Pinnularia undefined-1

Pinnularia sp 1 (Figs 44–47)

Valves linear with parallel or, in larger specimens, weakly undulated margins, and non-protracted, broadly rounded apices. Valve dimensions (n=11): length 25–35 µm, width 5.0– 6.5 µm. Axial area narrow, linear, clearly widening towards the central area. Central area forming a large rhombic fascia, about 1/3 to 1/4 of valve length, expanded to the margins. Raphe slightly curved. Central raphe endings extending into the central area well beyond the last central striae, unilaterally deflected and finishing in relatively distant drop-like pores. Terminal fissures almost indistinct,?-shaped. Striae strongly radiate in the middle, abruptly changing their direction to convergent toward the valve ends, 11–13 in 10 µm. Longitudinal lines absent.

Habitat and distribution:—This is an extremely rare species on Livingston Island. Only a few valves were found on Hurd Peninsula, in streams, in a vegetation of wet mosses close to streams, brooks or puddles.

Observations:— These Antarctic specimens belong to the complex of species around P. divergentissima (Grunow in Cleve & Möller 1879: 186) Cleve (1895: 77) sensu Krammer (2000). Pinnularia divergentissima and its varieties can be separated based on a different valve outline, having mostly convex margins and smaller subrostrate to capitate valve ends, as well as by a slightly higher number of striae (according to Krammer 2000: 12–14 vs 11–13 in 10 µm) and a smaller central area. The most similar species is P. krammeri Metzeltin (in Lange-Bertalot & Metzeltin 1996: 98), described from Finland. The main differences between the Antarctic population and P.krammeri include a larger valve width of P. krammeri (6.7–7.8 µm vs. 5.0– 6.5 µm), a comparably smaller central area and more closely spaced proximal raphe endings. Since only a few valves have been observed, it is too early to conclude whether they should be described as an independent species or belong to P. krammeri.