Phylum: Charophyta   Family: Desmidiaceae

Cosmarium ocellatum  B. Eichler & Gutwinski 1894

Most often found in acidic habitats but also occurs in circumneutral waters. It prefers the plankton and marginal vegetation of lochs. West & West (1905, p. 144) recorded it as unknown in Britain, which is surprising as it's frequently encountered in the Outer Hebrides. Zygospores are rarely encountered: one being found here in 2023. It was first recorded by Homfeld (1929, p. 47, pl. 6: 65).

References: 
Homfeld, H. (1929) Beitrag zur Kenntnis der Desmidiaceen Nordwestdeutschlands besonders ihrer Zygoten.
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.
West, W. & West, G.S. (1905) A Monograph of the British Desmidiaceae, Volume 2.