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Galeripora discoides
G. discoides

Galeripora discoides  (Ehrenberg, 1871) González-Miguéns et al., 2021
Basionym: Arcella discoides Ehrenberg, 1871
! A. discoidea Ehrenberg, 1843 (only a name without description)

Diagnosis: Test circular to elliptic in apertural view, flattened to a disc shape in lateral view, sometimes arched, arched tests look elliptical in apertural view. Aperture central, circular in most cases, elliptical in those who present an arched test, long axis of aperture is perpendicular to long axis of test), delimited by a small curled lip. Whole test made of chitinous material built from box-like, hollow, roughly hexagonal units. The conspicuous pattern created by the building units is not visible at the abapertural region due to an additional covering layer, which may cover the whole abapertural region evenly. At the apertural region, there is no covering layer, therefore, the pattern is visible, and the building units appear collapsed. A ring of pores, usually 3 μm in diameter each, located outside the aperture is often, but not always present. Test light yellow to deep brown. Variable number of nuclei (two or more).

Dimensions: Lahr and Lopes, 2009: “All measures are highly variable. For test diameter, 72.78% of measures are within 100–150 μm. For test height, 69.86% of measures are within 20–30 μm and for aperture diameter, 65.18% of measures are within 50–100 μm.”
Literature: shell diameter 70-157 µm; aperture diameter 21-52 µm.

Ecology and geographic distribution: Freshwater; common in pond water. The Americas, Europe, Africa, Asia and Australia.

Remarks:
Belonging to the G. discoides + G. polypora + G. megastoma complex:
G. discoides var. difficilis Deflandre, 1929
G. discoides var. scutelliformis Playfair, 1918
G. discoides var. foveosa Playfair, 1918
G. discoides var. pseudovulgaris Deflandre, 1928
G. discoides var. pseudovulgaris fma. undulata Deflandre, 1928
G. discoides var. pseudovulgaris fma. arcuata Deflandre, 1928: 261, Figs 346–348
G. polypora var. curvata Deflandre, 1928
G. megastoma fma. arcuata Deflandre, 1928

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

González-Miguéns, R., Soler-Zamora, C., Villar-Depablo, M., Todorov, M., Lara, E., 2021. Multiple convergences in the evolutionary history of the testate amoeba family Arcellidae (Amoebozoa: Arcellinida: Sphaerothecina): when the ecology rules the morphology. Zoological Journal of the Linnean Society zlab074. https://doi.org/10.1093/zoolinnean/zlab074

I have measured the diameter of the aperture and the shell of a number of discoid Arcella and Galeripora specimens from several locations in the Netherlands. The results are plotted in the graph below and shows the existence of at least two different groups, probably two species.

Arcella megastoma
Arcella discoides
Arcella discoides
G. discoides, lateral view
Arcella discoides
G. discoides, lateral view
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