Hippodonta costuloides (Skvortsov) 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

Hippodonta costuloides (Skvortsov) Kulikovskiy, Metzeltin & Lange-Bert. ( Figs 373–376 )

Valves broadly rhombic-lanceolate with broadly rounded valve ends that are not protracted. Valve length 33.5–48.0 µm, breadth 9.3–11.3 µm. A post-initial valve observed was 58.0 µm long and 12.6 µm wide. Axial area broad linear, becoming wider towards the centre, gradually expanding into quite large, lanceolate to rhombic central area. Central area well defined by continuous shortening of the striae, from both valve sides, at the middle of the valve. Terminal area distinct, large and triangular in shape. Raphe linear-filiform, with weakly pronounced, simple linear or teardrop-shaped, and distantly positioned central pores. Terminal pores of raphe distinct, equally pronounced as the central pores, simple circular-shaped and clearly advancing into the terminal area. Striae coarse, moderately radiate at the middle becoming weakly convergent near valve ends, 8 in 10 µm. Interstriae of equal or greater width than striae, more pronounced at the middle of the valve. Lineolae composing the striae quite densely spaced within a stria, vaguely distinguishable in LM, 7 or 8 within a stria at the middle of the valve.

Distribution:— Hippodonta costuloides has so far only been observed in Lake Baikal, Russia. Skvortsov (1937) in his treatise on the diatom flora of Olhon Gate of Lake Baikal lists this species within the group of “Siberian and subalpine elements”. In the present study H. costuloides was observed within the sediment of Tolsky (at 5 m of depth) and within the sand substrate 3 km east of Cape Tonkij (at 20 m of depth).

Observations:—With regard to the valve outline and the striae pattern, H. costuloides is quite similar to H. intermedia (Skab.) Kulikovskiy, Metzeltin & Lange-Bert. in Kulikovskiy et al. (2012: 148, figs 66: 4–10). Nevertheless, valves of H. intermedia are narrower (width = 5.0–8.0 µm) as well as the central pores of the raphe being more strongly pronounced and consistently teardrop-shaped. Regarding the valve shape and size, H. costuloides is only similar to H. pulchra and easily distinguishable from other Hippodonta taxa. Still, the remaining morphological features of H. pulchra (striae pattern and density, axial area width, and shape of the central area) contribute for a clear distinction between the two taxa.

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.