Hippodonta exigua 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 : 19

Hippodonta exigua A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 201–212 , 225 )

Type:— MACEDONIA. Lake Prespa , Bay of Stenje, littoral sediment, 28 June 2002, collector Z. Levkov. (Accession No. MKNDC 000142; holotype: slide MKNDC! 000142/1; isotype: slide BM! 101642) .

Valves strictly lanceolate, with narrow and long protracted and broadly rounded valve ends. Valve length 14.5–19.0 µm, width 4.0– 4.5 µm. Axial area narrow-linear throughout, expanding into a transverse small, irregularly shaped central area. Usually, central area defined by shortening of one central stria, from both valve sides. Terminal area clearly discernible in LM, small, semi-circular in shape. Raphe linear-filiform, with weakly pronounced, small teardrop-shaped central pores. Central pores closely spaced. Terminal pores of raphe discernible in LM, as round-shaped. Striae fine, moderately to strongly radiate at middle, becoming parallel towards valve ends, 14 in 10 µm. Interstriae of equal or higher width than striae, evenly pronounced throughout valve length. In longer valves, interstriae more pronounced than striae. Lineolae composing striae quite densely spaced within a stria and not clearly distinguishable in LM.

SEM: Externally, valve face flat ( Fig. 225 ). Terminal area expanding weakly onto valve face. Terminal pores of raphe simple teardrop-shaped and weakly pronounced, slightly advancing into the terminal area. Terminal pores of raphe slightly deflected towards one valve side. Striae uniseriate. Lineolae simple linear, moderately pronounced and short, c. 40 in 10 µm. Rows of lineolae positioned on apical valve mantle also uniseriate.

Distribution:— Hippodonta exigua has so far only been observed from Lake Prespa, Macedonia, where it occurs on organic sediment from various depths.

Observations:—A specimen depicted in Lange-Bertalot (2001: fig. 79: 6) named as “ Hippodonta spec. ”, from Lake Baikal, is quite similar to H. exigua with respect to valve outline and appearance of the striae. However, H. exigua has slightly more densely spaced striae (14 in 10 µm compared to 10 striae in 10 µm on the specimen from Lake Baikal) as well as somewhat more broadly protracted valve ends. In addition, and concerning the valve outline, H. exigua is also similar to H. coxiae Lange-Bert. (2001: 221, 222, figs 76: 29–33), but the latter species has shorter protracted, broadly rostrate and broadly rounded valve ends, when compared to the narrow and long protracted valve ends in H. exigua . Furthermore, the terminal area of H. coxiae is more strongly pronounced and the lineolae are clearly discernible in LM.

Figures

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.

FIGURES 219–225: SEM micrographs. Figs 219–222. Hippodonta abunda (type material). Fig. 219. External view of entire valve. Terminal pores of raphe are simple linear depressions. Fig. 220. External view of complete frustule showing broad, unornamented girdle. Fig. 221. Internal view of entire valve. Striae positioned in shallow depressions, helictoglossae in line with raphe slits. Fig. 222. Close-up of one half of valve, internal view. Lineolae covered by quite prominent elliptical volae. Fig. 223. Hippodonta rostratoides (type material). External view of entire valve. Lineolae quite prominent. Terminal pores of raphe slightly deflected towards one valve side. Fig. 224. Hippodonta affinis (type material). External view of entire valve. Lineolae narrow but long. Terminal pores of raphe simple linear. Fig. 225. Hippodonta exigua (type material). External view of entire valve. Lineolae weakly pronounced, short. Terminal pores of raphe slightly deflected towards one valve side. Scale bars in Figs 219, 223, 224 = 5 µm; Fig. 220 = 2 µm; Fig. 221 = 6 µm; Fig. 222 = 3 µm; Fig. 225 = 4 µm.