Hippodonta capitata (Ehrenb.) 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 : 20

Hippodonta capitata (Ehrenb.) Lange-Bert., Metzeltin & Witkowski ( Figs 226–238 , 272, 273 )

Valves elliptic-lanceolate, with strongly protracted, sub-capitate and broadly rounded valve ends. In some specimens observed, valve with nearly parallel margins. Smaller specimens with rostrate valve ends, their protraction not as strongly pronounced; longer specimens with valve ends nearly capitate. Valve length 13.5–24.5 µm, width 5.5–7.2 µm. Axial area narrow-linear throughout. Central area quite small, rhombic or apically elongated, defined by shortening of one stria from both valve sides. Terminal area strongly pronounced, clearly distinguishable with LM, broad, lunate and cap-like. Raphe linear-filiform, with closely spaced and weakly pronounced, simple linear or teardrop-shaped central endings. Terminal pores of raphe clearly discernible in LM, more pronounced than central pores, teardrop-shaped. Terminal pores of raphe positioned before terminal area or weakly advancing into it, slightly unilaterally curved. Striae coarse, moderately to strongly radiate at middle, becoming weakly to strongly convergent towards valve ends, 8–12 in 10 µm. Interstriae consistently narrower than striae. Lineolae composing striae quite densely spaced within a stria, not clearly discernible in LM.

SEM: Externally, valve face flat ( Fig. 272 ). Terminal area expanding strongly onto valve face. Terminal pores of raphe distinct, positioned just before the terminal area and not advancing into it. Terminal pores of raphe slightly deflected towards secondary valve side (where the Voigt discontinuities are present). Central raphe endings strongly pronounced, teardrop-shaped depressions. Striae biseriate, lineolae small, elliptical and broad, alternating along a stria, 30–40 in 10 µm. Some lineolae close to raphe positioned perpendicular towards remaining lineolae of stria. Rows of lineolae positioned on apical valve mantles, uniseriate or biseriate ( Fig. 272 ). Internally striae positioned in quite prominent and broad depressions, thus valve face not flat ( Fig. 273 ). Striae composed of two or three rows of lineolae. Lineolae internally covered by small volae, having somewhat irregular elliptical shape. Raphe sternum thickened a little towards central raphe endings. Central raphe endings simple linear. Linear raphe slits distally terminated with weak, semi-circular, helictoglossae. Helictoglossae positioned just before terminal area and in line with raphe slits. Terminal area internally quite prominent.

Distribution:—With regard to the ancient lakes subject of this study, Hippodonta capitata has only been observed in Lakes Dojran and Prespa, Macedonia. It has also been observed as quite abundant in eutrophic and polluted rivers in Europe ( Lange-Bertalot 2001).

Observations:—The slight deflection of the raphe slit towards the secondary valve side, observed by Cox (1999: fig. 56, as N. capitata ), was not found in these specimens.

The morphology and valve dimensions (valve size and stria density) of H. capitata from Lake Dojran match those depicted in Lange-Bertalot et al. (1996: fig. 4: 23) and Lange-Bertalot (2001: figs 75: 1–4), with the addition of shorter valves being observed in Lake Dojran. Lange-Bertalot (2001) notes that a population characterised by less capitate and nearly rostrate valve ends (as observed by Patrick & Reimer 1966: fig. 52: 2) questionably belongs to H. capitata and may be a new taxon. Rostrate valve ends within the population from Lake Dojran are characteristic only for the shortest size diminution stages, with the valve morphology throughout the size diminution clearly placing this population within the concept of H. capitata .

Metzeltin et al. (2005) described the new infraspecific taxon for H. capitata subsp. iberoamericana Metzeltin, Lange-Bert. & García-Rodríguez (2005: 102, figs 59: 1–13), taking into consideration the valve outline (quite weak protracted valve ends, never capitate) and the differences in valve width as the main distinctive characters. The distinctive characters between these two taxa are more clear and straightforward when compared to the differences between H. hungarica and H. uruguayensis ( Metzeltin et al. 2005: figs 59: 15–29).

Figures

FIGURES 226–271: LM micrographs. Figs 226–238. Hippodonta capitata. Figs 239–248. Hippodonta hungarica. Figs 249–271. Hippodonta linearis. Scale bar = 10 µm.

FIGURES 272–275: SEM micrographs. Figs 272, 273. Hippodonta capitata. Fig. 272. External view of entire valve. Striae biseriate throughout. Terminal pores of raphe slightly deflected towards secondary valve side. Fig. 273. Internal view of entire valve. Striae composed of two or three lines of pores, positioned in quite broad depressions. Raphe sternum thickened towards central raphe endings. Helictoglossae positioned just before terminal area, not advancing into it. Figs 274, 275. External view of entire valves of Hippodonta hungarica. Striae biseriate throughout; lineolae quite small; terminal pores of raphe weakly deflected towards secondary valve side. Scale bars in Figs 272–275 = 5 µm.