Phylum: Charophyta   Family: Closteriaceae

Closterium calosporum V.B. Wittrock 1869

First described and illustrated by Wittrock (1869: 23, pl. 1: 11). It needs to be compared with the similar C. parvulum in the vegetative state, although the zygospores are different.
Cell dimensions: L: 92-114 µm; B: 7.7-11.4 µm; Ap: 2.3-2.5 µm; Arc: 102-122°; L/B: 9-12.
The British distribution is rather vague: West & West (1904: 138) suggest it's a scarce species. They give the Outer Hebrides as one of the locations but it's rarely recorded. Most recent finds in the Outer Hebrides have been with mosses on wet rocks. This might suggest a different ecology from the much commoner variety brasiliense.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Van Westen, M. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1904). A Monograph of the British Desmidiaceae, Volume 1.
Wittrock, V.B. (1869). Anteckningar om Scandinavins Desmidiaceer.