The Choanoflagellates are a group of organisms belonging to the kingdom Protista and have among its features the presence of a scourge that helps them move. This group of organisms is considered to be the closest, from an evolutionary point of view, to true animals.
It presents two orders: Craspedida and Acanthoecida. Between them the more than 150 that belong to this class are distributed. It is interesting to observe and study the similarities between the structure of choanoflagellates and choanocytes (sponge cells).
Colony representation of Choanoflagellates. Source: By Iliá Méchnikov, via Wikimedia Commons
This group of organisms are of great interest to those who study the evolution of species, since from them it is possible to reconstruct the last unicellular ancestor of current animals. Without a doubt, choanoflagellates have been of great help in the various studies carried out on this subject.
Taxonomy
The taxonomic classification of choanoflagellates is as follows:
Domain: Eukarya
Kingdom: Protista
Phylum: Choanozoa
Class: Choanoflagellatea
Morphology
Organisms belonging to this taxonomic class display eukaryotic cells. This means that the genetic material (DNA and RNA) is enclosed in a structure very well delimited by a membrane, known as the cell nucleus.
Likewise, they are unicellular organisms, which means that they are made up of a single cell. This single cell has a characteristic morphology, presenting a shape similar to an oval, being sometimes spherical.
As their name implies, they present flagella, specifically a single flagellum. In the same way, they have a peduncle that serves to fixate to the substrate. From the bottom of this peduncle the scourge emerges.
Around the birth of the flagellum there is a kind of collar that surrounds it, made up of finger-shaped structures known as microvilli. These are full of a protein known as actin.
Inside the cell are certain organelles such as food vacuoles and basal bodies. Similarly, sometimes the body of these organisms is covered by a layer known as the periplast.
This is made up of proteins and, depending on the type of organism, it can present a diverse composition, in addition to presenting distinctive characteristics such as scales, among others.
The approximate diameter of the cells of the organisms that make up this class is 3-9 microns.
General characteristics
Choanoflagellates are a group of organisms of which many aspects are still unknown. Regarding their lifestyle, the vast majority of genres that make up this class are free-living.
However, some organisms that are closely related to this class have turned out to be parasites, so the description of parasitic choanoflagellate species as well is not ruled out in the future.
Likewise, many of the species are solitary, however, genera whose species form simple colonies have been described. Sometimes these colonies resemble bunches of grapes, in which each cell represents a grape and are attached to the same stem.
These organisms can live sessile lives or move in bodies of water. They can adhere to the substrate through a thin peduncle that they present. Those who move in the water do so thanks to the undulations of the only scourge they possess.
This movement of the flagellum develops currents of water that give impulse to the choanoflagellate, facilitating its movement.
This form of displacement allows them to be classified as opisthtocons, while most protists are called acrocons, since the flagellum they possess is located in front of them and in the displacement it seems to "tow" them.
Habitat
Choanoflagellates are a group of organisms that are found mainly in aquatic environments. They are known to have a predilection for fresh waters.
However, there are some species that also thrive in seawater. They live in this type of environment because that way they have access to their food source.
Nutrition
From a biological point of view, choanoflagellates are heterotrophic organisms. This means that they are not capable of synthesizing their own nutrients, so they must use other living beings to feed themselves, either from their own body or from organic substances manufactured by them.
Choanoflagellates feed mainly on organic particles that are free in water. When it moves through these, as a result of the movement of the flagellum, the debris and bacteria are trapped in the microvilli around the flagellum, which constitute the main food of these organisms. They are later ingested.
Once inside the body of the choanoflagellate, the food particle is encompassed within the food vacuole, in which a large amount of digestive enzymes are contained. These act on the food, breaking it down into its constituent elements.
Once this happens, the already fragmented nutrients are used by the cell in various processes, such as those that involve obtaining energy.
As expected, as a product of any digestive process there are also remnants of substances that were not assimilated. These wastes are released into the extracellular environment.
Breathing
Because of how simple these organisms are, they do not have specialized organs to carry out the uptake and transport of oxygen. Taking this into account, respiratory gases (oxygen and carbon dioxide) cross the cell membrane through a process of passive cellular transport, diffusion.
Through this process, oxygen enters the cell, in which there is little concentration of it, to be used in various metabolic processes.
At the end of these processes, carbon dioxide is obtained, which is released into the extracellular space, also through diffusion.
Reproduction
The type of reproduction of these organisms is asexual. This implies that the descendants will always be exactly the same as their parent. The process by which these living things reproduce is called binary fission.
The first thing that must happen to start the process is the duplication of the DNA present in the cell nucleus. Once duplication has occurred, each copy of the genetic material is oriented toward each pole of the cell.
Immediately the organism begins to divide longitudinally. Once the cytoplasm has undergone a complete division, two daughter cells are obtained exactly the same as the one that divided.
It is important to mention that in choanoflagellates, this type of division is known as symmetrogenic. This means that the two daughter cells that are obtained are mirror images of each other, that is, one appears to be the mirror of the other.
In these organisms the type of sexual reproduction has not been reliably established. It is believed that in some species this type of reproduction occurs, although this is still under study.
References
- Bell, G. (1988) Sex and Death in Protozoa: The History of an Obsession. Cambridge: University Press.
- Campbell, N. and Reece, J. (2007). Biology. Editorial Médica Panamericana. 7th edition.
- Fairclough S. and King, N. (2006). Choanoflagellates. Retrieved from: tolweb.org
- King, N. (2005) Choanoflagellates. Curr. Biol., 15 pp. 113-114
- Thomsen, H. and Buck, K. and Chavez, F. (1991) Choanoflagellates of the central California waters: Taxonomy, morphology and species assemblages. Ophelia, 33 pp. 131-164.