The Jellyfish That Can Reset Its Own Age

Off Sardinia in 1988, a German marine biology student named Christian Sommer kept a jar of plankton at room temperature and noticed something odd. The tiny jellyfish he was watching had shrunk. It had pulled its tentacles back in, dropped to the bottom, and turned into a polyp — the sessile, upside-down stage jellyfish are supposed to leave behind at birth.

The animal was Turritopsis dohrnii, a speck of gelatin a few millimeters across. Under stress — hunger, injury, old age — its cells can transdifferentiate, rewriting their identity on the fly. A muscle cell becomes a nerve cell; a tentacle reorganizes into a stalk. The medusa collapses, the polyp regrows, and the cycle begins again.

In the lab of Shin Kubota at Kyoto University, one colony has been cycling for more than a decade. In 2022, a team at the University of Oviedo sequenced the species' genome alongside a non-reverting cousin and found elevated activity in genes for DNA repair, telomere maintenance, and stem-cell regulation. It is as close as anyone has come to a genetic signature for the trick.

That is also why 'immortal' is a tabloid word, not a biological one. Out in the ocean, Turritopsis dohrnii gets eaten by fish, pinched by crabs, and infected by parasites just like every other cnidarian. The cell lineage can, in principle, persist indefinitely. The animal usually does not.

What makes the species interesting is less 'does it live forever' and more 'how does a fully differentiated cell walk backward.' Most vertebrate cells lose that option by the time they finish specializing. Cancer is what happens when they find it again, badly. Turritopsis finds it cleanly.