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Jellyfish are ancient creatures, so it is hardly surprising they have learned some new tricks along the way!
One specific species of jellyfish, Turritopsis dohrnii is of particular interest to scientists after the discovery that they are effectively ‘immortal’. This discovery was made by accident in 1988, by a German marine-biology student who was collecting tiny sea creatures off the coast of Italy. One of the species he collected was observed to behave very oddly while living in petri dishes, it seemed to be aging in reverse, getting younger and younger and refusing to die.
For most jellyfish their life cycle is one-directional. An egg is fertilized, becoming a ‘gamete’ and then develops into a ‘planula’ which is a small oval shaped structure that floats through the water and settles on the sea floor. The planula attaches to a hard surface and develops into a structure called a ‘polyp’ which is like a tube with small tentacles on top. As the polyp matures it begins to segment into discs, it looks almost like a stack of plates. These discs then separate from the polyp and develop into an immature jellyfish called an ‘ephyra’ before developing into the mature ‘medusa’ stage which is what we recognize as a jellyfish. In regular jellyfish these medusas mature, breed and then die. However, in the case of Turritopsis they can revert back to earlier developmental stages.
This video has been simplified to help illustrate the point that these ‘immortal’ jellyfish can ‘rewind the clock’ on their development. In truth these jellyfish can revert from their medusa form, folding in on themselves to form a ball of cells (one researcher calls this phase a ‘meatball’), and then turn into immature polyps again, shortcutting the gamete and planula phases of their life cycle. It’s sort of like a butterfly reverting back to being a caterpillar again. But why on earth would they want to do this? Well, if they are faced with a stressful environment, such as if they do not have enough food, there is a big change in water temperature, they become sick, or maybe something has tried to take a bite of them, instead of dying they can reset their development clocks and go back to their sexually immature stage and grow all over again.
How do they do it? Each cell of an organism contains the instruction manual for of how to build a new whole organism – these instructions are the DNA. Normally, as an organism develops from a fertilized egg, cells are able to grow into different parts of the body by reading from different parts of the instruction manual. For example, in humans, a nerve cell grows by using the instructions needed by a nerve cell, and a kidney grows by using the instructions needed by a kidney cell. This is not the case in the ‘immortal’ jellyfish though. All of their cells can read from any part of the instruction manual under the right circumstances. So, after damage, a medusa’s specialized tentacle cell can reset, and become any of the cells needed to become a polyp. Researchers are very interested in learning exactly what sort of genetic changes allow this to happen in these jellyfish, as they believe this could be very important for human ageing research given that there are a number of genetic similarities between jellyfish and humans.
While these jellyfish are capable of rejuvenating themselves over and over again, it is worth noting that they can still be killed, so they are not truly immortal. In their polyp stage they are susceptible to being eaten by sea slugs, and in their medusa form there are many predators who wish to eat them, including other jellyfish and penguins. Despite not being truly ‘immortal’ there is likely a great deal these tiny jellyfish can teach us about ageing and illness.
You can read a story from the New York Times about the discovery and ongoing research into Turritopsis dohrnii.