Turritopsis dohrnii

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.