Phylum: Charophyta   Family: Desmidiaceae

Staurastrum boreale  W. & G.S. West 1905

A widespread species but easily confused with the very similar Staurastrum gracile, both morphologically and in ecological preference. The latter species tends to be broader (longer processes) with little to no overlap within the Outer Hebridean populations. Also, S. gracile has parallel processes whereas S. boreale often has the processes slightly divergent. Also compare with another ‘catch-all’ species: S. polymorphum. This species is slightly smaller, has a more acute sinus and generally smaller granules.
The zygospore is rarely recorded and has 6–7 projections in face-view, with 3-4 apical appendages. S. gracile has 8-9 (-10). (This is based on a record by Homfeld in 1929 - see the entry for S. gracile.)
Cell dimensions: L: 29-33 µm; B: 35-45 µm; Is: 7.5-10.2 µm; L/B 0.7-0.8.
Zygospore: minus projections: 22-30 µm; overall: 38-50 µm.
A common species found in slightly acidic to slightly alkaline, 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.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.