Phylum: Charophyta   Family: Desmidiaceae

Cosmarium bioculatum J. Ralfs 1848

Following earlier confusion over this species, predating 1848, Ralfs (1848: 95) gave a more detailed description which should be regarded as the valid diagnosis of C. bioculatum. His cell dimensions (calculated from his fractions) are L: 17.9 µm; B: 14.3 µm; Is: 4.3 µm. (These are very precise dimensions which might suggest just a single specimen was involved.) The cells depicted here have the general characteristics of the species but are so like C. tenue. the identification cannot be taken for certain. The only certain way to tell them apart is via the zygospores: smooth in C. tenue, ornamented in C. bioculatum. Unfortunately, zygospores are rarely encountered. My dimensions below closely match Van Westen’s C. bioculatum morpha a (2024: 392).
Cell dimensions: L: 13.7-16 µm, B: 13.2-14.8 µm, Is: 5-6.1 µm; Th: 8-9 µm, L/B: 1.04-1.09.
Found in is slightly acidic, mesotrophic habitats. Because of the uncertainty over identification its status in the Outer Hebrides is unknown.

References: 
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Ralfs, J. (1848). The British Desmidieae.
Van Westen, M.C. (2024). Sieralgen in Drenthe.