Turritopsis dohrnii

Some jellyfish can live for a long time.

They live in warm ocean waters. These jellyfish are very small. They have many long tentacles.

If they get sick or old, they change. They turn back into tiny polyps. These polyps live on the sea floor.

This change lets them start life again. It is like a reset button.

This helps them stay alive. It is a very special way to live.

The *Turritopsis dohrnii* jellyfish is very special. People call it the immortal jellyfish. It can live many lifetimes. This happens through a way called transdifferentiation. This is a way for cells to change into new types.

Most jellyfish grow and then die. This jellyfish is different. If it gets sick or old, it can go back. It turns from a swimming jellyfish into a polyp. A polyp is a small part that lives on the sea floor.

This process works like a reset button. The jellyfish can do this many times. It can even do this if it lacks food. This helps the jellyfish survive in many oceans.

These jellyfish are very small. Adult ones have 80 to 90 tentacles. They eat tiny things in the water. They use stinging cells to catch food. They are mostly made of water. Most of them still die from being eaten by others. Scientists study them to learn about aging. They want to learn how cells fix themselves.

The *Turritopsis dohrnii* is a tiny jellyfish with a very big secret. Most people call it the immortal jellyfish. This is because it can do something no other animal can do. It can turn its body back into a younger version of itself. This helps it avoid death from old age. Scientists study it to learn how cells work. They want to understand how life can reset itself.

This amazing thing happens through a process called transdifferentiation. This is a way for cells to change their type. Imagine if a skin cell could suddenly turn into a heart cell. When the jellyfish faces hard jobs, it starts this change. These hard jobs include being sick or getting old. It can also happen if there is not enough food. The jellyfish first lets its bell and tentacles break down. It then turns into a small, cyst-like stage. Finally, it becomes a polyp colony on the sea floor.

Humans have been studying this creature for a long time. It was named in 1883 to honor Anton Dohrn. He was the founder of a research station in Naples, Italy. For a long time, people thought it was a different species. Scientists used to think it was the same as *T. nutricula*. However, a study in 2006 showed they are actually separate. Now we know *T. dohrnii* is its own unique group.

These jellyfish are quite small in real life. An adult jellyfish has a diameter of about 4.5 millimeters. It can have between 80 and 90 tentacles. Young ones only have about eight tentacles. They live in all kinds of ocean water. They can be found in both warm and cool areas. They are mostly made of water, with only 5% being other matter. They eat tiny things like plankton and fish eggs.

You can think of this jellyfish like a video game character. When the character loses all its health, it restarts at level one. The jellyfish does this to stay alive in the ocean. It can even travel to new places in ship water. This is called a silent invasion because they are so small. Even though they are immortal, they can still be eaten. Predators like sea turtles and sharks still hunt them. They are a tiny but mighty part of our blue planet.

The *Turritopsis dohrnii* is a small jellyfish with a remarkable biological capability. Often called the "immortal jellyfish," it belongs to the Medusozoa group. This species is unique because it can reverse its life cycle. Most animals grow from young to old and eventually die. However, *T. dohrnii* can revert from a mature adult back to a colonial polyp stage. This ability allows it to bypass death from old age. It is a vital subject for scientists studying aging and regenerative medicine.

To understand how this works, we must look at a process called transdifferentiation. This is a method where specialized cells change into entirely different types of cells. When the jellyfish faces extreme stress, it begins this transformation. Stressors include physical injury, sickness, old age, or lack of food. The jellyfish first experiences a deterioration of its bell and tentacles. It then enters a cyst-like stage. From there, it transforms into stolons and polyps on the sea floor. This creates a new colony of polyps that are clones of the original.

The life cycle of *T. dohrnii* follows several distinct stages. It begins as a tiny, free-swimming larva called a planula. Once the planula settles on the sea floor, it grows into a colony of polyps. These polyps form branched structures known as hydroids. From these hydroids, tiny medusae bud off and swim freely. These medusae eventually reach sexual maturity and hunt for food. While the medusa stage is the most recognizable, the polyp stage is essential for survival. This cycle allows the species to reset its biological clock indefinitely.

Scientists have spent many years classifying this species. It was named in 1883 to honor Anton Dohrn. He was the founder of the Stazione Zoologica Anton Dohrn in Naples, Italy. For a long time, researchers believed *T. dohrnii* was the same species as *T. nutricula*. A 2006 study corrected this, proving they are separate species. We also once thought *T. rubra* was the same, but it is distinct. Modern genomic analysis has since helped verify these species through DNA sequencing. This history shows how much our understanding of marine life has evolved.

Physical measurements show just how small these creatures are. An adult medusa has a maximum diameter of about 4.5 millimeters. They are roughly as tall as they are wide. Their bodies consist of three layers: the epidermis, the mesoglea, and the gastrodermis. The mesoglea is a jelly-like middle layer. Interestingly, the jellyfish is mostly water, containing only about 5% non-aqueous matter. An adult can have between 80 and 90 tentacles. In contrast, a young specimen may have only eight tentacles.

Despite their "immortality," these jellyfish face many dangers in the wild. They are part of the plankton and are often eaten by predators. Common predators include sea turtles, sharks, tuna, and even penguins. They also face threats from disease and environmental changes. In terms of distribution, they are found in temperate and tropical waters worldwide. They have spread globally through trans-Arctic migrations and ballast water discharge. This spread is sometimes called a "silent invasion" because they are so small. They can survive harsh conditions by regressing into cell clusters.

The study of *T. dohrnii* connects to many important scientific fields. Researchers use its genome to study DNA replication and repair. They are specifically interested in how it maintains telomere length. Telomeres are parts of cells that help prevent aging. By studying transdifferentiation, scientists hope to find ways to renew damaged tissue in humans. This makes the jellyfish a bridge between marine biology and human pharmaceutical research. Understanding how a tiny jellyfish resets its life could change medicine forever.