Arcella rotundata – Microworld

A. rotundata, after Playfair, 1918, and a 42 µm large shell

Arcella rotundata Playfair, 1918

Diagnosis: shell with a depressed dome in side-view, expanded at the sides above the baseline; no basal angles, the margin of the dome merging into the base in a single, broadly rounded curve.

Dimensions: According to Playfair (1918): Shell diameter 47—54 µm, height 25—30 µm, pseudostome 17 µm. My measurements: Diameter 47—62 µm, height 25—30 µm.

Remarks: 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
The main feature that could separate individuals of A. rotundata from those of A. hemisphaerica is the larger semi-circle contour in lateral view, but this could also fall within the natural variation. Further observations and measurements are needed.

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.

Playfair (1918) distinguishes two varieties: var. alta and var. scrobiculata.