Phylum: Charophyta   Family: Desmidiaceae

Cosmarium fastidiosum W. & G.S. West 1897

First described and illustrated by West & West (1897: 489, pl. 6: 11). It was only recorded from a small pond in Oxfordshire. Its site preferences are not defined for Britain. For Europe, Tsarenko, et al. (2014: 174) give "Planktonic, bethic in bogs, also in reservoirs; mesotrophic." This would suggest slightly acidic habitats. In the Outer Hebrides the two main collections have come from ephemeral winter floods and from a friend’s hand-built aquatic tank – both slightly alkaline. The zygospore, not previously known, is globose with robust digitate projections, truncate and with mamillate bases.
Cell dimensions: L: 34-42 µm, B: 30-37 µm, Is: 9-13 µm; Th: 18-22 µm, L/B: 1.1-1.2.
Zygospore: minus projections: 37 µm; overall: 47 µm.
A seamingly rare species.

References: 
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Tsarenko, P.M., Wasser, S.P. & Nevo, E. (editors)(2014). Algae of Ukraine, Volume 4: Charophyta.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.