Pinnularia gemella Van de Vijver

Pinnularia gemella Van de Vijver (in Van de Vijver et al. 2009: 432) (Figs 197–204)

Valves linear with strictly parallel margins and broadly rounded, non-protracted apices. Valve dimensions (n=20): length 43–52 µm, width 7.3–8.8 µm. Axial area moderately broad, linear to linear-lanceolate, not, or only very slightly, widening towards the central area. Central area forming a rectangular, usually asymmetrical fascia. Raphe lateral with deflected proximal raphe endings terminating in weakly expanded pores. Distal raphe fissures bayonet-shaped. Striae almost parallel throughout the entire valve. Longitudinal lines absent. One single row of small spines visible on the primary side, 9–10 in 10 µm. Valves usually found in pairs, connected by linking spines, lying in juxtaposition.

Habitat:— Pinnularia gemella was found in several larger lakes on the central plateau of Byers Peninsula. All lakes had a pH varying between 7.2 and 7.5 and a very low specific conductance level (<60 µS/ cm). The species seems to be absent from Hurd Peninsula.

Observations:— Pinnularia gemella cannot be mistaken with any other species of Pinnularia due to the presence of the spines on the primary side. Only a few Pinnularia species have spines or form colonies in juxtaposition. Van de Vijver et al. (2004a, 2009) reviewed all colony- and/or spine-forming species of Pinnularia. Pinnularia subantarctica var. elongata has a similar valve outline but higher stria density and lacks the typical spines of P. gemella.