Hippodonta arkonensis Lange-Bert., Metzeltin & Witkowski

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 : 28-29

Hippodonta arkonensis Lange-Bert., Metzeltin & Witkowski ( Figs 393–441 )

Synonym: Hippodonta jakubii Kulikovskiy, Lange-Bert. & Metzeltin in Kulikovskiy et al. (2012: 148, 149, figs 66: 28–32),

Hippodonta perarkonensis Kulikovskiy, Lange-Bert. & Metzeltin in Kulikovskiy et al. (2012: 154, figs 66: 33–35).

Frustules rectangular, narrow in girdle view. Valves rhombic-lanceolate with long-protracted, narrowly rounded valve ends. The protraction of the valve ends not as strongly pronounced in shorter cell sizes. Valve length 13.2–27.0 µm, breadth 3.3–6.7 µm. Axial area linear, quite narrow, weakly expanding into a quite small, apically elongated, elliptical central area. Terminal area quite small but clearly distinguishable in LM view, semi-circular in shape. Raphe linear-filiform, with pronounced teardrop-shaped and closely spaced central pores. Terminal pores of raphe quite small and not clearly discernible in LM view. Striae coarse, strongly radiate at middle, becoming clearly convergent toward valve ends, 12–16 in 10 µm. Interstriae consistently of lower width than striae, equally pronounced throughout valve length. Lineolae composing the striae quite densely spaced along a stria, not distinguishable in LM.

Distribution:—Described as fossil from freshwater deposits in the Arkona Bay of the Baltic Sea (Lange- Bertalot et al. 1996: 252, figs 4: 11–18). Lange-Bertalot (2001) notes a small number of specimens present in the recent flora of some rivers and lakes in Germany. Within Lake Baikal it was found as quite abundant in the littoral, on several localities at various depths (0.7 m at Kultuk, 20 m at Vydira and east of Cape Tonkij).

Observations:—The population of this species from Lake Baikal presents a somewhat broader range of the valve size than the one given in the protologue by Lange-Bertalot et al. (1996: 252, length 10–20 µm and breadth 3.7–4.8 µm). Nevertheless, the morphological features of the population from Lake Baikal correspond quite well with the protologue of the species.

Hippodonta arkonensis closely resembles H. humboldtiana ( Figs 213–218 ) with regard to the valve outline, the appearance of the striae, and the shape of the central area. The distinction between both taxa can be made due to the strongly radiate pattern of the striae at the middle of the valves in H. arkonensis as well as the higher density of the striae in H. humboldtiana (16–18/ 10 µm). Furthermore, valves of H. arkonensis having long-protracted valve ends (see Figs 418–429 ) appear similar to H. pseudorostrata Metzeltin, Kulikovskiy & Lange-Bert. in Kulikovskiy et al. (2012: 155, 156, figs 66: 24, 25) but can be distinguished from it by the rhombic central area and the more strongly pronounced interstriae in the latter taxon.

Furthermore, valves depicted in Kulikovskiy et al. (2012: figs 66: 28–35) named as H. jakubii Kulikovskiy, Lange-Bert. & Metzeltin in Kulikovskiy et al. (2012: figs 66: 28–32) and H. perarkonensis Kulikovskiy, Lange-Bert. & Metzeltin in Kulikovskiy et al. (2012: figs 66: 33–35) cannot be differentiated from H. arkonensis either by morphological or by numerical values, if the complete cell size range is considered rather than just a few valves. Therefore, both H. jakubii and H. perarkonensis are considered as a synonym of H. arkonensis .

Hippodonta baicalorostrata Metzeltin, Kulikovskiy & Lange-Bert. in Kulikovskiy et al. (2012: 146, figs 66: 11–15) ( Fig. 442 )

A single valve was observed. It was elliptic with strongly protracted, narrowly rostrate valve ends, 12.3 µm long and 4.5 µm wide. Axial area narrow-linear, expanding into small, rhombic central area. Terminal area quite small, not discernible in LM view. Raphe linear-filiform, with weakly pronounced, teardrop-shaped and closely spaced central pores. Terminal pores of raphe equally pronounced as central pores, distinguishable in LM view, simple linear in shape. Striae coarse, weakly radiate at middle, becoming parallel towards valve ends, 15 in 10 µm. Interstriae of equal or greater width than striae, more strongly pronounced at middle. Lineolae composing the striae quite densely spaced along a stria, not discernible in LM.

Distribution:— Hippodonta baicalorostrata has so far only been observed in Lake Baikal, Russia. In the present study this species has been observed only at one locality, 5 km east of Nizhneangarsk, on a sand substrate at 0.8 m depth.

Observations:—The morphology and the size of the valves found herein correspond quite well to those given in Kulikovskiy et al. (2012: figs 66: 11–15). Hippodonta baicalorostrata resembles slightly H. pseudorostrata with respect to valve outline and appearance and pattern of striae. Still, the former taxon has broader elliptic valves in addition to the narrower protracted and nearly subcapitate valve ends. With respect to valve outline, H. baicalorostrata is also similar to H. rostratoides from Lake Prespa ( Figs 180–191 ). Nonetheless, the latter taxon consistently has a fully developed fascia and acutely rounded valve apices in contrast to the rhombic central area and nearly subcapitate valve ends of H. baicalorostrata .

Figures

FIGURES 393–442: LM micrographs. Figs 393–441. Hippodonta arkonensis. Fig. 404. Complete frustule in girdle view. Fig. 442. Hippodonta baicalorostrata. Scale bar = 10 µm.

FIGURES 166–218: LM micrographs. Figs 166–179. Hippodonta subrostrata (type material). Figs 180–191. Hippodonta rostratoides (type material). Figs 192–200. Hippodonta affinis (type material). Figs 201–212. Hippodonta exigua (type material). Figs 213–218. Hippodonta humboldtiana (type material). Scale bar = 10 µm.