Hippodonta angustata 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 : 24

Hippodonta angustata A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 329–342 )

Type:— TANZANIA. Lake Tanganyika , tangled in trawl from 8 m depth, ¼ mile offshore between Kjiyi and Bangwe, 4 February 1953, collector R. Ross. (holotype: slide BM! 72160; isotype: slide MKNDC! 005850/A) .

Valves narrow, strictly lanceolate to narrow rhombic-lanceolate, with broadly cuneate or moderately rounded valve ends that are not protracted. Valve length 10.0–27.0 µm, width 3.3–5.3 µm. Axial area narrow-linear, central area forming panduriform fascia expanding to valve margins. In smaller valves, fascia nearly rectangular. Terminal area distinguishable with LM, broad, semi-elliptical. Raphe linear-filiform, with weakly pronounced, teardrop-shaped, closely spaced central endings. Terminal pores of raphe rarely discernible in LM, simple, circle-shaped and slightly advancing into the terminal area. Striae coarse, moderately to strongly radiate at middle, becoming parallel or weakly convergent towards valve ends, 10–14 in 10 µm. Interstriae of equal or lower width than striae. Lineolae composing striae quite densely spaced along a stria, not distinguishable in LM.

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

Observations:—With regard to valve outline, the appearance and pattern of the striae H. angustata closely resembles H. avittata and H. cf. avittata in Lange-Bertalot et al. (1996: figs 1: 35–40). Nonetheless, the central area of both H. avittata and H. cf. avittata is not a fully developed fascia, whereas in H. angustata this is a consistent feature, making the distinction between H. angustata and the two in Lange-Bertalot et al. (1996) clear and straightforward. Furthermore, the lineolae in H. angustata are quite densely spaced along a stria and never discernible in LM view, as compared to H. avittata and H. cf. avittata where the lineolae are clearly distinguishable in LM view. Concerning the valve outline H. angustata is somewhat similar to H. pseudacceptata . Still, the latter taxon has quite small and rectangular-shaped central area compared to the consistently fully developed fascia of H. angustata . In addition, the striae of H. pseudacceptata are weakly radiate at the middle or parallel throughout the valve length, whereas the striae of H. angustata are moderately to strongly radiate at the middle and more coarse. With regard to the valve outline and the shape of the central area, H. angustata resembles H. subrhombica ( Metzeltin et al. 2005: figs 59: 31–36) described from a freshwater habitat near La Pedrera in Uruguay. Nevertheless, the latter taxon has clearly finer striae, also having less radiate or nearly parallel pattern at the middle. Additionally, the valve ends of H. subrhombica are broadly rounded and never cuneate, compared to the moderately rounded or broadly cuneate valve ends in H. angustata . Hippodonta angustata is also similar to H. avittatiformis ( Figs 308–328 ) in regard to striae appearance, pattern and density. However, the valve ends in H. avittatiformis are consistently broadly protracted, as well as more broadly rounded compared to the unprotracted and broadly cuneate valve ends of H. angustata .

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.