Hippodonta avittatiformis 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 : 23

Hippodonta avittatiformis A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 308–328 )

Type:— TANZANIA. Lake Tanganyika , tangled in trawl from 8 m depth, 400 m offshore between Kjiyi and Bangwe, 4 February 1953, collector R. Ross. (holotype: slide BM! 72160; isotype: slide MKNDC! 005850/A) .

Frustules rectangular in girdle view. Valves lanceolate in larger specimens to rhombic-lanceolate in smaller specimens. Valve ends broadly protracted and broadly rounded. In shorter size diminution stages the protraction of valve ends not as clearly pronounced as in longer valves. Valve length 10.5–22.5 µm, width 3.9–4.9 µm. Axial area very narrow-linear, rarely observed as weakly broadened close to the mid-valve. Central area variable, transversally elongated to panduriform, defined by shortening of a single central stria from both valve sides. In some specimens, central area forming well-defined fascia, continuously broadening towards the margins. Terminal area clearly distinguishable throughout size diminution stages, quite broad, semi-elliptical in shape. Raphe linear-filiform, with teardrop-shaped, closely spaced central pores. Terminal pores of raphe distinguishable in LM view, equally pronounced as central pores. Striae coarse, moderately to strongly radiate in middle, becoming parallel or weakly convergent towards valve ends. Striae continuing on valve mantle with the same pattern as on valve face ( Figs 327, 328 ). Striae evenly spaced throughout, 10–14 in 10 µm. Interstriae of equal or greater width than the striae. Lineolae composing the striae quite densely spaced within a stria, not distinguishable in LM view.

Distribution:— Hippodonta avittatiformis has so far only been observed from Lake Tanganyika, East Africa. In the type material it is very frequent.

Observations:—With regard to the striae morphology and pattern and the valve outline, H. avittatiformis resembles H. avittata (Cholnoky) Lange-Bert., Metzeltin & Witkowski (1996: 253, 254, figs 1: 30–34) (≡ Navicula subcostulata var. avittata Cholnoky 1959: 51 , figs 6: 265–269) and a taxon named as “ H. cf. avittata ” in Lange-Bertalot et al. (1996: figs 1: 35–40). Nevertheless, the latter two taxa do not have protracted valve ends and have more rhombic valve outlines. Furthermore, the central area of both H. avittata and H. cf. avittata sensu Lange-Bertalot et al. (1996) is never expanded to the valve margins at the middle of the valve and is quite small and not clearly defined. The central area of H. avittatiformis is transversely elongated into a panduriform to unilateral fascia.

Hippodonta avittatiformis is also similar to H. angustata (this study, Figs 329–342 ) in regard to the appearance, pattern and density of the striae. Still, the valve ends of H. angustata are not protracted and broadly cuneate, and the central area is consistently a well-defined fascia.

Figures

FIGURES 308–364: LM micrographs. Figs 308–328. Hippodonta avittatiformis (type material). Figs 327, 328. Images of complete frustules in girdle view. Figs 329–342. Hippodonta angustata (type material). Figs 343–349. Hippodonta cocquytiae (type material). Figs 350–353. Hippodonta conspicua (type material). Figs 354–364. Hippodonta minuta (type material). Fig. 354. Complete frustule in girdle view. Scale bar = 10 µm.