Hippodonta subelegans Kulikovskiy, Lange-Bert. & Metzeltin

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 : 29-30

Hippodonta subelegans Kulikovskiy, Lange-Bert. & Metzeltin ( Figs 443–469 , 547, 548 )

Valves narrow-rhombic with narrow-cuneate to moderately rounded valve ends that are not protracted. Valve length 14.3–32.4 µm, breadth 3.3–6.0 µm. Axial area narrow-linear, weakly broadened towards middle of valve, expanding into small elliptic, apically elongated central area. Terminal area small but clearly distinguishable in LM view, having a lunar shape. Raphe linear-filiform, with pronounced teardrop-shaped and closely spaced central pores. Terminal pores of raphe equally or less pronounced than central pores, simple, linear and weakly advancing into the terminal area. Striae coarse, strongly radiate at middle, becoming strongly convergent towards valve ends, 10–16 in 10 µm. Interstriae of lower width than striae, except at middle of valve where they are quite strongly pronounced. Lineolae composing striae densely spaced within a stria, not discernible with LM.

SEM: Externally, valve face flat ( Figs 547, 548 ). Terminal area expanding strongly onto valve face. Terminal pores of raphe simple linear, weakly advancing into the terminal area. Central raphe endings pronounced, forming teardrop-shaped depressions. Striae uniseriate, lineolae quite narrow, long, rather boneshaped ( Fig. 548 ). Lineolae density c. 40 in 10 µ m. Rows of lineolae positioned on valve apices also uniseriate ( Fig. 547 ).

Distribution:— Hippodonta subelegans has so far only been observed in Lake Baikal, Russia and Lake Hövsgöl, Mongolia. This species is found as quite abundant in the littoral of Lake Baikal, at different localities and at various depths (0.8–20 m). In Lake Hövsgöl, Mongolia H. subelegans is found on shallow sandy substrates including Khilent Bulan bay (Edlund, unpublished observations).

Observations:—Both morphological features and measurements of specimens in the population of H. subelegans depicted herein correspond well with the images from the protologue given in Kulikovskiy et al. (2012: figs 67: 26–29). With regard to valve outline and appearance of the striae, H. subelegans is similar to H. neglecta Lange-Bert., Metzeltin & Witkowski (1996: 262, 263, figs 4: 8–15) and a taxon named “ H. cf. avittata ” in Lange-Bertalot et al. (1996: figs 4: 35–40). Still, the latter two taxa have less radiate striae at the middle as well as more strongly pronounced interstriae throughout the valve length.

Figures

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.

FIGURES 547–555: SEM micrographs. Figs 547, 548. Hippodonta subelegans. Fig. 547. External view of entire valve. Terminal pores of raphe are simple linear. Fig. 548. Detail of central area. Lineolae quite narrow and long, bone-shaped. Figs 549, 550. Hippodonta acuta (type material). Fig. 549. Internal valve view. Linear raphe slit distally terminated by semi-elliptical helictoglossa, positioned just before terminal area. Fig. 550. Detail of internal mid-valve. Striae positioned in prominent depressions. Lineolae covered by prominent, broadly elliptical volae. Figs 551–554. Hippodonta microcostulata. Fig. 551. External view of entire valve. Terminal pores of raphe distinct, advancing strongly into terminal area. Figs 552, 554. External view of two complete frustules, showing girdle unornamented. Fig. 553. Detail of mid-valve in fig. 552. Lineolae prominent and elliptical, longer near raphe and shorter towards valve mantle. Fig. 555. Hippodonta communis (type material). External view of complete frustule. Lineolae prominent and long, simple linear. Girdle quite broad and unornamented. Scale bars in Figs 547, 549, 554 = 5 µm; Figs 548, 550, 553 = 1 µm; Figs 551, 552 = 2 µm; Fig. 555 = 10 µm.