Phylum: Charophyta   Family: Closteriaceae

Closterium lunula Ehrenberg & Hemprich ex Ralfs 1848

Widespread and can be locally common in acidic peaty pools, bogs, and disused peat-cuttings. West & West (1904, p. 150, pl. 18: 8-9) specifically mention it being 'general' in the Outer Hebrides.

References: 
Ralfs, J. (1848) The British Desmidieae.
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1.
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.