The Immortal Jellyfish

The familiar concept of birth and death as a one-way finality does not apply to the Turritopsis dohrnii. Dubbed the immortal jellyfish, the Turritopsis dohrnii is a species of jellyfish in the hydrozoa class, characterized by its dominant polyp phase and small medusae form, that returns to its youth in a constant, renewing cycle.

The immortal jellyfish is quite small. Upon reaching maturity, it only extends to a height of about 4.5 millimeters, or 0.18 inches—small enough to fit on a pinky fingernail. Its body is translucent and bell-shaped, and in the center, the stomach is an eye-catching red. There are up to 90 thin, string-like tentacles extending from the muscle ring at the bottom of its bell.

Like all jellyfish, the Turritopsis dohrnii starts out as an egg and undergoes external fertilization, where the female jellyfish releases unfertilized eggs into the water. The eggs form planulae, the short-lived larvae stage of a jellyfish, after coming in contact with sperm released by the male. At this point, they look similar to bacteria: flat, oval-shaped, and propelled by cilia, small hairlike structures that protrude from their edges.

As planulae, they attach themselves to a solid surface and develop into their polyp phase, which is a stationary, cylindrical form that certain marine species undergo. In the first half of this phase, the planulae form a colony of polyps called a hydroid colony. A tubal structure, the hydroid colony stations itself using a stolon (a stem-like structure) and buds, branching out along its length like coral. The branches consist of polyps, which can be either male or female. Through asexual reproduction, the polyps detach from the parent colony and mature into medusae, the final form of the jellyfish that we are most familiar with.

In most species, the juvenile medusae will grow into their adult state, where they can sexually reproduce, release eggs or sperm, and die. For the Turritopsis dohrnii, however, it can revert back to its polyp phase when it is near the end of its lifespan. It does not even have to be in old age as the process can be conducted in response to suboptimal conditions such as starvation, physical damage, and physical threats at any time during its medusae phase. The jellyfish would first transform into a cyst with an exterior of chitin. It would remain in that form for 24 to 36 hours before becoming a polyp once again. This is possible due to a cellular process called transdifferentiation.

Cells can differentiate from stem cells into a wide variety of types with different functions, such as epithelial cells, blood cells, and fat cells. Stem cells are usually restricted in the type of cell into which they can differentiate by the area they are located in. For example, hematopoietic stem cells can only differentiate into cells of the blood and immune system. Transdifferentiation is when a matured, specialized cell switches to another specialization. In some transdifferentiation cases, a cell would need to dedifferentiate into an unspecialized stem cell before redifferentiating into some other specialization. In the Turritopsis dohrnii, the cells of its polyp and medusae forms contain completely different cell types. It is still unknown how the jellyfish are able to return to a previous stage in their life cycle, though it has been noted that a gene in the mRNA may be responsible for this transformation.

Ever since this special characteristic of the Turritopsis dohrnii was discovered, it has been of interest to the media and scientists in the field of regenerative medicine. In transplants, a transdifferentiated cell would be much more stable than a stem cell whose future trajectory is harder to control. It also has a high potential for treating neurodegenerative diseases like Parkinson’s disease. Through the jellyfish’s ability to recycle its cells, scientists can study ways to replace damaged cells and neurons. Today, scientists are continuing their studies on the Turritopsis dohrnii in hopes of unlocking the secret of their eternal youth.