Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus cuspidicurvatus Coesel & Meesters

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 than earlier

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