Phylum: Charophyta   Family: Desmidiaceae

Staurastrum pelagicum W. & G.S. West 1902

Similar to S. pseudopelagicum but the semicells of that species are more triangular whereas those of S. pelagicum are bowl-shaped. Some specimens can possess an extra spine at the end of the processes.
Overall cell dimensions: L: 41-47 µm; B: 47-53 µm; Is: 9.0-12 µm; L/B: 0.8-1.0.
A rare planktonic species with a seemingly Atlantic distribution. Found in circumneutral waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.