Hippodonta microcostulata Metzeltin, Kulikovskiy & 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 : 30-31

Hippodonta microcostulata Metzeltin, Kulikovskiy & Lange-Bert. ( Figs 138–155 , 482–519 , 551–554 )

Valves narrow-rhombic, with narrow, cuneate ends, not protracted. Valve length 10.3–23.6 µm, breadth 2.8–4.6 µm. Axial area narrow-linear throughout, expanding into a prominent, rectangular to panduriform central area. Central area forming fully developed fascia, rarely interrupted by shortened striae at middle. Terminal area quite small but clearly distinguishable in LM view, triangular or lunar-shaped, and cap-like. Raphe linear-filiform, with weakly pronounced, simple, round, and closely spaced central pores. Central pores of raphe distantly positioned in longer valves only. Terminal pores of raphe equally pronounced as central pores, simple linear, weakly advancing into terminal area. Striae fine, of variable pattern at middle, weakly to strongly radiate, becoming parallel or weakly convergent towards valve ends, 12–15 in 10 µm. Interstriae of equal or greater width than striae. Lineolae composing striae discernible with LM, 3 or 4 along a stria at the middle of the valve.

SEM: Externally, valve face flat ( Figs 551 ). Terminal areas expanding strongly onto valve face ( Figs 551, 552 ). Terminal pores of raphe distinct, advancing strongly into terminal areas. Central raphe endings strongly pronounced, teardrop-shaped depressions ( Fig. 553 ). Striae uniseriate throughout valve face and mantle. Striae continuing part way on valve mantle. Lineolae prominent and elliptical, longer near the raphe, shorter towards the mantle. Lineolae density c. 40 in 10 µm. Rows of areolae that go around valve apices also uniseriate ( Figs 551, 552 ). Prominent hyaline area present throughout valve mantle, absent of any structure ( Figs 552, 553 ). Girdle band unornamented ( Figs 552, 554 ).

Distribution:— Hippodonta microcostulata has been observed in Lake Baikal, Russia, Lake Ohrid, Macedonia and Lake Hövsgöl, Mongolia. In Lake Baikal it was observed within the sediment of Vydira at 20 m depth. In Lake Hövsgöl, Mongolia, it was found on sand at 70 cm depth (M. Edlund, unpublished observations). In Lake Ohrid it was observed from a submerged carbonate springs (ca. 3 m depth) in St. Naum. St. Naum is a small and shallow lake-like area, formed from submerged springs before making an inflow in Lake Ohrid. Most of the water of these springs originates from Lake Prespa, the sister lake of Lake Ohrid, through an underground hydrological connection between the lakes ( Anovski et al. 1992, Matzinger et al. 2006, 2007).

Observations:—The population of H. microcostulata depicted herein corresponds well with the morphology of the specimens from the protologue depicted in Kulikovskiy et al. (2012: figs 66: 26, 27). However, more specimens are depicted in the present study, representing more of the size diminution series.

Hippodonta microcostulata appears slightly similar to H. abunda ( Figs 102–137 ) and H. media ( Figs 156 – 165 ) with regard to valve outline and striae pattern. Still, the striae of H. abunda are finer with vaguely discernible and more densely spaced lineolae, whereas the valves of H. media are wider for any given valve length. Considering the valve outline, H. microcostulata is also slightly similar to H. geocollegarum (Lange- Bertalot et al. 1996: figs 4: 1–25). The former taxon can be distinguished by the finer striae and less pronounced interstriae throughout the valve length.

Hippodonta kornevae Kulikovskiy, Lange-Bert. & Metzeltin in Kulikovskiy et al. (2012: 150, 151, figs 67: 5–12) ( Figs 520–525 )

Frustules rectangular, broad in girdle view. Valves narrow-lanceolate, with not protracted and acutely rounded valve ends. Valve length 17.6–32.4 µ m, breadth 4.4–6.3 µ m. Axial area linear and broad throughout, expanding into a prominent, rectangular to weakly panduriform central area. Central area consistently forming fully developed fascia. Terminal area small but clearly distinguishable with LM, semi-circular in shape. Raphe linear-filiform, with weakly pronounced, small, simple, round, distantly positioned central pores. Terminal pores of raphe equally pronounced as central pores, small and circle-shaped, weakly advancing into the terminal area. Striae quite coarse, weakly radiate at middle, becoming parallel to weakly convergent towards valve ends, 6–9 in 10 µm. Interstriae of equal or lower width than striae, equally pronounced throughout. Lineolae composing striae clearly discernible with LM, quite large, 3 or 4 within a stria at the middle of the valve.

Distribution:— Hippodonta kornevae has so far only been observed in Lake Baikal, Russia.

Observations:—With regard to the valve outline and the appearance of the striae H. kornevae is similar to H. certa (this study, Figs 526 – 534 ) and H. communis (this study, Figs 535 – 543 ). It can still be distinguished from the two by the coarser striae and the rectangular form of the fascia. The fascia in H. certa and H. communis is consistently panduriform.

Figures

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.

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 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.