Animal
Immortal jellyfish
The immortal jellyfish is a tiny jelly. It is smaller than your little fingernail. When it is very hungry, it can turn back into a baby and start again! It can still be eaten, so it does not live forever.
The immortal jellyfish is a tiny sea jelly only about 3 millimeters across. Most jellyfish die after they grow up and make babies, but this one can do something amazing: a starving adult can turn back into a polyp, the young stage, and grow up all over again. It can still be eaten or get sick, so it does not really live forever.
Turritopsis dohrnii is a hydrozoan whose medusae, even after reproducing, can revert to the polyp stage through a cyst, a process called life cycle reversal. Scientists describe it as potentially or biologically immortal, with the caveat that individuals still die from predation and disease. First described from Naples, it has spread with people to other warm seas.
Photo: Dr. Karen J. Osborn, CC0 1.0, via Wikimedia Commons
Meet the immortal jellyfish
The immortal jellyfish is tiny. It is even smaller than a grain of rice.
It has a clear bell and lots of thin tentacles. It has a special trick. It can turn back into a baby and grow up again! But other animals can still eat it.
The immortal jellyfish is a tiny hydrozoan, a small relative of jellyfish and corals. A grown-up one is only about 3 millimeters across, smaller than a grain of rice, with a see-through bell and up to 32 thin tentacles.
It is famous for one amazing trick. Most animals grow up, have young and then grow old. But when an adult immortal jellyfish is starving, it can turn back into its young stage, a polyp, and start growing up again. That is why people call it immortal, although it can still be eaten or die from illness.
Turritopsis dohrnii (Weismann, 1883) is a hydrozoan in the family Oceaniidae. The Smithsonian's NEMESIS database describes adult medusae up to 2.7 millimeters high and 3.3 millimeters wide with 14 to 32 tentacles, a red, cross-shaped manubrium ending in four lips, and brownish gonads.
In 1996 Piraino and colleagues reported that all stages of the medusa, from newly released to fully mature, could transform back into colonial hydroids, "escaping death and achieving potential immortality." They called it the first metazoan known to revert to a juvenile, colonial form after reaching sexual maturity. They studied it under the name T. nutricula; NEMESIS notes that most Mediterranean reports of T. nutricula probably refer to T. dohrnii.
Quick facts
- Group
- Invertebrate
- Length
- up to 3.3 mm
- Height
- up to 35 mm
Where it lives
Immortal jellyfish live in warm seas.
Their baby polyps grow on rocks and docks. Some now live in seas far away. Scientists think people carried them there.
The immortal jellyfish was first found near Naples, in Italy, and its polyps are common on rocks in the western Mediterranean and Adriatic Seas.
Today it also lives far from there: in Florida, on both coasts of Panama, in Japan, and in California and Texas. Scientists think people carried it to these places, though exactly how is not clear. Its polyps grow on rocks, docks, wooden posts in the water and even the bottoms of ships.
NEMESIS describes T. dohrnii as a common hydroid on rocky substrates in the western Mediterranean and Adriatic, where its status is considered cryptogenic (of uncertain origin). Genetic surveys by Miglietta and colleagues found it in Florida's Indian River Lagoon (Fort Pierce, 2006), on the Caribbean and Pacific coasts of Panama and in Japan, with a disjunct distribution and genetic similarity that point to human-mediated introduction; later records include Newport Beach, California (identified genetically in 2019) and Galveston Bay, Texas.
The hydroid grows on rocks, docks, pilings and ships' hulls in warm-temperate to tropical water. NEMESIS stresses that Turritopsis species can only be told apart reliably by DNA, so more hidden species and "silent invasions" are possible.
Did you know?
Even a grown-up immortal jelly can turn back into a polyp.
In 1996, scientists showed that immortal jellyfish of every age, even fully grown ones, could turn back into polyps.
Piraino and colleagues (1996) found all medusa stages could transform into colonial hydroids, directly or after a resting period.
Where can you find this animal?
- Brought here by people · Countries and seas named by a scientific source
- Lives here naturally · Countries and seas named by a scientific source
- Sightings recorded · Records of wild sightings (GBIF)
Brought here by people · Countries and seas named by a scientific source
Introduced populations identified genetically in Florida's Indian River Lagoon, on the Caribbean and Pacific coasts of Panama and in Japan; later records from Newport Beach, California, and Galveston Bay, Texas.
Places: United States, Panama and Japan.
Source: https://invasions.si.edu/nemesis/species_summary/-502 · checked October 2026
Lives here naturally · Countries and seas named by a scientific source
First described from Naples, Italy; a common hydroid on rocky coasts of the western Mediterranean and Adriatic, where its origin is considered cryptogenic (uncertain). Only Italy is shaded; other western Mediterranean and Adriatic coasts are not named country by country in the source.
Places: Italy.
Source: https://invasions.si.edu/nemesis/species_summary/-502 · checked October 2026
Sightings recorded · Records of wild sightings (GBIF)
Counts of GBIF records of wild observations (human and machine) and preserved specimens. Records include introduced populations, vagrants, escaped or released animals and misidentifications, so a country with records is not necessarily part of the native range.
Wild sightings have been recorded in Mexico, Spain, Italy, Egypt, United States, Indonesia, Atlantic Ocean, Pacific Ocean and 3 more places.
Source: gbif · checked October 2026
Range maps show where scientists say an animal lives, often country by country. An animal may live in only part of a country shown, and sightings records can include animals that wandered far from home.
Tiny food for a tiny hunter
Immortal jellyfish eat tiny animals that float in the sea.
They catch them with their thin tentacles.
The immortal jellyfish is a carnivore. It eats zooplankton, the tiny animals that drift in the sea, many too small to see without a magnifying glass.
Its polyps feed too. Each polyp has a ring of 12 to 20 thread-like tentacles around its mouth for catching food from the water as it drifts past.
NEMESIS lists the species as a carnivore feeding on zooplankton. Polyp hydranths are spindle-shaped with 12 to 20 filiform tentacles spread along most of their length and a conical hypostome around the mouth; medusae catch plankton with their marginal tentacles.
Food matters for the life cycle too: NEMESIS describes starved medusae settling and reverting to polyps, and in the laboratory experiments of Pascual-Torner and colleagues (PNAS, 2022) reversal was started by starving medusae for a day before a five-hour chemical treatment.
Did you know?
Baby immortal jellies have four glowing yellow spots.
Newly released medusae have four fluorescent-yellow pads inside the bell.
Newly released medusae bear four fluorescent-yellow pads between the roots of the interradial canals (NEMESIS).
A tiny, clear bell
The immortal jellyfish has a clear, round bell.
Thin tentacles grow all around the edge. In the middle there is a red tube with its mouth.
A grown-up immortal jellyfish has a clear, bell-shaped body about 3 millimeters wide and a little less tall. Around the edge grow 14 to 32 thin tentacles.
In the middle of the bell hangs a red tube with the mouth at its end, called the manubrium. Brownish parts that make eggs or sperm sit around it. When a baby medusa first leaves its polyp, it has only 8 to 12 tentacles and four glowing yellow pads inside.
NEMESIS (after Bouillon and colleagues and Schuchert) gives adult medusae up to 2.7 millimeters high and 3.3 millimeters in diameter, with 14 to 32 tentacles that sometimes have terminal swellings; tentacle number varies by region, from 8 in Bocas del Toro, Panama, to 12 to 24 in Italy and 14 to 24 in Japan, probably partly because of environmental differences.
The manubrium is red and cross-shaped in section, ending in four lips and reaching the bell margin; gonads are brownish with four rust-colored dots between the radial canals. Newly released medusae have 8 to 12 tentacles and four fluorescent-yellow pads between the roots of the interradial canals.
Growing up, and growing young again
An immortal jellyfish starts as a polyp stuck to a rock.
The polyp grows baby jellies that swim away and grow up. If a grown-up jelly is starving, it shrinks into a tiny ball. Then the ball grows into a new polyp, and life starts again!
Like many of its relatives, the immortal jellyfish has two forms. Branching colonies of polyps grow on rocks and docks, and they bud off tiny swimming jellies called medusae. Grown-up medusae are either male or female; they release eggs and sperm, and the young larvae settle down to grow into new polyps.
Here is the amazing part. When a grown medusa is starving, it can settle to the bottom, shrink into a little blob called a cyst and then grow into a new polyp colony, starting life over. Scientists have seen even fully grown medusae that had already reproduced do this. In theory it could happen again and again, but in the sea immortal jellyfish can still die, eaten by other animals or from disease.
NEMESIS describes branched hydroid colonies up to 35 millimeters high that bud medusae; medusae have separate sexes and release gametes, which form planulae that settle as new hydroids. Starved medusae can instead settle as an undifferentiated cyst that grows into a new polyp, restarting the cycle and making each individual potentially immortal. Pascual-Torner and colleagues (PNAS, 2022) describe the medusa shrinking until its structures disappear into an opaque cyst, from which a stolon grows and polyps bud.
Those authors call T. dohrnii the only species that keeps its high rejuvenation potential after reproducing, "reaching biological immortality"; two other Turritopsis can reverse after reproduction, but their ability drops sharply with maturity. As Discover Wildlife notes, immortal jellyfish can still die from predation and disease, so the immortality is potential, not a guarantee.
What scientists are learning
Scientists study the immortal jellyfish to learn how its body can start over.
They look at its genes, the tiny instructions inside every living thing.
How can an animal turn back into its younger self? To find out, scientists in Spain read the full set of genes, called the genome, of the immortal jellyfish and of a close relative that cannot do the trick.
They found differences in genes that help copy and repair DNA, protect the ends of chromosomes and keep cells able to become many different kinds of cell. These may help explain its powers. The authors say its abilities challenge what scientists understand about aging.
In 2022 Pascual-Torner and colleagues at the University of Oviedo published genome assemblies of T. dohrnii and of T. rubra, a congener that cannot rejuvenate at mature stages, plus gene-activity profiles across the stages of life cycle reversal. They reported variants and expansions of genes linked to DNA replication and repair, telomere maintenance, redox balance, stem cell populations and cell communication, and, during reversal, silencing of polycomb repressive complex 2 targets and activation of pluripotency targets, which they propose as key drivers.
These are the authors' proposals from comparative data; the paper has since been corrected twice and has drawn published comment, so the mechanism is still being studied.
Did you know?
Immortal jellyfish polyps can grow on the bottoms of ships.
Immortal jellyfish polyps grow on rocks, docks, wooden posts and even ships' hulls.
Hydroids settle on rocks, docks, pilings and hulls; the species' disjunct, genetically similar populations indicate human-mediated introduction, though the route is unclear (NEMESIS).
Tiny, tough and spreading
Immortal jellyfish are not in danger.
They are so small that most people never see them. They live in seas in many parts of the world.
The immortal jellyfish has not been assessed for the IUCN Red List. It is not known to be in danger, and it has spread to new seas around the world.
In places where it is new, scientists have not reported any harm from it so far. But because there are several look-alike species that can only be told apart by their DNA, scientists keep watching to learn where each one really lives.
Turritopsis dohrnii has not been evaluated for the IUCN Red List (GBIF's Red List copy: Not Evaluated). NEMESIS reports no impacts from introduced populations, though in Panama Bay it was among the seven most abundant hydromedusan taxa.
Because Turritopsis species are cryptic and identifiable reliably only by molecular means, the true ranges of T. dohrnii and its relatives remain uncertain, and other unnoticed introductions are possible (NEMESIS, after Miglietta and Lessios).
Did you know?
An immortal jellyfish is smaller than a grain of rice.
A grown-up immortal jellyfish is only about 3 millimeters across.
Adult medusae measure up to 3.3 mm across and 2.7 mm high (NEMESIS).
Amazing facts
-
An immortal jellyfish is smaller than a grain of rice.
A grown-up immortal jellyfish is only about 3 millimeters across.
Adult medusae measure up to 3.3 mm across and 2.7 mm high (NEMESIS).
-
Baby immortal jellies have four glowing yellow spots.
Newly released medusae have four fluorescent-yellow pads inside the bell.
Newly released medusae bear four fluorescent-yellow pads between the roots of the interradial canals (NEMESIS).
-
Even a grown-up immortal jelly can turn back into a polyp.
In 1996, scientists showed that immortal jellyfish of every age, even fully grown ones, could turn back into polyps.
Piraino and colleagues (1996) found all medusa stages could transform into colonial hydroids, directly or after a resting period.
-
Immortal jellyfish polyps can grow on the bottoms of ships.
Immortal jellyfish polyps grow on rocks, docks, wooden posts and even ships' hulls.
Hydroids settle on rocks, docks, pilings and hulls; the species' disjunct, genetically similar populations indicate human-mediated introduction, though the route is unclear (NEMESIS).
Words to know
- polyp
- a small, tube-shaped stage that stays stuck in one place
- medusa
- the bell-shaped, swimming stage of a jellyfish
- cyst
- a small resting blob that can grow into something new
- immortal
- never dying of old age
- genome
- the full set of genes, the instructions inside a living thing
- introduced
- brought by people to a place where it did not live before
Discovery challenge
-
What can a starving immortal jellyfish turn back into?
What can a starving adult immortal jellyfish turn back into?
Into which stage can a starved medusa of Turritopsis dohrnii revert?
- A polyp
- A fish
- A sea star
Show the answer
A polyp
It can turn back into a polyp and grow up again.
It shrinks into a cyst, which grows into a new polyp, starting the life cycle again.
It reduces to an undifferentiated cyst from which a stolon and new polyps grow.
-
Can an immortal jellyfish still be eaten?
Does an immortal jellyfish really live forever?
Which statement about the 'immortality' of Turritopsis dohrnii is accurate?
- No, it can still be eaten or die from disease
- Yes, nothing can ever hurt it
- Yes, it never even gets hungry
Show the answer
No, it can still be eaten or die from disease
It can still be eaten or get sick, so it does not really live forever.
It can restart its life cycle, but predators and disease can still kill it.
Its immortality is potential: rejuvenation avoids aging, but predation and disease still kill individuals.
-
How big is an immortal jellyfish?
About how wide is a grown-up immortal jellyfish?
What is the maximum bell diameter of an adult T. dohrnii medusa?
- About 3 millimeters
- About 3 meters
- About 30 centimeters
Show the answer
About 3 millimeters
It is tiny, smaller than a grain of rice.
About 3 millimeters across, with 14 to 32 tentacles.
About 3.3 mm in diameter and 2.7 mm high (NEMESIS).
-
Where was the immortal jellyfish first found?
Where was the immortal jellyfish first described?
From which place was Turritopsis dohrnii first described?
- Naples, Italy
- The North Pole
- A lake in Canada
Show the answer
Naples, Italy
It was first found near Naples, in Italy.
It was described from Naples, Italy, and later found in Florida, Panama, Japan and elsewhere.
Naples; it is cryptogenic in the western Mediterranean and Adriatic and introduced elsewhere.
Places where you might see it
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For parents & teachers
What children will learn
- Describe the polyp and medusa stages of the immortal jellyfish's life cycle.
- Explain life cycle reversal and why the jellyfish is called potentially, not truly, immortal.
- Understand how people can spread sea animals to new places.
- See how scientists use genomes to study aging.
Recommended ages: 3 to 17 · About 12 minutes
Talk about it
- If an animal can restart its life, why do most immortal jellyfish still die in the sea?
- How might a tiny polyp travel across an ocean?
- Why do scientists study unusual animals to learn about aging in people?
Try at home or in class
- Draw the life cycle as a circle, then add an arrow going backward from the adult jelly to the polyp.
- Measure 3 millimeters on a ruler and compare it with a grain of rice.
Key vocabulary: polyp, medusa, cyst, immortal, genome, introduced
Where these facts come from
- GBIF Backbone Taxonomy and GBIF Occurrence Search , Global Biodiversity Information Facility (CC0 1.0 (backbone); occurrence counts are facts derived from records under CC0, CC BY and CC BY-NC)
- Turritopsis dohrnii (Immortal Jellyfish) species summary , NEMESIS, Smithsonian Environmental Research Center
- Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Piraino et al., Biological Bulletin 1996) , PubMed
- Comparative genomics of mortal and immortal cnidarians unveils novel keys behind rejuvenation (Pascual-Torner et al., PNAS 2022) , PNAS, via PubMed Central
- Immortal jellyfish , Discover Wildlife (BBC Wildlife)
- Wikidata , Wikimedia Foundation (CC0 1.0)
- Wikimedia Commons , Wikimedia Foundation (Per file: Public domain, CC0, CC BY or CC BY-SA only (credited on each page))
Checked, not yet human-reviewed. Last updated October 27, 2026. Spotted a mistake? Tell us.