The immortal jellyfish

Meet the jellyfish that can cheat death by regenerating its body and starting all over again.

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The immortal jellyfish (Turritopsis dohrnii)

The concept of eternal life has captured human imaginations for centuries and for some it is the ultimate goal to be able to cheat death. Over time we have sought immortality in a number of ways including religion, medicine, cryogenics and, in the case of Indianna Jones, by searching for magical artefacts such as the Holy Grail. But while we’ve been unsuccessfully searching  an unlikely candidate has been floating around the ocean right under our noses. The immortal jellyfish (Turritopsis dohrnii) is one of several remarkable jellies that can cheat death by reversing their own life cycle. In times of crisis these gelatinous blobs can revert to their juvenile state and begin again like a butterfly changing back into a caterpillar. This incredible evolutionary adaptation is not only extremely interesting but could hold the key to countless medical advancements and maybe even the secret to everlasting life, but I wouldn’t get your hopes up anytime soon.

The life of a jellyfish

The popular image that comes to mind when you think of a jellyfish, a translucent bell-shaped body with tentacles sprawled out beneath it, is what is known as a medusa. But it is actually the third and final stage in their life cycle. A jellyfish begins life as a tiny larvae that looks more like a furry tic-tac than an ocean dweller. During this time the planktonic larvae will float aimlessly in the water column until it finds a rock or other suitable surface to settle on. When it has settled the larvae then morphs into a polyp, the second stage of the jellyfish life cycle. In its polyp stage the jellyfish strongly resembles a sea anemone with its body anchored on the ground with tiny tentacles floating overhead. After a while when the conditions are right the polyp undergoes budding where part of the polyp divides itself from the structure and becomes a tiny free swimming medusa that eventually grows into a fully grown jellyfish. The adult jellyfish will then reproduce to create more tiny larvae and eventually die thus completing their lifecycle. However for a few species this is not the end but instead a chance for a fresh start.

Cheating death

The immortal jellyfish are a tiny species only reaching a size of around 4-5mm but can be vibrantly colourful. It was discovered in 1883 but only gained its name in the early 1990’s when something remarkable happened after it appeared to die. A marine biology student from Italy collected a medusa from the sea near Genoa and left it in a bowl of seawater to study. In a fortunate twist of fate the student forgot to refrigerate the specimen over the weekend as they had intended and when they returned were amazed to find the medusa had disappeared. But the bowl was not empty in the spot where the medusa had laid lifeless was a brand new polyp. After exploring alternative hypotheses the only explanation that remained was that the medusa had regenerated itself into a previous form. So it seems rather than dying, in times of chronic stress, starvation and injury, these jellies are capable of reversing their own life cycle.

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Diagram illustrating the life cycle of the immortal jellyfish (http://discovermagazine.com/2017/dec/the-immortal-jellyfish)

But how does a creature with no blood, no heart and no brain manage to pull off something so seemingly impossible. The secret lies within the cells of the jellyfish that have some very special properties. All cells within an organisms are differentiated into a certain type which is controlled by the switching on and off of certain genes. This is what makes a muscle cell so different from a nerve cell even though they both started out as the exact same thing. What makes the immortal jellyfish’s cells so remarkable is their ability to revert back to a basic stem cell and change into a different type of cell in what is called transdifferentiation. So when a dying medusa sinks to the ocean floor it becomes a blob of undifferentiated cells that can reform into a polyp that produces a new genetically identical medusa. This can be done multiple times making the jellyfish functionally immortal.

An exclusive club

Although it is named the immortal jellyfish T.dohrnii is not the only species of jellyfish that can prolong their lives in this way. It is believed that there are actually five species capable of regeneration. The most recent and well known of these species is the moon jellyfish (Aurelia aurita) which was also discovered by a marine biology student but this time in China in 2011. However this time it was not an accident but deliberately tested for. The moon jellyfish is a considerably larger measuring at between 25-40cm wide, when it died and was left in a tank for three months, multiple polyps began growing from its tissue. This is a much more slow acting form of regeneration that is perhaps less common in nature as the jellyfish’s body is unlikely to remain undisturbed for so long. However it demonstrates that multiple species have managed to evolve this incredible adaptation that was once thought impossible in nature.

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A swarm of moon jellyfish (Aurelia aurita)

Uses in medicine

Although they are unlikely to help us achieve immortality these jellyfish species could still be very useful in advancing modern medicine, in particular their ability of transdifferentiation. The switching on and off of genes in a cell to reprogram it could have near endless possibilities in curing diseases. For example rather than fighting a cancerous tumour with drugs or radiotherapy imagine if you could simply reprogram those cells to turn back into healthy tissue. The same could be done in the brain to cure things like Parkinson’s and Alzheimer’s diseases. There may also be implications for treating injuries or enhancement by cultivating a greater number of a particular cell, the potential uses could be endless. However first we must figure out how jellyfish are able to do it and whether or not it is transferable to humans. Unfortunately this is an incredibly hard thing to study due to the complexity of the genetics involved and the many differences between jellyfish and us.

Swarming clones

As well as extending their lifespans the process of reverting back into a polyp can also be used to create multiple clones of the original jellyfish. This is because during the polyp stage of a jellyfish’s life cycle the polyp can create copies of itself creating a bloom of genetically identical polyps that can completely cover the surface that the first polyp settled on. This coupled with the ability to continually return to a polyp before death means that a single immortal jellyfish has the potential to create an exponential swarm of clones of itself during its extended lifespan. Recently researchers have found that the immortal jellyfish living off Japan, Panama, Florida, Spain and Italy were all nearly identical genetically. This is partly due to the exponential spread of clones but is also because these tiny creatures are transported around the world in the ballast water of shipping vessels, a major reason for many marine invasions.

Are these jellyfish truly immortal?

As interesting and impressive as these jellyfish are they do not actually live forever, what has been described as immortality should perhaps more accurately be labelled serial regeneration. As summarised by researcher Stefano Piraino who works closely with immortal jellyfish “If they were truly immortal, the ocean would be completely full of Turritopsis, and we don’t see that”. It is not known if there is a limit to the number of times these jellyfish can regenerate, in a laboratory the record is 12 although it is likely a higher number could be achieved in the wild. But whilst they can cheat death under certain circumstances they can still be killed by predation and infections especially in the polyp stage when they are particularly vulnerable.

Due to their ability to regenerate and create identical copies of itself the immortal jellyfish can keep its genes in the forefront of its species gene-pool without having to spend all of its time and energy on reproduction. For that reason some people wrongly assume that immortality would be the ultimate evolutionary adaptation. However if the immortal jellyfish were to be truly immortal and abandon reproduction altogether it would spell the end for its species, because without reproduction and death a species would be unable to adapt to the challenges it could face in the future. It is perhaps for the best then that as humans we are unable to achieve everlasting life.

Links

http://discovermagazine.com/2017/dec/the-immortal-jellyfish

https://news.nationalgeographic.com/2016/02/160302-jellyfish-immortal-science-animals-oceans-deadpool/

https://www.bbcearth.com/blog/?article=the-jellyfish-that-never-dies&ocid=twert&fbclid=IwAR0IgUh7UpaUEQFk3-DHfxbUeVG3Ix3ilhW927GhSPN0K3r5Ir5KZEIpGTc


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