Arcella discoides Ehrenberg, 1843
Diagnosis: test generally circular in apical view, plano-convex in lateral view, sometimes arched; diameter about three to four times the height; without a distinct rim or border. Two nuclei or more. Aperture circular, invaginated, bordered by a shallow lip, usually surrounded by a ring of numerous small pores. Ratio diameter aperture/shell usually 0.30-0.45. Height of the shell about 1/3 that of the diameter. Shell yellow or brown.
Dimensions: literature: shell diameter 70-157 µm; aperture diameter 21-52 µm.
Ecology: Common in pond water
Remarks: this species forms together with A. megastoma and A. polypora a species complex. They are hard to separate from each other. I think A. polypora isn't a true species. I have based my division between A. discoides and A. megastoma on morphometric data. A. megastoma has a larger apertural diameter, though there is some overlap with A. discoides. I sampled my data from numerous individuals on several locations in the Netherlands. The number of nuclei could be a taxonomic character, but as most observed shells are empty, it is hard to get sufficient data. It is probably a variable character, because the number of nuclei apparently increases with the size of the test.
I have measured the diameter of the aperture and the shell of a number of discoid Arcella specimens from several locations in the Netherlands. The results are plotted in the graph below and shows the existence of at least two different groups, probably two species, Arcella discoides and Arcella megastoma.