Hippodonta lueneburgensis (Grunow) Lange-Bert., Metzeltin & Witkowski

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 : 12

Hippodonta lueneburgensis (Grunow) Lange-Bert., Metzeltin & Witkowski ( Figs 1–26 , 67 )

Navicula hungarica var. lueneburgensis Grunow 1882: 156 , figs 30: 43, 44

Valves rhombic-lanceolate to lanceolate with narrowly rounded ends, not protracted. Valve length 21.5–48.8 µm, valve width 6.0– 8.7 µm. Axial area narrow, linear, expanding into a distinct, round or transapically elongated elliptic central area. Central area well-defined by the continuous shortening of striae, from both valve sides, at the middle of the valve. Terminal area narrow, semi-elliptical in shape, clearly distinguishable in LM. In smaller valves, terminal area less pronounced. Raphe straight or weakly curved, lateral, with large, teardrop-shaped central pores. Central pores distantly positioned. Terminal pores distinct. Striae coarse, radiate at middle becoming parallel or convergent towards valve apices. Striae evenly spaced throughout, 7–9 in 10 µm. Interstriae narrow and evenly pronounced throughout valve length. Lineolae composing the striae not discernible in LM.

SEM: Externally, valve face flat, with only central area slightly depressed ( Fig. 67 ). Terminal area quite prominent but not expanding onto valve face. Terminal pores of raphe distinct but not advancing into terminal area. Central raphe endings prominent and linear following the depression of central area. Striae uniseriate throughout valve face and mantle. Lineolae rather broad, bone-shaped, slightly longer on valve face than on valve mantle, 30–35 in 10 µm. The row of lineolae positioned on apical valve mantle are also uniseriate. Quite prominent hyaline area present throughout valve mantle, absent of any structure. Girdle band quite broad, unornamented.

Observations:—The specimens of H. lueneburgensis depicted in Lange-Bertalot et al. (1996, figs 1: 3, 4) and Lange-Bertalot (2001, figs 74: 5, 6, 8–10) possess transverse and a somewhat rectangular-shaped central area. The valves of the population observed in Lake Ohrid are characterised by round or apically elongated elliptical central areas. Nevertheless, with respect to the remaining characters of valve morphology, H. lueneburgensis from Lake Ohrid conforms to the concept outlined in the protologue of Grunow (1882, figs 30: 43, 44) for Navicula hungarica var. lueneburgensis (the basionym for H. lueneburgensis ).

Distribution: — Reported as cosmopolitan for the northern hemisphere, occurring in electrolyte-rich to slightly brackish waters ( Lange-Bertalot 2001). In this study it was observed only in Lake Ohrid on sand substrate near the shore (depth 1–5 m). According to Edlund et al. (2006b) it is found in ancient Lake Hövsgöl, Mongolia.

Figures

FIGURES 1–26: LM micrographs of Hippodonta lueneburgensis. Scale bar = 10 µm.

FIGURES 64–68: SEM micrographs. Figs 64–66. Hippodonta latelanceolata (type material). Fig. 64. External view of entire valve. Terminal pores of raphe deflected towards secondary valve side. Fig. 65. Close-up of central area of valve in Fig. 64. Central raphe endings with pronounced teardrop-shaped depressions. Fig. 66. External view of valve depicting complete mantle. Pronounced hyaline area present throughout valve mantle. Fig. 67. Hippodonta lueneburgensis. External view of complete frustule showing broad unornamented girdle. Fig. 68. Hippodonta costulatiformis var. costulatiformis. External view of complete frustule showing prominent hyaline area present on valve mantle and broad, unornamented girdle. Scale bars in Figs 64, 66, 68 = 5 µm; Fig. 67 = 10 µm; Fig. 65 = 2 µm.