Arcella hemisphaerica

Arcella hemisphaerica, shell 60 µm in diameter

Arcella hemisphaerica Perty, 1852

Diagnosis: Shell circular in apertural view, hemispherical or somewhat domed in lateral view. Aboral hemisphere turns into ventral surface at an acute angle. The aperture is circular, with a small collar.

Dimensions: According to Tsyganov et al. (2006): Shell diameter 38—68 µm; height 23—37 µm; aperture diameter 10—20 µm; length of pre-apertural cavity 6—9 μm; shell height/diameter ratio 0.48—0.75. My measurements: shell diameter 36—56 µm.

Remarks: Tsyganov et al. (2006) distinguish two subspecies:
Arcella hemisphaerica undulata Deflandre, 1928 differs from the type by the presence of depressions on the aboral hemisphere.
Arcella hemisphaerica tuberculata Stepanek, 1963 differs from the type by the tuberculate aboral hemisphere.

Lahr and Lopes (2009) showed that Arcella hemisphaerica and A. rotundata are indistinguishable. They consider both species as a single entity with 9 non-distinguishable nominal taxa within. These are:
Arcella hemisphaerica Perty, 1852
Arcella hemisphaerica Perty, 1852
Arcella hemisphaerica forma undulata Deflandre, 1928
Arcella hemisphaerica var. depressa Playfair, 1918
Arcella hemisphaerica var. tuberculata Stepanek, 1963
Arcella rotundata Playfair, 1918
Arcella rotundata var. stenostoma Deflandre, 1928
Arcella rotundata var. stenostoma fma undulata Deflandre,1928
Arcella rotundata var. aplanata Deflandre, 1928
Arcella rotundata var. alta Playfair, 1918
Arcella rotundata var. alta fma undulata Stepanek,1963

References:
Tsyganov, A., Mazei, Y., 2006. Morphology and biometry of Arcella intermedia (Deflandre, 1928) comb. nov. from Russia and a review of hemispheric species of the genus Arcella (Shellcealobosea, Arcellinida). Protistology 4, 361–369.

Lahr, D.J.G., Lopes, S.G.B.C., 2009. Evaluating the Taxonomic Identity in Four Species of the Lobose Testate Amoebae Genus Arcella Ehrenberg, 1832. Acta Protozool. 48, 127–142.

The presence or absence of depressions is a natural variation of tests, as I could observe in several natural populations. Therefore it is not a qualitative taxonomic character. The shells below were all photographed in one drop of a sample. Their size is about 49 µm in diameter. There is no doubt that all these shells are related. It shows the variation in shape within a natural population. Three shells, to the right, show a distinct basal rim. One shell, the second to the left, has depressions.