Phylum: Charophyta   Family: Desmidiaceae

Staurastrum sexangulare (O. Bulnheim) L. Rabenhorst 1863

A large, striking, cell with usually 6 radiating processes. Hence the name.
Overall cell dimensions: L: 64-75 µm; B: 76-112 µm; Is: 12-13 µm; L/B: 0.82-0.85.
West, West & Carter (1923: 194) 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 found in the plankton of mesotrophic waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.