Phylum: Charophyta   Family: Desmidiaceae

Cosmarium margaritiferum J. Ralfs 1848

A variable species in terms of its granulation with populations in the Outer Hebrides having quite a large central papilla surrounded by a gradation of granules, the disposition of which is extremely variable. In apical and side views the protuberance clearly shows the papilla. Similar in size and shape to C. kirchneri Børgesen 1889 and C. luxuriosum (Kouwets) Coesel 1989 but those species have defining characteristics.
Cell dimensions: L: 49-61 µm; B: 42-56 µm; Is: 14-17.6 µm; Th: 28-35.6 µm; L/B: 1.12-1.21.
Zygospore dimensions: 52-68 µm.
A common desmid from vegetated loch-margins and moorland pools, and occasionally with Sphagnum. It prefers less acidic and more mesotrophic conditions.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
West, W. & West, G.S (1908). A Monograph of the British Desmidiaceae, Volume 3.