Phylum: Charophyta   Family: Desmidiaceae

Staurastrum gladiosum W.B.Turner 1885

This is superficially similar to S. teliferum but that species has an acute-angled sinus whereas in S. gladiosum it is more closed. In apical view the sides are concave in S. gladiosum but almost straight in S. teliferum. Also, the L/B ratio (without spines) of S. teliferum is less than 1.1, while that of S. gladiosum is greater than 1.1. A final difference is in the ecology; S. teliferum is an acidophile mainly found in oligotrophic waters whereas S. gladiosum prefers circumneutral to slightly alkaline habitats. Most of these distinctions are from Van Westen (2024).
Cell dimensions: L: 36-39 µm; B: 33-39 µm; Is: 11-13 µm; Sp: 4-6 µm; Overall LxB: c.43 x 42; L/B minus spines: 1.0-1.04.
A rare species found in circumneutral to slightly alkaline waters such as loch-margins, especially in the sandy machair lochs. West, West & Carter (1923: 58) only record two sites for the whole of Britain.

References: 
Van Westen, M.C. (2024). Sieralgen in Drenthe.