Is the Immortal Jellyfish Really Immortal?

What Is the Immortal Jellyfish?

The immortal jellyfish is Turritopsis dohrnii, a tiny hydrozoan about 4.5 millimeters wide and tall, or roughly 0.18 inches. That is likely smaller than the nail on your little finger.^[1] The species was described in 1883, and its strange ability was found by accident in the 1980s.^[1]

It is not immortal in the everyday sense: it can reverse its life cycle, but it can still be eaten or get sick and die. The nickname points to one real and very unusual trick.

What Are the Normal Life Stages?

Turritopsis dohrnii starts out much like its relatives, moving through four main stages:^[2][1]

1. Medusa. This is the free-swimming, bell-shaped adult that releases eggs or sperm.
2. Planula larva. A fertilized egg produces a larva that swims until it settles on a hard surface.
3. Polyp. The larva becomes a polyp, a form that stays attached and grows into a colony.
4. New medusae. The polyp colony buds off young medusae, which mature and close the loop.

Most jellyfish medusae die after reproducing.^[7]

How Does It Go From Medusa Back to Polyp?

Reverse development can start when a medusa is physically damaged, senescent (showing its age) or facing harsh conditions.^[2] Starvation is a common example.^[1] Researchers can trigger it on purpose, too: one team starved newly released medusae in glass bowls, and they formed cysts within 24 to 48 hours.^[2]

1. The medusa shrinks. It absorbs its tentacles and loses the ability to swim.^[1][2]
2. It settles as a cyst. It sinks to the bottom and becomes a cluster of poorly differentiated cells with a thin outer envelope, showing no clear medusa or polyp features.^[2]
3. Polyp features appear. Over the next 24 to 36 hours, the cyst develops into a polyp, starting with root-like stolons that anchor it.^[2][1]
4. The cycle restarts. The new polyp can produce new medusae that are genetically identical to the original.^[1]

What Is Transdifferentiation?

The cell-level process usually named here is transdifferentiation.^[1] Transdifferentiation means specialized cells get reprogrammed into other cell types. A cell committed to a medusa job changes its gene expression and takes on a new role in the polyp body.^[2]

A 2019 study in G3 says transdifferentiation has been reported in this species, but that dedifferentiation, where cells first step back to a less specialized state, cannot be ruled out and may happen alongside it.^[2] So the broad outline is well supported, while the exact cellular route is still being worked out.

That same study looked at which genes were active in polyps, medusae and cysts. In the cyst, categories tied to DNA repair and telomere maintenance were more represented, while cell signaling, cell division and differentiation were less represented. The authors call those results preliminary and say they need confirmation with replicate samples.^[2]

What Does "Biologically Immortal" Mean?

A 2022 paper in PNAS uses the phrase "biologically immortal" for this species because its medusae can rejuvenate repeatedly, even after reproducing.^[3] London's Natural History Museum puts it more carefully: in perfect conditions, these jellyfish may never die of old age.^[1]

Here is what the phrase does not mean:

• It cannot escape being eaten. Fish and turtles prey on the medusae, and sea slugs and crustaceans eat the polyps.^[1]
• It cannot escape disease. Marine biologist Helen Scales writes that immortal jellyfish can still die from predation and disease.^[5]
• Most probably do not get a second act. National Geographic reported in 2009 that "most of the time they die the old-fashioned way too."^[6]
• Endless repeats have not been shown. The highest count we found was up to 10 rejuvenations in captive colonies over two years.^[1] That is impressive, but it is far from forever.

What Did the 2022 PNAS Genome Study Find?

A team at the University of Oviedo in Spain sequenced the genomes of T. dohrnii and a close relative, T. rubra, then compared them with each other and with other cnidarians. They also tracked gene activity as T. dohrnii went through reversal, which they triggered by soaking starved medusae in a cesium chloride solution for five hours.^[3]

They reported differences across several kinds of genes:^[3]

• DNA copying and repair. T. dohrnii had extra copies of genes in both groups.
• Telomeres. Differences appeared in genes tied to maintaining chromosome ends.
• Redox balance. It carried five copies of the gene TXN, against two in T. rubra.
• Cell communication and stem cells. It had eight copies of BMP7, against five in T. rubra, and a duplicated GLI3.

During reversal, genes normally silenced by a protein complex called PRC2 were less active, especially in the cyst, and some genes linked to pluripotency changed too. The authors propose these as candidate parts of the process.^[3]

The study's central comparison is disputed. The authors treated T. rubra as the "mortal" species, saying they found no reports of it rejuvenating after reproduction. In a published 2023 letter, Maria Pia Miglietta called that premise incorrect. She pointed to earlier studies of a Japanese Turritopsis, presumably T. rubra, that reported rejuvenation before and after reproduction at a lower rate. If both species rejuvenate, she argues, "comparing their genomes cannot identify the keys to rejuvenation." She also says the paper gives no details on how its specimens were identified, and that misidentifications are frequent in this genus.^[4]

The authors flagged limits of their own, including the distance between the two species and the relatively low quality of both genome assemblies.^[3] A sensible reading is that the study offers candidate genes, not a finished explanation.

Myths vs Facts

What We Still Can't Tell You

We did not run any experiments or observe these animals ourselves. Everything here comes from the published work and institutions cited.

We cannot tell you how often wild immortal jellyfish actually pull off this reversal. The studies we read describe reversal induced in the lab or seen in captive colonies, and we did not find a direct count from the open ocean. Texas A&M says the mechanism isn't fully understood yet, and Miglietta calls herself one of the only people working on the species, so the evidence base is small.^[7]

We also cannot say how many times one lineage could repeat the cycle, which cells do what during the change, or which of the reported genes actually matter.

Nothing in this article suggests a treatment, supplement or anti-aging method for people. This is a story about jellyfish biology.