Phylum: Charophyta   Family: Desmidiaceae

Actinotaenium cruciferum (A. de Bary) Teiling 1954

A small species with a slight, but obvious, median constriction. The chloroplast is stelloid with usually eight strands as seen in apical view with one pyrenoid per semicell. Just a single, abundant, population has been found, which is at the upper limits of the size range.
Cell dimensions: L: 20-28 µm; B: 13-14 µm; L/B: 1.7-2.0.
Found in oligotrophic, slightly acidic habitats. It is proving to be rare in the Outer Hebrides.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Ruzicka, J. (1981). Die Desmidiaceen Mitteleuropas, Volume 1, Part 2.
Van Westen, M.C. (2024). Sieralgen in Drenthe.