The immortal jellyfish can revert its life cycle to the polyp stage.
Why it is true
When facing physical stress or starvation, Turritopsis dohrnii transforms its adult cells back into stem cells, becoming a polyp again and starting life anew.
Source: Marine Biology
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.
Related facts
Animals
The immortal jellyfish can reverse its life cycle, making it biologically immortal.
Animals
The jellyfish Turritopsis dohrnii can reverse its life cycle and become biologically immortal.
Animals
There is a jellyfish that can revert to a juvenile state and avoid death.
Animals
The hydra has a life expectancy of about 20 years.