Hippodonta cocquytiae 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 : 24-25

Hippodonta cocquytiae A. Pavlov, Levkov, D.M. Williams & Edlund , sp. nov. ( Figs 343–349 )

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 lanceolate to elliptic-lanceolate, with broadly rounded valve ends. Valve length 13.5–17.0 µm, width 4.2–5.2 µm. Axial area very narrow-linear, central area forming well defined fascia, expanding to valve margins at middle of the valve, continuously broadened towards the margins. Terminal area clearly distinguishable in LM, broad and semi-circular or semi-elliptical in shape. Raphe linear-filiform, with moderately pronounced teardrop-shaped and closely spaced central pores. Terminal pores of raphe discernible in LM view, simple circle-shaped and clearly advancing into terminal area. Striae coarse, consistently moderately radiate at middle, becoming parallel or weakly convergent towards valve ends, evenly spaced throughout, 10–12 in 10 µm. Interstriae consistently of lower width than the striae. Lineolae composing the striae densely spaced within a stria and not discernible in LM view.

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

Observations:—With regard to valve outline and appearance and pattern of the striae H. cocquytiae is most similar to H. conspicua (this study, Figs 350–353 ) from Lake Tanganyika and H. umbilicatissima (Reichardt) Reichardt in Lange-Bertalot et al. (1996: 265, fig. 1: 42) (≡ Navicula umbilicatissima Reichardt 1983: 411 , 412, figs 2: 29; 3: 34, 35). Nevertheless, the central area of H. conspicua is never a fully developed fascia, whereas the fascia is a consistent character throughout the size diminution stages of H. cocquytiae . The fascia of H. cocquytiae is less pronounced than in H. umbilicatissima . Additionally, H. umbilicatissima has remarkable strongly pronounced and quite distantly positioned central raphe endings, compared to the moderately pronounced and closely spaced central raphe endings in H. cocquytiae . Furthermore, the valves of H. umbilicatissima are longer than in specimens of H. cocquytiae (21.0–29.0 µm in H. umbilicatissima as compared to 13.5–17.0 µm in H. cocquytiae ).

With regard to the presence of a prominent fascia and the valve outline, H. cocquytiae has a slight resemblance to H. pseudopinnularia Lange-Bert. (2001: 103, 222, figs 77: 1–8). Nonetheless, H. pseudopinnularia has a more linear valve outline in addition to the strongly curved, distinctly deflected and distantly positioned central raphe endings, compared to the consistently linear raphe system in H. cocquytiae , with equally pronounced central and terminal pores. More so, the valve length of H. pseudopinnularia (20.0–25.0 µm) is larger than the valve length of H. cocquytiae (13.5–17.0 µm).

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.