Arcella crenulata Deflandre, 1928
Diagnosis: shell circular with a domed aboral region, in lateral view there usually appears to be a basal collar at the border of the shell; shell surface smooth or irregular and can have numerous small pores. The aperture is invaginated, circular had has a denticular (crenulated) margin.
Dimensions: in literature diameter of shell 113-160 µm; height 52-58 µm; diameter of aperture 24-53 um; my measurements: shell diameter 112-224 µm (mean 157 µm); diameter of aperture 31-104 µm (mean 53 µm), ratio diameter of shell - aperture 0.3-0.4 µm.
Ecology: common in fresh water, often abundant in water with Sphagnum-contact.
Remarks: The species was described by Deflandre (1928) as a variety of Arcella vulgaris. Several observers consider the difference in the shape of the aperture sufficient to treat A. crenulata as a valid species. I am not quite convinced, though there is is a difference with specimens which I consider as A. vulgaris, having a smaller aperture. Deflandre describes 'numerous small pores', but I have never observed any pores. I have also seen multinucleate specimens of A. crenulata. Just like A. vulgaris, it always has a basal collar or rim, sometimes very hard to detect in lateral view. Only in samples from Austria the basal rim is absent.