Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subcucumiforme  F.A.C. Kouwets 2025

This recently described species, along with C. subcucumoides Kouwets 2025, are based on the poorly described C. subcucumis Schmidle 1893. The latter is the smaller of the two, but dimensions do overlap a little. More significantly, the latter species has several small pyrenoids per semicell whereas C. subcucumiforme has two large ones.
Cell dimensions: L: 63-71 µm, B: 41.46 µm, Is: 17-21 µm; Th: c.31.5 µm, L/B: 1.50-1.66.
Due to taxonomic confusion, the status in the Outer Hebrides is unknown. It would seem to favour mesotrophic, slightly acidic environments.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.