Arcella
Encyclopedia
Arcella is a genus
of testate amoebae
or Arcellinida, usually found in freshwater
s and moss
es, and rarely in soils. A key characteristic of Arcella is the circular test
with a hole on its center from where finger-like pseudopods emerge. It is one of the largest testacean genera.
ous, umbrella-shaped test
(or shell) that has a single central aperture through which the pseudopod
s – which are used for locomotion – extend out. In some species the aperture is surrounded by a ring of pores. The test is composed of organic material with a diameter of up to 300 µm and is transparent or light-yellow-colored in young Arcella, but browns while aging due to the progressive deposition of iron
and manganese
compounds. Contrary to other genera, no gravel chips or other foreign bodies are inserted to strengthen the housing.
Different species of Arcella can have different number of nuclei, ranging from a single nucleus, as in some A. hemisphaerica, up to 200 nuclei, as in A. megastoma, though the majority is binucleate. They also have many contractile vacuole
s, and can develop vacuoles of carbon dioxide in their cytoplasm to float up to the surface of the water.
pools, eutrophic waters, marsh
es, moss
es, as well as wet foliage. Few species can also be found in soils. They nourish on diatom
s, unicellular green algae
or animal protozoa such as flagellate
s and ciliate
s.
Most species are worldwide-distributed, but some have restricted distributions, e.g. A. brasiliensis and A. rota which are endemic to South America.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of testate amoebae
Testate amoebae
Testate amoebae are single-celled protists partially enclosed in a simple test .They are commonly found in soils, leaf litter, peat bogs and near/in fresh water....
or Arcellinida, usually found in freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
s and moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es, and rarely in soils. A key characteristic of Arcella is the circular test
Test (biology)
A test is a term used to refer to the shell of sea urchins, and also the shell of certain microorganisms, such as testate foraminifera and testate amoebae....
with a hole on its center from where finger-like pseudopods emerge. It is one of the largest testacean genera.
Anatomy
An Arcella is typically enclosed in a chitinChitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous, umbrella-shaped test
Test (biology)
A test is a term used to refer to the shell of sea urchins, and also the shell of certain microorganisms, such as testate foraminifera and testate amoebae....
(or shell) that has a single central aperture through which the pseudopod
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
s – which are used for locomotion – extend out. In some species the aperture is surrounded by a ring of pores. The test is composed of organic material with a diameter of up to 300 µm and is transparent or light-yellow-colored in young Arcella, but browns while aging due to the progressive deposition of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
compounds. Contrary to other genera, no gravel chips or other foreign bodies are inserted to strengthen the housing.
Different species of Arcella can have different number of nuclei, ranging from a single nucleus, as in some A. hemisphaerica, up to 200 nuclei, as in A. megastoma, though the majority is binucleate. They also have many contractile vacuole
Contractile vacuole
A contractile vacuole is a sub-cellular structure involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole....
s, and can develop vacuoles of carbon dioxide in their cytoplasm to float up to the surface of the water.
Nutrition and distribution
Arcella inhabit freshwaterFreshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
pools, eutrophic waters, marsh
Marsh
In geography, a marsh, or morass, is a type of wetland that is subject to frequent or continuous flood. Typically the water is shallow and features grasses, rushes, reeds, typhas, sedges, other herbaceous plants, and moss....
es, moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es, as well as wet foliage. Few species can also be found in soils. They nourish on diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s, unicellular green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
or animal protozoa such as flagellate
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
s and ciliate
Ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...
s.
Most species are worldwide-distributed, but some have restricted distributions, e.g. A. brasiliensis and A. rota which are endemic to South America.
Species
Over 130 species and subspecies of the genus Arcella have been described till now. Species include:- Arcella arenaria
- Arcella artocrea
- Arcella catinus
- Arcella conica
- Arcella dentata
- Arcella discoides
- Arcella excavata http://www.nencki.gov.pl/pdf/ap/ap683.pdf
- Arcella gibbosa
- Arcella hemisphaerica
- Arcella megastoma
- Arcella vulgaris