Phylum: Charophyta   Family: Closteriaceae

Closterium gracile  J. Ralfs 1848

It has been thought for a long time that this species was synonymous with Closterium lundellii. Recent work by (Müller 2002) has given a means to separate these taxa in the vegetative state. Van Westen (2024: 143) has succinctly described it: the ratio of apex breadth/cell breadth in Cl. lundellii is less than 0.5 and in Cl. gracile it is greater than 0.5. If you are fortunate enough to find zygospores, the difference is distinctive. Cl. lundellii has zygospores with projections that extend into the empty cell halves. The zygospores of Cl. gracile are round to elliptical.
Cell dimensions (2): L: 128-274 µm; B: 4.9-5.3 µm; Ap: 2.7-3.0 µm; ApexB/CellB: 0.56; L/B: 24-56.
Very rare in the Outer Hebrides, found in acidic, mesotrophic waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Müller, M. (2002). Closterium cornu Ehrenb. ex Ralfs var. lundellii (Lagerh.) Willi Krieg. 1935 (Desmidiaceae): eine Taxon-Revision.
Van Westen, M. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1904). A Monograph of the British Desmidiaceae, Volume 1.