Phylum: Charophyta   Family: Desmidiaceae

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

Predominantly a triradiate species with a wide-open sinus with a rounded extremity, which can be slightly elongated. Semicells triangular to bowl-shaped with the lateral-apical extremities often mamillate supporting long spines pointing obliquely upwards.
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: 19), 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 to slightly alkaline lochs with open water.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Teiling, E. (1967). The Desmid Genus Staurodesmus. A taxonomic study.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.