Pinnularia laterotundata Van de Vijver & Zidarova 2011, sp. nov.

Pinnularia laterotundata Van de Vijver & Zidarova, sp. nov. (Figs 39–44)

Valvae moderate grandes, lineares marginibus stricte parallelis, apicibusque late rotundatis, numquam protractis. Dimensiones (n=8): longitudo 44–55 µm, latitudo 11.5–13.5 µm, ratio longitudinis/latitudinis 3.6–4.2. Area axialis potius angusta, linearis-lanceolata, 1/6–1/7 latitudinis valvae, leviter dilatans ad aream centralem. Area centralis rectangularis. Fascia abest. Raphe moderate lateralis. Rami externi raphis moderate curvati. Terminationes raphis proximales leviter unilateraliter deflexae. Fissurae distales unciformes. Striae transapicales latae, leviter radiatae in media parte valvae, ad parallelae in apices, (4) 5–6 in 10 µm.

Valves moderately large, linear with strictly parallel margins and broadly rounded, never protracted apices. Valve dimensions (n=8): length 44–55 µm, width 11.5–13.5 µm, length/width ratio 3.6–4.2. Axial area rather narrow, linear-lanceolate, 1/6–1/7 of valve width, slightly widening towards central area. Central area rectangular, lacking fascia since asymmetrically bordered by 2–3 shortened striae on each side. Raphe moderately lateral with outer raphe fissures moderately curved. Proximal raphe endings unilaterally slightly deflected, pore-like. Distal fissures sickle-shaped. Transapical striae broad, weakly radiate in middle to parallel near apices, (4–) 5–6 in 10µm.

Type:— Byers Peninsula, Livingston Island, South Shetland Islands, sample BYS012, leg. B. Van de Vijver, coll. date 14/01/2009, slide no. BR-4235 (holotype BR), slide PLP-185 (isotype University of Antwerp, Belgium), slide BRM-7/98 (isotype BRM).

Habitat and Distribution:— Pinnularia laterotundata is quite rare on Livingston Island and only found in a few samples from Byers Peninsula. The largest population was observed between small stones in a seepage area near Limnopolar Lake, continuously moistened by flowing water. It was also observed on James Ross Island (Van de Vijver & Kopalová, unpubl. results).

Etymology:—The specific epithet refers to the shape of the apices that are broadly (latin ‘ latus ’ meaning broadly) rounded (latin ‘ rotundatus ’ meaning rounded)

Observations:— Pinnularia laterotundata has a very typical, almost rectangular, outline and cannot be confused with any other members of the Distantes section (Table 2). It differs from P. rabenhorstii (including the variety subantarctica) in having smaller valve dimensions (length 44–55 µm vs. 50–90 µm), strictly parallel margins and broadly rounded apices, as opposed to P. rabenhorstii that has cuneate to round, subrostrate apices. Pinnularia borealis (including almost all its varieties, with the exception of var. islandica) has less parallel margins and usually narrower valves (width up to 10 µm). Metzeltin et al. (2009) identified a valve from the Jargalant river in Mongolia as P. rabenhorstii var. franconica, which has some similar features to Pinnularia laterotundata but can easily be separated on the basis of the typical lanceolate outline, in contrast to the strictly linear outline of P. laterotundata, with broadly rounded, slightly narrower heads and the clearly lanceolate axial area, in contrast to P. laterotundata, which has a linear, straight axial area.