ARCELLA: CHARACTERISTICS, TAXONOMY, MORPHOLOGY, NUTRITION - BIOLOGY - 2025

Arcella is a genus of Amoebozoa from the Protista Kingdom, made up of eukaryotic unicellular organisms that have a characteristic and distinctive element, a kind of cover or shell that covers the entire cell and provides protection.

It was discovered and described by the German naturalist Christian Ehrenberg in 1832. They are organisms that still need to know and study their properties and characteristics.

Arcella specimen. Source: By ja: User: NEON / commons: User: NEON_ja, from Wikimedia Commons

The genus Arcella includes approximately 50 species, which are ubiquitous, that is, they are distributed throughout the world geography. They belong to the Amoebozoa phylum, so they bear certain similarities with other genera such as Difflugia. Likewise, they are free-living organisms, not pathogenic for humans or animals.

Taxonomy

Arcella's taxonomic classification is as follows:

Domain: Eukarya

Kingdom: Protista

Phylum: Amoebozoa

Class: Tubulínea

Order: Arcellinida

Suborder: Arcellina

Family: Arcellidae

Genus: Arcella

Morphology

The organisms that belong to the genus Arcella are characterized by presenting a kind of circular shell or cover. However, it is not complete, that is, it does not cover it completely, but has a centrally located opening which is extremely important, since it serves as an exit hole for the pseudopods that the cell uses to move.

Likewise, it has been observed in a large number of Arcella species that this opening is surrounded by pores. The texture of the shell or "shell" is made of organic material and is chitinous in appearance.

In young organisms, the shell is light yellowish and even transparent. As it matures and ages, and thanks to the progressive depositions of iron and manganese compounds, it solidifies its color, becoming completely golden in adulthood.

With regard to the cell interior, it can be seen that, like all eukaryotic organisms, there is the presence of a cell nucleus. Most of the species that make up the genus Arcella are binucleate, that is, they have two nuclei. However, there are others that have more, such as Arcella megastoma, which can have up to 200.

Similarly, the existence of contractile-type vacuoles can be seen in the cell cytoplasm. Likewise, it has been established that some species develop vacuoles that contain carbon dioxide (CO2), in order to float and stay on the surface of the water, which is their habitat.

However, it is important to clarify that the Arcella cell itself does not occupy the entire interior of the shell, but rather adheres internally to it by means of small pseudopods.

General characteristics

The Arcella genus is made up of unicellular organisms that are located within the group of eukaryotes, this means that their cells have a cell membrane, cytoplasm and cell nucleus. In the nucleus the genetic material is stored in the form of DNA and RNA.

The members of this genus are free-living, that is, they are not fixed to any substrate, but float freely in bodies of water, without establishing dependency relationships with any other organism. They generally do not form colonies.

To move through the environment in which they inhabit, the cell emits a series of extensions that are known as pseudopods. These allow you to move calmly and slowly through the water, waiting for the capture of some food that is within reach.

Habitat

These types of organisms can be found mainly in freshwater bodies, as well as in moist mosses and in the soil.

Similarly, there are some species that are specific to certain places, for example Arcella arenaria is found in dry mosses only.

Nutrition

Organisms of this genus are heterotrophs. This means that they are not able to synthesize their own nutrients, as some organisms do through photosynthesis. Due to this inability, they must feed on other living beings, or on substances made by others.

The diet of this organism is herbivorous, based mainly on the ingestion and processing of algae, fungi and some bacteria.

The process by which they feed is known as phagocytosis. Through this process the cells encompass the food particle and incorporate it into them to initiate digestion.

In the particular case of the genus Arcella, using pseudopods, the organism surrounds the food particle, enclosing it in a kind of capsule that floats within the cytoplasm.

Here it comes into contact with the lysosomes that are present and that contain the various digestive enzymes that are going to be responsible for degrading and digesting the nutrients.

The ingested food is then subjected to the action of digestive enzymes and is fragmented, degraded and converted into much simpler molecules that can be used by the cell for various essential processes.

As in all digestive processes, once this has occurred, various waste substances remain that will not be used by the cell, since they are not useful. In this sense, these substances are released to the external environment.

Breathing

Taking into account that the organisms of the genus Arcella are primitive, it is to be expected that they do not have a specialized structure or organ for the respiration process. They do not have lungs, like mammals, tracheas, like some insects, or gills like fish.

Because of this, they do a fairly simple type of breathing known as direct breathing. In this, the gases of respiration freely cross the cell membrane of the organism through a simple process of passive transport known as simple diffusion.

Oxygen enters the cell through the cell membrane down the concentration gradient. This means that you go from a place where you are very focused to another where you are not.

Inside the cell, oxygen is used by the various cellular mechanisms in processes that are of vital importance to it. As a product, carbonic anhydride (CO ₂) is generally formed, which is sometimes toxic to cells, so it must be expelled out of them.

The way to expel it is similar to the route that oxygen takes to enter. Carbon dioxide diffuses outside the cell through the simple diffusion process, being released into the environment to be used by other types of living beings that require it for their metabolic processes.

Reproduction

There are really very few studies about the process of genesis of this type of organisms. However there are some specific facts that have been established.

First, organisms of the genus Arcella reproduce asexually. This implies that there is no type of exchange of genetic material with other cells.

Likewise, knowing that there are a large number of asexual reproduction methods, it is important to emphasize that the members of Arcella reproduce, like the vast majority of the members of the Protista Kingdom, through binary fission.

In this process, a cell divides into two cells exactly the same as it, with the same genetic information.

In protists of other genera, whose body does not have a protective shell, this process is quite simple. Not so in Arcella, since in them the reproduction encompasses on the one hand the generation of the shell and on the other the duplication of the cell itself.

The first step then for the reproduction of these organisms is the generation of the shell. Depending on the type of cover, the mineral components and the cement are organized around a cytoplasmic extension.

When it is complete, the DNA of the cell nucleus is duplicated and the cell divides into two exactly the same.

The specific mechanism by which all this occurs is not very well elucidated, although they have been studied since the 1970s.

Classification

The genus Arcella comprises a total of 22 species, the oldest being discovered in 1832 and the most noble in 2016. This genus has been divided into two large complexes:

1. Arcella hemisphaerica complex - Arcella rotundata
2. Arcella dscoides complex - Arcella megastoma - Arcella polypora

Likewise, according to the diameter - height relationship, four groups have been established:

1. Vulgaroides group: hemisphaerica - A. gibbosa - A. vulgaris - A. conica - A. brasiliensis.
2. Arenoid group: arenaria - A. catinus - A. dentata
3. Discoid group: discoid - A. megastoma
4. Altoides group: mitrata - A. apicata

Some relevant aspects of some of the species that are integrated into this genus are:

Arcella brasiliensis

This type of Arcella has a circular shell with a distinct marginal edge. Its surface has a wavy appearance because it has a large number of undulations. It also has a circular opening, which is delimited by a curled lip.

Arcella excavata

Adult individuals have an intense brown coat. The opening of the shell is bordered by a small lip.

It has a surface with a deeply invaginate opening that presents two protrusions in its lower part. Likewise, the wall of its shell is made up of numerous alveoli that are forming a uniform layer.

Arcella dentata

It presents its characteristic shell, which has an intense brown color. Fifteen to seventeen thorns emerge from it. It also has a serrated edge (hence its name). Its ventral part that is in contact with the substrate is shaped like an inverted funnel, with its circular opening in the center.

Arcella rotundata

They are organisms that have a dome expanded to the sides at the base line. The margin of the dome is fused with the base.

When viewed from the side, its semicircular outline can be seen in all its glory. This allows them to be differentiated from other quite similar species.

Arcella vulgaris

It has certain distinctive features, such as a dome that is uniformly convex and has a distinct basal edge. The surface of its shell can be smooth or with regular waves. Its opening is circular and is bordered by a small lip.

Arcella conica

It is made up of a hemispherical shell. Its dorsal surface presents angular facets that have six or more prominent folds on the edge. The opening has a small invagination, is circular and is bordered by a small collar.

Arcella megastoma

One of its essential characteristics is that it has a large number of cores. It can reach up to 200. Its shell is flattened and has a fairly wide opening.

Arcella discoides

It has two or more nuclei. The shell from the apical angle appears circular, however, in the lateral view it can be seen arched.

The opening is circular, bordered by a shallow lip that is surrounded by a ring of small pores. The shell has an intense brown color.

Arcella gibbosa

It has a circular-looking shell in dorsal view, which in lateral view has a domed appearance. It has a central opening, circular in shape, invaginate with a distinct lip. In the aboral region it presents regular depressions that are easily distinguishable.

Arcella arenaria

It has a circular shell, which in lateral view can be seen in the shape of a dome. It has some folds on its dorsal surface and a small, circular opening. Around this a large number of pores are appreciated. They also have several nuclei, their pseudopods are small and have several vacuoles.

References

1. Bless, E. Arcella, A study in cell Physiology. Retrieved from: jcs.biologists.org
2. Cairns, J.; Ruthven, JA (1972). A test of the cosmopolitan distribution of fresh-water protozoans. Hydrobiology, 39: 405-427
3. Meisterfeld, R. and Mitchell, E. Retrieved from: tolweb.org/Arcella
4. Ogden, CG & Hedley, RH (1980). An Atlas of Freshwater Testate Amoebae. Oxford University Press, Oxford.
5. Yaeger, RG (1989). Protozoa: structure, classification, growth, and development. In: Tropical Medicine and Parasitology. Heyneman, R. and Goldsmith, R. (Eds.). Appleton and Lange. California. USES