Jellyfish that cheats death and gets young again

There's a jellyfish that, when injured or starving or simply old, sinks to the seafloor, melts into a blob, and grows back as a juvenile.

It's called Turritopsis dohrnii. About the size of a fingernail, transparent, unremarkable to look at. But under stress, its adult cells reverse course — a process biologists call transdifferentiation. Muscle cells become nerve cells. Reproductive cells become structural cells. The jellyfish reabsorbs its tentacles, collapses into a featureless mass of tissue, and a few days later sprouts back as a polyp, the larval form. Then it grows up again.

It can, in theory, do this forever. It is one of the only known animals capable of completely rewinding its life cycle from a mature adult back to a juvenile stage. Most cells, once they've committed to being a heart cell or a skin cell, stay that way until they die. Turritopsis cells appear to ignore that rule entirely.

In practice it still dies — eaten by fish, hit by disease, crushed by surf. So no individual lives forever. But the failure point is environmental, not biological. There is no internal clock telling it to age out.

Researchers in Japan and Italy have kept colonies alive in lab tanks for over a decade, repeatedly stressing them and watching them regenerate from scratch. The same jellyfish, biologically speaking, has been reborn dozens of times. Genome sequencing in 2022 revealed that Turritopsis dohrnii has duplicate copies of several DNA-repair genes — far more than its closest non-immortal relatives.

The frustrating part is that we still don't really know how it works. The animal is tiny and the colonies are notoriously hard to keep alive. But somewhere in that fingernail-sized blob of cells is an instruction manual for getting younger, written in protein, that we haven't finished reading.