Phylum: Charophyta   Family: Desmidiaceae

Staurastrum gracile J. Ralfs 1848

It can be confused with the similar S. boreale but that species is smaller with little overlap. Also, S. gracile has parallel processes whereas S. boreale not infrequently has slightly divergent processes. The only known zygospores are by Homfeld (1929) with 9-10 projections in face view (6-7 in S. boreale) and is larger.
Cell dimensions: L: 30-34 µm; B: 50-53 µm; Is: 8-9 µm; L/B: 0.6-0.7.
A widespread species on the continent but rarely found in the Outer Hebrides. The habitat is slightly acidic, mesotrophic waters such as shallow vegetated pools and loch-margins.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Homfeld, H. (1929). Beitrag zur Kenntnis der Desmidiaceen Nordwestdeutschlands besonders ihrer Zygoten.