Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tenue  W. Archer 1868

Archer's original, rather sketchy description has been adapted and broadened by later authors. This is very similar to C. bioculatum and the only certain way to tell them apart is via the zygospores: smooth in C. tenue, ornamented in C. bioculatum.
Cell dimensions: L: 13.7-17.3 µm, B: 13.2-16.9 µm, Is: 5-6.4 µm; Th: 7-9 µm, L/B: 1.03-1.19.
Found in slightly acidic, mesotrophic habitats. Because of the uncertainty over identification its status in the Outer Hebrides is unknown.

References: 
Archer, W. (1868). Quarterly Journal of Microscopical Science, New Series 8: 293.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Van Westen, M. (2024). Sieralgen in Drenthe.