The jellyfish that grows young again

There is a tiny jellyfish, Turritopsis dohrnii, nicknamed "the immortal jellyfish." And the nickname is not entirely an exaggeration.

When it is injured, goes hungry or simply grows old, this jellyfish can reverse its life cycle: instead of dying, its cells transform and it returns to its juvenile stage, the polyp stuck to the seafloor. From there it can grow again and repeat the process, in theory, many times over.

That does not mean it is truly immortal: in the sea other animals devour it, diseases kill it, or the environment changes. But, free of those dangers, it has no clear biological expiry date, something extremely rare in the animal kingdom.

Source: American Museum of Natural History