Genus Arcella Ehrenberg, 1832
Diagnosis: Species with a more or less circular shell, aperture central, invaginated, in many species surrounded by a tubus or a circle of pores. Test completely organic, composed of box-like building units arranged in a single layer and cemented together, resulting in an areolar surface. Size of the building units varies between species. Young shells are colorless, older become brown due to iron and manganese storage into the building units. Most species are binucleate, but several species have more; A. megastoma may have up to 200. These nuclei are always vesicular. Several contractile vacuoles. Cell does not fill the test, fixed with small epipodes at the shell wall. Cyst round, within test.
Arcella is a genus of testate amoebae and one of the largest and most common testacean genera. Over 50 species, subspecies and varieties have been described, but many descriptions are not adequate or based on artificial criteria. Therefore it is not always easy to identify species. An important characteristic is the shape of the test in lateral view, which is in practice not so easy to observe.
Arcella species have more or less umbrella-shaped shells with a central invaginated aperture from where lobopodia, finger-like pseudopods, emerge. In dorsal view the shell ranges from circular or broad elliptical up to an irregular square shape. In lateral view the test varies from flattened discoid till spherical. Most species have hemispherical shells. The aperture is normally circular and in some species elliptical, in many species surrounded by a tube and/or a ring of pores.
The test is completely composed of organic material, and is transparent in young Arcella, but browns while aging due to the progressive deposition of iron and manganese compounds. In the building process organic granules are arranged in a single layer en cemented together, resulting in a hexagonal honeycomb like structure. Contrary to other genera, no sand grains or other foreign bodies are inserted to build the test.
The nucleus is always vesicular. Most species are binucleate, though some species can have different number of nuclei, up to 200 nuclei as in A. megastoma. There are also many contractile vacuoles present. Arcella species can develop vacuoles of carbon dioxide in their cytoplasm to float up to the surface of the water.
Arcella species inhabit freshwater pools, eutrophic waters, marshes, mosses, as well as wet foliage. Few species can also be found in soils. They nourish on diatoms, unicellular green algae or animal protozoa such as flagellates and ciliates.
I made a key for Arcella species which I have found in the Netherlands since 1971. It is a key to the most common species and it can be used worldwide. But a key is a tool, just a guide to orientate yourself, a compass to show the direction. It is no guarantee for an definite identification. In fact it is often very difficult to identify a species based on one shell. You have to know the variation within a population at a specific location. And, to be honest, we don't know how many species there are and what separates them.
It is very hard to identify a species on the basis of morphology alone, because most original descriptions are inadequate. A number of species is not distinguishable. For the moment these non-distinct taxa are grouped in complexes. Two large complexes are:
- A. hemisphaerica - A. rotundata complex
- A. discoides - A. megastoma - A. polypora complex
Each 'species' contain also several varieties and forms.
Deflandre (1928) distinguishes four groups, based on the diameter-height ratio:
Group Vulgaroides : | hemisphaerica | gibbosa | vulgaris | conica | brasiliensis |
Group Arenoides : | arenaria | catinus | dentata |
Group Discoides : | discoides | megastoma |
Group Altoides : | mitrata | apicata |