Arcella intermedia  (Deflandre, 1928) Tsyganov & Mazei, 2006
Basionym: Arcella hemisphaerica intermedia undulata Deflandre, 1928)

Diagnosis: Shell colorless, light yellow or brown, circular in apertural view, domed or hemispherical with an implicit basal border in lateral view. Apertural surface and basal border are smooth, but the dorsal surface has a number of regular depressions. Aperture circular with or without a distinct collar.

Dimensions: According to Tsyganov and Mazei (2006): Shell diameter 65—80 μm; shell height 35-50 μm; aperture diameter 16—23 μm; aperture diameter/shell diameter 0.35—0.55; shell height/diameter ratio 0.5—0.7. My measurements: shell diameter 52—71 μm.

Habitat: Freshwater, between algae and beteen the debris of shallow waters.

Remarks: Tsyganov and Mazei (2007) distinguish a subspecies, Arcella intermedia laevis (Deflandre, 1928)  (Syn.: Arcella hemisphaerica intermedia Deflandre, 1928). Porfírio-Sousa et al. ( 2017), evaluated the morphometrical and morphological variation of the closely  related and morphologically similar taxa Arcella intermedia laevis Tsyganov and Mazei, 2006 and  Arcella intermedia (Deflandre, 1928) Tsyganov and Mazei, 2006 in nature and in cultured individuals and their correlation with molecular data. Their results demonstrated that phenotypic plasticity in Arcella intermedia makes morphological distinctions impossible in both taxa. Arcella intermedia and Arcella intermedia laevis are molecularly identical for SSU rDNA and a mitochondrial molecular marker (NAD9/7). The authors concluded that morphological techniques alone cannot identify phenotypic plasticity from natural populations. More work is clearly needed to better understand the morphological, morphometric and molecular variability in these organisms.

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.

Porfírio-Sousa, A.L., Ribeiro, G.M., Lahr, D.J.G., 2017. Morphometric and genetic analysis of Arcella intermedia and Arcella intermedia laevis (Amoebozoa, Arcellinida) illuminate phenotypic plasticity in microbial eukaryotes. European Journal of Protistology 58, 187–194. https://doi.org/10.1016/j.ejop.2016.11.003