Phylum: Charophyta   Family: Desmidiaceae

Staurastrum controversum var. semivestitum (West) Coesel

Only made a named variety in 1996 so little distribution data available for the Outer Hebrides. It has the same habitat requirements as the nominate

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus