The Radiolaria They are a set of protozoa of marine life formed by a single cell (unicellular organism), which present very varied forms, and a highly complex endoskeleton of siliceous origin.
The various species of Radiolaria are part of the marine zooplankton and owe their name to the presence of radial extensions in their structure. These marine organisms live floating in the ocean but when their skeletons die they settle to the bottom of the sea, being preserved as fossils.
This last characteristic has made the presence of these fossils useful for paleontological studies. In fact, more is known about fossilized skeletons than about living organisms. This has been due to how difficult it is for researchers to reproduce and keep alive the entire food chain of radiolaria in a way in vitro.
The life cycle of radiolaria is complex, since they are voracious predators of large prey, that is, they need to eat other microorganisms of equal size or greater than theirs every day or every other day. That is, it would be necessary to keep the Radiolaria, their prey and the plankton that eat their prey viable..
Radiolaria are believed to have a half-life of two to 4 weeks, but this has not been proven. It is also believed that the life span may vary depending on the species, as well as it is possible that other factors such as food availability, temperature and salinity may influence..
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The first fossil records of radiolaria date from the Precambrian Era, that is, 600 million years ago. At that time the Radiolarians of the order prevailed Spumellaria and the order appeared in the carboniferous Nesselaria.
Later the Radiolarians during the late Paleozoic showed a progressive decrease until reaching the end of the Jurassic, where they underwent an accelerated diversification. This coincides with the increase in dinoflagellates, important microorganisms as a food source for Radiolaria..
In the Cretaceous, the skeletons of radiolaria became less robust, that is, with much finer structures, due to competition in the capture of silica from the environment with the appearance of diatoms..
The Radiolaria belong to the Eukaryotic Kingdom and the Protista Kingdom, and according to the mode of locomotion they belong to the group of the Rhizopods or Sarcodinos characterized by moving by pseudopods.
Likewise, they belong to the class Actinopoda, which means radial feet. From there, the rest of the classification of subclass, superorders, orders, family, genera and species differs enormously between different authors..
However, the 4 main groups that were initially known were: Spumellaria, Nassellaria, Phaeodaria and Acantharia. Later 5 orders were described: Spumellaria, Acantharia, Taxopodida, Nassellaria and Collodaria. But this classification is constantly evolving.
Most Radiolaria are composed of a very compact silica skeleton, such as the order Spumellaria, characterized by having concentric, ellipsoid or discoidal spherical shells that fossilize upon death.
While, the order Nasselaria, it is characterized by adopting elongated or conical shapes due to the arrangement of several chambers or segments along its axis, and is also capable of forming fossils.
However, there are some exceptions. For example, Acantharia was classified as a different subclass to Radiolaria, because it has a skeleton of strontium sulfate (SrSO4), a substance soluble in water, therefore its species do not fossilize.
Likewise, the superorder Phaeodaria, Although their skeleton is made of silica, their structure is hollow and filled with organic material, which also dissolves in seawater once they die. This means that they do not fossilize either.
Collodaria meanwhile, it includes species with colonial lifestyles and without silicification (that is, they are naked).
For a single-celled organism, Radiolaria have a fairly complex and sophisticated structure. Their diverse forms and the exceptional nature of their designs have made them look like small works of art, which has even inspired many artists..
The body of a Radiolaria is divided into two parts by a capsular central wall. The innermost part is called the central capsule and the outermost outer capsule.
It is composed of the endoplasm, also called intracapsular cytoplasm, and the nucleus.
In the endoplasm there are some organelles such as mitochondria, Golgi apparatus, vacuoles, lipids and food reserves.
That is, in this part is where certain vital functions of its life cycle are carried out, such as respiration, reproduction and biochemical synthesis..
It contains the ectoplasm, also called extracapsular cytoplasm or calima. It has the appearance of an enveloping foamy bubble with many alveoli or pores and a crown of spicules that can have different arrangements depending on the species.
Some mitochondria, digestive vacuoles, and symbiotic algae are found in this part of the body. That is, the functions of digestion and waste elimination are carried out here..
Spicules or pseudopods are of two types:
The long and stiff ones are called axopods. These start from the axoplast located in the endoplasm, which crosses the central capsular wall through its pores..
These axopods are hollow, which resembles a microtubule that connects the endoplasm with the ectoplasm. On the outside they have a mineral structure coating.
On the other hand, there are the finest and most flexible pseudopods called philopods, which are found in the outermost part of the cell and are made up of organic protein material..
The skeleton of Radiolaria is of the endoskeleton type, that is, no part of the skeleton is in contact with the outside. This means that the entire skeleton is covered.
Its structure is organic and it is mineralized through the absorption of silica dissolved in the environment. While the Radiolaria is alive the siliceous structures of the skeleton are transparent, but once it dies they become opaque (fossil).
The radial shape of its structure is the first characteristic that favors the flotation of the microorganism. Radiolaria also have intracapsular vacuoles full of lipids (fats) and carbon compounds that help them float..
Radiolaria take advantage of ocean currents to move horizontally, but to move vertically they contract and expand their alveoli.
The flotation alveoli are structures that disappear when the cell is agitated and appear again when the microorganism has reached a certain depth..
Finally, there are the pseudopods, which at the laboratory level could be observed that can cling to objects and make the cell move on a surface, although this has never been seen directly in nature..
Not much is known about this aspect, but scientists believe that they may have sexual reproduction and multiple fission.
However, it has only been possible to verify reproduction by binary fission or bipartition (type of asexual reproduction).
The bipartition process consists of the division of the cell into two daughter cells. The division starts from the nucleus to the ectoplasm. One of the cells retains the skeleton while the other must form its own.
The proposed multiple fission consists of a diploid fission of the nucleus, which generates daughter cells with the complete number of chromosomes. Then the cell breaks down and distributes its structures in its offspring.
For its part, sexual reproduction could occur through the process of gametogenesis, in which swarms of gametes are formed with only one set of chromosomes in the central capsule..
Subsequently, the cell swells and breaks to release the biflagellate gametes; later the gametes would recombine to form a complete adult cell.
So far it has been possible to verify the existence of biflagellate gametes, but their recombination has not been observed..
Radiolaria have a voracious appetite and their main prey are represented by: silicoflagellates, ciliates, tintinids, diatoms, copepod crustacean larvae and bacteria.
They also have several ways to feed and hunt.
One of the hunting systems used by the Ridiolarios is of the passive type, that is, they do not chase their prey, but instead remain floating waiting for some other microorganism to meet them.
By having the prey close to their axopods, they release a narcotic substance that paralyzes the prey and leaves it attached. Subsequently, the philopods surround it and slowly slide it until they reach the cell membrane, forming the digestive vacuole.
This is how digestion begins and ends when the Radiolaria completely absorbs its victim. During the process of hunting and engulfing the prey the Radiolarius is completely deformed.
Another way they hunt prey is through the formation of colonies.
The colonies are made up of hundreds of cells interconnected by cytoplasmic filaments wrapped in a gelatinous layer, and can acquire multiple forms.
While an isolated Radiolarian oscillates between 20 to 300 microns, the colonies measure centimeters and exceptionally they can reach several meters.
Some Radiolaria have another way of nourishing themselves when food is scarce. This alternate system of nutrition consists of the use of zooxanthellae (algae that can live inside the Radiolaria) creating a state of symbiosis.
In this way the Radiolario is able to assimilate the COtwo using light energy to produce organic matter for food.
Under this feeding system (through photosynthesis), Radiolaria moves to the surface where they remain during the day, and subsequently descends to the ocean floor, where they remain throughout the night..
In turn, the algae also move within the Radiolarium, during the day they are distributed on the periphery of the cell and during the night they are positioned towards the capsular wall.
Some Radiolaria can have up to several thousand zooxanthellae at the same time, and the symbiotic relationship is terminated prior to the reproduction of the Radiolaria or upon its death, through digestion or expulsion of the algae..
Radiolaria have served as a biostratigraphic and paleoenvironmental tool.
That is, they have helped to order rocks according to their fossil content, in the definition of biozones, and in the preparation of paleotemperature maps on the sea surface..
Also in the reconstruction of marine paleocirculation models and in the estimation of paleodepths.
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