Phylum: Charophyta   Family: Mesotaeniaceae

Tortitaenia obscura (J. Ralfs) A.J. Brook 1998

First described and illustrated by Ralfs (1848: 179, pl. 34: 2 a-e) as Spirotaenia obscura. Much later, Brook (1998: 146) moved it to the genus Tortitaenia. There is much similarity between this species and Spirotaenia erythrocephala, having fusiform bodies and spiralled chloroplasts with red poles. However, Tortitaenia obscura is much longer with little overlap and broader without overlap.
Cell dimensions: L: 50-210 µm; B: 15-30 µm; L/B: 3.0-6.0.
Infrequently recorded in oligo-mesotrophic habitats such as moorlands pools and loch-margins.

References: 
Brook, A.J. (1998) Tortitaenia nom. nov. pro Polytaenia Brook, a name of a genus of saccoderm desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Ralfs, J. (1848) The British Desmidieae.
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1.