Phylum: Charophyta   Family: Desmidiaceae

Cosmarium ocellatum  B. Eichler & R. Gutwinski 1894

An extended isthmus gives the sinuses a gaping appearance. Semicells are reniform to varying degrees with a rounded apex or narrowly truncate. A quite large scrobiculus appears centrally. Zygospores are rarely encountered: one being found in the Outer Hebrides in 2023. It’s multifaceted (polyhedral).
Cell dimensions: L: 24.6-30.4 µm; B: 21.7-27.7 µm; Is: 5.4-6.8; Th: c.10-13 µm; L/B: 1.06-1.29.
Zygospore dimensions: 23.3 x 33 µm.
Most often found in acidic habitats but also occurs in circumneutral waters. It prefers the plankton and marginal vegetation of lochs. West & West (1905) recorded it as unknown in Britain, which is surprising as it's occasionally encountered in the Outer Hebrides.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
West, W. & West, G.S. (1905). A Monograph of the British Desmidiaceae, Volume 2.