Hippodonta rostratoides 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 : 17-18

Hippodonta rostratoides A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 180–191 , 223 )

Type:— MACEDONIA. Lake Prespa , Bay of Asamati, littoral mud, 30 November 2002, collector Z. Levkov. (Accession No. MKNDC 000288; holotype: slide MKNDC! 000288/1; isotype: slide BM! 101640) .

Valves elliptic, with acutely rounded, long and narrowly protracted valve ends. Protraction of valve ends equally pronounced throughout size diminution stages. Valve length 15.0–24.0 µm, valve width 5.0– 6.5 µm. Axial area linear and moderately broad, expanding into panduriform central area. Central area forming welldefined fascia, expanding to valve margins and continuously broadened toward margins. Terminal area rarely distinguishable in LM, as quite small and semi-circular in shape. Raphe curved, weakly lateral, with weakly pronounced, small, teardrop-shaped central endings. In most specimens observed, central endings closely spaced. Terminal pores of raphe rarely discernible in LM. Striae coarse, moderately to strongly radiate at middle, becoming parallel towards valve ends. Striae evenly spaced throughout, 8–12 in 10 µm. Interstriae of equal or lower width than striae, evenly pronounced throughout valve length. Lineolae composing striae quite large and discernible with LM, 4–6 within a stria at middle of valve.

SEM: Externally, valve face flat ( Fig. 223 ). Terminal area weakly expanding onto valve face. Terminal pores of raphe small and teardrop–shaped, slightly advancing into the terminal area. Terminal pores of raphe slightly deflected towards one valve side. Striae uniseriate, lineolae simple, linear and strongly pronounced, 30 in 10 µm.

Distribution:—Besides the specimen depicted in Lange-Bertalot (2001: fig. 78: 16) having an unknown origin, H. rostratoides , with respect to this study, has only been observed in Lake Prespa, Macedonia. It occurs in the sediment and macrophytes on various depths.

Observations:—Illustrations of type specimens of Navicula subcostulata var. rostrata Hustedt in A. Schmidt et al. (1874 –1959: fig. 402: 65) were given by Simonsen (1987: 193, figs 303: 11–16), depicting two valves (holotype and isotype). Later, Lange-Bertalot et al. (1996: 263) transferred Navicula subcostulata var. rostrata to Hippodonta rostrata (Hust.) Lange-Bert., Metzeltin & Witkowski (1996: 263) using only the holotype specimen (valve depicted in Simonsen 1987: figs 303: 11–13). The isotype specimen depicted by Simonsen (1987: figs 303: 14–16) was treated as a different species and described by Lange-Bertalot (2001: 101, figs 78: 1–10) as Hippodonta kissii Lange-Bert. (2001: 101) .

Hippodonta rostrata in Lange-Bertalot (2001: figs 78: 11–16) is presented with six valves, with figs 78: 11–14 being similar to the original drawning and holotype specimens. Nonetheless, it is somewhat questionable whether the valves depicted in Lange-Bertalot (2001: figs 78: 15, 16) are conspecific with H. rostrata . Both specimens have significantly shorter protracted valve ends and higher density of the striae. The specimen depicted with fig. 78: 16 in Lange-Bertalot (2001) from BRM, with an unknown origin, corresponds quite well with the morphology of H. rostratoides from Lake Prespa. On the other hand, the specimen depicted in Lange-Bertalot (2001: fig. 78: 15), originating from the Albanian part of Lake Prespa, very likely belongs to H. affinis (compare with Figs 192–200 ; see comment under H. affinis ).

Hippodonta rostratoides closely resembles H. rostrata sensu stricto with respect to valve outline, protraction of valve ends and appearance of striae. Nonetheless, H. rostratoides has shorter protracted valve ends, less pronounced central pores of the raphe as well as a narrower axial area compared to H. rostrata . In addition, the interstriae of H. rostrata are more pronounced and broader than the striae in comparison to the narrow interstriae in H. rostratoides . Hippodonta kissii has much larger valves (length = 28.0–40.0 µm; width = 8.5–11.0 µm), a central area bordered with one shortened central stria (fascia is absent), broader axial area (irregularly lanceolate) and indiscernible lineolae. These features enable easy differentiation from H. rostratoides .

Hippodonta rostratoides from Lake Prespa is, with respect to valve outline and appearance of striae, most closely similar to H. affinis ( Figs 192–200 ) and H. media ( Figs 156–165 ), both from Lake Prespa. Still, the valve ends of H. rostratoides are narrower and longer protracted as a clear and straightforward distinguishing character.

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.

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.