Hippodonta communis 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 : 32-33

Hippodonta communis A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 535–543 , 555 )

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 narrow, rhombic-lanceolate, with acutely rounded valve ends, not protracted. Valve length 19.6–26.6 µm, valve breadth 4.0–5.0 µm. Axial area narrow, linear throughout, rarely observed as broad, expanding into a prominent, panduriform central area. Central area consistently forming fully developed fascia. Terminal area small, clearly distinguishable with LM, lunar-shaped, and cap-like. Raphe linear-filiform, with pronounced teardrop-shaped and distantly positioned central pores. Terminal pores of raphe less pronounced than central pores, vaguely distinguishable with LM, simple linear, weakly advancing into the terminal area. Striae coarse, of variable pattern at middle, weakly to strongly radiate, becoming parallel to weakly convergent towards valve ends, 9–12 in 10 µm. Interstriae of lesser width than striae, equally pronounced throughout. Lineolae composing the striae clearly discernible with LM, large, 3 or 4 along a stria at middle of valve.

SEM: Externally, valve face flat ( Fig. 555 ). Terminal areas weakly expanding onto valve face. Striae uniseriate. Lineolae prominent and long, but simple linear. Row of areolae positioned on apical valve mantle, also uniseriate. Prominent hyaline area present throughout valve mantle, absent of any structure. Girdle band quite broad and unornamented.

Distribution:— Hippodonta communis has so far only been observed in Lake Baikal, Russia. Quite abundant at the type locality.

Observations:—With respect to valve outline and striae pattern, H. communis is most similar to H. certa ( Figs 526 – 534 ). Still, H. communis has a lower valve width when valves of equal length are compared, as well as fewer lineolae within a stria at the middle of the valve compared with the number of lineolae in H. certa (4–6 within a stria).

Hippodonta acus Kulikovskiy, Metzeltin & Lange-Bert. in Kulikovskiy et al. (2012: 143, figs 67: 13, 14) ( Figs 544–546 )

Valves quite narrow-rhombic, with weakly protracted and acutely rounded valve ends. Valve length 24.4–35.4 µm, breadth 3.7–5.3 µm. Axial area narrow linear throughout, expanding into a prominent panduriform central area. Central area consistently forming fully developed fascia. Terminal area prominent, clearly distinguishable with LM, triangular in shape. Raphe linear-filiform, with pronounced teardrop-shaped, closely spaced central pores. Terminal pores of raphe less pronounced than central pores, clearly distinguishable with LM, simple-circular, not advancing into the terminal area. Striae coarse, rarely observed as fine, weakly radiate at middle, becoming parallel towards valve ends, 7–11 in 10 µm. Interstriae of equal or greater width than striae, pronounced stronger towards valve centre. Lineolae composing striae, vaguely discernible with LM, 3 or 4 along a stria at middle of valve.

Distribution:— Hippodonta acus has so far only been observed in Lake Baikal, Russia. In the present study this species was found in a 0.8 m deep sand sample, 5 km east from Nizhneangarsk.

Observations:— Hippodonta acus has characteristically narrow and acutely rounded valve ends, thus showing a resemblance only to H. acuta ( Figs 470–481 ). Hippodonta acus can be distinguished by the consistent presence of a well-developed panduriform fascia as well as by the more strongly pronounced interstriae throughout the valve length.

Figures

FIGURES 499–546: LM micrographs. Figs 499–519. Hippodonta microcostulata. Figs 520–525. Hippodonta kornevae. Fig. 525. Complete frustule in girdle view. Figs 526–534. Hippodonta certa (type material). Fig. 533. Complete frustule in girdle view. Figs 535–543. Hippodonta communis (type material). Figs 544–546. Hippodonta acus. 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 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.