Diploneis exigua, Jovanovska & Levkov & Edlund, 2015

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 : 230

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: M272 A, 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 .

Figures

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).

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).