Of course, not all jellyfish are immortal, but Turritopsis nutricula is a very special jellyfish that seems to repeat its life cycle over and over again without dying.

The cell development process of reverting to a younger form is called transdifferentiation, and just how it works is still largely a mystery.

During normal growth and development, genes are turned on or off in a particluar pattern, so being able to effectively undo the instructions and start again is a remarkable process.

But never mind the DNA of the genes, and how are genes controlled - the fact that the jellyfish are immortal has other implications...

... once an inhabitant of the Carribean only, it is now spreading around the world, hitching a ride in tankers, and of course it never dies! 

Hmmm.  I hope it is friendly!  But don't worry - at only 5mm long, it is unlikely to disturb you while you swim!

Action:

Jellyfish are not really fish at all!  Do some research about what kind of animals they are and make a jellyfish model here

Want to keep a jellyfish as a pet?  It's not so easy - read here first to find out how molecules can be a problem before making big plans!