Immortal jellyfish

The creature that turns back into a 'baby' when it ages

This jellyfish is the size of a fingernail — but it doesn't die! When it grows old or gets injured, it 'rewinds' itself: its body shrinks, sinks to the seabed and becomes a 'baby' again — a polyp that grows up anew. It's like a butterfly turning back into a caterpillar and becoming a butterfly again! Scientists study it to help people.

Turritopsis dohrnii is a 4.5 mm hydrozoan jellyfish, originally from the Mediterranean, spread worldwide in ships' ballast water. It is the only known 'biologically immortal' animal: under stress, ageing or injury, through transdifferentiation — mature cells changing into other cell types — the medusa reverts to the polyp stage and the cycle restarts, theoretically forever. In the lab it has been observed up to 10 times. In practice most are eaten or fall ill. Its genome was sequenced in 2022: DNA-repair and telomerase genes are duplicated. It is a model for ageing and regeneration research. Japanese scientist Shin Kubota has studied it for 20 years and sings a song about the 'immortal jellyfish'.

Fun facts

• This jellyfish is the only 'immortal' animal — when it ages, it returns to childhood.
• It's the size of a fingernail — 4.5 mm.
• Jellyfish are 95% water and have no brain.