Phylum: Charophyta   Family: Desmidiaceae

Xanthidium scrobiculatum (A.M. Scott & R. Grönblad) J. Stastny, P. Skaloud & J. Neustupa 2013

A medium-sized cell with subpyramidate to hexagonal semicells. Each semicell has paired spines at the median and apical angles and a single spine at the basal angle. The centre of the semicells is coloured orange and has a circle of scrobiculae surrounding a small protuberance.
A quote from Stastny et al. (2013: 12): “The molecular data revealed three well-resolved clades within traditional X. cristatum that corresponded to the traditional varieties X. cristatum var. cristatum, X. cristatum var. scrobiculatum, and X. cristatum var. uncinatum. This classification pattern was also confirmed by the geometric morphometric analysis that illustrated highly significant semicell shape differences among these three taxa.”
Single cell dimensions: L: 61 µm; B: 43 µm; Is: 16 µm; Sp: 10-12 µm; overall LxB: 79 x 64.
A very rare desmid found predominantly in North America including the southern part of Canada. There is just one record for the Outer Hebrides, which was found in the benthic layer of a very shallow and turbid loch.

References: 
Stastny, J., Skaloud, P., Langenbach, D., Nemjova, K. & Neustupa, J. (2013). Polyphasic evaluation of Xanthidium antilopaeum and Xanthidium cristatum (Zygnematophyceae, Streptophyta) species complex.