The jellyfish known as Immortal Jellyfish (*Turritopsis dohrnii*) is about the same size as the pinky fingernail on a full-grown human, but despite its small size, it has an incredible adaptation that allows it achieve biological immortality. 

I know many people have a hard time believing this, but continue to read the post, and I will explain the details on how this jellyfish can accomplish this amazing feat. By the way, we are talking about real immortality, with no upper limit to their age, not the vague definition of immortality that only requires the animal to not have decreased life expectancy after getting older. 

https://upload.wikimedia.org/wikipedia/commons/7/74/Turritopsis_dohrnii.jpg
*The immortal jellyfish, here in the hydra stage. [Image by Bachware, posted with the Creative Commons Attribution-Share Alike 4.0 International license](https://commons.wikimedia.org/wiki/File:Turritopsis_dohrnii.jpg)*.

## The life cycle of the Turritopsis dohrnii
The immortal jellyfish is born from fertilized eggs produced by jellyfish in their medusae stage. These hatch into free-swimming larvae that will eventually settle down onto the sea floor and develop into polyp colonies. The jellyfish is called a hydroid during this life stage, in which it is attached to the sea floor. 

During the hydroid stage, the polyp will bud new jellyfish, about one millimeter in size. Each polyp can produce multiple jellyfish, but these will all be identical to each other in terms of genes, so they are essentially clones. The hydroid stage will begin to produce medusae after a few days, and can keep producing more parent clones of itself after releasing the initial clones. 

The jellyfish stage is known as the medusa stage, and after budding from the polyp, they will reach sexual maturity after a few weeks, and can reproduce to start the life cycle again. 

## How they can achieve biological immortality
This is the really interesting part. During their medusa / jellyfish stage, they have the ability to revert back to the polyp stage! This is a very special adaptation that is found in this species along with a few others, and medusae at all ages can revert back to the polyp stage. 

The trigger to get back to the polyp stage is physical damage or stress. This can include getting sick, or even being too cold. The way this functions is called **transdifferentiation**, in which a cell type transform itself into another cell type. It can then transform its medusa cells back into cells needed by the polyp, and thus returning to the hydroid stage.

At the hydroid stage, it can bud new medusae, which are then able to return to a polyp stage again. This process can keep going on forever, rendering the organism to be biologically immortal. 

https://upload.wikimedia.org/wikipedia/commons/9/93/Nutting_1906_Turritopsis_minor.png
*This is a sketch of a Turritopsis sp. polyp. This belongs in the same genus as the Immortal Jellyfish, but it is only a closely related species, not a sketch of the immortal jellyfish itself. However, the polyp looks sort of like this, so you get an idea what the polyp looks like. [Image by Charles Cleveland Nutting, posted as Public Domain](https://commons.wikimedia.org/wiki/File:Nutting_1906_Turritopsis_minor.png)*. 

## So one individual can live forever?
In theory, it is possible that a single individual of the immortal jellyfish can live forever, but life for the jellyfish is not that easy, and most will be dead before they can revert into the polyp stage again. There are several predators that eat the immortal jellyfish, and it is known to get sick from diseases, so the average life span of a single jellyfish is not that long. 

## Is this really true?
As I said above, a lot of people have problems with accepting this fact, but there have been several different studies that have shown that all the hydrae are capable to revert back into a polyp. The scientist Shin Kubota kept a colony of these jellyfish in an aquarium for 2 years, and said the colony rebirthed itself a total of 11 times during these two years. 

The reversion of the medusae into a polyp stage has never been seen in a natural setting, and only in laboratory settings. The reason behind this is that the reversion is fast (I could unfortunately not dig up a source giving a better estimation), so it is unlikely to see the process in the wild. 

## Thanks for reading
Thank you for reading my post about the immortal jellyfish! I find this species extremely fascinating, and I’m sure that I am not the only one. 

## Sources
I mainly used the [Wikipedia entry on the Immortal Jellyfish]( https://en.wikipedia.org/wiki/Turritopsis_dohrnii) for this post, and did some fact-checking by going to a few of the citations. 

## Amazing Animal Adaptations
This post is part of a series called *Amazing Animal Adaptations*. I try to highlight some cool adaptations that we find in the nature, so make sure to follow me if you like these kinds of posts. 
Previous entries in the series:
1) [The incredible strength of the Coconut crab](https://steemit.com/nature/@valth/the-incredible-strength-of-the-coconut-crab-amazing-animal-adaptations-1).
2) [The biological secrets behind the hibernation of brown bears](https://steemit.com/nature/@valth/the-biological-secrets-behind-the-hibernation-of-brown-bears-amazing-animal-adaptations-2).
3) [How clownfish changes their sex depending on the social group](https://steemit.com/nature/@valth/how-clownfish-changes-their-sex-depending-on-the-social-group-amazing-animal-adaptations-3).
4) [Buffalo herds use a voting system to select their next grazing spot](https://steemit.com/nature/@valth/buffalo-herds-use-a-voting-system-to-select-their-next-grazing-spot-amazing-animal-adaptations-4).
5) [How the Greater honeyguide bird lead humans to find honey](https://steemit.com/nature/@valth/how-the-greater-honeyguide-bird-lead-humans-to-find-honey-amazing-animal-adaptations-5).