Diploneis boldtiana Cleve, 1891

Jovanovska, Elena, Levkov, Zlatko & Edlund, Mark B., 2015, The genus Diploneis Ehrenberg ex Cleve (Bacillariophyta) from Lake Hövsgöl, Mongolia, Phytotaxa 217 (3), pp. 201-248 : 238

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 .

Figures

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).