Didinium is a genus of ciliated protists characterized by its particular barrel shape. They are usually found in freshwater habitats and were first described in 1859 by the Danish naturalist Otto Friedrich Müller.
Throughout history it has undergone various reclassifications. It was not until 1974 that the order Haptorida was created to group what he called "predatory carnivorous forms"
Scheme of a Didinium. Source: By V. Schewiakoff, via Wikimedia Commons
The genus Didinium encompasses a total of 10 species known so far. Of all of them, the most studied and representative of the genus is Didinium nasutum.
One of the most representative characteristics of the genus and that still amazes specialists is the voracious behavior that they exhibit when feeding, since it attacks the prey and swallows it, regardless of whether it is larger.
Taxonomy
The taxonomic classification of the genus Didinium is as follows:
Domain: Eukarya
Kingdom: Protista
Phylum: Ciliophora
Class: Litostomatea
Order: Haptorida
Family: Didiniidae
Genus: Didinium
Morphology
Members of the genus Didinium are single-celled organisms that have various shapes: barrel, round, or oval. The cell body is surrounded by two bands known as pectinels, which are nothing more than rows of cilia. These have the function of promoting the movement of the organism in the water.
In the anterior part, a cone-shaped protrusion is seen, in which the opening of the cytostome, or mouth opening is located. It is important to note that this hole is not permanent, but only appears when the body is going to eat some food. It has a capacity to expand in large dimensions.
Cells have an average size of between 50 and 150 microns. In the same way, it has a macronucleus with an elongated appearance. Contractile vacuoles can be seen at the posterior end of the cell, as well as an anal opening.
General characteristics
The genus Didinium is made up of eukaryotes, which means that it has a structure in which the contained genetic material is found.
They are free-living organisms, that is, they do not need to establish symbiotic or commensal relationships with any other living being. They are not parasites nor are they responsible for any type of pathology in large mammals or humans.
Organisms of this genus are characterized by their rapid mobility in the aqueous medium, thanks to the action of the numerous cilia in the bands around the body.
Habitat
The vast majority of members of this genus are found freely in fresh and brackish water bodies. However, so far three species have been discovered in marine habitats.
Nutrition
Those of the genus Didinium are heterotrophic organisms, that is, they do not synthesize their nutrients by any process, but must feed on other living beings. In this sense, members of this genus are extremely carnivores. They are known predators of some ciliates, especially those belonging to the genus Paramecium.
In fact, the trophic relationship that they establish with the Paramecium has been extensively studied over the years. It is particularly surprising that Didinium can ingest a Paramecium that sometimes greatly exceeds its size.
When a Didinium perceives a Paramecium, it expels trichocytes, which are a kind of poison dart with which it manages to paralyze its prey. Likewise, it expels the so-called union lines, with which it manages to attract the Paramecium towards itself and begin to swallow it through the cytostome, which expands to a great extent to allow the entry of such a large prey.
Once ingested as a prey, in the cytoplasm it is enveloped by a food vacuole, in which a large number of enzymes are contained. These are responsible for degrading and fragmenting the food to transform it into much smaller particles and molecules. Those molecules that are better assimilated will be used in other cellular processes.
The residues that remain product of this digestion process are stored and are expelled to the outside environment through a posterior hole known as the anal pore.
Organisms of the genus Didinium are consummate predators that know how to adapt their food requirements to the prey that is available in the environment in which they develop.
Breathing
As in all members of the phylum Ciliophora, those of the genus Didinium do not have specialized structures for the respiratory process, to capture and process oxygen. However, oxygen is necessary for various processes. Therefore, the cell must make use of other mechanisms to acquire them.
Those of the genus Didinium have a type of direct respiration, which uses a type of passive cellular transport; simple diffusion. Through this process, oxygen diffuses through the cell membrane in favor of the concentration gradient. That is, from the outside of the cell, where it is concentrated, to the inside of the cell, where it is found in little quantity.
Once inside the cell, oxygen is used in various internal cellular processes. As a product of the use of oxygen, carbonic anhydride (CO2) is generated, which must be expelled from the cell, since it is to a certain extent toxic to it.
Through the same simple diffusion mechanism, it is released to the outside of the cell.
Reproduction
These organisms present two types of reproduction: asexual and sexual. In the first there is no union of sex cells or exchange of genetic material.
There are various asexual reproductive mechanisms. In the case of the genus Didinium, asexual reproduction occurs through binary fission. In this process, a cell divides into two exactly the same cells.
The first step that must occur is DNA duplication. This is so because each daughter cell must receive the same genetic load as the parent.
Once the DNA has been duplicated, a transverse division of the cytoplasm begins to take place, up to the point where both the cytoplasm and the cell membrane complete the division, thus generating two cells that are genetically and morphologically equal to the progenitor cell.
In the case of sexual reproduction, it occurs through a process known as conjugation. During conjugation, two cells exchange their genetic material, specifically the micronucleus.
References
- Audesirk, T., Audesirk, G. and Byers, B., Biology: life on Earth. 9th edition.
- Beers, C. (1925). Encystment and the Life Cycle in the Ciliate Didinium Nasutum. Proceedings of the National Academy of Sciences of the United States of America. 11 (9). 523-528
- Khana, D. (2004). Biology of Protozoa. Discovery Publishing House. 1st edition.
- Mast, SO (July 1917). "Conjugation and encystment in Didinium nasutum with special reference to their significance". Journal of Experimental Zoology. 23 (2): 340
- Wessenberg, H. and Antipa, G. (2007). Capture and Ingestion of Paramecium by Didinium nasutum. Journal of Eukaryotic Microbiology. 17 (2). 250-270