Phylum: Charophyta   Family: Desmidiaceae

Haplotaenium rectum  (G.B. Delponte) T. Bando 1988

First described and illustrated by Delponte (1877: 225, pl. 20: 8-11) as Pleurotaenium rectum. Much later it was moved to Haplotaenium by Bando (1988: 176). The main differences separating this genus from Pleurotaenium are the lack of terminal vacuoles, an axile chloroplast, a slight, but discernible, basal inflation and a smooth cell-wall. The similar Haplotaenium minutum is generally slightly smaller in length and doesn’t have a basal inflation, but still shows an obvious sinus.
Cell dimensions: L: 220-350 µm; B: 16-23 µm; L/B: 11-18.
Fairly common in peaty moorland pools and disused peat-cuttings.

References: 
Bando, T. (1988). A revision of the genera Docidium, Haplotaenium and Pleurotaenium (Desmidiaceae, Chlorophyta) of Japan.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Delponte, G.B. (1877) Specimen Desmidiacearum subalpinarum.
Ruzicka, J. (1977) Die Desmidiaceen Mitteleuropas, Volume 1, Part 1.