Phylum: Charophyta   Family: Desmidiaceae

Desmidium aptogonum L.A. de Brébisson ex F.T. Kützing 1849

West, West & Carter (1923: 242) reported it as widespread in suitable habitats but not common. This would still seem to be the case for the Outer Hebrides. Undisturbed chains an be exceedingly long.
Cell dimensions: L: 12.6-22 µm; B: 19.6-36 µm; Is: 15-31.
An acidophile of moorland bogs and pools, also shallow lochans.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.