Phylum: Charophyta   Family: Desmidiaceae

Staurastrum arachne  J. Ralfs 1848

A small cell with an open, acute-angled sinus, semicells cup-shaped. Processes are long and convergent with numerous concentric series of granules, tipped with about three small spines. Apices slightly convex. In apical view predominantly 5-radiate with slightly concave sides, within the margins are five strong teeth.
Cell dimensions: L: 20-30 µm; B: 33-55 µm; Is: 8-9 µm.
It prefers vegetated loch-margins and in the plankton. Infrequently recorded in the Outer Hebrides but can be locally abundant.

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.