Animals That Cheat Death

The hydra is a freshwater creature smaller than your fingernail, related to jellyfish. It shows no measurable signs of aging at all. Its stem cells keep dividing and quietly replace every cell in its body, so the whole animal is rebuilt roughly every few weeks. An old hydra is no more likely to die than a young one — it is, in practice, biologically immortal.

The jellyfish Turritopsis dohrnii goes one step further. A normal animal can only grow up. This one can grow back down: when life gets hard, it returns to its baby form and starts over. People call it the immortal jellyfish, and you will see exactly how the trick works later in this lesson.