Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tetraophthalmum J. Ralfs 1848

Debate has been running for some time regarding the granulation size and pattern of this species. Kouwets (2025) has now created a new species: Cosmarium tetraophthalmoides to accommodate the cells with smaller granules. He also notes that the latter species is smaller with no overlap in the species length. However, the Outer Hebridean populations do overlap in the size of the length. Nevertheless, they are easily separated on the size and distribution of the granules.
Cell dimensions: L: 95-110 µm; B: 70-78 µm; Is: 26-29 µm; Th: c.48-56.4; L/B: 1.33-1.43.
West & West (1908: 270) note “Very common in the Outer Hebrides ... is found generally distributed over the whole of the British Islands, but more especially in Sphagnum-bogs and in peaty moorland ditches”. It is still relatively common and can also be found in less acidic waters such as vegetated lochs.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.