Hippodonta acuta A. Pavlov, Levkov, D.M. Williams & Edlund, 2013

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 : 30

Hippodonta acuta A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 470–481 , 549, 550 )

Type:— RUSSIA. Lake Baikal , Vydira, sediment, 20 m depth, 23 June 1998, collector D.M. Williams. (holotype: slide BM! 100465; isotype: slide MKNDC! 005845/A) .

Valves rhombic-lanceolate, with narrow, cuneate valve ends that are not protracted. Valve length 16.5–31.2 µm, breadth 3.8–6.3 µm. Axial area narrow-linear, expanding into a prominent, transverse, rectangular to panduriform central area. Central area forming not a fully developed fascia. One shortened stria is present on both valve sides at middle of valve. Terminal area quite small, but clearly distinguishable with LM, triangular in shape. Raphe linear-filiform, with weakly pronounced, simple linear and closely spaced central pores. Central pores of raphe rarely distantly positioned. Terminal pores of raphe equally pronounced as central pores, simple linear and weakly advancing into the terminal area. Striae fine, weakly radiate at middle, becoming parallel or weakly convergent towards valve ends, 12–14 in 10 µm. Interstriae of equal or lower width than striae, except at middle where they are more strongly pronounced. Lineolae composing striae clearly discernible with LM, small, 5–7 along a stria at middle of valve.

SEM: Valve ultrastructure only observed internally ( Figs 549, 550 ). Striae positioned in prominent depressions, thus valve face not flat. Striae uniseriate. Lineolae density: 30 in 10 µm. Lineolae covered by quite prominent and broad volae ( Fig. 550 ). Shape of volae mostly elliptical, rarely circular. Internal central raphe endings small, dot-like, distantly positioned. Linear raphe slit distally terminated by small, semielliptical helictoglossa ( Fig. 549 ). Helictoglossa positioned just before terminal area and in line with raphe slit. Terminal area strongly pronounced internally.

Distribution:— Hippodonta acuta has so far only been observed in Lake Baikal, Russia. Found as quite abundant in the type locality.

Observations:—With regard to the valve outline and the appearance of the striae, H. acuta is similar to H. microcostulata ( Kulikovskiy et al. 2012: figs 66: 26, 27) from Lake Baikal as well as H. abunda ( Figs 102–137 ) from Lake Ohrid. Nonetheless, both H. microcostulata and H. abunda have a transverse central area as a well-defined fascia, expanding to the valve margins at the middle of the valve. Quite rarely the fascia of these taxa is interrupted by shortened striae. On the contrary, the central area of H. acuta does not expand to the valve margins. In addition, considering the acutely rounded valve ends, H. acuta is also similar to H. acus Kulikovskiy, Metzeltin & Lange-Bert. in Kulikovskiy et al. (2012: 143, figs 67: 13, 14), but can be distinguished from it due to the completely developed fascia and the more strongly pronounced interstriae of the latter taxon.

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.

FIGURES 102–165: LM micrographs. Figs 102–137. Hippodonta abunda (type material). Figs 138–155. Hippodonta microcostulata. Figs 156–165. Hippodonta media (type material). Scale bar = 10 µm.