There are several organisms that have reached what is known as biological immortality , being able to remain alive for an indefinite time once they reach the adult stage of their development. Of course, the fact of having biological immortality does not imply that the animal cannot die from causes other than cellular aging. One of the best known cases of this phenomenon is that of a jellyfish with a very peculiar life cycle.
The jellyfish of eternal youth
Jellyfish, like gorgonians, corals, and sea anemones, belong to the phylum of cnidarians . A common characteristic of all cnidarians is that their life cycle is divided into two well differentiated parts: one in which the individual is sessile and remains anchored to the substrate (polyp) and another in which the organism floats freely in the water column. (jellyfish).
The difference between the different cnidarians is that jellyfish reach their sexual maturity during the jellyfish phase, while gorgonians, corals and anemones do so during the polyp phase. While adult jellyfish resort to sexual reproduction to produce new polyps, each polyp can asexually give rise to several juvenile individuals, called ephras, which mature into reproductive jellyfish. All the ephras produced by a polyp are genetically identical clones.
There are some jellyfish that have the ability to return to the polyp stage once they have reached the adult stage of their life cycle. This phenomenon is known as transdifferentiation and consists of the transformation of a cell that is not a stem cell into another type of cell.
The main characteristic of stem cells is precisely that they can transform into any cell type, a characteristic that other cell types lack once they have specialized. Hence, this phenomenon described in some jellyfish is so unique, since there are no known other organisms capable of making this alteration in their body . This occurs in response to stress, be it an environmental threat, illness, or old age.
Some species of jellyfish capable of carrying out this process are Turritopsis dohrnii (previously known as T. Nutrula and commonly called “immortal jellyfish”, as it was the first in which this phenomenon was discovered), Laodicea undulata or various species of Aurelia sp.
Thus, the characteristic of these jellyfish is that they are capable of reversing their life cycle and returning to a juvenile stage once they have reached maturity . Once in polyp form, each individual can asexually give rise to several ephrae, which will mature into adult jellyfish. So, in a way, these jellyfish can be considered to have achieved biological immortality.
Other immortal animals
There are several types of organisms that have developed strategies to avoid cellular senescence, being able to remain alive indefinitely once they reach maturity and are often considered immortal animals .
One of these organisms is the freshwater hydra . Hydras, like jellyfish, are a type of cnidarian, reaching sexual maturity during the polyp phase, so they spend most of their lives anchored to the substrate. Hydras are capable of reproducing asexually through budding, giving rise to genetically identical organisms. The age of these organisms does not appear to have a negative effect on them, but the exact mechanism by which hydras prevent cellular senescence is not yet known .
Immortal animals: planarians
Another organism that is immune to the effects of aging is the planarians , a type of flatworm with amazing regenerative abilities that can reproduce sexually or asexually. They can also survive indefinitely once they have reached their adult form, thanks to the fact that aging cells can be continuously replaced by young cells. The replication of cells indefinitely is possible in these organisms thanks to the fact that they have an enzyme called telomerase, also present in stem cells, which allows their cells to divide in an unlimited way.
Planarians reproduce asexually in a controlled way, by fission, or by regeneration of fragments after an external agent has damaged their body. These organisms are capable of regenerating their entire body from a few cells, thanks to the existence of cells called neoblasts, which can differentiate into different cell types and account for 20% of the cells of an adult planarian. The regenerative mechanisms of planarians can give rise to peculiar situations. Thus, if they suffer a longitudinal cut on their head, the individual will produce two healthy heads and continue to live .
Lobsters: Another “Immortal” Animal
All the examples we have seen so far are organisms of limited complexity and small size. Undoubtedly the most surprising case of an “immortal” animal is sea lobsters . These crustaceans, like the planarians, also possess the enzyme telomerase, which allows their cells to divide in an unlimited way, which makes them immune to senescence. However, as we mentioned at the beginning of this article, cellular aging is not the only factor that causes the death of an organism.
Lobsters, like other crustaceans, need to shed their shells in order to grow. This process consumes a great deal of energy and resources, so eventually a lobster can die of exhaustion during one of its molts . On the other hand, it has been observed that older lobsters can stop this process and maintain a stable size (which indicates a certain degree of control over cell multiplication).
However, by not changing its exoskeleton, it deteriorates allowing the entry of parasites and causing infections, which end up killing the animal. Therefore, although lobsters do not undergo cellular aging, they are not immortal.
As explained, biological immortality refers to the ability of some species to avoid the negative effects of aging at the cellular level.
In all the aforementioned examples, except in the case of locusts, asexual reproduction allows the effect of factors other than senescence to be camouflaged, giving the (false) impression that some animals can survive for an unlimited time, when really what Lasts are the clones of the original individual and therefore, they are not immortal animals.