Investigations of two-celled colonies of Synura formerly described as Chrysodidymus with descriptions of two new species

The two-celled colonial synurophyte genus Chrysodidymus was originally distinguished from its close relative Synura on the basis of the colonies always being of two cells, the shape of the cells and the odd swimming nature of the colony. Recently, based on gene sequence data, Chrysodidymus was found to be deeply nested within the Synura clade. As a result Chrysodidymus was placed in synonymy with Synura and a new combination, S. synuroidea, was made for the basionym C. synuroideus. Based on our observations and findings of two-celled colonies originally described under the genus Chrysodidymus, we propose descriptions of two new species Synura papillosa and Synura prowsei and an emended description for Synura synuroidea. Scales of S. synuroidea are significantly smaller than those of the two proposed species and possess an exceptionally wide posterior rim that is perforated with large holes of uneven diameters. Scales of the new species are significantly larger with narrow posterior rims that lack holes. Scales of S. prowsei possess anterior ribs that connect the ends of the posterior rim to the base of the spine and lack surface papillae, whereas scales of S. papillosa have surface papillae, but lack the anterior ribs. In addition, the base plate pore patterns are distinctive for each species. We further designate a neotype for S. synuroidea since the original holotype has been lost. All three species favour dilute, shallow, highly acidic water bodies often with highly coloured dissolved organic matter (CDOM).