One jellyfish can reverse its own ageing and start over as a baby

Turritopsis dohrnii does something no other known animal can do: when it is starving, injured, or simply old, it collapses into a ball of cells, sinks to the seafloor, and regrows into the juvenile stage it started from. Scientists call the result biological immortality, though the ant-sized jellyfish still dies plenty of other ways — mostly by getting eaten.

Dr. Maria Miglietta, an associate professor of marine biology at Texas A&M University at Galveston, is one of the few researchers in the world who can reliably find the species in the wild. Her two decades studying the animal have helped turn it from a curiosity into a serious research subject for scientists trying to understand why most life ages and dies at all.

A Medusa That Refuses to Die

Most jellyfish live once. A larva settles on the seafloor, grows into a stalk-like polyp, buds off swimming medusae, and those medusae die after they reproduce. Turritopsis dohrnii is the only known metazoan able to rejuvenate repeatedly after its medusae reproduce, reverting to the polyp stage instead of dying and starting the cycle over with the same genome intact. Researchers describe the jellyfish’s rejuvenation probability as climbing as high as 100%, with no observed limit on how many times an individual can repeat the cycle.

The trigger can be starvation, physical injury, or nothing more dramatic than the ordinary aging of its tissue. “Even if you starve it, bring it to 100 degrees Celsius, cut it, or even use UV light — things that would normally kill an animal — it doesn’t die,” Miglietta said, describing how a stressed medusa instead collapses into a cyst-like ball and falls to the ocean floor to begin again as a polyp. Texas A&M reported that the entire reversal, from a dying adult to a newly forming polyp, takes less than 24 hours — faster, Miglietta said, than any other organism known to reprogram its own cells.

The Genome Behind an Impossible Reset

The biology behind that reset only became clear once researchers could compare the jellyfish’s genome against a close relative that cannot repeat the trick. A team led by Carlos López-Otín at the Universidad de Oviedo in Spain, with Maria Pascual-Torner as lead author, sequenced the full genome of Turritopsis dohrnii and set it against Turritopsis rubra, a mortal cousin that dies the way ordinary jellyfish do. The Oviedo team found that the immortal species carried roughly double the number of genes associated with DNA repair and protection, along with mutations that curb uncontrolled cell division and keep its telomeres — the protective caps on chromosome ends — from breaking down over repeated cycles.

The same genome project, published in the Proceedings of the National Academy of Sciences, traced what happens at the molecular level during the reversal itself: a wave of silencing across polycomb repressive complex 2 targets paired with activation of genes tied to pluripotency, the property that lets a specialized cell become any other cell type. That combination is what allows a fully formed medusa’s swimming or digestive cells to, in effect, forget their assigned job and revert to stem cells capable of building an entirely new body plan.

The comparison is not quite as clean as “immortal versus mortal.” A later commentary on the Oviedo genome study noted that Turritopsis rubra, the mortal jellyfish used as the control species, has itself been observed rejuvenating both before and after reproduction, just at a lower rate than Turritopsis dohrnii. That leaves Turritopsis dohrnii’s advantage as one of degree — a far higher, seemingly unlimited rejuvenation rate — rather than a total monopoly on the ability itself.

One Immortal Polyp, Hundreds of Clones

Turritopsis dohrnii was first described in the Mediterranean Sea in the 1880s, but its reversal trick was not documented until the 1990s, and the “immortal jellyfish” label only stuck afterward. A fully grown medusa is roughly 4.5 millimeters across — smaller than a pinky nail, according to the American Museum of Natural History — with a bright red stomach visible through its transparent bell and as many as 90 fine white tentacles ringing its edge.

Because the reversal produces a polyp colony rather than a single organism, one survivor can flood its surroundings with genetically identical offspring. “When the season is right, one Turritopsis dohrnii polyp can bud off multiple jellyfish,” Miglietta said. “So from one jellyfish that doesn’t die, you can have hundreds of new jellyfish injected into the environment.” Every one of those medusae carries the same DNA as the original animal, without a brain, a nervous system, lungs, or a single dedicated organ managing the process.

A Blueprint for Regenerative Medicine

What makes the jellyfish valuable to medicine has less to do with immortality than with the mechanism behind it. Transdifferentiation — the process by which a mature, specialized cell reverts to an unspecialized stem cell and then becomes something else entirely — is exactly the trick regenerative medicine needs to repair tissue damaged by injury or disease in humans, who cannot naturally do this at anywhere near the same scale. “Understanding how cells can reprogram themselves to become something else that is needed is at the base of regenerative medicine,” Miglietta said. “And this jellyfish is the only creature that can do it in less than 24 hours.”

None of that has translated into a treatment yet, and Miglietta is candid about how little of the underlying biology is settled. What is established is narrower and still striking: in her lab, the jellyfish can theoretically go on resetting indefinitely, while in the ocean, the cycle usually ends the ordinary way, in the mouth of a predator, before old age or injury ever gets the chance to trigger a reset at all.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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