Phylum: Charophyta   Family: Desmidiaceae

Staurastrum reductum (Messikommer) Coesel 2007

A biradiate species first described as a variety of Staurastrum dimazum by Messikommer (1927, p. 345, pl. 2: 15). It was later raised to species level by Coesel (2007, p. 10, fig. 23). In their later publication (Coesel & Meesters 2013, p. 142, pl. 105: 5-7) They describe the apical ornamentation as pronounced emarginate verrucae. However, in the Outer Hebrides’ cells, they are displayed as three coronas either side of the apex, the central one being largest with six small teeth, the two on either side have four teeth. Found in the plankton of mesotrophic waters. Infrequently recorded and with an unknown distribution in the Outer Hebrides.

References: 
Messikommer, E. (1927) Beiträge zur Kenntnis der Algenflora des Kantons Zürich II. Die Algenvegetation des Böndlerstück.
Coesel, P.F.M. (2007) Taxonomic notes on Dutch desmids IV: new species, new names, new combinations.
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.