Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus cuspidicurvatus P.F.M. Coesel & K.J Meesters 2013

Predominantly a triradiate species with fairly long spines pointing obliquely upwards. The isthmus can be slightly extended.
Cell dimensions: L: 28-29 µm; B: 25-36 µm; Is: 6-7 µm; Sp: 9-18 µm; Overall LxB: 42-48 x 50-67.
West, West & Carter (1923), as Staurastrum curvatum, note “… is most commonly found in the western part of the British Isles, and is frequent in the plankton …”. More sampling is required for the Outer Hebrides, but it would seem to be much scarcer now. Found in slightly acidic lochs with open water.

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.