Valvae lineares marginibus paene parallelis, apicibus late rotundatis, non-protractis. Longitudo 44.7–72.3 µm, latitudo 10–12.8 µm. Area axialis potius lata, linearis-lanceolata, dilatans in aream centralem. Area centralis formans fasciam rhombicam latam, maculis rotundatis in marginibus. Raphe lateralis, ramis leviter undulatis. Terminationes raphis proximales leviter deflexae cum poris indistinctis. Pori centrales annicibus lateralis. Fissurae raphis distales unciformes. Striae moderate ad fortiter radiatae in media parte valvae, graduatim moderate ad fortiter convergentes ad apices, 11–12 in 10 µm. Lineae speciosae longitudinales nullae.
Valves linear with almost parallel margins and very broadly rounded, non-protracted apices. Valve dimensions (n=53): length 44.7–72.3 µm, width 10.0– 12.8 µm. Axial area linear-lanceolate, rather broad, widening toward the central area. Central area forming a large, rhomboid fascia with rounded thickenings at the margin, visible in LM. Raphe lateral, only slightly undulating with straight to only very weakly deflected proximal raphe endings and inconspicuous central pores, bearing small lateral annexes. Distal raphe fissures bajonetshaped. Striae moderately to strongly radiate in the middle, gradually becoming moderately to strongly convergent towards the ends, (10) 11–12 in 10 µm. Longitudinal lines absent.
Type:— Hurd Peninsula, Livingston Island, South Shetland Islands, sample 5b/05, leg. R. Zidarova, coll. date 25/01/2005, slide no. BR-4259 (holotype BR), slide PLP-204 (isotype University of Antwerp, Belgium), slide ZU8/22 (isotype BRM).
Habitat:— Pinnularia livingstonensis is most abundant in a sample taken on Hurd Peninsula from very wet mosses close to a brook running down on a rocky surface, at about 70m from the sea shore near Caleta Argentina Bay. The species was previously misidentified and reported as P. divergens var. sublinearis P.T. Cleve (1895: 79) (Zidarova 2008: 30, figs 25 & 26) whereas the larger forms were earlier reported as P. divergens f. biconstricta (Cleve-Euler 1939: 15) Cleve-Euler (1955: 53), which probably represent initial cells of the same species.
Etymology:— The specific epithet refers to Livingston Island where the species was first discovered.
Observations:— Pinnularia livingstonensis belongs to the P. divergens -complex based on the central rounded thickenings and the annexes to the proximal raphe pore (Krammer 2000). Pinnularia divergens var. subbacillaris Krammer (2000: 62) has a similar valve outline but is narrower (width 7.6–9.0 µm vs. 10.0– 12.8 µm) with a higher stria density (12–14 vs. 11–12 in 10 µm). Pinnularia canadodivergens has slightly larger dimensions, a lower stria density (8–10 vs. 11–12 in 10 µm) and a smaller fascia. Pinnularia divergens var. sublineariformis , described from Bavaria, Europe, has comparable dimensions but a lower stria density (10 vs. 11–12 in 10 µm). All other varieties of Pinnularia divergens, listed in Krammer (2000), are easily distinguished from the Antarctic species by outline, dimensions and shape of the central area. Somewhat similar in outline to P. livingstonensis is P. parvulissima Krammer (2000: 95) but the latter has typical depressions in the central area and lacks the rounded thickenings.