In July 2026, I found several specimens of Mesostigma viride in plankton samples from the pond of the waste disposal company Constance. The pond is heavily eutrophicated.

Mesostigma viride has an unusual body structure. The cells are not flattened laterally, but from the cell poles. That is, the anterior and posterior ends are brought together. As a result, the flagella and the eyespot are now more or less in the middle of the flattened disc. In addition, this disc is curved in the shape of a saddle, with two upturned edges. Thus, the saddle shape has a concave and a convex side. The two flagella of Mesostigma viride arise from a depression on the convex side, while the large brick-red eyespot lies approximately in the middle of the concave side (s. fig. 4 a-c). The chloroplast is yellow-green in color. Two pyrenoids are located at opposite ends of the cell (s. figs. 1 b and 5 a).

The periplast of Mesostigma viride is covered by a clearly visible layer of scales. The form of these scales was examined by electron microscopy by Manton & Ettl (1965). They are essentially cup-shaped and sit on a base of plate-like scales. Under the light microscope, the scales appear to consist of short spines (s. figs. 5 b and 6). However, these are the adjoining walls of the “cups.” In apical view, the scales appear approximately circular and lie close together.

In my population, I was able to find cells 18–30 µm in length, which is considerably larger than the range of 15–18 µm given by Huber-Pestalozzi (1961). However, culture experiments by Manton & Ettl (1965) have shown that cell dimensions can be variable. The cells kept in culture were only 7.7–12.5 µm long. The size of the cells in my population may also be attributable to the highly eutrophic habitat.