Hippodonta intermedia (Skab.) Kulikovskiy, Metzeltin & Lange-Bert.

Pavlov, Aleksandar, Levkov, Zlatko, Williams, David M. & Edlund, Mark B., 2013, Observations on Hippodonta (Bacillariophyceae) in selected ancient lakes, Phytotaxa 90 (1), pp. 1-53 : 27-28

Hippodonta intermedia (Skab.) Kulikovskiy, Metzeltin & Lange-Bert. ( Figs 377–392 )

Navicula hungarica var. intermedia Skab. 1936: 712 , 720, fig. 1: 12

Frustules rectangular in girdle view. Valves narrow-lanceolate to narrow rhombic-lanceolate, with cuneate to broadly rounded valve ends, not protracted. Valve length 22.0– 31.6 µm, breadth 5.0–8.0 µm. Axial area narrow-linear, weakly broadened towards the middle, gradually expanding into large rhombic central area. The central area not as clearly defined in shorter cell sizes. Terminal area quite small but clearly distinguishable in LM view, triangular in shape. Raphe linear-filiform, with pronounced teardrop-shaped central pores, distantly positioned, rarely closely spaced. Terminal pores of raphe clearly distinguishable in LM view, small, teardrop-shaped, and clearly advancing into the terminal area. Striae coarse, weakly to moderately radiate at the middle, becoming clearly convergent near valve ends, 10–12 in 10 µm. Interstriae of equal or lower width than the striae, more strongly pronounced at the middle of the valve. Lineolae composing the striae quite densely spaced within a stria, rarely discernible in LM, 5 or 6 within a stria at the middle of the valve.

Distribution:— Hippodonta intermedia has so far only been observed in Lake Baikal, Russia. In the present study this species has been observed within the sediment of the lake’s littoral at various depths (5 m at Tolsky and 20 m at Vydira) as well as on a silt substrate at Kultuk at 0.7 m of depth.

Observations:—It is somewhat questionable whether Navicula hungarica var. intermedia and Navicula costuloides ( Skvortsov 1937: fig. 7: 22) represent different taxa. The drawing of Skabichevskij of N. hungarica var. intermedia does not correspond well with the dimensions given in the protologue. If this drawing is compared with the one of N. costuloides by Skvortsov, at the same magnification, they quite closely resemble each other. Therefore, regarding the concept of H. intermedia , we follow the guidelines given by Kulikovskiy et al. (2012). Furthermore, Skabichevskij (1936: 39) lists N. costulata var. baikalensis Skvortsov (1937 : fig. 7: 4) as a synonym of his N. hungarica var. intermedia which is unlikely to be the case. Regarding the valve outline and the striae pattern, H. intermedia quite closely resembles H. costuloides and H. subelegans ( Kulikovskiy et al. 2012: figs. 67: 26–29; this study Figs 443–469 ). Still, H. costuloides has broader valves (9.3–11.3 µm) as well as less pronounced and mostly simple, linear central pores of the raphe. Valves of H. subelegans are characterised by more acutely rounded valve ends in addition to more strongly radiate striae pattern at the middle, these being the main characters that distinguish it from H. intermedia .

Figures

FIGURES 365–392: LM micrographs. Figs 365–372. Hippodonta pulchra (type material). Figs 366, 367. Images of same valve at different focus. Fig. 372. Complete frustule in girdle view. Figs 373–376. Hippodonta costuloides. Fig. 373. Image of broken initial cell. Figs 377–392. Hippodonta intermedia. Fig. 380. Complete frustule in girdle view. Scale bar = 10 µm.

FIGURES 443–498: LM micrographs. Figs 443–469. Hippodonta subelegans. Figs 470–481. Hippodonta acuta (type material). Figs 482–498. Hippodonta microcostulata. Scale bar = 10 µm.