Phylum: Charophyta   Family: Closteriaceae

Closterium incurvum  L.A. de Brébisson 1856

Closterium tumidulum is similar but slightly larger and with less curvature.
Cell dimensions: L: 40-80 µm; B: 6-12 µm; Ap: 1.0-2.0 µm; Arc: 180; L/B: 5-7.
West & West (1904: 136) describe it as a rare desmid in Britain with no mention of the Outer Hebrides. It favours a range of habitats with varying degrees of acidity but infrequently recorded.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
West, W. & West, G.S. (1904). A Monograph of the British Desmidiaceae, Volume 1.