Surirella subrotunda Cvetkoska, Levkov & Hamilton 2014, sp. nov.

Surirella subrotunda Cvetkoska, Levkov & Hamilton sp. nov. (Figs 1–15) Valves isopolar about apical axis, rhombic-elliptic to rhombic with acute apices. Valve length ranges between 35–55 µm and width 22–30 µm. Valve face undulated in apical direction (Figs 1–9), extending from valve margin to median area (Fig. 10), parallel in center to strongly radial towards valve apices (Figs 1–9). Each undulation ridge aligned with alar canal and depressions between connected to fenestrae. Alar canals narrower than fenestrae (Figs 10, 11), 20–30 in 100 µm. Striae fine, not discernible in LM. In SEM, striae uniseriate throughout (Figs 12, 13, 15); stria density 34–38/ 10 µm. Internally, striae present on entire valve face, continuing to space between alar canals (Figs 13, 15). Puncta externally open as simple rounded pores, internally puncta rimmed (Fig. 15). Median area widely lanceolate, short and terminates before valve apices, with fine striation sometimes not visible in LM. Small internal circular to elliptical depression present in median area (Figs 2, 3, 5, 7, 11). Median area a lanceolate raised ridge, covered with short conic spines. Externally, conic spines present on undulations, somewhat longer conic tubes appear at mantle side of alar canals (Fig. 15). Wing projection vertical, positioned close to valve face. Rounded keel with 3–4 silicate grooves extending along both sides of raphe (Figs 12, 14). Externally, raphe fissures deflected towards mantle (Fig. 14); internally, raphe fissures with simple ends (Fig. 13). Fenestrae rectangular, occluded by 4–5 fenestral bars (Fig. 12). Internally, alar canals cylindrical (Figs 11, 13), alar openings simple elliptical portulae (Figs 11, 13). Type:— MACEDONIA: Lake Prespa, sediment, core Co1215, core depth 249 cm, ca. 13.7 cal ka BP, collection date: October 2009 (holotype! slide MKNDC 005419/A, holotype! designated here, Accession No. MKNDC 005419, holotype specimen = Fig. 3; isotypes BRM ZU9/34, CANA 93609). Etymology: —the specific epithet (subrotunda) refers to its similarity to S. rotunda. Observations: — Surirella subrotunda may be compared to S. rotunda Jurilj (1948: 178, fig. 5), an endemic species for Lake Ohrid. The original description of Jurilj (1948) translates in English as: “Valve shape almost circular, valve apices projected. Valve length 80–100 µm, width 60–70 µm. Costae weakly developed, reaching the median line, 14–18 / 100 µm. Valve almost flat, fine striae cover the costae. Additional punctae are irregularly arranged on the valve”. In his description, Jurilj provided a single line drawing of the taxon (reproduced here as Fig. 16). Surirella rotunda was recorded during the observation of Jurilj’s samples from Lake Ohrid (Figs 17–19). The two species can be separated from each other by their valve size: S. subrotunda is much smaller (L= 35–55 µm, B= 22–30 µm) compared to S. rotunda (L= 90–120 µm, B= 65–70 µm). The number of alar canals is 20–30 in 100 µm in S. subrotunda vs. 14–18 in 100 µm in S. rotunda. The shape and size of the median area is smaller and lanceolate to round in S. subrotunda, compared to the linear-lanceolate shape in S. rotunda. The presence of the small circular to elliptical depression in the median area can be considered a unique character for S. subrotunda. S. subrotunda also resembles S. turgida W.Sm. (1853: 31, fig. 9:60), however, it is distinguished by the smaller valves and its isopolar valve morphology. Surirella turgida sensu Krammer & Lange-Bertalot (1988, figs 152: 1, 1A) is easily differentiated by the valve size (L= 70 µm, B= 44 µm in S. turgida). Surirella turgida sensu Hartley et al. (1996, fig 280: 6) has a lanceolate shape and larger valve length (L= 58–80 µm) and narrow linear median area. Surirella turgida sensu Foged (1981, fig. 60: 6, 7) has a comparable valve size, but can be distinguished by the narrow, linear median area. Surirella helissella Jurilj (1954:164 = Helissella glabra Jurilj 1948:16, 17, figs 8a, b) has a comparable valve shape, but has larger valves (L= 95–250 µm, B= 65–100 µm) and conspicuous valve torsion. Surirella bifrons Ehrenberg (1843: 388, figs 3/5: 5, 4/3: 1) sensu Krammer & Lange-Bertalot (1988, figs 145: 2–4, 146: 1–4, 147: 1–5) very likely represents a heterogeneous group of species. The specimen depicted in Krammer & Lange-Bertalot (1988, fig. 146: 3) has a comparable valve outline and shape of the median area, but differs from S. subrotunda by its valve size (L= 130 µm, B= 67 µm) and rostrate apices. There are a number of Surirella taxa with this valve outline, but few SEM images are available to document the species (Gaul et al. 1993, Henderson & Reimer 2003). A complete examination of these elliptical-rhombic taxa is required. Specimens depicted as S. bifrons in the recent diatom flora of Lake Prespa, Levkov et al. (2007, fig 204: 1–6) resemble S. subrotunda in valve length (L= 45–70 µm in S. bifrons sensu Levkov et al. 2007) and the presence of a circular to elliptical depression in the median area. However, the specimens can be distinguished by the valve outline, rhombic-elliptic to rhombic in S. subrotunda and linear-lanceolate in S. bifrons sensu Levkov et al. (2007) and the shape of the median area, widely lanceolate, short in S. subrotunda and linear-lanceolate in S. bifrons sensu Levkov et al. (2007). Surirella subrotunda has been observed only in the fossil diatom assemblages from the sediment sequence Co1215, retrieved from the central northern part of Lake Prespa. According to the age model (Aufgebauer et al. 2012), samples in which S. subrotunda has been observed are dated between ca. 14.8 and 13.2 cal ka BP, 273– 240 cm core depth and correspond to the Bølling/Allerød (B/A) interstadial (cf. Björck et al. 1998).

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