Phylum: Charophyta   Family: Desmidiaceae

Staurastrum lunatum J. Ralfs 1848

This is similar to S. avicula in shape, size and ecology but the defining characteristic is: two spines at the apical angle in S. avicula and a single spine in S. lunatum. Coesel & Meesters (2013: 68) made this species a variety of S. avicula noting that ‘intermediate’ cells have been observed and noted in the literature. West, West & Carter (1923: 30) note: “The granules in the vicinity of the angles often tend to become developed into small spicules in this species, so that the angular spines may seem to be duplicated.” It is also worth noting that recently divided cells develop their spines last, giving a misleading appearance in immature cells. Some species with spines such as S. pelagicum occasionally grow an additional spine – this phenomenon I would describes as an anomaly. I have found nothing in the Outer Hebrides to suggest intermediate forms so, at least for the time being, I prefer to keep them as separate species.
Cell dimensions: L: 30-36 µm; B: 33-39 µm; Is: 10-13 µm; L/B: 0.8-1.0.
A common species found in circumneutral waters such as loch-margins, moorland pools and abandoned peat-cuttings.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Ralfs, J. (1848). The British Desmidieae.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.