One jellyfish species can reverse its own ageing and start life over

Turritopsis dohrnii, a jellyfish barely 4.5mm across, can respond to injury or starvation by reverting its adult cells back into an earlier polyp stage — biologically restarting its life cycle instead of dying. It's the only animal known to do this repeatedly, which is why biologists sometimes call it 'biologically immortal.'

— Stefano Piraino et al., Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula — The Biological Bulletin, 1996