Arcella crenulata

 

 

Arcella crenulata Ø 122 µm

 

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 shell diameter/aperture 0.3-0.4.

 

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 was absent.

 

Shell 100 µm, Tirol
Arcella crenulata
Shells in lateral view
Arcella crenulata
Arcella crenulata
Shells in lateral view
Arcella crenulata
Aperture
Arcella crenulata
Arcella crenulata, 147 µm in diameter, with numerous epipodia - Tirol, Austria.
Arcella crenulata
Arcella crenulata in lateral view, on top of an Amoeba proteus.
Arcella crenulata