Hippodonta naviculiformis 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 : 14-15

Hippodonta naviculiformis A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 84–95 , 98–101 )

Type:— MACEDONIA. Lake Ohrid, Bay of Trpejca , mud + sand, 14 m depth, 2 April 2007, collector Z. Levkov. (Accession No. MKNDC 001212; holotype: slide MKNDC! 001212/1; isotype: slide BM! 101636) .

Valves narrow, elliptic-lanceolate, with acutely rounded valve ends that are not protracted. In shorter size diminution stages valve ends more broadly rounded. Valve length 11.0– 22.7 µm, breadth 3.5–5.0 µm. Axial area quite narrow-linear, expanding into quite small, transverse, rhombic to panduriform central area. Central area defined by shortening of one stria at middle, from both valve sides, never constituting a fascia. Terminal area quite small, vaguely distinguishable in LM, triangular in shape. Raphe linear-filiform, with weakly pronounced, simple teardrop-shaped central pores. Central pores of raphe closely spaced. Terminal pores of raphe indistinct. Striae fine, weakly radiate at middle, becoming parallel towards valve ends, (12) 16–20 in 10 µm. Lineolae composing striae densely spaced within a stria, vaguely discernible with LM.

SEM: Externally, valve face flat ( Figs 98, 100, 101 ). Terminal area distinct, weakly expanding onto valve face. Terminal pores of raphe weakly pronounced, small teardrop-shaped and slightly deflected towards one valve side. Terminal pores of raphe weakly advancing into the terminal area. Central raphe endings small, forming teardrop-shaped depressions. Striae uniseriate throughout valve face and mantle. Most of the lineolae have sinusoid shape, moderately broad, c. 40 in 10 µm. The rows of lineolae that go around the valve apices also uniseriate ( Figs 98, 100, 101 ). Hyaline area present throughout valve mantle, moderately pronounced. Internally, each lineola positioned in shallow elliptical depression, thus valve face not flat ( Fig. 99 ). Central raphe endings simple linear, distantly positioned. Linear raphe slits distally terminated by weak helictoglossae. Distal raphe endings also deflected towards one valve side and strongly advancing into the terminal area. Terminal area strongly pronounced internally.

Distribution:— Hippodonta naviculiformis has so far only been observed from the type locality, Lake Ohrid, Macedonia. Found within a mixed sand and clay substrate in the deeper part of the lake littoral (14 m), where the bottom is dominated by Chara spp.

Observations:—With respect to valve outline, valve size and appearance of striae, H. naviculiformis is most similar to H. abunda (this study, Figs 102–137 ) and H. microcostulata Metzeltin, Kulikovskiy & Lange- Bert. in Kulikovskiy et al. (2012: 151, 152, figs 66: 26, 27). However, the latter two taxa have completely developed fascia as a central area and lower density of striae (12–16/ 10 µm in H. abunda and 10–12/ 10 µm in H. microcostulata ) as the main distinctive features. In addition, the striae of H. microcostulata are clearly coarser and broader than the striae of H. naviculiformis . Regarding the appearance of the striae and the shape of the central area, H. naviculiformis also resembles H. exigua (this study, Figs 201–212 ), but can be distinguished from it due to the long protracted valve ends and lower stria density (14–16/ 10 µm) in H. exigua .

Figures

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.

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.

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.