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

Pinnularia microstauroides sp. nov. (Figs 102–125)

Valvae lineares marginibus plerumque undulatis, parallelis ad leviter convexis, apicibus late rotundatis, leviter protractis, rostratis ad subcapitatis. Longitudo 26–50 µm, latitudo 5.3–6.7 µm. Area axialis moderate lata, lanceolata, dilatans in aream centralem. Area centralis formans fasciam angustam ad moderate latam, rhomboidem, asymmetricam. Raphe filiformis, ramis leviter curvatis. Terminationes raphis proximales leviter unilateraliter deflexae cum poris expansis, guttiformibus. Fissurae raphis distales falcatae. Striae moderate radiatae in media parte valvae, convergentes ad apices, 12–14 in 10 µm. Lineae speciosae longitudinales nullae.

Valves linear with usually undulating, parallel to weakly convex margins and weakly protracted, rostrate to subcapitate, broadly rounded apices. Larger specimens with undulated margins. Valve dimensions (n=25): length 26–50 µm, valve breadth 5.3–6.7 µm. Axial area relatively large, lanceolate, widening toward the central area. Central area forming a narrow to moderately broad, rhomboid, usually asymmetrical fascia. Striae gradually becoming shorter towards the valve middle. Raphe filiform with slightly curved raphe branches. Proximal raphe endings clearly deflected with expanded droplike pores. Terminal fissures distinct, sickle-shaped. Striae moderately radiate in the middle, convergent at the ends, 12–14 in 10 µm. Longitudinal lines absent

Type:— Byers Peninsula, Livingston Island, South Shetland Islands, sample BY003, leg. B. Van de Vijver, coll. date 09/01/2009, slide no. BR-4262 (holotype BR), slide PLP-207 (isotype University of Antwerp, Belgium), slide ZU8/24 (isotype BRM).

Habitat:— Pinnularia microstauroides was frequently observed in larger lakes on Byers Peninsula and only occasionally on Hurd Peninsula, where it was previously reported from as P. microstauron var. rostrata Krammer (Zidarova 2008: 30, fig. 27). The largest populations were found in coastal lakes on Byers Peninsula with a higher specific conductance (150–250 µS/cm) and higher nutrients due to the presence of marine mammals and penguins.

Observations:— Pinnularia microstauroides clearly belongs to the complex of species around P. microstauron (Ehrenb. 1843) Cleve (1891: 28). The latter is a highly variable species and probably represents a complex rather than a single species. Pinnularia microstauron, based on the account in Krammer (2000), is larger with a valve breadth of 10.0– 12.4 µm and usually with a lower stria density (9–11 in 10 µm vs. 12–14 in 10 µm). The undulating margins in P. microstauroides are also rarely observed in P. microstauron. A large number of varieties (later raised to the species level by Kulikovskiy et al. 2010), have already been described within the P. microstauron -complex, usually reflecting the local variability of the species. However, according to the original description in Krammer (2000), some of these varieties have a higher valve width and/or a lower stria density: P. nanomicrostauron Kulikovskiy, Lange-Bert. & Metzeltin (2010: 361) has a width of 6.0–7.0 µm with 10–11 striae in 10 µm whereas P. microstauropsis Kulikovskiy, Lange-Bert. & Metzeltin (2010: 361) has a width of 6.5–8.0 µm and 10–12 striae in 10 µm.