Arcella Ehrenberg 1832

Introduction

Arcella is one of the larger genera of Arcellinida. About 50 species and many more varieties and forms are known. The type species is A. vulgaris Ehrenberg, 1832.

Characteristics

Species with a more or less circular shell, aperture central, invaginated, in many species surrounded by a tubus or a circle of pores (Fig. 1).

Fig. 1: Arcella catinus. The aperture is surrounded by a short tube and a ring of pores. © Ralf Meisterfeld

Fig. 2: Organic building units (Arcella hemisphaerica). © Ralf Meisterfeld

Test completely organic, composed of box-like building units arranged in a single layer and cemented together resulting in an areolar surface (Fig. 2). Size of the building units varies between species. Young shells are colourless, older become brown, due to iron and manganese storage into the building units. Most species are binucleate, but A. hemisphaerica can have one (see title illustration) and several species have more; A. vulgaris multinucleata (see title illustration) has 7-10 and A. megastoma may have up to 200. These nuclei are always vesicular (Fig. 3).

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Fig. 3: Vesicular nuclei (Arcella vulgaris multinucleata). © Ralf Meisterfeld

Arcella species have several contractile vacuoles. The cell does not fill the test, sometimes it is fixed with small epipodes at the shell wall. The diameter of the tests varies between 25 and 300 µm. The cysts are round and are built within the test. In his monograph Deflandre (1928a) has subdivided this genus into four artificial sections and four derived groups on the basis of the height/diameter ratio, the presence of angular facets or a flat circular rim.

Ecology

Species of Arcella can be found in all freshwater biotopes, wet and dry mosses and, with a few species, in soils. Some species are characteristic for specific habitats – e.g. Arcella arenaria is common in dry mosses and Arcella artocrea pseudocatinus in wet Sphagnum.

Feeding

Arcella sp. are mainly herbivores (algae, fungi or bacteria).

Geographical distribution

Most species have a worldwide distribution, but Arcella brasiliensis and Arcella rota are only known from South America, and Arcella formosa from Canada, Germany and Hungary!

References

Deflandre, G. 1928a. Le genre Arcella Ehrenberg. Morphologie - Biologie. Essai phylogénetique et systématique. Arch. Protistenk., 64:152-189.

Decloitre, L. 1976a. Le genre Arcella EHRENBERG. Compléments à jour au 31. décembre 1974 de la monographie du genre parue en 1928. Arch. Protistenk., 118:291-309.

Decloitre, L. 1979b. Mises à jour au 31.12.1978 des mises à jour au 31.12.1974 concernant les genres Arcella, Centropyxis; Cyclopyxis; Euglypha et Nebela. Arch. Protistenk., 122:387-397.

Decloitre, L. 1982. Compléments aux publications précédentes. Mise à jour au 31.XII. 1981 des genres Arcella, Centropyxis, Cyclopyxis, Euglypha, Nebela et Trinema. Arch. Protistenk., 126:393-407.

Decloitre, L. 1986. Compléments aux publications précédentes. Mise à jour au 31. XII. 1984 des genres Arcella, Centropyxis, Cyclopyxis, Euglypha et Nebela. Arch. Protistenk., 132:131-136.

Netzel, H. & Grunewald, B. 1977. Morphogenesis in the shelled rhizopod Arcella dentata. Protistologica, 13:299-319.

Ogden, C. G. & Hedley, R. H. 1980. An Atlas of Freshwater Testate Amoebae. Oxford University Press, Oxford.