The Immortal Jellyfish

Turritopsis dohrnii, commonly called the immortal jellyfish, is a small hydrozoan rather than a true jellyfish, and it occurs in temperate to tropical waters around the world. The species is notable among animals for reverting completely to a sexually immature, sedentary colonial polyp after it has already matured as a free-living individual. That capacity for reversal, rather than any exemption from being eaten or killed, is what the name commemorates.

The adult medusa is a small animal, its bell reaching about 4.5 mm (0.18 in) across and standing roughly as tall as it is wide. A bright red cruciform stomach shows through the body, and up to 90 white tentacles run along the bell margin. The adult tentacle count is given as 80–90. Newly released young medusae carry far fewer, about eight tentacles, and the full complement develops as the animal matures.

The life cycle of the immortal jellyfish proceeds from a settled planula to a branching polyp colony that can stand up to 35 mm tall, and these colonies bud one-millimetre medusae. The medusa buds in 3 to 4 weeks, taking up to 30 days at 20 degrees Celsius and as little as 18 days at 22 degrees Celsius. Sexual maturity is reached in 18–30 days depending on temperature. The species survives in water of 14–25 °C (57–77 °F).

Reversal to the polyp form can be triggered by low food, sudden temperature or salinity change, or physical damage. The medusa maintains telomere length during its life-cycle reversal, effectively resetting cellular ageing. Sequencing of polyp, medusa and cyst stages yielded tens of thousands of contigs each and implicated telomerase and DNA repair in the reversal. Tissue lacking interstitial cells can still revert, so the process depends on transdifferentiation rather than a dedicated stem-cell pool.

A companion transcriptome study found the cyst stage enriched for genes tied to ageing control, transposable elements, DNA repair and ubiquitin processes. In the related Xiamen species, cutting the medusa into upper and lower parts showed the two halves differ significantly in their ability to become polyps. The reverse-development literature dates the concept to a paper on Turritopsis published in the Biological Bulletin in 1996 by Piraino and colleagues. Britannica stresses the escape from ageing depends on favourable conditions and the individual not being eaten or falling to disease.

Under stress the medusa absorbs its tentacles, loses the ability to swim and sinks as a cyst, which becomes a new polyp within two days once stolons have fed. Starvation, sudden temperature change, lowered salinity or cut bells all trigger the transformation, and every medusa stage can revert.

Transcriptome work shows that tissue lacking interstitial cells can still revert, so the process depends on transdifferentiation rather than on a dedicated stem-cell pool.

The scientific record of the species begins with August Weismann, whose 1883 monograph described it as Dendroclava dohrnii and remains the original description. The capacity for which the animal is now known entered the literature by a different route entirely: in the 1980s the British students Christian Sommer and Giorgio Bavestrello observed the reversal by accident, when stressed medusae held in a jar turned into polyps without passing through larvae.

A 2008 study in Biological Invasions examined Turritopsis populations from Spain, Italy, Japan, Florida and Panama and found them genetically identical, a pattern implying transport by ship rather than drift. Genetic work on the animal continued into the following decade, with a 2022 genome study published in PNAS 119(36): e2118763119.

• 1883
  Original description
  August Weismann describes the species as Dendroclava dohrnii in a monograph that remains the original description^{8Turritopsis dohrnii (Weismann, 1883) · World Register of Marine Species ↗}.
• 1980s
  Accidental discovery of reversal
  Christian Sommer and Giorgio Bavestrello observe stressed medusae in a jar turning into polyps without larvae^{9Alan Turing · Natural History Museum ↗}.
• 2008
  Genetic uniformity across oceans
  A study in Biological Invasions finds Turritopsis from Spain, Italy, Japan, Florida and Panama genetically identical, implying ship transport rather than drift^{10"Immortal" Jellyfish Swarm World's Oceans · National Geographic ↗}.
• 2022
  Genome study
  A genome study appears in PNAS 119(36): e2118763119^{7'Immortal Jellyfish' Could Spur Discoveries About Human Aging · Smithsonian Magazine ↗}.

Sequencing of the polyp, medusa and cyst stages yielded tens of thousands of contigs for each and implicated telomerase and DNA repair in the reversal. A co-author cautioned that it is a mistake to think people will share this jellyfish's immortality, though the work may inform age-related disease.

Reverse development is not unique to Turritopsis: earlier work described it in the scyphozoan Chrysaora and in several hydrozoans under adverse conditions. At least three Turritopsis species show inverted metamorphosis, turning pelagic medusae back into benthic polyps.

The species is now seen as an aggressive hitchhiker, carried across oceans in ship ballast water and now found worldwide. It is a hydrozoan rather than a true jellyfish, and it likely spread worldwide from the Mediterranean in the early twenty-first century via ballast water. Populations differ in form as they spread, with tropical swarms having about 8 tentacles and temperate ones 24 or more.

Immortal jellyfish still die: fish and turtles eat the medusae, and sea slugs and crustaceans prey on the practically defenceless polyps. In nature most individuals die before ever reverting, so 'immortality' means evading old age rather than escaping death. Immortality only kicks in as an emergency response to starvation, damage or other crisis; most of the time the animals reproduce and die conventionally.

The medusa begins life with roughly eight tentacles, while the adult form carries 80 to 90 along the bell margin ^{1Turritopsis dohrnii · Wikipedia ↗}. Sexual maturity arrives quickly: planulae settle and grow into branching polyp colonies, which bud one-millimetre medusae that mature in weeks rather than years ^{1Turritopsis dohrnii · Wikipedia ↗}. When stressed, the medusa absorbs its tentacles, loses the ability to swim and sinks as a cyst, becoming a new polyp within two days once stolons have fed ^{1Turritopsis dohrnii · Wikipedia ↗}. The longest documented captive record is Kubota's observation of 11 rebirths in a two-year period ^{1Turritopsis dohrnii · Wikipedia ↗}.

The immortal jellyfish is classified as Turritopsis dohrnii, a small hydrozoan occurring in temperate to tropical waters around the world ^{1Turritopsis dohrnii · Wikipedia ↗}. WoRMS records the accepted name as Turritopsis dohrnii (Weismann, 1883) within the family Oceaniidae, with a small hydrozoan polyp colony from Italy serving as the reference ^{8Turritopsis dohrnii (Weismann, 1883) · World Register of Marine Species ↗}. The species was first described as Dendroclava dohrnii by Weismann, whose 1883 monograph remains the original description ^{8Turritopsis dohrnii (Weismann, 1883) · World Register of Marine Species ↗}.