatom herbaria at the California Academy of Sciences, the National University of Mongolia (Botany Faculty), and in the Dr. Mark Edlund collection at the Science Museum of Minnesota, USA. 204 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. Samples were treated using acid digestion with nitric acid, followed by six rinses with distilled water to remove the oxidation byproducts, and the cleaned material mounted on microscope slide using Naphrax® or Zrax®. Digital light micrographs were made on a Nikon Eclipse 80i using a Nikon Coolpix 6000 digital camera at the Faculty of Natural Sciences in Skopje, Macedonia; Zeiss Axioscope using a Cannon EOS 60D digital camera at the University of Texas, Austin, USA; and on a Olympus BX53 with a Olympus DP73 digital camera at the University of Colorado, Boulder, USA. For scanning electron microscopy (SEM), cleaned diatom material was prepared by drying diluted sub-samples of material onto cover slips that were attached with carbon tape to SEM stubs and coated with 50 nm gold–palladium (Polaron SC7640 sputter coater, Quorum Technologies, Ashford, UK). Scanning electron microscopy (SEM) was performed using a Cambridge S4 Stereoscan (Cambridge Instruments Ltd, Cambridge, UK) at the Friedrich Hustedt Study Centre for Diatoms (BRM) in Bremerhaven, Germany and a field emission scanning electron microscope FEM [FEI Quanta FEG 450 (Oregon, USA)] at the USGS Denver Microbeam Laboratory in Denver, Colorado, USA. Holotype slides for the new species are deposited in the Diatom Herbarium at the Academy of Natural Sciences of Drexel University (ANSP) in Philadelphia, USA. Isotypes are designated accordingly, hosted in the Diatom Collection at the California Academy of Sciences (CAS) in California, USA. Descriptive terminology follows Idei & Kobayasi (1986a, b; 1988; 1989a,b), Round et al. (1990), Idei (2013) and Jovanovska et al. (2013a, b). Results The genus Diploneis in Lake Hövsgöl and Arkhangai province is represented with 25 species (Table 2), five of which are new to science (Diploneis hoevsgoelensis, Diploneis stoermeri, Diploneis paraparma, Diploneis soninkhishigae, Diploneis exigua). Four additional species are not identified to the species level, either because of insufficient number of observed valves or because of lack of LM or SEM analysis. Hustedt’s type material for D. dimorpha was observed and compared with Lake Hövsgöl populations. Type material for the species recently described from Mongolian lakes (D. mongolica, D. spectabilis, D. eximia, D. curiosa, D. heteromorphiforma, D. linearielliptica; Metzeltin et al. 2009) were also observed and compared with Lake Hövsgöl Diploneis specimens. A few materials from Arkhangai region were analyzed and compared to some of the characteristic Mongolian species. Detailed LM and SEM observations are provided. Species descriptions and comments are included for all taxa including comparisons to morphologically related species. TABLE 2. Main morphological features of the genus Diploneis from Lake Hövsgöl and region; n.d. = no data Species Length Width Striae in Areolae in Valve outline Stria structure (µm) (µm) 10 µm 10 µm 43.0–106.5 25.0–44.5 8–10 6–10 rhombic-elliptic to uniseriate Diploneis hoevsgoelensis elliptic-circular 39.5–69.0 18.0–26.0 8–9 8–9 elliptic-lanceolate uniseriate Diploneis praeclara Diploneis elliptica 21.0–40.0 13.0–21.0 8–11 8–15 elliptic Diploneis paraparma 19.5–42.0 12.5–16.5 12–15 15–20 elliptic Diploneis stoermeri 19.5–44.5 12.5–22.0 12–15 10–20 elliptic-lanceolate Diploneis krammeri 23.0–34.0 13.0–17.0 11(12) 13–15 elliptic-lanceolate Diploneis soninkhishigae 14.5–24.5 8.0–12.0 11–15 10–15 elliptic-lanceolate Diploneis exigua 14.5–17.0 9.0–10.5 15–16 16–20 elliptic DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Areola structure cribra cribra uniseriate/ cribra biseriate (on mantle at apex) uniseriate cribra /biseriate (on mantle) uniseriate/ cribra biseriate (on mantle) uniseriate/ cribra biseriate (on the mantle) uniseriate cribra /biseriate uniseriate/ cribra biseriate (on mantle) ...continue on next page Phytotaxa 217 (3) © 2015 Magnolia Press • 205 TABLE 2. (Continued) Species Length (µm) 24.5–29.0 Width (µm) 9.5–11.0 Striae in 10 µm 12 (13) Areolae in 10 µm 12 Valve outline Stria structure linear-elliptic uniseriate Areola structure n.d 29.5–36.5 10.5–11.0 12 (13) 20–25 linear-elliptic biseriate n.d Diploneis subovalis 20.0–33.0 9.0–17.0 10–14 20–25 linear-elliptic biseriate n.d Diploneis mongolica 34.5 15.5 18 12 elliptic uniseriate n.d Diploneis spectabilis 28.5–38.0 13.0–18.0 18 20 elliptic uniseriate n.d Diploneis linearielliptica 21.0–34.5 7.0–10.5 15–17 15–20 elongate-elliptic uniseriate n.d Diploneis aff. fontanella 17.0–22.0 8.0–9.0 16–20 25–30 elliptic uniseriate n.d Diploneis aff. heteromorphiforma Diploneis boldtiana 16.0–29.0 8.5–11.5 9–11 25–30 linear-elliptic cribra 16.5–26.0 8.0–9.0 15–18 lanceolate-elliptic Diploneis oculata 13.0–24.5 5.0–7.5 22–24 elongate-elliptic uniseriate vola Diploneis peterseni 13.5–21.0 6.0–7.0 26–27 elongate-elliptic uniseriate vola Diploneis cf. eximia 67.5 27.5 10 not discernible not discernible not discernible 10 uniseriate /biseriate biseriate elliptic uniseriate n.d. Diploneis cf. minuta 18.0 7.5 22 elongate-elliptic uniseriate vola Diploneis sp. 1 13.5–15.0 4.0–4.5 29–30 lanceolate-elliptic uniseriate vola Diploneis sp. 2 23.5–34.0 9.5–15.0 13–16 not discernible not discernible 14–16 lanceolate-elliptic uniseriate n.d Diploneis sp. 3 39.0 20.5 11 15 elliptic uniseriate n.d Diploneis sp. 4 14.0 5.5 23 not discernible linear-elliptic uniseriate n.d Diploneis elliptica cf. var. ladogensis Diploneis pseudovalis cribra Diploneis hoevsgoelensis sp. nov. (Figs 1–40) Valves are broadly lanceolate to rhombic-elliptic becoming circular with smaller cell size. Valve apices are bluntly round (Figs 1–8, 24–30, 36, 39, 40). Valve length is 43.0–106.5 µm and valve breadth is 25.0–44.5 µm. The axial area is linear to lanceolate, expanding toward the round to transapically elongated central area. Internally, a thick rhombic silica plate covers the whole length of the longitudinal canal (Figs 14, 35). Externally, the central area is round to transapically elongate, 4.5–9.5 µm wide (Figs 9, 36, 39). Externally, the longitudinal canal is lanceolate and expanded in the middle of the valve with two to five rows of areolae narrowing into two to one at the valve apices (Figs 1–3, 24, 25, 36). From outside the areolae of the longitudinal canal are covered with cribrate occlusions similar to the external opening of the striae, from which they are separated by a thin hyaline line hardly visible on SEM (arrow on Figs 9, 36). Internally, the longitudinal canal is closed with a thick silica plate throughout the whole length. The silica plate that encloses the canal is raised above the level of the raphe, leaving the raphe in a deep “depression” (Figs 14–16, 31, 32, 35). Externally, the raphe is straight, with expanded drop-like proximal ends that are bent to the same side of the valve, positioned within a small depression (Figs 8, 9, 13, 36, 37). The distal raphe ends terminate with short terminal fissures that are bent to the same side of the valve (Figs 8, 10, 36, 38, 39). Internally, the raphe is straight and placed in the depression formed by the longitudinal canal with simple proximal and distal raphe ends that are slightly elevated (Figs 14–16, 31, 32, 35). The striae are parallel in the middle becoming radiate towards the valve apices, 8–10 in 10 µm. The alveolate striae are composed of one row of round to rectangular areolae, 6–10 in 10 µm. Externally, the areolae are covered with cribra (Figs 11, 12), becoming larger towards the valve margins (Figs 8, 10, 13, 36, 38). Internally, each alveolus opens through a single elongate aperture covered with perforate siliceous layer (arrow on Fig. 11; Figs 14, 17, 18, 31–35). In the broken valves where the perforated silica layer is corroded and/or destroyed the alveolus becomes clearly visible, which opens to the exterior through one row of areolae (Figs 17, 33, 34). Herein a detailed illustration of initial valve is presented (Figs 19–23). The raphe system and the other morphological features are not 206 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. well developed (Figs 19, 20). The initial valve length is 91.0 µm and the valve breadth is 21.0 µm. The striae are parallel in mid-valve becoming slightly radiate towards the valve apices, 9 in 10 µm. Each alveolate stria is composed of one row of areolae, externally covered with weakly developed cribrate occlusions (Figs 21, 22). FIGURES 1–7. Diploneis hoevsgoelensis morphotype 1, LM valve views. Fig. 2. Specimen from the holotype slide M280A. Scale bar = 10 µm. DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 207 FIGURES 8–13. Diploneis hoevsgoelensis morphotype 1, SEM external valve views. Fig. 8. Whole valve. Fig. 9. View of the central area with proximal raphe endings; white arrow showing the narrow hyaline area between the longitudinal canal and the striae. Fig. 10. Distal raphe endings with deflected terminal fissures. Fig. 11. The openings of the alveolus, external cribrate and internal perforate silica layer (see white arrow). Fig. 12. Close view of the structure of the depressed cribrate areolae. Fig. 13. External and internal view of the whole frustule. Scale bars = 10 µm (Figs 8, 13); 5 µm (Figs 9, 10); 1 µm (Figs 11, 12). 208 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. FIGURES 14–18. Diploneis hoevsgoelensis morphotype 1, SEM internal valve views. Fig. 14. View of the whole valve. Fig. 15. Central area with simple proximal raphe endings. Fig. 16. Distal raphe endings. Fig. 17. Structure of the single elongate opening of the alveolus. Fig. 18. Alveoli covered with thin layer of perforate silica. Scale bars = 10 µm (Fig. 14); 5 µm (Figs 15, 16); 2 µm (Fig. 17); 1 µm (Fig. 18). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 209 FIGURES 19–23. Diploneis hoevsgoelensis morphotype 1, SEM external view of initial valve. Fig. 19. View of entire valve. Fig. 20. Central area with proximal raphe endings. Fig. 21. Structure of the alveolate striae. Fig. 22. Close view of the cribra. Fig. 23. Distal raphe endings. Scale bars = 20 µm (Fig. 19); 5 µm (Fig. 20); 2 µm (Figs 21, 23); 1 µm (Fig. 22). 210 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. FIGURES 24–30. Diploneis hoevsgoelensis morphotype 2, LM valve views. Figs 28, 30. Specimens from the holotype slide, M280A. Scale bar = 10 µm. DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 211 FIGURES 31–35. Diploneis hoevsgoelensis morphotype 2, SEM internal valve views. Fig. 31. View of the valve central area. Fig. 32. Distal raphe endings. Fig. 33. Broken valve, showing the structure of the alveolate striae. Fig. 34. Close view of the structure of the single openings of the alveoli. Fig. 35. View of the whole valve. Scale bars = 10 µm (Fig. 35); 5 µm (Figs 31–34). 212 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. FIGURES 36–40. Diploneis hoevsgoelensis morphotype 2, LM and SEM external valve views. Fig. 36. View of the entire valve, white arrow showing the narrow hyaline area between the longitudinal canal and the striae. Fig. 37. Proximal raphe endings. Fig. 38. Distal raphe endings with terminal fissures. Fig. 39. View of the whole valve. Fig. 40. LM view of D. hoevsgoelensis morphotype 1 and morphotype 2, showing the differences in the valve shape. Scale bars = 10 µm (Figs 36, 39, 40); 5 µm (Figs 37, 38). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 213 Type:—MONGOLIA, Lake Hövsgöl (Hövsgöl National Park), Port of town of Hanh, at 1688 m elevation. Coordinates: 51°30.407’ N; 100°39.236’ E., marl from 5 m depth (accession number: M280A, M.B. Edlund Collection, Science Museum of Minnesota, collected by Mark B. Edlund and Nergui Soninkhishig, 19 July 1998) (Slide M280A, ANSP GC-36351, GCM-24053), holotype, designated here; example specimens on Figs 2, 3; (Slide 919080, CAS, isotype, designated here; Slide 919076, CAS, paratype, designated here). Etymology:—The species name refers to the type locality, Mongolia’s ancient Lake Hövsgöl. Observations:—Diploneis hoevsgoelensis seems to be quite variable in valve outline: lanceolate-rhombic to elliptic-circular with bluntly rounded ends (Fig. 40). In all other characters these two shape trajectories are rather indistinguishable, and therefore we consider the population to contain two morphologically different races (morphodemes). With cell size reduction both morphodemes have the tendency to become elliptical in valve outline, and therefore a clear morphological cutoff is difficult at smaller cell sizes when using the outline to distinguish them as separate entities. In order to clarify the identity and potential separation of the morphodemes, detailed morphometric analyses, including shape analysis, should be included in future studies. Such increased attention can potentially provide important insights into the phenotypic plasticity or evolutionary trajectories of D. hoevsgoelensis. To attempt to reduce the subjectivity in morphologically based analyses, a molecular study and culture study are also justified in order to reveal the key drivers for shape variation in D. hoevsgoelensis. Such an approach could potentially clarify the identity of the proposed morphotypes within D. hoevsgoelensis. Interestingly, both morphodemes are found sympatric in Lake Hövsgöl collections, and populations of D. hoevsgoelensis have also been observed in Lake Baikal, having more or less similar morphological variations (Kulikovskiy et al. in prep.). In order to test the conspecificity of Baikal and Hövsgöl populations a comparative analysis is needed. Diploneis curiosa is morphologically similar to D. hoevsgoelensis. In the original description, Metzeltin et al. (2009) gives very narrow size range for D. curiosa [80–120 µm], without providing LM illustrations. Therefore, it is difficult to compare D. hoevsgoelensis with D. curiosa. Further analyses on the type material of D. curiosa are necessary in order better characterize the identity of this taxon. Taking the valve outline into account D. hoevsgoelensis can easily be compared with Diploneis balcanica Ognjanova-Rumenova & Butczkó (2010: 171, figs 14–28). The measurements of D. hoevsgoelensis fit the original description given for D. balcanica. Interestingly, Ognjanova-Rumenova & Butczkó (2010) illustrates two different valve shapes when describing D. balcanica: a rhombic and an elliptical form. The rhombic form can potentially be allied with D. hoevsgoelensis. However, the differences in the structure of the longitudinal canal and the presence of apertures along the whole length of the raphe distinguish D. balcanica (Ognjanova-Rumenova & Butczkó 2010: 169; fig. 18). Diploneis clevei Fontell (1917: pl. 1, fig. 2) and D. hoevsgoelensis have uniseriate striae, while Diploneis finnica (Ehrenberg) Cleve (1891: pl. 2, fig. 11) and Diploneis duplopunctata Fontell (1917: pl. 1, fig. 4) have biseriate striae throughout the whole length. According to Metzeltin et al. (2009), the biseriate pattern can be noticed in larger specimens in D. curiosa (see p. 658, fig. 2), a feature not observed in D. hoevsgoelensis. The structure of the longitudinal canal (lanceolate and wide throughout in D. clevei vs. lanceolate, expanded in the middle of the valve, which gradually narrowing at the valve apices in D. hoevsgoelensis) clearly distinguishes these two species. Diploneis hoevsgoelensis can be compared with Diploneis baicalensis Skvortzow & Meyer (1928: 11, pl. 1, fig. 31), from which it differs by the narrower longitudinal canal. The drawing of Diploneis elliptica var. ostracodarum f. baicalensis Skvortzow & Meyer (1928: pl. 1, fig. 30) also closely fits D. hoevsgoelensis, from which it slightly differs in the stria density (7 in 10 µm vs. 8–10 in 10 µm) and in the valve width (54.0 µm vs. 25.0–44.7 µm). Similarities in the valve outline can be observed in Diploneis skvortzovii Skabichevskii (1936: pl. 1, fig. 7), although the difference in striae structure (uniseriate vs. biseriate) separates them. Ecology and Distribution:—M248A; M276A; M280A; M281A; M329A; M330A; M331A: distributed in shallow waters of central and northern Lake Hövsgöl in sediments and on marl and rocks. Diploneis praeclara (Pantocsek) Cleve-Euler (Figs 41–53) Valves are elliptical-lanceolate to elliptic with convex margins and bluntly rounded ends (Figs 41–49). The valve length is 39.5–69.5 µm, and the valve breadth is 18.0–26.0 µm. The axial area is narrow, linear to lanceolate, expanding toward the central area. The central area is transapically expanded, 4.0–6.0 µm wide. The longitudinal canal is lanceolate, expanded in the middle with two rows of areolae, that coalescence into one at the valve apices (Figs 41–49). Externally, the areolae of the longitudinal canal open in depressions slightly lower then the rest of the non-porous valve surface (Figs 49, 51). Externally, the areolae of the canal are each covered with a cribrum similar to those of the striae (Figs 49, 50, 53). Externally, the raphe 214 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. is straight with expanded proximal ends, which are positioned within a small depression (Figs 49, 53). Distally, the raphe branches terminate with short terminal fissures that are bent to the same side of the valve (Figs 49, 51, 52). The stria are radiate, 8–9 in 10 µm, composed of round to rectangular areolae, 8–9 in 10 µm. Externally the alveolate striae are covered with cribrate occlusions that are placed in depressions slightly lower then the rest of the non-porous silica (Fig. 50). FIGURES 41–48. Diploneis praeclara, LM valve views. Scale bar = 10 µm. Observations:—The measurements of Lake Hövsgöl’s Diploneis praeclara (Pantocsek) Cleve-Euler (1934: pl. 5, fig. 151), fully fit the original description given by Pantocsek (1892, 1905). Diploneis praeclara is described as a fossil from Köpecz, the Neogene fossil deposits in Romania (Pantocsek 1892, 1905). In the modern flora it was reported from Lake Ohrid, reassigning its status from fossil to relic (Jurilj 1954). Recently, Jovanovska et al. (2013a) documented a population with a smaller size range in both modern and fossil sediments in Lake Ohrid (length: 31.0–39.7 μm; breadth: 16.3–18.3 μm). The illustrations given by Jovanovska et al. (2013a) show differences in the valve outline between Lake Ohrid and Köpecz populations (elliptical with acute apices vs. elliptical with bluntly rounded apices). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 215 The areola density is slightly different between lakes Hövsgöl and Ohrid populations (8–9 in 10 µm vs. 10–12 in 10 µm). In the original description Pantocsek (1905) gives a stria density of 10 in 10 µm, which fully fits the Lake Ohrid population. On the other hand, Jurilj (1954) in his description gives a stria density of 7–8 in 10 µm, similar to Lake Hövsgöl population. Due to this, using the stria density as a distinguishing feature is unclear. FIGURES 49–53. Diploneis praeclara, SEM external valve views. Fig. 49. View of the entire valve. Fig. 50. Close view of the cribrate occlusions. Fig. 51. Distal raphe endings. Fig. 52. One row of areolae of the longitudinal canal at the valve apices. Fig. 53. Central area with proximal raphe endings. Scale bars = 10 µm (Fig. 49); 5 µm (Figs 51–53); 2 µm (Fig. 50). 216 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. Followed by the recent typification of D. praeclara (Jovanovska et al. 2013b) it seems like the population from Lake Hövsgöl more closely resembles the type population rather then the one from Lake Ohrid. However, the presence of the biseriate pattern in the middle of the longitudinal canal makes the separation of Lake Ohrid’s population rather difficult, and we currently consider this morphological variation to be within the concept of D. praeclara. This study expands the distribution of D. praeclara, previously reported only from the Neogene fossil deposits in Romania (Pantocsek 1892, 1905), Scandinavian fossil deposits (Cleve-Euler 1934, 1953) and Lake Ohrid (Jurilj 1954, Jovanovska et al. 2013). Ecology and Distribution:—M274A; M247A; M248A; M284A; M288A; M329A; M330A: found in shallow to deep habitats in northern and central Lake Hövsgöl on sediments and marl deposits. FIGURES 54–64. Diploneis elliptica, LM and SEM internal valve views. Figs 54–61. LM valve views. Figs 62, 63. Internal views of the entire valve. Fig. 64. Internal view of the central area with the proximal raphe endings. Scale bars = 10 µm (Figs 54–63); 2 µm (Fig. 64). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 217 FIGURES 65–69. Diploneis elliptica, SEM external valve views. Figs 65, 67. View of the whole valve. Fig. 66. Close view of the cribrate occlusions. Fig. 68. Valve central area with proximal raphe endings. Fig. 69. Distal raphe endings with terminal raphe fissures. Areolae biseriate formation (black arrow on the cribra structure at the valve apices). Scale bars = 10 µm (Figs 65, 67); 2 µm (Figs 66, 68, 69). Diploneis elliptica (Kützing) Cleve (Figs 54–69) Valves are elliptical with convex margins and rounded ends (Figs 54–61). The valve length is 21.0–40.0 µm and the valve breadth is 13.0–21.0 µm. The axial area is linear, slightly expanding towards the central area. The central area is round to slightly transapically elongate, 2.5–5.0 µm wide. From inside a thick lanceolate to rhombic silica plate covers the whole length of the longitudinal canal (Figs 63, 62). Externally, the longitudinal canal is linear and slightly expanded in the middle with one, rarely two rows of areolae (Figs 65, 66, 68). The external openings of the longitudinal canal are covered with cribra similar to those of the striae, from which they are separated with a thick hyaline area (Figs 65, 66, 68). Externally the 218 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. raphe is straight with expanded proximal ends, bent to the same side of the valve and positioned within a small depression (Figs 67, 68). Distally, the raphe branches end with short terminal fissure some distance from the valve mantle (Figs 65, 69). From inside the raphe is straight, placed in the “depression” formed by the longitudinal canal (Figs 62, 63). The proximal ends reach the height of the canal itself (Figs 62, 64), whereas the distal raphe endings are slightly elevated but not reaching the level of the canal (Figs 62, 63). The striae are radiate in mid-valve becoming strongly radiate towards the valve apices, 8–11 in 10 µm. Externally the alveolate striae are composed of large rectangular to round areolae, 8–15 in 10 µm. Striae are uniseriate, becoming biseriate towards the valve apices and the valve mantle (black arrow on Fig. 69). The areolae are occluded by cribra, which are slightly lower than the rest of the non-porous valve surface (Figs 65, 67, 68). Such cribrate depressions are not present at the valve apices (see arrow on Fig. 69). Internally the alveoli open via a wide single opening, covered with fine silica layer (Figs 62–64). Ecology and Distribution:—M063A; M247A; M248A; M262A; M272A; M273A; M274A; M280A; M284A; M287A; M288A; M289A; M290A; M291A; M329A; M330A: widely distributed throughout Lake Hövsgöl on habitats ranging from sand, epipelon, sediments, marl, and epiphytic on Chara. Diploneis paraparma sp. nov. (Figs 70–79) Valves are elliptic to linear-elliptic with convex margins and rounded ends (Figs 70–76, 79). The valve length is 19.5–42.0 µm and the valve breadth 12.5–16.5 µm. The axial area is narrow, linear to lanceolate and slightly expanding into a lanceolate and weakly asymmetric central area. From inside a thick silica plate covers the whole length of the longitudinal canal (Fig. 78). The central area is lanceolate and slightly asymmetrical, 1.5–3.0 µm wide. Externally, the longitudinal canal appears narrow, lanceolate to linear and slightly expanded in the middle of the valve. Two rows of areolae open externally, slightly widening into one row of larger areolae towards the valve apices (Figs 76, 79). The external openings of the canal are occluded with cribra (Figs 76, 77, 79), which are open in a depression slightly lower that the rest of the non-porous silica. From inside the longitudinal canal is covered by a thick silica plate (Fig. 78). The heavily siliceous plate forms a “trench” along the whole length of the valve in which the raphe is situated (Fig. 78). From outside the raphe is straight and simple with drop-like proximal ends that are bent to the same side of the valve, positioned with an expanded depression (Fig. 77). Distally, the raphe branches are bent to the same side of the valve into drop-like short terminal fissures (Figs 76, 79). The striae are parallel in mid-valve, becoming radiate towards the distal ends of the valve, 12–15 in 10 µm. Striae are biseriate on the mantle and valve margins, alternately positioned (arrow on Fig. 78) and becoming uniseriate toward the axial area. In some valves the biseriate pattern is present throughout the whole striae length (Figs 71, 75). Each stria is composed of small round to elliptical areolae, 15–20 in 10 µm. Externally, the areolae are covered with cribrate occlusions, increasing in size towards the valve margins (Figs 76, 79). Internally, the alveoli open via a single elongate opening covered with a fine silica layer (Fig. 78). The structure of the alveolus can be seen through the fine siliceous layer (Fig. 78). Type:—MONGOLIA, Lake Hövsgöl (Hövsgöl National Park), south end of lake near Hatgal. Coordinates: 50°25.704’ N; 100°9.137’ E, epipelon from 0.1 m depth (accession number: M129A, M.B. Edlund Collection, Science Museum of Minnesota, collected by Mark B. Edlund, Eugene F. Stoermer and Nergui Soninkhishig, 13 June 1996) (Slide M129A, ANSP GC-36352, GCM-24054), holotype, designated here; example specimen on Fig. 70; (Slide 918029, CAS, isotype designated here). Etymology:—The species name refers to this taxon’s close relationship and confusion with Diploneis parma. Observations:—Diploneis paraparma can easily be associated with D. parma. However, the population from Lake Hövsgöl is characterised with an elongated outline compared to the elliptical outline in the D. parma lectotype given by Idei & Kobayasi (1986a: figs 1, 2). Hustedt (1937) depicts two different shapes for D. parma, an elliptic and an elongate-elliptic, arguing that these differences in outline do not merit taxonomic separation. However, the observations of the type material (Idei & Kobayasi 1986a) did not show specimens with a linear-elliptic form; their analysis of Cleve’s type material showed no relation to later reports for D. parma. It appears that a later D. parma concept has been erroneously perpetuated. Therefore, Hustedt’s concept for two shapes within D. parma is rather questionable because the type material has only a broadly elliptic coarse form. There are no reports for the two different shapes in one locality, and the same is true for Lake Hövsgöl population (this study). Based on historical documentation and our analysis, the concept for two different shapes is poorly supported and therefore we recognize the linear-elliptic form as a new species, Diploneis paraparma. In addition to Hustedt’s misguided and expanded concept, the striae structure is usually reported as uniseriate becoming biseriate towards the valve margins, which was a key feature for further widening the concept of this species. In addition to valve shape differences between D. parma and D. paraparma, morphological differences can be observed in: the structure of the external raphe branches-i) proximal ends (straight without central pores in D. parma vs. straight with drop-like central pores, slightly bent to the same side of the valve in D. paraparma) and ii) distal ends (deflect and straight in D. parma vs. bent into dropDIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 219 like short terminal fissures in D. paraparma); the longitudinal canal (lanceolate expanded in the middle of the valve, composed of one to three areolae in D. parma vs. lanceolate to linear slightly expanded in the middle of the valve, composed of one or two areolae in D. paraparma); and the valve width (16.0–27.5 µm in D. parma, sensu Idei & Kobayasi 1986a vs. 12.5–16.5 µm in D. paraparma). Even though D. parma was described from lakes in Finland and Sweden (Cleve 1891, Idei & Kobayasi 1986a), reports of D. parma exist from different localities worldwide. Some reports likely belong to the now newly described D. paraparma. Further detailed analyses are necessary to ascertain the identity of the many populations widely reported as D. parma. FIGURES 70–79. Diploneis paraparma, LM and SEM valve views. Figs 70–75. LM valve views. Fig. 70. Specimen from the holotype slide, M129A. Figs 76, 79. External view of the whole valve. Fig. 77. External view of the central area with deflected proximal raphe ends. Fig. 78. Internal view of the entire valve. Scale bars = 10 µm (Figs 70–75); 5 µm (Figs 76, 78, 79); 2 µm (Fig. 77). 220 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. Ecology and Distribution:—M068A; M077A; M121A; M129A; M248A; M262A; M329A; M331A; M351A: distributed in central and southern Lake Hövsgöl in the sediments, marl, and in epipelic, epilithic, periphytic communities from 10 cm to 40 m depths. FIGURES 80–90. Diploneis stoermeri morphotype 1, LMs and SEM external valve view. Figs 80–89. LM valve views. Figs 80, 84, 87–89. Specimens from the holotype slide, M068A. Fig. 90. External valve view. Scale bars = 10 µm (Figs 80–89); 5 µm (Fig. 90). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 221 FIGURES 91–96. Diploneis stoermeri morphotype 1, SEM external and internal valve views. Fig. 91. External view of the whole valve. Fig. 92. External view of the distal raphe endings with short terminal fissure. Fig. 93. Internal view of the entire valve. Fig. 94. Close view of the external cribrate occlusions. Fig. 95. Detailed external view of the mid valve. Fig. 96. External view of the proximal raphe endings, bent to the same side of the valve. Scale bars = 5 µm (Figs 91, 93, 95); 2 µm (Figs 92, 94, 96). 222 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. FIGURES 97–112. Diploneis stoermeri morphotype 2, LM and SEM external valve views. Figs 97–108. LM valve views. Figs 109, 111. External view of the whole valve. Fig. 110. External view of the proximal raphe endings. Fig. 112. Distal raphe endings. Scale bars = 10 µm (Figs 97–108, 111); 5 µm (Figs 109, 112); 2 µm (Fig. 110). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 223 Diploneis stoermeri sp. nov. (Figs 80–112) Valves are elliptical-lanceolate becoming almost circular with size reduction, with convex margins and broadly round ends (Figs 80–91, 97–109, 111). The valve length is 19.5–44.5 µm, and the valve breadth is 12.5–22.0 µm. The axial area is linear to lanceolate, expanding towards the round-elongate to rectangular and slightly asymmetric central area. From inside, a thick linear silica plate covers the whole length of the longitudinal canal (Fig. 93). The central area is round-elongate to rectangular and slightly asymmetric, 1.5–4.0 µm wide. Externally, the longitudinal canal is lanceolate to linear, expanded in the middle of the valve with two to three areolae that coalesce into one areola covered with large and complex cribra toward the valve apices (Figs 90, 106, 107, 111, 112). The external openings of the canal are covered with cribrate occlusions similar to those occluding the striae from which they are separated by a thin hyaline area (Figs 90, 91, 109, 110); cribra are located in a depression slightly lower that the rest of the non-porous silica. Internally, the longitudinal canal is covered with silica plate throughout the whole length, forming a “trench” structure housing the raphe slits (Fig. 93). Externally, the raphe is straight and simple with drop-like proximal ends that are deflected to the same side of the valve and positioned within an expanded depression (Figs 90, 95, 96, 109, 110). Towards the central area the raphe branches are located in a slight “depression” below the surface of the rest of the non-porous silica (Figs 90, 95, 109, 111) and surrounded with a silica ridge (Figs 96, 110). Distally, the raphe branches are bent into a short drop-like terminal fissures (Figs 91, 92, 109, 111, 112). Internally, the raphe is straight with simple proximal and distal ends, inserted in a slightly elevated sternum inside the “trench” formed by the longitudinal canal (Fig. 93). The proximal raphe ends are the only raphe structure that reaches the height of the longitudinal canal (Fig. 93). The striae are uniseriate, radiate, 12–15 in 10 µm, composed of round to rectangular areolae, 10–20 in 10 µm. In some valves there is a slight biseriate striae pattern toward the margins (Figs 95, 102). Externally, the alveolate striae open through one row of areolae covered with cribrate occlusions. The cribrate occlusions increase in size and at the same time become more complex in structure toward the valve margins (Figs 90, 94, 95, 109, 110). Internally, the alveoli open through a single linear opening covered with a fine silica layer (Fig. 93). Type:—MONGOLIA, Lake Hövsgöl (Hövsgöl National Park), Hatgal. Coordinates: 50°28.052’ N; 100°10.336’ E, Ulothrix/epilithic from 0.1 m depth (accession number: M068A, M.B. Edlund Collection, Science Museum of Minnesota, collected by Mark B. Edlund, Eugene F. Stoermer, and Nergui Soninkhishig, 11 June 1996) (Slide M068A, ANSP GC-36353, GCM-24055), holotype, designated here; example specimens on Figs 80, 84, 87–89; (Slide 917068, CAS, isotype, designated here; Slide 919072, CAS, paratype, designated here). Etymology:—The species name honors Dr. Eugene Stoermer, University of Michigan, who initiated the research collaboration with the National University of Mongolia with a collecting expedition in 1996. Observations:—Diploneis stoermeri appears to be morphologically variable in shape of the central area (rectangular and slightly asymmetric to round) and in the structure of the longitudinal canal (lanceolate, expanded in the middle of the valve to linear-lanceolate and slightly asymmetrical; compare Figs 80–90 with Figs 97–109). Variability in other distinguishing features, such valve shape, the structure of the striae, and the density of striae and areolae was not observed, but based on the above noted variable structures, we consider the Lake Hövsgöl populations to be a morpholgical complex represented by at least two different phenodemes (Figs 80–96, morphotype 1 and Figs 97–112, morphotype 2). Sympatry among these phenodemes makes their separation even more difficult and opens further questions into their identity and phylogenetic relationship. Such morphological patterns might be correlated with /related to physiological, ecological and/or genetic traits. Therefore, additional analyses including paleolimnology, morphometry and genetic information could provide further support in revealing the patterns and mechanisms for such morphological variations within the D. stoermeri complex, or could simply explain the sympatric coexistence of two genotypically distinct entities (Mann 1999). Diploneis stoermeri can easily be confused with D. paraparma. These two species share similar morphological features, such as the central area, the axial area, and the structure of the longitudinal canal, but differ in striae structure: uniseriate, rarely biseriate towards the valve margins in D. stoermeri and biseriate becoming uniseriate towards the axial area in D. paraparma (compare Figs 80–91 with Figs 70–76). However, using the striae as a distinguishing feature might be a challenge due to the tendency of the cribrate occlusions to become complex in structure towards the valve margins (see Figs 76, 79, 95), which can leave the impression of a biseriate pattern. Hence the cribrate occlusions cover the alveolate striae, the number of areolae is difficult to observe, and sometimes can lead to misinterpretation. Therefore, when using the striae for identification, the uni- and biseriate pattern should be taken as a main feature in separating D. stoermeri from D. paraparma. Diploneis parma differs from D. stoermeri in: the valve outline (broadly elliptical vs. elliptical-lanceolate); the central area (round vs. rectangular and slightly asymmetric); and the striae structure (uniseriate becoming biseriate towards the valve margins vs. uniseriate, rarely biseriate towards valve margins). Diploneis stoermeri closely resembles 224 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. D. krammeri, though the stria density separates them [12–15 in 10 µm vs. 11(12) in 10 µm]. Additionally, the structure of the longitudinal canal further supports the differences between D. krammeri and D. stoermeri (one to two rows of areolae in the middle of the valve narrowing into one at the valve apices vs. two rows of areolae continuing into one that is covered with a large cribrum). Moreover, the extended depression where the proximal raphe ends are placed (Figs 90, 96, 111) is barely present in D. krammeri (Lange-Bertalot & Reichardt 2000; this study, Figs 129, 130, 133). Diploneis elliptica is another morphologically similar taxon, differing in stria density (8–11/10 µm in D. elliptica vs. 12–15/10 µm in D. stoermeri). Ecology and Distribution:—M063A, M068A; M077A; M247A; M248A; M272A; M273A; M274A; M276A; M280A; M284A; M287A; M289A; M291A; M329A; M351A: distributed in shallow to deep waters in the northern and southern areas of Lake Hövsgöl on epipelon, epiphyton, epilithon, marl accretions, and sediment habitats. FIGURES 113–128. Diploneis krammeri, LM and SEM valve views. Figs 113–124. LM valve views. Fig. 125. Internal view of the whole valve. Fig. 126. Detailed view of the internal proximal raphe ends. Fig. 127. Internal view of the distal raphe endings. Fig. 128. Close view of the external cribrate occlusions. Scale bars = 10 µm (Figs 113–124); 5 µm (Figs 125, 127); 2 µm (Fig. 126); 1 µm (Fig. 128). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 225 FIGURES 129–134. Diploneis krammeri, SEM external valve views. Fig. 129. View of the whole valve. Fig. 130. Proximal raphe endings. Fig. 131. View of the whole frustule. Fig. 132. Showing the structure of the striae at the valve mantle. Fig. 133. Detailed view of the valve central area. Fig. 134. Distal raphe endings with short deflected terminal fissures. Scale bars = 5 µm (Figs 129, 131); 2 µm (Figs 130, 132–134). 226 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. Diploneis krammeri Lange-Bertalot & Reichardt (Figs 113–134) Valves are elliptical-lanceolate with convex margins and round ends (Figs 113–124). The valve length is 23.0–34.0 µm, and the valve breadth is 13.0–17.0 µm. The axial area is linear to lanceolate, expanding toward the round to slightly elongate central area. Internally a thick silica plate covers the longitudinal canal throughout (Fig. 125). Externally the central area is elongate, 2.5–3.5 µm wide. From outside the longitudinal canal is linear to lanceolate, expanded in the middle of the valve with one to two rows of areolae. The external openings of the canal areolae are covered with cribra similar to those on the striae, from which the canal areolae are separated with a thick hyaline area (Figs 129, 130, 133). From inside, the longitudinal canal is covered with thick silica forming a “depression” where the raphe is placed (Figs 125, 156). Externally, the raphe is straight with expanded proximal ends slightly bent to the same side of the valve and positioned within a small depression (Figs 129, 130, 133). From outside the distal raphe endings finish with short terminal fissures bent to the same side of the valve (Figs 129, 131, 132, 134). Internally, the raphe is straight and simple, placed in the “depression” formed by the longitudinal canal (Fig. 125). The proximal raphe endings are the only raphe section reaching the height of the longitudinal canal in contrast to the only slightly elevated distal ends (Figs 125–127). The striae are radiate, 11(12) in 10 µm, with round areolae covered with cribra (Fig. 128), 13–15 in 10 µm. The external areolar openings increase in size and become more complex in structure, or are biseriate toward the valve margins (Figs 129, 131, 132, 134). Internally the alveoli open via a single continuous and elongate opening covered with a fine silica layer (Figs 125, 126). Observations:—In the original description of D. krammeri the longitudinal canal is composed of one row of areolae throughout the whole length (Lange-Bertalot & Reichardt 2000: pl. 4, figs 1–10, 12). However, on the SEM illustrations the external openings of the longitudinal canal are composed of two rows of areolae in the middle of the valve narrowing into one at the valve apices (Lange-Bertalot & Reichardt 2000: pl. 5, figs 1–5). The longitudinal canal in Lake Hövsgöl populations are composed of one row of areolae throughout, which unambiguously fits the LM illustrations given by Lange-Bertalot & Reichardt (2000). Ecology and Distribution:—M063A; M248A; M273A; M274A; M280A; M329A: distributed in shallow to deep benthic habitats throughout Lake Hövsgöl in the sediments and marl. Diploneis soninkhishigae sp. nov. (Figs 135–151) The valves are elliptical-lanceolate with convex margins and round ends (Figs 135–148). The valve length is 14.5–24.5 µm, and the valve breadth is 8.0–12.0 µm. The axial area is narrow, linear to lanceolate expanding into a small elongate central area. From inside, a wide linear silica plate covers the longitudinal canal throughout the whole valve length (Fig. 149). The central area is small and elongate, 1.5–2.5 µm wide. Externally, the longitudinal canal is narrow slightly expanded in the middle of the valve with one row of areolae throughout the whole valve length (Figs 135–148). The areolae of the longitudinal canal are covered with cribra similar to the striae (Fig. 150). In the middle of the valve the cribra of the canal are simple, becoming wider and more complex in structure towards the valve apices (Fig. 148). From inside, the longitudinal canal is closed with thick silica plates, forming a “trench” where the raphe is inserted. Externally, the raphe is simple and straight with drop-like proximal ends that are slightly bent, positioned within an expanded depression (Figs 148, 150). Distally, the raphe branches finish with short drop-like terminal fissures, bent to the same side of the valve (Fig. 148). The distal raphe ends terminate some distance from the valve mantle. Internally, the raphe is straight with simple proximal raphe ends, placed in the “trench” formed by the longitudinal canal (Fig. 149). The proximal raphe endings are raised in the central area, while the distal raphe ends are also slightly raised but terminate into small helictoglossae (Fig. 149). Striae are usually uniseriate, becoming biseriate toward the valve margins, 11–15 in 10 µm, and composed of round areolae, 10–15 in 10 µm. Externally, the areolae are covered with cribrate occlusions (Fig. 150), increasing in size towards the valve margins (Fig. 151). From inside, each alveolus opens through a single and continuous opening covered with a thin silica layer (Fig. 149). Type:—MONGOLIA, Lake Hövsgöl (Hövsgöl National Park). Coordinates: 50°55.056’ N; 100°37.912’ E, Sediments from 40 m depth (accession number: M248A, M.B. Edlund Collection, Science Museum of Minnesota, collected by Mark B. Edlund and Nergui Soninkhishig, 18 July 1998) (Slide M248A, ANSP GC-36354, GCM-24056), holotype, designated here; example specimens on Figs 136, 137; (Slide 919048, CAS, isotype designated here). Etymology:—The species is named in honor of Dr. Nergui Soninkhishig, National University of Mongolia, Botany Faculty, for her pivotal role in developing diatom research in Mongolia and our long friendship. Observations:—Diploneis soninkhishigae closely resembles Diploneis vetusa Jovanovska, Nakov & Levkov (2013a: figs 106–121), which is characterized with biseriate striae throughout the whole length. In contrast, Diploneis soninkhishigae has uniseriate striae becoming biseriate towards the valve margins. A population of D. soninkhishigae was also observed in Lake Ohrid, Macedonia (Figs 142–147). The striae pattern is more visible in the Lake Ohrid DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 227 population (Figs 145, 147). D. fontanella has a linear-elliptical valve shape in contrast to the elliptical-lanceolate outline in D. soninkhishigae. In addition, D. fontanella has a higher stria and areola density (striae: 16–18 in 10 µm; areolae: 22–25 in 10 µm), uniseriate striae and a round central area (Werum & Lange-Bertalot 2004). Diploneis separanda Lange-Bertalot (2004: pl. 76, fig. 1–16; pl. 77, fig. 1–5) is another linear-elliptic taxon, with uniseriate striae becoming biseriate towards the valve margins. However, in addition to differences in valve shape, there are also differences in the stria density (11–15 in 10 µm in D. soninkhishigae vs. 19–21 in 10 µm in D. separanda) and the areola density (10–12 in 10 µm in D. soninkhishigae vs. 30–35 in 10 µm in D. separanda). Additionally, SEM images show that the cribrate occlusions in D. soninkhishigae are more numerous and more complex in structure than those in D. separanda (Werum & Lange-Bertalot: 338, pl. 76, figs 16, 15). FIGURES 135–151. Diploneis soninkhishigae, LM and SEM valve views. Figs 135–141. LM valve views, Lake Hövsgöl. Figs 142–147. LM valve views, Lake Ohrid specimens. Figs 136, 137. Specimens from the holotype slide M248A. Fig. 148. External valve view. Fig. 149. Internal view of the whole valve. Fig. 150. View of the valve central area with proximal raphe ends. Fig. 151. Close-up view of the external openings of the alveolate striae, covered with cribrum. Scale bars = 10 µm (Figs 135–147); 5 µm (Figs 148, 149); 2 µm (Figs 150, 151). 228 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. Ecology and Distribution:—M248A; M272A; M273A; M274A; M287A; M289A; M291A: found throughout Lake Hövsgöl in shallow to deep water habitats including marl, sediments, and epipelon. FIGURES 152–165. Diploneis exigua, LM and SEM valve views. Figs 152–159. LM valve views. Figs 153, 156, 158. Specimens from the holotype slide, M272A. Fig. 160. External view of the entire valve. Fig. 161. External distal raphe endings with short terminal fissure. Fig. 162. Broken valve, showing the external and internal opening of the alveolate striae. Fig. 163. Internal view of the distal raphe endings. Fig. 164. Showing the simple proximal raphe endings from the interior. Fig. 165. Internal view of the whole valve. Scale bars = 10 µm (Figs 152–160, 165); 3 µm (Fig. 161); 2 µm (Fig. 162); 1 µm (Figs 163, 164). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 229 Diploneis exigua sp. nov. (Figs 152–165) Valves are elliptical with convex margins and round ends (Figs 152–160). The valve length is 14.5–17.0 µm, and the valve breadth is 9.0–10.5 µm. The axial area is narrow, linear, and expands into a small rectangular central area. From inside, a thin silica plate covers the whole length of the longitudinal canal (Fig. 165). From outside, the central area is small and rectangular, ca. 1.5 µm wide. Externally, the longitudinal canal appears linear with one to two rows of small areolae in the middle of the valve continuing into one or two areolae covered with a large cribrum towards the valve apices (Figs 160, 161). The areolae of the longitudinal canal are covered with cribra morphologically identical to those on the striae, from which they are separated with a narrow hyaline area (Fig. 161). In the middle of the valve the areolae of the canal are covered with small and simple cribra that become larger and irregularly polygonal towards the valve apices (Fig. 160). Internally the longitudinal canal is closed with a thick linear silica plate forming a “depression” where the raphe is placed (Fig. 165). Externally, the raphe is straight and simple with expanded drop-like proximal ends that are bent to the same side of the valve and positioned within an expanded depression (Fig. 160). Toward the central area the raphe branches drop into a “depression” below the rest of the non-porous silica, and are surrounded with a thick silica ridge (Fig. 160). The depression is deepest and widest at the central area and gradually narrows toward the raphe distal ends. Distally, the raphe branches are bent into a short drop-like terminal fissures deflected to the same side of the valve (Figs 160, 161). Internally the raphe is straight with simple proximal ends, inserted in a slightly elevated sternum inside the “depression” formed by the longitudinal canal (Fig. 165). Distally, the raphe endings are slightly bent to the same side of the valve into very short terminal fissures (Fig. 163). Striae are parallel in the middle of the valve, becoming radiate at the distal ends of the valve, 15–16 in 10 µm, and composed of round areolae, 16–20 in 10 µm. Striae are uniseriate becoming biseriate and/or complex in structure towards the valve margins. From outside, the areolae are covered with cribrate occlusions, which increase in size towards the valve margins, becoming largest at the mantle (Figs 160–162). Internally the alveoli open via single continuous and elongate openings covered with a fine silica layer (Figs 162–165). Type:—MONGOLIA, Lake Hövsgöl (Hövsgöl National Park), Station anchored off Hanh Gol mouth. Coordinates: 51°27.268’ N; 100°43.11’E, Chara from 4 m depth (accession number: M272A, M.B. Edlund Collection, Science Museum of Minnesota, collected by Mark B. Edlund and Nergui Soninkhishig, 19 July 1998) (Slide M272A, ANSP GC-36355, GCM-24057), holotype, designated here; example specimens on Figs 153, 156; (Slide 919072, CAS, isotype designated here). Etymology:—The species name refers to the taxon’s small size. Observations:—With respect to the valve size, Diploneis exigua is very similar to D. soninkhishigae, but differs in: valve shape (elliptical vs. elliptical-lanceolate); central area (rectangular vs. elongate); striae density (15–16 in 10 µm vs. 11–15 in 10 µm); and areola density (16–20 in 10 µm vs. 10–12 in 10 µm). Diploneis exigua has a similar central area as D. stoermeri morphotype 1 (compare Figs 152–159 with Figs 80–89). However, the size range (length: 14.5–17.0 µm in D. exigua vs. 19.5–44.5 µm D. stoermeri; breadth: 9.0–10.5 µm in D. exigua vs. 12.5–22.0 µm in D. stoermeri) separates these two species. Diploneis paraparma differs from D. exigua by the greater size range (length: 19.5–42.0 µm vs. 14.5–17.0 µm; breadth: 12.5–16.5 µm vs. 9.0–10.5 µm) and the striae structure (uniseriate becoming biseriate towards the valve margins vs. uniseriate). Ecology and Distribution:—M248A; M272A; M273A; M274A; M291A: found in depths from 4 to 40 m in the central and northern areas of Lake Hövsgöl in sediment and epiphytic on Chara. Diploneis elliptica cf. var. ladogensis Cleve (Figs 166–169) Valves are linear-elliptic with weakly convex margins and round ends. The valve length is 24.5–29.0 µm and the breadth is 9.5–11.0 µm. The axial area is narrow, expanding into a small, round to slightly elongate central area. The central area is ca. 1.5 µm wide. The longitudinal canal is narrow, linear and slightly expanded in the middle, with a single row of areolae throughout the whole length. Raphe branches are straight and simple, positioned with an expanded depression. Striae are parallel in mid-valve, becoming slightly radiate towards the distal ends of the valve, 12(13) in 10 µm, with round to rectangular areolae that are secondarily aligned in wavy longitudinal rows, 12 in 10 µm. Observations:—Cleve (1891) drew Diploneis elliptica var. ladogensis with an elliptical outline and moderately large and round central area. Later, Hustedt (1930, 1937) provided drawings of Diploneis elliptica var. ladogensis with a more or less rhombic outline and round central area. The population observed from Lake Hövsgöl is characterized with an elliptical to linear-elliptic outline and a small and round to slightly elongate central area. However, in this study additional differences can be observed between Cleve’s drawing and the population from Lake Hövsgöl: size range (length: 60.0 µm vs. 24.5–29.0 µm; breadth: 25.0 µm vs. 9.5–11.0 µm); stria density (9 in 10 µm vs. 12(13) in 10 µm); and areola density (10 in 10 µm vs. 12 in 10 µm). These differences might be as a result of the size dimensions given from a single valve (Cleve 1891), and/or because of the small number of observed valves in this study. Further analyses 230 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. could potentially provide a greater size range in Lake Hövsgöl, which might overlap the measurements given in the original description. Diploneis elliptica var. ladogensis closely resembles Diploneis meyeri Skabichevskii (1936: 709, pl. 1, fig. 56). Both species are similar in valve outline, but differ in the size range (length: 33.7–48.6 µm in D. meyeri vs. 60.0 µm in D. elliptica var. ladogensis; breadth: 18.0–23.4 µm in D. meyeri vs. 25.0 µm in D. elliptica var. ladogensis), and in the structure of the longitudinal canal (wide in D. meyeri vs. narrow in D. elliptica var. ladogensis). Additionally, thick silica furrows are present on the valve face in D. meyeri, interrupting and crossing the striae, which are not present in D. elliptica var. ladogensis. A year later, Skvortzow (1937) described a different unrelated species under the same name D. meyeri (an illegitimate later homonym). Due to the broader size range and the width of the longitudinal canal and the absence of silica furrows on the valve face, the population from Lake Hövsgöl is better situated as D. elliptica var. ladogensis (see Krammer & Lange-Bertalot 1986, fig. 108: 5, p. 658); however, due to size differences with the type we retain the “cf.” designation in our identification. FIGURES 166–183. LM valve views. Figs 166–169. Diploneis elliptica cf. var. ladogensis. Figs 170–172. Diploneis pseudovalis. Figs 173–178. Diploneis subovalis. Fig. 179. Diploneis mongolica. Figs 180–183. Diploneis spectabilis. Scale bar = 10 µm. DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 231 Ecology and Distribution:—M247A; M276A; M289A: found on rocks and sediments in 5 to 40 m deep collections from central and northern Lake Hövsgöl. Diploneis pseudovalis Hustedt (Figs 170–172) Valves are linear-elliptic with slightly convex margins and round ends. The valve length is 29.5–36.5 µm and the valve breadth is 10.5–11.0 µm. The axial area is narrow, linear to linear-lanceolate expanding into a moderately large central area. The central area is elongate-ovoid and 3.0–3.5 µm wide. The longitudinal canal is linear to lanceolate with two rows of areolae throughout its whole length, expanding in the middle of the valve. The raphe branches are straight and simple, positioned with an expanded depression. Striae are biseriate, radiate throughout, 12(13) in 10 µm, consisting of small round areolae, 20–25 in 10 µm along a single row of the biseriate pattern. Ecology and Distribution:—M272A; M289A; M290A: found in northern Lake Hövsgöl on Chara and sediment collections taken from 4 to 30 m depth. Diploneis subovalis Cleve (Figs 173–178) Valves are broadly elliptic in larger specimens to linear-elliptic in smaller specimens with convex margins and rounded apices. The valve length is 20.0–33.0 µm and the breadth is 9.0–17.0 µm. The axial area is linear to lanceolate expanding into a large elongate central area. The central area is elongate, 2.0–4.0 µm wide. The longitudinal canal is narrow and expanding in the middle of the valve; linear to lanceolate with two rows of areolae throughout its whole length. The raphe is straight and simple, positioned with an expanded depression. Striae are radiate and biseriate, 10–14 in 10 µm, composed of small round areolae, 20–25 in 10 µm, counting along one row of the biseriate pattern. Ecology and Distribution:—M063A; M272A; M273A; M247A; M276A; M280A: found throughout Lake Hövsgöl at depths of 4 to 40 m on rocks, Chara, epipelon, marl, and sediments. Diploneis mongolica Metzeltin, Lange-Bertalot & Nergui (Fig. 179) The valve is elliptical with convex margins and bluntly rounded apices. The valve length is 34.5 µm and the valve breadth is 15.5 µm. The axial area is narrow and lanceolate, expanding into a large elongate central area. The central area is elongate-oval, 4.5 µm wide. The longitudinal canal is lanceolate with two rows of areolae in the middle of the valve, gradually narrowing into one row toward the valve apices. The raphe is straight and simples, positioned with an expanded depression. The striae are uniseriate and radiate, 18 in 10 µm, composed of round to rectangular areolae, 12 in 10 µm. Observations:—Diploneis mongolica was described by Metzeltin et al. (2009) from Lake Khangal, Mongolia. The type material of D. mongolica was observed and compared with the specimen from Arkhangai region. In contrast to the type population, where D. mongolica is quite dominant, in Arkhangai region this taxon is extremely rare with only one specimen found. Even though larger valves of the given size range were not observed, the observed specimen fully fits the original description given by Metzeltin et al. (2009). Ecology and Distribution:—M166A; M167A: examined for comparison, not found in Lake Hövsgöl but distributed in lakes and streams in Arkhangai and Khentii provinces (Metzeltin et al. 2009). Diploneis spectabilis Metzeltin, Lange-Bertalot & Nergui (Figs 180–183) Valves are elliptical with convex margins and broadly round apices becoming bluntly rounded with the valve size reduction. The valve length is 28.5–38.0 µm, and the valve breadth is 13.0–18.0 µm. The axial area is very narrow and linear, expanding towards the large almost circular to rectangular central area. The central area is 4.5–5.0 µm wide. The longitudinal canal is narrow, linear with a single row of areolae throughout the whole length. The central area is broadly expanded. The raphe is straight and simples, positioned with an expanded depression. Striae are uniseriate and radiate, 18 in 10 µm, composed of round areolae, 20 in 10 µm. Observations:—Diploneis spectabilis was described from Lake Khangal, Mongolia (Metzeltin et al. 2009). Observations on the type material were performed in order to ascertain the identity of Arkhangai population. Detailed LM comparisons of the type population and Arkhangai population revealed morphological differences in: valve size (length: 36.0–37.0 µm vs. 28.5–38.0 µm; breadth: 19.0–31.0 µm vs. 13.0–18.0 µm); stria density (15–16 in 10 µm vs. 18 in 10 µm); and areola density (16–18 in 10 µm vs. 20 in 10 µm). In Arkhangai materials D. spectabilis was very rare, 232 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. while in Lake Khangal was quite frequent. The differences in size dimensions might be a result of the small number of specimens in Arkhangai material or ecophenotypic variation. However, in most specimens (there is some variability in valve end shape; compare Fig. 181 with Figs 180, 183) the valve outline in Arkhangai’s population fits the original description given by Metzeltin et al. (2009). The differences between D. mongolica and D. spectabilis (see Metzeltin et al. 2009) are based on the size range (38.0–58.0 µm vs. 36.0–72.0 µm) and stria density (12.5–14 in 10 µm vs. 15–16 in 10 µm). Additionally, the structure of the longitudinal canal (one row of areolae throughout the whole length in D. spectabilis vs. two to three rows of areolae in the mid-valve narrowing into two to one at the valve apices in D. mongolica) also separates these species. However, further SEM analyses are necessary to better distinguish differences between D. spectabilis and D. mongolica and within populations of each, and should be supported by additional morphological data. Ecology and Distribution:—M166A; M167A: examined for comparison, not found in Lake Hövsgöl but more broadly distributed in lakes and streams in Arkhangai and Khentii provinces (Metzeltin et al. 2009). Diploneis linearielliptica Metzeltin, Lange-Bertalot & Nergui (Figs 184–196) The valves are linear-elliptical with slightly convex margins and round ends (Figs 184–191). The valve length is 21.0–34.5 µm, and the valve breadth is 7.0–10.5 µm. The axial area is linear to lanceolate, merging with and into a distinctly enlarged round central nodule. From inside, a silica plate covers the whole length of the lanceolate longitudinal canal (Figs 192, 194). The central area is 3.0–4.5 µm wide. From the internal side, the central nodule is raised and distinct (Figs 192–194). The longitudinal canal is narrow and linear, expanded in the middle of the valve, with one row of areolae throughout the whole valve length. From inside the longitudinal canal is covered with a silica plate, forming a “depression” where the raphe is placed (Figs 192, 194). Externally, the raphe is straight and simple, positioned with an expanded depression. Internally, the raphe is straight, inserted in a slightly elevated sternum inside the “depression” formed by the longitudinal canal (Figs 192, 194). The proximal raphe ends are simple and terminate in small drop-like terminal fissures at the margins of the central area, but are not raised onto the central area itself (Figs 193, 194). Distally, the raphe endings are raised into small helictoglossae (Figs 195, 196). The striae are radiate, 15–17 in 10 µm, composed of round areolae, 15–20 in 10 µm. Internally, the alveoli each open via a single oval and continuous opening (Figs 192, 194). This species is characterized with small hyaline area near the longitudinal canal (see arrows on Figs 186, 188). Observations:—Diploneis linearielliptica is described from Mongolian lakes in Khentii province (Metzeltin et al. 2009). The population from the type locality is characterized by a narrow size range in terms of that observed in Lake Hövsgöl and Arkhangai populations (length: 32.0–46.0 µm vs. 21.0–34.5 µm, breadth: 10.0–14.0 µm vs. 7.0–10.5 µm). The stria density is an additional character that distinguishes these populations (15–17 in 10 µm vs. 11–14 in 10 µm). However, these differences may be the result of an insufficient number of observed valves in the material. SEM analyses of the valve exterior are needed to examine the potential narrow hyaline structure alongside the longitudinal canal. Ecology and Distribution:—M052A; M166A; M167A; M262A: found in shallow habitats (<1 m depth) in central and southern Lake Hövsgöl and also in streams in Arkhangai province and lakes of Khentii province (Metzeltin et al. 2009). Diploneis aff. fontanella Lange-Bertalot (Figs 197–205) The valves are elliptical with slightly convex margins and broadly rounded ends. The valve length is 17.0–22.0 µm, and the valve breadth is 8.0–9.0 µm. The axial area is narrow, linear to lanceolate expanding toward the central area. The central area is round, 2.5–3.5 µm wide. The longitudinal canal is narrow, linear to lanceolate, and expanded in the middle of the valve, with one row of areolae throughout the whole length. The raphe is straight and simple, positioned with an expanded depression. The striae are radiate, 16–20 in 10 µm, composed of round areolae, 25–30 in 10 µm. Observations:—The observed population is very similar to Diploneis fontanella, from which it differs in stria density (16–20 in 10 µm vs. 16–18 in 10 µm) and areola density (25–30 in 10 µm vs. 22–25 in 10 µm). The valve shape is an additional feature when distinguishing this Arkhangai population from the type (elliptical vs. elliptical to linearelliptic). In Khentii province Metzeltin et al. (2009) identified a population of D. fontium Reichardt & Lange-Bertalot (2004: pl. 4, fig. 1–6, pl. 5, fig. 1–7), which morphologically is similar to D. aff. fontinella. The valve measurements of the Arkhangai population more closely resemble D. fontanella than D. fontium, therefore we have identified it as D. aff. fontinella. In order to test the conspecificity between these two species, SEM analyses are necessary and should be the subject of further studies. DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 233 Ecology and Distribution:—M166A; M167A: not found in Lake Hövsgöl nor previously reported from Mongolia. Collections are from a small stream in Arkhangai province. FIGURES 184–205. LM and SEM internal valve views. Figs 184–196. Diploneis linearielliptica. Figs 184–191. LM valve views. Figs 192, 194. View of the whole valve. Fig. 193. Central area with proximal raphe endings. Figs 195, 196. Distal raphe endings. Figs 197–205. Diploneis aff. fontanella, LM valve views. Scale bars = 10 µm (Figs 184–192, 197–205); 5 µm (Fig. 194); 2 µm (Figs 193, 195, 196). 234 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. FIGURES 206–210, 213–219. Diploneis aff. heteromorphiforma, LM and SEM valve views. Figs 211, 212. Diploneis dimorpha, type slide (O4/78) from Hustedt collection, the Friedrich Hustedt Study Centre for Diatoms in Bremerhaven, Germany. Figs 206–212. LM valve views. Fig. 213. Valve central area with proximal raphe endings. Fig. 214. View of the whole valve. Fig. 215. Close view of the external distal raphe endings. Fig. 216. Internal valve view. Fig. 217. Internal view of the central area with proximal raphe ends. Fig. 218. Showing the distal raphe endings from the interior. Fig. 219. View of the alveoli structure in a corroded specimen. Scale bars = 10 µm (Figs 206–212, 216); 5 µm (Figs 214, 219); 2 µm (Figs 213, 215, 217, 218). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 235 Diploneis aff. heteromorphiforma Metzeltin, Lange-Bertalot & Nergui (Figs 206–210, 213–219) The valves are linear-elliptical with slightly convex margins and round ends (Figs 206–210). The valve length is 16.0–29.0 µm, and the valve breadth is 8.5–11.5 µm. The axial area is narrow, lanceolate, and expands toward the round to elongate central area. From inside, a thick lanceolate silica plate covers the longitudinal canal (Fig. 216). The central area is round to elongate, 2.5–4.5 µm wide. The longitudinal canal is narrow, linear to lanceolate, expanded in mid-valve, composed in parts with two rows of areolae and in parts with one row of areolae (Fig. 214). The external openings of the canal areolae are covered with cribra, similar to those in the striae and from which they are separated by a thin hyaline area (Figs 214, 215). Internally, the longitudinal canal is closed with thick silica plate throughout its length, forming a “depression” where the raphe is located (Fig. 216). Externally, the raphe is simple and straight with slightly curved and expanded central pores (Figs 213, 214). The distal raphe branches are deflected, and the terminal fissures are slightly bent into a drop-like shape (Figs 214, 215). Internally, the raphe is straight with simple and slightly expanded proximal ends, inserted in a slightly elevated sternum inside the wide “depression” formed by the longitudinal canal (Fig. 216). The distal raphe endings are slightly curved (Fig. 218). Only the proximal raphe endings reach the height of the canal itself (Fig. 217). The striae are radiate and uniseriate becoming biseriate toward the valve margins, 9–11 in 10 µm. The striae areolae are round and occluded externally with cribra, 25–30 in 10 µm. The areolae are barely visible with LM observations, leaving the impression that each alveolate striae is composed of two rows of areolae (Fig. 206). In highly corroded valves it seems that from the valve exterior the alveolus opens though a single opening covered with cribrate occlusions (Fig. 219). The alveoli each open to the valve interior via a single continuous opening (Figs 216–218). However, from the inside, it appears that each alveolus initially opens toward the valve exterior via three areolae aligned in staggered transapical rows (Figs 217, 218), then transition to the cell exterior through a layer of one or two cribrate occlusions that must each cover multiple internally staggered areolae (213–215). The external openings of the areolae (uniseriate becoming biseriate) and the LM observations (biseriate throughout) is misleading as to the detailed structure of the alveolate striae and their areolae. Observations:—Diploneis aff. heteromorphiforma closely resembles D. heteromorphiforma, described from Zyyn byrkh River in Khentii province in Mongolia (Metzeltin et al. 2009). Morphologically D. aff. heteromorphiforma is almost identical to D. heteromorphiforma, but differs in size range (length: 16.0–29.0 µm vs. 22.0–43.0 µm; wide: 8.5– 11.5 µm vs. 10.0–15.0 µm) and in the shape of the axial area (narrow-lanceolate vs. wide to narrow-lanceolate). Stria density is an additional character that separates D. heteromorphiforma [“8–9 (not 10)”] from D. aff. heteromorphiforma (9–11 in 10 µm). When describing D. heteromorphiforma, Metzeltin et al. (2009) illustrated two different morphotypes [narrow, lanceolate axial area (pl. 108, figs 1–3, 16) vs. wide, lanceolate axial area (pl. 108, figs 4–15)], and designated the wide axial area form as the type (pl. 108, fig. 4). However, when illustrating type specimens they referred to both narrow and wide axial area specimens (pl. 108, figs 1–3, 6, 10–12, 14). Due to this, it is rather difficult to characterize D. heteromorphiforma, and an emended typification is in order to clarify the identity of the given morphs. Furthermore, Metzeltin et al. (2009) were not consistent in designating the type locality for D. heteromorphiforma. In the description they gave Zyyn byrkh River as a type locality, while in the illustrations they point to Jargalant River (pl. 108, fig. 4). Additionally, Metzeltin et al. (2009) argue that the population from Zyyn byrkh River is characterized with morphological variation within the axial area, explaining the variation as possible for abnormal clones or maybe a separate species. Such morphological variations are observed neither in Lake Hövsgöl nor in the Arkhangai province. Furthermore, in the description Metzeltin et al. (2009) compared D. heteromorphiforma to a non-existent D. heteromorpha Hustedt, when they should have referred to Diploneis dimorpha Hustedt (1937: pl. 1, fig. 18–19). Keeping in mind that D. dimorpha is characterized by heterovalvar frustules and has a distribution restricted to marine and brackish environments, it is rather unlikely that D. dimorpha should be compared with D. heteromorphiforma and D. aff. heteromorphiforma (Hustedt 1937, Witkowski et al. 2000, Witon et al. 2006). The heterovalvar frustule is nicely visible in Simonsen’s illustrations (1987, pl. 680, figs 12–14). In order to support the comparisons and therefore to observe the heterovalvar feature, the Hustedt type material of D. dimorpha (O4/78) was analysed and included in this study (Figs 211, 212). According to the comparisons, no Mongolian population seems to be allied with the type of D. dimorpha. When describing D. heteromorphiforma, Metzeltin et al. (2009) mentioned Genkal’s report for D. dimorpha (Lange-Bertalot & Genkal 1999: 194, pl. 43, figs 12, 13), characterized with frustules that have isomorphic valves and suggested their separation into a new species. According to the comparisons, D. aff. heteromorphiforma appears to be different from both D. heteromorphiforma morphotypes (Metzeltin et al. 2009) but closely allied to Genkal’s description of D. dimorpha (Lange-Bertalot & Genkal 1999). Further analysis is therefore preferable in order to clarify the morphological variation within the D. heteromorphiforma complex and possibly to ascertain any phylogenetic relationships with D. dimorpha. The original drawings (iconotype) of D. robusta Cleve-Euler (1953, fig 622 a–c) are very similar to D. aff. 236 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. heteromorphiforma, whose numerical features unambiguously fit the measurements given in the original description of D. robusta. However, additional analyses such as observation of D. robusta type material are needed in order to test the similarity, or the possible conspecificity of these species. Ecology and Distribution:—M068A; M166A; M167A: found in a shallow epilithic collection from southern Lake Hövsgöl and also in streams of Arkhangai province. FIGURES 220–235. Diploneis boldtiana, LM and SEM valve views. Figs 220–230. LM valve views. Fig. 231. External valve view. Fig. 232. Internal view of the entire valve. Fig. 233. Detailed view of the external irregular and round areolar pores/openings. Fig. 234. External central area with proximal raphe endings, which are bent to the same side of the valve. Fig. 235. View of the distal raphe endings with a short terminal fissure. Scale bars = 10 µm (Figs 220–232); 2 µm (Figs 233–235). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 237 Diploneis boldtiana Cleve (Figs 220–235) Valves are lanceolate-elliptic with slightly convex margins and bluntly rounded ends (Figs 220–231). The valve length is 16.5–26.0 µm, and the valve breadth is 8.0–9.0 µm. The axial area is very narrow, slightly expanding into a small to indistinguishable central area, 1.5–2.0 µm wide (Figs 222–225). From inside, a thick and lanceolate silica plate closes the longitudinal canal throughout its whole length (Fig. 232). Externally, the longitudinal canal appears narrow, lanceolate to linear, slightly expanded in the middle of the valve with one to two rows of areolae becoming three towards the valve apices (Figs 231, 234). In LM it seems like the longitudinal canal is composed of one row of areolae throughout the whole length of the canal (Figs 220–230). From the exterior, the areolae of the canal are covered with simple cribra formed by irregular, small and round (3–10) pores, arranged in groups (Figs 234, 235). The structure of areolar cribra is morphological similar to those from the striae, from which they are separated with a narrow hyaline area (Figs 231, 233–235). Internally the longitudinal canal is closed with a thick silica plate, forming a “depression” where the raphe is inserted (Fig. 232). From the outer wall the raphe is straight with expanded proximal ends, bent to the same side of the valve (Figs 231, 234). The distal raphe branches are bent into a short drop-like terminal fissure (Fig. 235). Internally, the raphe is straight with simple proximal and distal ends, inserted in a slightly elevated sternum inside the “trench” formed by the longitudinal canal (Fig. 232). The proximal raphe endings reach the height of longitudinal canal and the valve itself (Fig. 232). Striae are parallel in the middle becoming radiate at the valve apices, 15–18 in 10 µm. Each alveolate stria is composed of two rows of areolae, covered with simple cribrate occlusions (Figs 231, 235). The irregular and round cribrate pores (4–5), are increasing in number towards the valve margins (7–9). The perforations of the cribra are more regularly scattered then those of the longitudinal canal, where the perforations are more densely arranged (Figs 233, 235). At the valve apices the last two-three alveolate striae have a triangular shape (arrow on Fig. 235). Internally each alveolus opens through a single continuous opening covered with a fine silica layer (Fig. 232). The single continuous opening has a clavate shape in the middle of the valve, becoming elongate in shape towards the valve apices. Observations:—Cleve (1891: 43, pl. 2, fig. 12 ) in the original description gives only a single valve measurement for D. boldtiana (length: 30.0 µm, breadth: 12.0 µm, and stria density: 14 in 10 µm). Later, Hustedt (1937) illustrated a broader size range (length: 23.0–38.0 µm, breadth: 10.0–12.0 µm, and stria density: 14–15 with 27–30 areolae in 10 µm). According to the typification provided by Idei & Kobayashi (1989b), D. boldtiana unambiguously fits the measurements given by Cleve (length: 30.0 µm, breadth: 11.0 µm, and stria density: 14 in 10 µm). In addition, Idei & Kobayashi (1989b) observed a population of D. boldtiana in the Finnish lakes, though with greater valve lengths (20.0–30.0 µm), but similar valve widths (11.0–12.0 µm) and stria densities (13/14 in 10 µm). The smaller size range observed in Lake Hövsgöl could be result of insufficient number of observed valves. The populations from Lake Hövsgöl nicely fit the LM and SEM illustrations provided by Idei & Kobayashi (1989b). Ecology and Distribution:—M063A; M166A; M247A; M272A; M273A; M280A; M330A: found in shallow to deep habitats throughout Lake Hövsgöl including epipelon, Chara, marl, and sediments. Also found in streams in Arkhangai province. Diploneis oculata (Brébisson) Cleve (Figs 236–245) Valves are elongate-elliptical with parallel to slightly convex margins and bluntly round ends (Figs 236–244). The valve length is 13.0–24.5 µm, and the breadth is 5.0–7.5 µm. The axial area is linear, expanding into a small rectangular central area, ca. 0.5–1.0 µm wide. The longitudinal canal is linear, with a rectangular central nodule (Figs 243–245). At the valve apices the axial area expands slightly on the both sides of the valve (Figs 243, 244). Externally, the canal is composed of one row of areolae throughout the whole length, covered with volate occlusions (Figs 243–245). The raphe is straight and slightly expanding into small drop-like central pores (Figs 244, 245). The distal raphe endings are simple and slightly expanded, terminating some distance from the valve margins (Figs 243, 244). The striae are parallel in the middle of the valve, becoming slightly radiate at the valve apices, 22–24 in 10 µm. The areolae of the alveolate striae are not discernible with LM and SEM observations, due to the volate occlusions that occupy less then half of the valve width (Figs 244, 245). Ecology and Distribution:—M068A; M077A; M248A; M273A; M287A: found in shallow to deep habitats including epilithon and sediments throughout Lake Hövsgöl. Diploneis peterseni Hustedt (Figs 246–252) The valves are lanceolate-elliptical with convex margins and bluntly round ends (Figs 246–249, 252). The valve length is 13.5–21.0 µm, and the breadth is 6.0–7.0 µm. The axial area is linear throughout the whole length. The central area is small and rectangular, ca. 0.5–1.0 µm wide. The longitudinal canal is lanceolate, slightly expanding at the valve apices (Figs 251, 252). The external openings of the canal are covered with volate occlusions (Fig. 252). Externally, the raphe is straight with 238 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. simple and slightly expanded proximal and distal ends (Figs 250–252). Distally, the raphe branches terminate some distance from the valve margins (Fig. 251). The striae are parallel in the middle of the valve, becoming radiate toward the valve apices, 26–27 in 10 µm. The single areola in each stria is covered with a volate occlusion that occupies less then half of the valve width (Fig. 252). The hyaline area that covers part of the alveolate striae is lanceolate and broadly widens at the middle of the valve (Fig. 252). FIGURES 236–252. LM and SEM external valve views. Figs 236–245. Diploneis oculata. Figs 236–242. LM valve views. Fig. 243. View of the whole valve overlaid by the girdle bands. Fig. 244. View of the entire valve. Fig. 245. Proximal raphe endings and detailed view of the external alveolate openings covered with vola. Fig. 246–252. Diploneis peterseni, LM and SEM external valve views. Figs 246–249. LM valve views. Fig. 250. Valve central area with simple proximal raphe endings. Fig. 251. Distal raphe endings. Fig. 252. View of the entire valve. Scale bars = 10 µm (Figs 236–242, 246–249); 5 µm (Fig. 252); 2 µm (Figs 243–245, 250); 1 µm (Fig. 251). DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 239 Ecology and Distribution:—M166A; M068A: found on rocks in shallow habitats in southern Lake Hövsgöl and in small streams in Arkhangai province. Diploneis cf. eximia Metzeltin, Lange-Bertalot & Nergui (Fig. 253) The valve is elliptical to linear-elliptic with slightly convex margins and bluntly rounded ends. The valve length is 67.5 µm, and the breadth 27.5 µm. The axial area is narrow and parallel, slightly expanded near the large transapically elliptical central area, which is 7.0 µm wide. The longitudinal canal is lanceolate to linear expanding in the middle with two to three rows of areolae that coalescence into one to two towards the valve apices. The raphe branches are straight and simple with distinctly expanded proximal ends. The striae are radiate, 10 in 10 µm, composed of one row of round to rectangular areolae, 10 in 10 µm. Observations:—LM images of Diploneis eximia are not included in the original description, and therefore it is rather difficult to make comparisons (Metzeltin et al. 2009). Due to the similar valve outline and the structure of the longitudinal canal the population from Lake Hövsgöl is identified as Diploneis cf. eximia. Further analysis of the type material is preferable in order to fully understand the identity of D. eximia and its potential relationship to populations here identified as D. cf. eximia. Ecology and Distribution:—M331A: found in shallow sediments of central Lake Hövsgöl and tychoplanktonic in a lake in Khentii province (Metzeltin et al. 2009). Diploneis cf. minuta Petersen (Figs 254, 255) The valves are linear-elliptical with parallel margins and round ends. The valve length is 18.0 µm and the valve breadth is 7.0 µm. The axial area is linear, slightly expanded into a small elongate central area, and also expanded at the valve apices. The central area is elongate, 1.4 µm wide. The longitudinal canal is linear, slightly expanded in the middle of the valve, and composed of one row of areolae throughout the whole length. Externally, the openings of the canal are covered with volate occlusions (Figs 254, 255). At the valve apices the longitudinal canal is extended, circling the distal raphe endings (Fig. 254). The raphe is straight with slightly expanded proximal ends, bent to the same side of the valve (Fig. 255). Distally, the raphe endings are simple and slightly expanded, terminating some distance from the valve mantle (Fig. 254). The striae are parallel, becoming radiate towards the valve apices, 22 in 10 µm. Externally, the alveoli open via a single areola covered with volate occlusions that occupy less than half of the valve width (Fig. 254). The other part of the alveoli is covered by a lanceolate hyaline area, merging with the longitudinal canal (Fig. 255). Observations:—The population from Lake Hövsgöl is characterized with narrow and linear longitudinal canal very similar to the longitudinal canal of Diploneis minuta Petersen (1928: 381, fig. 6). In the original description, Petersen gives a single valve measurement for D. minuta (length: 13.0 µm and breadth: 4.4 µm). However, the valve found in Lake Hövsgöl has larger dimensions (length: 18.0 µm and breadth: 7.0 µm), and as such is identified as D. cf. minuta. Unfortunately the lack of additional LM observations makes more detailed comparison rather difficult. The SEM illustrations for D. minuta given in Werum & Lange-Bertalot (2004) nicely fit the population from Lake Hövsgöl. Further LM and SEM analyses are necessary to properly identify this taxon. Ecology and Distribution:—M276A: found only epilithic at 5 m depth in northern Lake Hövsgöl. Diploneis sp. 1 (Figs 256–259) The valves are lanceolate-elliptical with slightly convex margins and round ends (Figs 256, 259). The valve length is 13.5– 15.0 µm, and the breadth is 4.0–4.5 µm. The axial area is very narrow and linear toward the central area. The central area is quadratic to slightly rectangular, ca. 0.5 µm wide. The longitudinal canal is narrow and linear, composed of one row of areolae throughout the whole length (Fig. 259). The external openings of the canal are covered with volate occlusions (Figs 257, 259). At the valve apices the areolae of the canal are expanded, and surround the distal raphe endings (Figs 258, 259). Externally, the raphe is straight with expanded small drop-like proximal ends (Figs 257, 259). The distal raphe endings are simple and slightly expanded, terminating some distance from the valve margins (Figs 258, 259). The striae are parallel, becoming slightly radiate at the valve apices, 29–30 in 10 µm. Each alveolate stria opens to the exterior via a single opening covered with volate occlusions that occupy less than half of the valve width (Fig. 259). Observations:—Diploneis peterseni is the most similar taxon to Diploneis sp. 1. Diploneis peterseni is characterized by a greater size range (length: 15.0–19.0 µm vs. 13.5–15.0 µm) and a lanceolate hyaline area covering the alveolate striae from the exterior in contrast to a linear hyaline area in Diploneis sp. 1 (compare Fig. 252 with Fig. 259). 240 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. Diploneis minuta closely resembles Diploneis sp. 1, from which it differs in: valve outline (linear-elliptical with parallel margins vs. lanceolate-elliptical with slightly convex margins); valve size (length: 13.0–18.0 µm vs. 13.5–15.0 µm; breadth: 3.5–4.0 µm vs. 4.0–4.5 µm); and stria density (35 in 10 µm vs. 29–30 in 10 µm). Diploneis oculata differs from Diploneis sp. 1 in stria density (22–24 in 10 µm vs. 29–30 in 10 µm) and valve size (length: 15.0–20.0 µm vs. 13.5–15.0 µm; breadth: 6.0–7.0 µm vs. 4.0–4.5 µm). Taking the valve shape into account, Diploneis marginestriata Hustedt (1922: pl. 3, fig. 5) morphologically can be allied to Diploneis sp. 1, from which it differs in the external openings of the alveoli (biseriate simple and round openings vs. volate occlusions). Similar populations are reported from Lake Ohrid (Jovanovska et al. 2013a: figs 156–168), and Lake Prespa (Levkov et al. 2007: pl. 124, figs 17–19). Both populations are identified as D. peterseni. However, the detailed analyses on lakes Ohrid, Prespa and Hövsgöl populations show no relation to D. peterseni. In Lake Ohrid, Diploneis sp. 1 seems to be quite frequent in both modern and fossil sediments, while is Lake Hövsgöl is extremely rare and here represented by only two valves. Ecology and Distribution:—M273A: found only associated with chironomid tubes collected from 4 m depth in northern Lake Hövsgöl. Diploneis sp. 2 (Figs 260–269) The valves are elliptic-lanceolate with weakly convex margins and rounded ends. The valve length is 23.5–34.0 µm and the valve breadth is 9.5–15.0 µm. The axial area is narrow lanceolate, expanding toward the small, round to elongate central area, 2.0–2.5 µm wide. The longitudinal canal is narrow, linear to lanceolate, expanded in the middle of the valve with two rows of areolae. The raphe is straight with expanded proximal ends. Striae are radiate, 13–16 in 10 µm, with round areolae, 14–16 in 10 µm. Observations:—The similarities in valve outline and in the structure of the longitudinal canal makes Diploneis sp. 2 closely allied to D. stoermeri. The structure of the central area (2.0–2.5 µm in Diploneis sp. 2 vs. 1.5–4.0 µm in D. stoermeri morphotype 2) and the stria density (14–16 in 10 µm in Diploneis sp. 2 vs. 12–15 in 10 µm in D. stoermeri morphotype 2) somehow separates them. Diploneis krammeri is another similar taxon, differing in the structure of the longitudinal canal and in the striae density (14–16 in 10 µm in Diploneis sp. 2 vs. 11–12 in 10 µm in D. krammeri). Additional LM and SEM analyses are necessary to determine the identity of Diploneis sp. 2. Ecology and Distribution:—M272A; M274A; M280A; M281A; M284A; M291A: distributed in shallow (4 m) to deep (20 m) sediment, Chara, and marl collections from northern Lake Hövsgöl. Diploneis sp. 3 (Fig. 270) The valve is elliptical with convex margins and round ends. The valve length is 39.0 µm, and the valve breadth is 20.5 µm. The axial area is linear to lanceolate, expanding toward the round central area. The central area is 4.5 µm wide. The longitudinal canal is lanceolate, expanded in the mid-valve, with two rows of areolae that coalesces into one towards the valve apices. The raphe is straight with expanded proximal ends. The striae are radiate, 11 in 10 µm, composed of round areolae, 15 in 10 µm. Observations:—With respect to the valve outline, Diploneis sp. 3 can be associated with D. spectabilis and D. mongolica. However, the small number of observed valves does not enable critical comparisons to morphologically allied species. Therefore, additional observations are needed in order to define Diploneis sp. 3. Ecology and Distribution:—M068A: found in one epilithic collection at 10 cm depth in southern Lake Hövsgöl. Diploneis sp. 4 (Fig. 271) The valve is linear-elliptical with convex margins and round ends. The valve length is 14.0 µm, and the valve breadth is 5.5 µm. The axial area is very narrow, linear to lanceolate, expanding towards the round central area. The central area is ca. 1.0 µm wide. The longitudinal canal is very narrow, linear to lanceolate, expanded in the middle of the valve, with one row of areolae throughout the whole length. The raphe is straight with expanded proximal ends. Striae are parallel in the middle of the valve, quickly becoming radiate, 23 in 10 µm. Areolae not discernable with LM observations. Observations:—The morphological features, such as valve outline, valve size, and structure of the longitudinal canal make Diploneis sp. 4 morphologically closely related to D. separanda and D. fontanella. More valves are necessary to make detailed comparisons and full description of Diploneis sp. 4. DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 241 Ecology and Distribution:—M167A: found only in a peat collection submerged in a small stream in Arkhangai province. Not encountered in Lake Hövsgöl. FIGURES 253–271. LM and SEM external valve views. Fig. 253. Diploneis cf. eximia, LM valve view. Figs 254, 255. Diploneis cf. minuta, SEM external valve views. Fig. 254. Valve view. Fig. 255. Proximal raphe endings. Figs 256–259. Diploneis sp. 1. Fig. 256. LM valve view. Fig. 257. Central area with proximal raphe endings, SEM. Fig. 258. Distal raphe endings, SEM. Fig. 259. View of the entire valve, SEM. Figs 260–269. Diploneis sp. 2, LM valve views. Fig. 270. Diploneis sp. 3, LM valve view. Fig. 271. Diploneis sp. 3, LM valve view. Scale bars = 10 µm (Figs 253, 256, 260–271); 5 µm (Figs 254, 259); 2 µm (Figs 255, 257, 258). 242 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. Discussion During its long limnological and geological history, Lake Hövsgöl has developed an extraordinary diversity in different groups of organisms, albeit not nearly as much diversity as its sister, Lake Baikal (Goulden et al. 2006, Kozhov 1963). Both lakes lie in the Baikal Rift Zone at a relatively close geographical distance, yet they significantly differ in the level of diversity and endemism (Edlund et al. 2006b, Goulden et al. 2006). Such differences could either be a result of their separate and unique geological, limnological and biological histories (e.g., age, glacial history, depth, elevation), or could simply reflect the scarce knowledge of Lake Hövsgöl’s biodiversity (Goulden et al. 2006). However, recent investigations on Lake Hövsgöl have significantly revamped past observations, especially when it comes to diatoms (Edlund et al. 2006b). To date, existing studies on Lake Hövsgöl’s diatom flora reveal the presence of: i) widespread species; ii) new forms, potentially endemic; and iii) species flocks (Dorogostaïsky 1904; Ostenfeld 1907; Østrup 1908; Edlund et al. 2006a, b; Levkov 2009). Some groups of diatoms identified from Lake Hövsgöl have received intensive investigations, such as Cyclotella (Kützing) Brébisson (1838: 19), Amphora Ehrenberg ex Kützing (1844: 107), Navicula Bory (1822: 128), and Hippodonta Lange-Bertalot, Witkowski & Metzeltin (1996: 249) (Edlund et al. 2003, 2006b, 2009b; Edlund & Soninkhishig 2009; Pavlov et al. 2013), while many others remain less studied (e.g. Diploneis, Neidium). For example, until the current study only seven widespread Diploneis taxa were recognized in Lake Hövsgöl (D. elliptica var. ladogensis, D. oculata, D. ovalis, D. parma, D. puella, D. smithii var. pumila). Based on this study the number of Diploneis species in this region significantly increased, with a total of 25 taxa, five of which are described as new. Four species could not be identified to the species level due to low abundances or the lack of LM and/or SEM observations. The observed Diploneis species composition in Lake Hövsgöl reveals a mixture of: widespread species (e.g. D. krammeri, D. elliptica, D. oculata); species recently described from other Mongolian lakes and regions (D. linearielliptica, D. aff. heteromorphiforma, D. cf. eximia); and new forms, potentially endemic (Diploneis hoevsgoelensis, D. stoermeri, D. paraparma, D. soninkhishigae, D. exigua). Of the ten widespread species that were identified (D. elliptica, D. krammeri, D. elliptica cf. var. ladogensis, D. subovalis, D. pseudovalis, D. aff. fontanella, D. boldtiana, D. oculata, D. peterseni, D. cf. minuta), six of them are documented for the first time in Lake Hövsgöl. Edlund et al. (2006b) listed D. ovalis from the lake, but the recent observations of the type material (Lange-Bertalot & Reichardt 2000), suggested that the population identified as D. ovalis in Lake Hövsgöl most probably belongs to D. krammeri. Additionally, Edlund et al. (2006b) identified specimens as D. parma, which was most probably inaccurate due to a long-held incorrect species concept (Idei & Kobayashi 1986a). The specimens are here identified as a new taxon, D. paraparma. Intriguingly, a population of D. praeclara was observed in Lake Hövsgöl, a taxon known as a relict species from Lake Ohrid. Diploneis praeclara was originally described from Neogene fossil deposits in Romania (Pantocsek 1892, 1902). After a few decades an extant population of D. praeclara was reported from Lake Ohrid (Jurilj 1954), reassigning the species from fossil to a relict. Interestingly, the population from Lake Hövsgöl is morphologically more closely related to the type population, rather than the one from Lake Ohrid (Jovanovska et al. 2013b). Additional analyses are necessary to test its identity and more clearly ascertain the relict status of the Lake Ohrid population. Such analyses could preferably include neighboring localities in order to establish the phylogenetic position and the biogeography distribution of D. praeclara. Other recent observations on Mongolian lakes and habitats (Metzeltin et al. 2009), revealed six new Diploneis species (D. spectabilis, D. eximia, D. linearielliptica, D. mongolica, D. curiosa and D. heteromorphiforma). In this study only one of these species were reported from Lake Hövsgöl (D. linearielliptica) and two morphologically similar (D. aff. heteromorphiforma and D. cf eximia), but four of the new taxa were reported from two samples we analyzed from Arkhangai province (D. spectabilis, D. linearielliptica, D. aff. heteromorphiforma and D. mongolica), which is located about 350 km south of Lake Hövsgöl. Comparisons between the populations from Lake Hövsgöl and the type collections of the new species described by Metzeltin et al. (2009) showed a low abundance of the new taxa in the Lake Hövsgöl, despite high abundance in their type localities. Unfortunately, the species earlier described from the Mongolian lakes are rather difficult to observe and compare due to the lack of SEM or LM microphotographs for many of the new taxa (Metzeltin et al. 2009). Moreover, the often corroded SEM valves used in Metzeltin et al. (2009), made comparisons even more difficult. In this study a population with similarities to D. curiosa was observed in Lake Hövsgöl, and here described as D. hoevsgoelensis. However, the lack of LM illustrations, poor SEM quality (Metzeltin et al. 2009), and lack of specimens in loaned type material prevented more detailed morphological comparisons between D. curiosa and D. hoevsgoelensis. DIPLoNEIS EHRENBERG Ex CLEVE FROM MONGOLIA Phytotaxa 217 (3) © 2015 Magnolia Press • 243 Additionally, this study significantly increased the number of new and potentially endemic Diploneis species. These new Lake Hövsgöl species might be a result of in-lake speciation by adaptive radiation and diversification within the lake. The idea of ongoing intra-lacustrine speciation could potentially gain support from the wide morphological variation within the newly described species, e.g., D. stoermeri. According to Droop et al. (2000), the morphological variations within Diploneis appear to be stable with respect to distance and time (150 years). Unfortunately, our results cannot mechanistically explain such wide morphological variation, and therefore further studies need to be focused on each species phenotypic plasticity and genotypic diversity (e.g. Droop 1994, Droop et al. 2000). Analyses on physiological, genetic and breeding level are rather challenging due to the difficulties in cultivating species from the genus Diploneis (Droop et al. 2000). Therefore, at this point of time, we are limiting our data only to morphology and distribution, but further evidence of new and different aspects of Diploneis species biology are recommended. Such analyses could offer insights into the potential endemicity of the newly described species and could potentially clarify the biological meaning of their morphodemes. Tests of endemicity would also value from analysis of other mid-Asian habitats where the new Hövsgöl taxa might also be found. The taxa we identify as new and potentially endemic have not been encountered in other sampling in Mongolian areas such as lakes in Arkhangai and the Darkhad basin west of Lake Hövsgöl. The presence of: i) widespread; ii) endemic; and iii) characteristic species including relicts and flocks, seems to be a unique feature not only of Lake Hövsgöl, but for long-lived lakes in general, such as lakes Ohrid, Baikal, and the African rift lakes (Skvortzow & Meyer 1928, Jurilj 1954, Williams & Reid 2005, Edlund & Soninkhishig 2009, Levkov & Williams 2012, Jovanovska et al. 2013a, Cocquyt 1998). In contrast to nearby Lake Baikal (Skvortzow & Meyer 1928, Skvortzow 1937), relict species have not been reported from Lake Hövsgöl (excluding D. praeclara). The absence of Tertiary diatom relicts might simply be a result of scarce data about extant diatoms, or little investigation on fossil deposits in areas surrounding Lake Hövsgöl. Further intensive investigations on the recent and fossil diatom flora could potentially establish presence of relict species in Lake Hövsgöl and/or in the Mongolian region. Although other taxa have been shown to be represented by species flocks or cryptic diversity in Lake Hövsgöl [e.g., Aneumastus Mann & Stickle in Round, Crawford & Mann (1990: 663), Amphora, Navicula reinhardtii Grunow in Cleve & Möller (1877: 25), Cyclotella ocellata Pantocsek (1902: 104); Edlund et al. 2003, 2006; Edlund et al. 2009; Levkov 2009], the diversity of Diploneis does not appear to follow this pattern. The new and potentially endemic Diploneis species in Hövsgöl do not form a presumptive monophyletic group that can be readily diagnosed by one or more synapomorphic characters. Similarly, Diploneis in Lake Baikal, although diverse, does not have clear flocks as noted in other Baikalian groups such as Didymosphenia Schmidt in Schmidt et al. (1899: 214), Gomphoneis Cleve (1894: 73), Navicula, and Gomphonema Ehrenberg (1832: 87) (Skvortzow 1937, Mann 1999, Kociolek et al. 2013, Kulikovskiy et al. 2012, Metzeltin & Lange-Bertalot 2014). With a total number of 25 identified Diploneis species, Lake Hövsgöl is the second most diverse lake for the genus. The highest species diversity is reported from its sister Lake Baikal, with a total of 37 species, one third of which are endemic (Skvortzow 1937, Zabelina et al. 1951). Even though the lakes are geographically close, they significantly differ in their species composition within the genus Diploneis, with only six species in common for both lakes: D. subovalis, D. pseudovalis, D. elliptica, D. elliptica var. ladogensis, D. oculata, D. boldtiana (this study, Zabelina et al. 1951). In contrast, some species similarity has also been documented between lakes Hövsgöl and Ohrid, where only 15 taxa have been reported, four of which are present in both lakes (D. oculata, D. praeclara, D. krammeri and Diploneis sp. 1). The similarity between Diploneis species is even lower with other ancient lakes, for instance the Malili lakes in Indonesia and Lake Tanganyika in Africa (Cocquyt & Vyverman 1994, Cocquyt 1998, 1999, Bramburger et al. 2004, Hamilton et al. 2006). An increasing knowledge of the genus Diploneis is the basis for comprehensive and more detailed analyses not only in ancient lakes, but also in surrounding habitats (this study, Jovanovska et al. 2013a, b). While we are still in a necessary period of description and documentation of alpha diversity in many of the world’s understudied habitats, future analyses need to more rigorously explore phylogenetic and biogeographical components of diversity, in order to determine the number, distribution, and relationship among the potentially endemic, relict, and shared taxa in the Eurasian lakes. Acknowledgments EJ expresses her gratitude to the Friends of Iowa Lakeside Laboratory (ILL) for providing the Kingston Diatom Fellowship for the ILL Diatom Class in 2009 and to Edward Theriot and his laboratory at the University of Texas 244 • Phytotaxa 217 (3) © 2015 Magnolia Press JOVANOVSKA ET AL. at Austin where part of the work was performed. Part of the work was supported thanks to EJ’s Fulbright Research Fellowship. ZL would like to thank the Alexander von Humboldt Foundation for the financial support in obtaining the SEM microphotographs. Special thanks to Friedel Hinz from the Friedrich Hustedt Diatom Study Centre, Bremerhaven, Germany for the access to some materials and literature that were used in this study. Some of the literature was observed and included in this study thanks to Luc Ector, Luxembourg Institute of Science and Technology, Belvaux, Grand-duchy of Luxembourg. The authors express a deep gratitude to N. Soninkhishig and Ts. Jamsran of the National University of Mongolia and E.F. Stoermer of the University of Michigan; without their support this work would not have been possible. Bukhchuluun Tsegmid provided key information for naming the new species. Some of the material from Mongolia was observed thanks to Ditmar Metzeltin. The authors express their gratitude to Sarah Spaulding, US Geological Survey, Boulder, Colorado, USA for the useful comments and suggestion during the manuscript preparation and for her help and support in obtaining SEM images. 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