Phylum: Charophyta   Family: Desmidiaceae

Staurastrum sexangulare (O. Bulnheim) L. Rabenhorst 1863

First described and illustrated by Bulnheim (1861: 51, pl. 9A: 1) as Didymocladon sexangularis. Shortly after Rabenhorst (1863: 621) moved it to the Staurastrum genus. West, West & Carter (1923: 194, pl. 157: 2-3) describe its distribution as “not an uncommon species in the plankton of some lakes in the western part of the British Isles, but otherwise very rare.” One hundred years later, it's proving scarce even in the west. A species largely confined to Atlantic regions.
Overall cell dimensions: L: 60-100 µm; B: 80-125 µm; Is: 13-22 µm.
A rare species found in the plankton of mesotrophic waters.

References: 
Bulnheim, O. (1871). Beiträge zur Flora der Desmidieen Sachsens.
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Rabenhorst, L. (1863). Kryptogamen-Flora von Sachsen, Ober-Lausitz, Thüringen und Nord-Böhmen, mit Berücksichtigung der benachbarten Länder: erste Abtheilung: Algen im weitesten Sinne, Leber und Laubmoose.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.