Phylum: Charophyta   Family: Desmidiaceae

Staurastrum arachne  J. Ralfs 1848

First described and illustrated by Ralfs (1848: 136, pl. 23: 6). Not a common species but more frequent in the north and west of the British Isles. The Scottish areas of Sutherland and the Outer Hebrides seeing most records.
Cell dimensions: L: 20-30 µm; B: 33-55 µm; Is: 8-9 µm.
It has a preference for vegetated loch-margins and in the plankton.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Ralfs, J. (1848) The British Desmidieae.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.