Pinnularia strictissima Manguin 1964

Pinnularia strictissima Manguin (1964: 84, pl. 16, fig. 8) (Figs 36–43)

Valves linear with almost straight margins and non-protracted, cuneate obtuse apices. Valve dimensions (n=10): length 14–23 µm, width 4.3–5.0 µm. Axial area narrow, linear, not widened towards the central area. Central area forming a large rectangular, asymmetrical fascia due to irregular shortening of striae near the central area. Raphe straight. Proximal raphe endings clearly deflected with expanded, drop-like raphe pores. Distal raphe fissures moderately large, sickle-shaped. Striae parallel, weakly convergent towards the ends 10– 12 in 10 µm. Central striae bordering the central area, weakly radiate. Longitudinal lines absent.

Habitat:— Pinnularia strictissima has been only found in one sample on Byers Peninsula. This population was sampled from the sandy bottom of a small pool near the southern beaches, with an almost circumneutral pH (7.2) and a low specific conductance (<100 µS/cm) value.

Observations:— The Antarctic population of Pinnularia strictissima matches exactly the description provided by Manguin (1964), described from the Peruvian Andes in a swamp at an altitude between 4000 and 4100 m. The species can be confused with several small-celled Pinnularia species with a low stria density, a rather large fascia and an almost parallel striation pattern, such as P. intermedia (Lagerstedt 1873: 23) Cleve (1895: 80), P. lagerstedtii (Cleve 1895: 87) Cleve-Euler (1934: 57), P. schimanskii Krammer (2000: 32), P. cuneorostrata (Manguin in Bourrelly & Manguin 1954: 36) Van de Vijver & Le Cohu (in Van de Vijver et al. 2002: 83), P. incognita Krasske (1939: 397), P. palatina Lange-Bert. & W. Krüger (in Werum & Lange-Bert. 2003: 171) and even P.obscura. The most similar species is P. intermedia but the latter can be distinguished in having typical capitate to rostrate apices (in contrast to the cuneate obtuse apices in P. strictissima), large valve dimensions (length: 18–40 µm vs. 14–23 µm) and less distinct distal raphe fissures. Moreover, the striae in P. intermedia are radiate in the middle whereas in P. strictissima, the striae are almost entirely parallel. Pinnularia lagerstedtii, P. schimanskii and P. incognita have usually longer, more elongated valves (up to 35 µm) with more broadly rounded valve apices. Pinnularia cuneorostrata, up to now only found on the sub- Antarctic islands in the southern Indian Ocean, has a lower valve width (2.5–4.5 µm vs. 4.3–5.0) resulting in more elongated, thinner valves. Recently P. palatina was described from a spring in Germany but represents valves with bluntly rounded valve apices, a more elongated valve outline and less parallel striae. Finally, P. obscura has a different striation pattern with clearly radiate striae changing into distinctly convergent near the apices. The valve outline differs with typical rostrate apices.