The Immortal Jellyfish

Turritopsis dohrnii (not classified on the IUCN red list)

Although the species T. dohrnii was first discovered in 1883, its amazing ability to regenerate was not discovered until the 1990s. The immortal jellyfish was formally thought to be the Turritopsis nutricula, a similar but different species of jellyfish. The 'immortal jellyfish' is now classified as Turritopsis dohrnii, which is mainly found in the Mediterranean sea and the oceans surrounding Japan. The species is now being found more widely around the planet as they travel long distances by attaching themselves to cargo ships. Their preferred temperature is water around 22 degrees celcius, however, they are also found in colder water. They are made up of a clear bell-shaped structure filled with jelly. They have a bright red stomach which can be seen from the outside. This looks like a red cross from underneath. 

Red stomach cross-section

Once fully grown they are still incredibly small at 4.5mm in diameter and are only just visible without the use of a microscope. The first article published on the amazing ability of the immortal jellyfish classified it as a Turritopsis nutricula however this was corrected later on. The dohrnii has many similar features and genetic code to the T. nutricular and T. rubra. It has yet to be proven that these species also have the ability to 'regenerate' but it is thought that this could be possible. The dohrnii have nematocysts (also known as cnidoblasts or cnidocytes) on the tentacles which allow it to immobilise its prey before eating. These will not immobilise humans, but will cause a sting. Unlike other species, such as the box jellyfish, the dohrnii sting in not venomous. Jellyfish do not have a heart, brain or bones but the T. dohrnii do have light-sensitive receptors which allow it to detect light from the surface, helping it to recognise up and down directions.

The regeneration process

The process that the jellyfish undergoes to sustain its 'immortality' is known as transdifferentiation. Transdifferentiation is a type of metaplasia cell transformation which allows one type of specialised cell to turn into another. Although this process is found in other animal species such as newts, it is no where near as specialised as the process which occurs in the immortal jellyfish. The lens cells in the eye of a newt can become damaged. When this happens an iris cell within the eye can change into a stem cell. Stem cells have the ability to replicate any other cell and can then revert into a lens cell to replace the damaged/missing ones. In the immortal jellyfish, cells change directly into the required cell. The jellyfish can carry out this process in its entire being whereas a newt can not carry out differentiation in every cell of its body. The immortal jellyfish can even convert muscle cells into nerve or reproduction cells. 

 - Reproduction

The jellyfish are first born when a male ejaculates sperm into the water and some find their way into a female by chance. The fertilised eggs will stay on the surface of the female until they release free-swimming larvae known as planula

 - Planula

These planula have tiny cilia (hair-like structures) along their surface which allows a minimal amount of propulsion, but not enough for them to swim against currents.They will drift/swim through the water until they land on a firm surface such as the ocean floor or a rock. It will then anchor itself to this surface and transform into a polyp. 

 - Polyps

Polyp Stage

Polyps are cylindrical-shaped, stalk-like structures. They feed on small organisms such as zooplankton and plankton. Polyps use asexual reproduction to multiply. Eventually the polyps grow into a branch-like structure where the colony acts as a single organism (also known as hydroids). Polyps can live for 5 years and will eventually develop buds at its tips which will develop into medusae.

 - Medusae

The jellyfish that form from the buds of the polyps are also known as medusae. When the medusa buds off from the polyp it is only 1mm in diameter with just 8 tentacles. Once they are fully matured they are still incredibly small at about 4.5mm in diameter but they have 80-90 tentacles. It takes them about 2-4 weeks from budding to reach maturity. They will feed on plankton, tiny molluscs, larvae and fish eggs. 

 - Stimuli

The medusa will not automatically begin the regeneration process at sexual maturity. During its medusa stage the jellyfish may be exposed to environmental stress, injury, starvation, or it will eventually reach old age. This will stimulate the regeneration process. 

 - Retraction

Once the process is initiated, the medusa tentacles will retract and its body will begin to shrink in size. It will then drop down to the ocean floor and attach itself to a firm surface as a polyp again. The jellyfish then begins to mature all over again from the polyp stage. 

Transdifferentiation Cycle

Could these cells benefit humans?

A man named Shin Kubota is an associate professor at the Kyoto University in Japan, who has dedicated his life to studying the immortal jellyfish. These jellyfish are incredibly difficult to keep in captivity due to the fact that they have to be hand fed, given clean water daily, and must be constantly monitored. Shin Kubota is currently the only person who has managed to keep these jellyfish in captivity for a prolonged period of time. This species has inspired some scientists to research a new method of using stem cells to replace damaged cells, such as cancer cells, in humans. These jellyfish are more genetically similar to humans than simplistic species such as insects or worms. Despite this, the jellyfish are still a primitive species in comparison to humans. Without a brain, heart or bones it is much easier for these cells to perform transdifferentiation. We can not even be sure whether this process is possible in these organs. The process would not allow humans to revert back to the stage of being a baby but could be used in small collections of cells to repair damage. Overall, the structure of polyps and medusae are incredibly different to that of humans and this process is almost certainly not going to be seen in the near future. 

Are they immortal or just clones?
Although this process is described as 'immortality' it is actually closer to a cloning system that allows the jellyfish to replicate itself continuously, thus creating the possibility for immortality. It does not protect the species against other causes of death such as predation. This process can only be carried out during the medusa stage of life. Therefore, the species can die like any other animal from injury or disease during the polyp stage. 

The regeneration process has only been witnessed in captive jellyfish and never in the wild. This could suggest that it does not occur in their natural environment, however, 100% of specimens in captivity demonstrated the regenerative ability. Kubota stated that in two years, his colony re-birthed itself 11  times. The process is quite rapid, going from medusa to polyp in just three days. These jellyfish are so difficult to see that it is unlikely to observe them at the right time outside of captivity. They are likely to succumb to predation or disease during the polyp stage and are therefore highly unlikely to survive forever. They will be preyed upon by fish species such as tuna as well as other, larger species of jellyfish. Rebecca Helm from DeepSeaNews said that it is possible new clones may become more fragile from errors when copying the genetic code during the polyp stage. Mutations and errors in genetic code can occur during reproduction which could build up over time, causing the specimen to become weaker. It may eventually reach a stage where it can no longer complete the regeneration process, rendering it mortal after a certain life span. Due to the large number of varying factors and lack of observational research, it is impossible to determine an accurate lifespan for this species. 


More information:
-http://immortal-jellyfish.com/
-https://www.youtube.com/watch?v=2kLSiE-eNjw
-https://www.rsb.org.uk/membership/individual-membership/158-biologist/features/1510-everlasting-life-the-immortal-jellyfish