Phylum: Charophyta Family: Desmidiaceae
Staurastrum simonyi var. sparsiaculeatum (Schmidle) M.Hirano
Differs from the nominate variety in have additional lateral spines (more obvious in apical view) and generally more pronounced spines. Found in acidic oligotrophic waters and said to be widespread. More sampling is needed in the Outer Hebrides to establish its status. I'm grateful to Marien van Westen for guidance with this variety. He used the word 'probably' which emphasises the difficulty in separating this from the nominate
References:
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus