Phylum: Charophyta   Family: Desmidiaceae

Cosmarium regnesiforme M.C. van Westen & P.F.M. Coesel 2021

This recently described desmid is very similar to C. tritum (West) Kouwets 2025. Both are triangular at the base of the semicells but differs in having noticeably large granules on the lateral margins and on the apex. It is also smaller overall, with some overlap with the length and breadth but none with the thickness and breadth of isthmus. The L/B ratio is also smaller.
Cell dimensions (3): L: 6.7-9.4 µm, B: 7.1-9.3 µm, Is: 2.9-4.8 µm; Th: c.4 µm, L/B: 0.95-1.01.
This will have been confused with similar species in the past so distribution in the Outer Hebrides is unknown. It has been found in slightly acidic, oligo-mesotrophic waters.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Westen, M.C. (2024). Sieralgen in Drenthe.