Pinnularia hamiltonii Zidarova & Kopalová & Vijver 2012, sp. nov.

Pinnularia hamiltonii sp. nov. (Figs 126–134)

Valvae ellipticae-lanceolatae ad anguste ellipticae marginibus convexis, graduatim decrescentibus in apices, apicibus non-protractis,obtuse rotundatis. Longitudo 30–53 µm, latitudo 7.3–9.3 µm. Area axialis moderate lata, lanceolata, clare dilatans in aream centralem. Area centralis formans fasciam cuneiformem potius latam. Raphe filiformis, ramis leviter curvatis. Terminationes raphis proximales deflexae cum poris expansis, guttiformibus. Fissurae raphis distales falcatae. Striae fortiter radiatae in media parte valvae, graduatim sed non abrupte convergentes ad apices, 9–10 in 10 µm. Lineae speciosae longitudinales nullae.

Valves elliptic-lanceolate to narrowly elliptic in smaller valves with convex margins gradually tapering towards the bluntly rounded, non-protracted apices. Valve dimensions (n=10): length 30–53 µm, width 7.3– 9.3µm. Axial area moderately broad, lanceolate, clearly widening towards the central area. Central area forming a bowtie-shaped, rather large fascia. Raphe filiform with slightly curved raphe branches. Proximal raphe endings deflected terminating in droplike, expanded pores. Distal raphe fissures sickle-shaped. Striae strongly radiate near the valve middle, gradually but not abruptly becoming convergent towards the apices, 9– 10 in 10 µm. Longitudinal lines absent.

Type:— Byers Peninsula, Livingston Island, South Shetland Islands, sample BYS012, leg. B. Van de Vijver, coll. date 14/01/2009, slide no. BR-4255 (holotype BR), slide PLP-200 (isotype University of Antwerp, Belgium), slide ZU8/21 (isotype BRM).

Habitat:— Small populations of P. hamiltonii have been observed in several samples collected in a wide variety of habitats on Byers Peninsula ranging from small pools and streams to lakes and seepage areas with a pH-range of 7.3–7.9 and a specific conductance <100 µS/cm.

Etymology:— The species is named after our colleague Paul Hamilton (Museum of Nature, Ottawa, Canada) in recognition of his taxonomic work on diatoms.

Observations:— Pinnularia hamiltonii can hardly be confused with other Pinnularia taxa. Only a few species are similar to Pinnularia hamiltonii: P. suchlandtii Hust. (in A. Schmidt 1934: 388), P. superdivergentissima Chaumont & Germain (1976: 587–588), P. cuneola, P. acutobrebissonii Kulikovskiy, Lange-Bert. & Metzeltin (2010: 361) and P. crozetii Van de Vijver & Le Cohu (in Van de Vijver et al. 2002: 82). The most similar species is probably P. crozetii, described from the sub-Antarctic Crozet archipelago (Van de Vijver et al. 2002), but the latter differs in being usually larger (valve width up to 10.5 µm) with gradually shortening striae in the central area whereas P. hamiltonii lacks these shorter striae. P. acutobrebissonii is larger (width 10–11.4 µm vs. 7.3–9.3 µm) with more acutely rounded apices and a smaller central area (Krammer 2000). P. superdivergentissima has strictly linear valves with parallel margins (Chaumont & Germain 1976) and both P. suchlandtii and P. cuneola have a more rhombic-lanceolate valve outline (Krammer 2000).