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. Aperture invaginated, circular with a denticular (crenulated) margin.

Dimensions: In literature: diameter of  the shell 113—160 µm; height 52—58 µm; diameter of the aperture 24—53 um. My measurements: Shell diameter 112—224 µm (mean 157 µm); diameter of the aperture 31—104 µm (mean 53 µm), ratio shell diameter/aperture 0.3—0.4.

Habitat: 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. 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 was absent.