Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus robustus (E. Messikommer) P.F.M. Coesel & K.J Meesters 2023

Coesel & Meesters (2023: 205) raised this taxon to species rank mainly on the results of scanning electron microscopy, which revealed essential differences in the cell sculpturing. Other features include an elongated isthmus and a convex apex.
Cell dimensions: L: 24-30 µm; B: 22-29 µm; Is: 4.7-7.6 µm; Sp: 1.4-4.1 µm; Overall B: 28-34.
This species is proving to be quite scarce in the Outer Hebrides. It occurs in circumneutral water such as loch-margins and moorland pools. Interestingly, it has been found with moss on wet rocks.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.