Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subabbreviatum F.A.C. Kouwets 2025

Until recently this was known as Cosmarium abbreviatum var. planctonicum W. & G.S. West 1905, a larger version of the nominate variety. Much confusion and misidentifications have arisen in the past prompting Kouwets (2025) to rename it as a full species. Quite morphologically variably (Kouwets provides 30 images to illustrate this).
Cell dimensions: L: 15-23.6 µm, B: 15-25.8 µm, Is: 5.8-8 µm; Th: 9-12.1 µm, L/B: 0.9-1.0.
Because of taxonomic confusion under its previous title, the Outer Hebridean distribution and status is unclear. The preferred habitat is mesotrophic lochs that are mildly acidic to alkaline.

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