Hippodonta latelanceolata 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 : 12-14

Hippodonta latelanceolata A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 27–44 , 64–66 )

Type:— MACEDONIA. Lake Ohrid, Bay of Ljubanishta , mud and sand, 13 m depth, 2 April 2007, collector Z. Levkov. (Accession No. MKNDC 1202; holotype: slide MKNDC! 001202/1; isotype: slide BM! 101633) .

Frustules rectangular in girdle view. Valves broadly elliptic to rhombic-lanceolate with not to weakly protracted, moderately rounded valve ends. Valve length 21.5–28.5 µm, width 7.0– 9.5 µm. Axial area linear, broad, expanding abruptly into a rectangular central area. Central area forming broad fascia, expanded to valve margins. Terminal area moderately large, clearly distinguishable in LM view, semi-circular in shape. Raphe linear to weakly curved, filiform, with distinct, distantly positioned and teardrop-shaped central endings. Terminal pores indistinct. Striae coarse, strongly radiate in the middle, becoming parallel to convergent near valve ends. Striae distantly spaced throughout, 8–10 in 10 µm. Interstriae broad, of the same width as striae or even wider in longer valves. Lineolae composing the striae not discernible in LM view.

SEM: Externally, valve face flat. Terminal area expanding onto valve face ( Fig. 64 ). Terminal pores of raphe advancing into the terminal area and slightly deflected towards secondary valve side (where Voigt discontinuities are present). Central raphe endings prominent teardrop depressions ( Fig. 65 ). Striae biseriate, with lineolae alternating along the stria. Lineolae simple, linear and quite narrow, of equal length on valve face and mantle, 45–50 in 10 µm. Some lineolae, close to raphe, positioned perpendicular with respect to the rest of the lineolae within the stria. Biseriate structure of striae continuing onto valve mantle. The row of lineolae going around valve apex, positioned on apical valve mantle, rarely observed as biseriate. Quite prominent hyaline area present throughout valve mantle, absent of any structure ( Fig. 66 ).

Distribution:— Hippodonta latelanceolata has so far only been observed in Lake Ohrid, Macedonia. It was usually observed on silt substrate in deeper parts, between 10– 40 m.

Observations:— Hippodonta latelanceolata resembles H. costulatiformis var. costulatiformis Lange- Bert., Metzeltin & Witkowski in Lange-Bertalot et al. (1996: 254, 255, figs 1: 27–29; this study Figs 45–63 ) with respect to valve morphology (valve shape, striae pattern, axial and central area). Differences can be found in valve width ( H. latelanceolata being broader for any given valve length than H. costulatiformis var. costulatiformis ). However, the main differential character is the striae structure: in H. latelanceolata the striae are biseriate, while in H. costulatiformis var. costulatiformis the striae are uniseriate, a character observable only in SEM.

Hippodonta costulatiformis var. costulatiformis Lange-Bert., Metzeltin & Witkowski (1996: 254, 255, figs 1: 27–29; 4: 21, 22) ( Figs 45–63 , 68 )

Frustules rectangular in girdle view. Valves narrowly lanceolate to strictly lanceolate, gradually narrowing towards valve apices. Valve apices broadly cuneate, not protracted. Valve length 18.0– 35.5 µm, width 5.6–7.0 µm. Axial area narrow, abruptly expanding into a transverse rectangular central area. Central area forming well-defined fascia, expanding to valve margins at middle of valve, in some specimens broadened near valve margins. Rarely, the fascia is interrupted by one shortened stria on one valve side. Terminal area moderately large, distinguishable in LM, triangular in shape. Raphe linear, filiform, with distantly positioned, small teardrop-shaped central endings. Terminal pores of raphe, when discernible in LM, small and round. Striae coarse with a variable pattern, parallel to strongly radiate at middle, becoming parallel to weakly convergent towards valve ends. Striae evenly spaced throughout, 7–10 in 10 µ m. Interstriae broad, clearly more pronounced than striae, only in shorter valves the interstriae of the same width as striae. Lineolae composing striae not discernible in LM.

SEM: Externally, valve face flat ( Fig. 68 ). Striae uniseriate throughout valve face and mantle. Lineolae quite narrow, slightly longer on valve face than on valve mantle, c. 50 in 10 µm. Row of lineolae positioned on apical valve mantle, also uniseriate. Quite prominent hyaline area present throughout valve mantle, absent of any structure. Girdle band quite broad, unornamented.

Distribution:—According to Lange-Bertalot et al. (1996) and Lange-Bertalot (2001) this taxon was reported only from Northern Germany and near Berlin. Among the ancient lakes, H. costulatiformis var. costulatiformis was observed only from Lake Ohrid, Macedonia, on silt or sand substrate on depths greater than 10 m.

Observations:— Hippodonta costulatiformis var. costulatiformis resembles H. costulatiformis var. densistriata (this study, Figs 69–83 ) from Lake Prespa with respect to valve outline. Nonetheless, the striae in H. costulatiformis var. densistriata are more densely spaced and the central raphe endings consistently closer compared to the distantly positioned central endings of the raphe in H. costulatiformis var. costulatiformis . Furthermore, H. costulatiformis var. costulatiformis also resembles H. latelanceolata ( Figs 27–44 ) in valve outline, striae pattern and the presence and shape of the fascia. Still, the latter taxon has a clearly more rhombic valve outline in addition to broader valves (7.0– 9.5 µm in H. latelanceolata compared to 5.6–7.0 µm in H. costulatiformis var. costulatiformis ). Moreover, the striae of H. latelanceolata are more pronounced and have a coarser appearance.

Hippodonta costulatiformis var. densistriata A. Pavlov, Levkov, D.M. Williams & Edlund , var. nov. ( Figs 69–83 , 96, 97 )

Type:— MACEDONIA. Lake Prespa , Bay of Konjsko, mud, 2 m depth, 25 March 2003, collector Z. Levkov. (Accession No. MKNDC 000537; holotype: slide MKNDC! 000537/1; isotype: slide BM! 101634) .

Frustules rectangular in girdle view. Valves elliptic-lanceolate, with broadly cuneate to broadly rounded, but not protracted valve ends. Valve length 17.0–29.0 µm, width 4.7–5.8 µ m. Axial area narrow, slightly broadened close to central area. Central area transverse, forming rectangular to panduriform fascia, expanding to valve margins. Terminal area small and rarely discernible in LM. Raphe linear-filiform, with small, closely spaced, teardrop–shaped central pores. Terminal pores indistinct. Striae coarse, weakly to moderately radiate at middle, becoming strongly convergent near valve ends. Striae on valve mantle ( Figs 82, 83 ) have same orientation as valve face. Striae evenly spaced throughout, 12–13 in 10 µm. Lineolae not discernible in LM.

SEM: Externally, valve face flat ( Fig. 96 ). Terminal area slightly expanding onto valve face. Terminal pores of raphe simple linear, advancing into terminal area. Striae uniseriate throughout valve face and mantle. Lineolae quite prominent, of equal length on valve face and mantle, c. 40 in 10 µm. The row of lineolae positioned on the apical valve mantle is uniseriate ( Fig. 96 ). Hyaline area present throughout valve mantle, moderately pronounced. Girdle band broad, unornamented. Internally, each stria positioned in its own depression, thus valve face not flat ( Fig. 97 ). Lineolae covered by elliptical volae. Central raphe endings simple linear, distantly positioned. Linear raphe slits distally terminated by weak, semi-circular helictoglossae. Helictoglossae positioned just before the terminal area, in line with raphe slits. Internally, terminal area strongly pronounced.

Distribution:— Hippodonta costulatiformis var. densistriata has so far only been observed in littoral sediments in Lake Prespa, Macedonia.

Observations:— Hippodonta costulatiformis var. densistriata resembles H. costulatiformis var. costulatiformis (Lange-Bert. et al. 1996: figs 1: 27–29) with respect to valve outline, striae pattern and the presence of a well-defined fascia. Nevertheless, the latter taxon is consistently broader than H. costulatiformis var. densistriata for any given valve length. However, the most significant differential character between these two taxa is the stria density, being more densely spaced in H. costulatiformis var. densistriata (12–13 in 10 µm) compared to H. costulatiformis var. costulatiformis (8–10 in 10 µm). According the valve outline, H. costulatiformis var. densistriata also resembles H. costuloides (Skvortsov) Kulikovskiy, Metzeltin & Lange- Bert. in Kulikovskiy et al. (2012: 146, figs 66: 1–3). Nonetheless, a fully developed fascia is never observed in H. costuloides and the presence of one shortened stria from both valve sides in the middle of the valve is consistent throughout the size diminution stages.

Figures

FIGURES 27–63: LM micrographs. Figs 27–44. Hippodonta latelanceolata (type material). Fig. 30. Complete frustule in girdle view. Figs 45–63. Hippodonta costulatiformis var. costulatiformis. Fig. 63. Complete frustule in girdle view. 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.

FIGURES 69–95: LM micrographs. Figs 69–83. Hippodonta costulatiformis var. densistriata (type material). Figs 82, 83. Images of complete frustule in girdle view. Figs 84–95. Hippodonta naviculiformis (type material). Scale bar = 10 µm.

FIGURES 96–101: SEM micrographs. Figs 96, 97. Hippodonta costulatiformis var. densistriata (type material). Fig. 96. External view of complete frustule showing narrow hyaline area present on valve mantle and the unornamented girdle. Fig. 97. Internal view of entire valve; striae positioned in shallow ridge depressions; interstriae strongly pronounced; helictoglossae in line with raphe slits. Figs 98–101. Hippodonta naviculiformis (type material). Figs 98, 100. External view of entire valve. Terminal pores of raphe deflected towards one valve side. Fig. 99. Internal view of entire valve. Lineolae positioned in shallow elliptical depressions. Central raphe endings simple linear and distantly positioned. Fig. 101. External view of broken frustule showing girdle unornamented. Scale bars in Fig. 96 = 2 µm; Fig. 97 = 5 µm; Figs 98–101 = 2 µm.