To be forever young. That has been the elusive dream for people since the dawn of humanity, sparking such myths as the “Fountain of Youth.” Yet for one tiny jellyfish, that dream is merely the way of life.
Completely grown-up, adult jellyfish belonging to the genus Turritopsis possess an amazing ability: They can reverse their life cycle and turn back into their “newborn” form. This genus belongs to a group called the hydrozoans, which includes some small, predatory jellyfish, hydra, and the Portuguese Man o’ War, among others.
While most hydrozoans die after reproducing, Turritopsis has other plans. In fact, they’re the only animals in the world that have been found to be able to reverse their life cycle like this.
To truly appreciate this remarkable feat, it’s important to understand the basic, “normal” life cycle of the Turritopsis jellyfish. Turritopsis’s life cycle is made up of two basic stages: an immature polyp and a sexually mature medusa (also commonly referred to as the jellyfish). When the Turritopsis jellyfish create a fertilized egg, it settles to the ocean floor, in shallow or deep waters. The eggs can hatch within a few days. A single egg turns into a polyp, a cup-shaped, sessile (rooted in place), immature form that has tentacles on its raised mouth to help it catch food, and a large central cavity to then digest its food. Many polyps cluster together to form interconnected polyp colonies.
So how do the polyps turn into the adult medusae? Fascinatingly, individual polyps actually form a bud that breaks off and turns into an adult medusa. This is done asexually, without any of the polyps exchanging genetic material. The medusa looks like the classic jellyfish form we’re so familiar with: solitary, somewhat umbrella-shaped, and adorned with many tentacles.
That’s the normal, tried-and-true life cycle for many jellyfish, but Turritopsis nutricula and a related species, Turritopsis dohrnii, can do this and much more.
T. nutricula is quite small, about only one-fifth of an inch in diameter, with transparent walls and a vibrantly red-colored large stomach. It can have an astonishing number of tentacles, around 80 to 90.
But its appearance isn’t what’s gained it so much attention; it’s what it does. Under certain conditions, T. nutricula can successful transform to its juvenile stage 100 percent of the times tested in laboratories.
What does this mean in terms of the seemingly “bizarre” jellyfish lifestyle? The sexually mature, adult medusa can actually transform into a ball of tissue, like a cyst. It does this by somehow reabsorbing the external parts of its body. The cyst attaches to the ocean floor and, within a few days, depending on the temperature, grows into a colony of immature cylindrical polyps, and the cycle starts all over again; new medusae bud off of the polyps, forming genetically identical jellyfish.
The cysts can even wait for months before turning into polyps, if temperatures are very cool. Consequently, they’re virtually immortal. But this reversion doesn’t happen all the time: It only happens when the jellyfish are starved, physically injured, or under other stresses, and most likely many jellyfish die before undergoing this transformation.
So how exactly does the Turritopsis get rid of its wrinkles and gray hairs? It’s still a big mystery, but current thought suggests that the process of transdifferentiation is involved. Transdifferentiation is when an adult, mature cell turns into a completely different kind of cell. In the Turritopsis nutricula medusa, it’s thought, certain adult tissue layers transform into different, specific tissue layers found in the new cyst. While this is somewhat understood on a general tissue level, on the cellular level there is much that remains to be elucidated. Additionally, there is some debate over whether stem cells are necessary for the process of transdifferentiation, and how it works in Turritopsis is even more unclear.
Transdifferentiation occurs in other animals as well, though usually only in regrowing parts of the animal; Turritopsis is probably the only known case of an entire organism undergoing transdifferentiation (though there are other contenders). A long-standing classic example of transdifferentiation is the salamander: Cut off a leg and it regrows a new one, presumably from the mature cells on the leg stump. However, this was recently challenged by research done by Elly Tanaka (at the Max Planck Institute of Molecular Cell Biology and Genetics and the Center for Regenerative Therapies at the University of Technology, both in Dresden, Germany); it now appears that salamanders use several different kinds of limited stem cells (not mature cells) to regenerate their severed limbs. Clearly this is a rapidly evolving field as our understanding of these amazing abilities grows.
As we learn more about how animals such as Turritopsis can revert all of their cells to a much earlier stage in life, it may help us better understand human diseases and just the basic definition of what it is to be a cell. Transdifferentiation occurs in humans, but mostly in diseases such as cancer. But this is just one of the many hints, including the existence of Turritopsis, that the cells in our adult bodies are not necessarily fixed in their ways. Given the right conditions, they can change their very identities.
And the tiny Turritopsis is greatly profiting from its innate knowledge. Turritopsis have actually been leading a “silent invasion” of the world’s oceans. Research done by Dr. Maria Pia Miglietta (Pennsylvania State University) revealed that, just in recent times, the animals’ numbers have been significantly increasing. They have also been rapidly spreading around the world, most likely from water released from the ballasts of ships in ports. While they may only be one-fifth of an inch in diameter and lack a central nervous system, these jellyfish sure seem to have a lot of things figured out.
For more on transdifferentiation and Turritopsis’s ability to change from an adult to a juvenile, see Discover magazine’s blog post “The Curious Case of the Immortal Jellyfish,” Stefano Piraino’s article on “Reversing the Life Cycle: Medusae Transforming into Polyps…,” Maria Pia Miglietta’s article on “A Silent Invasion,” Shifaan Thowfeequ’s article on “Transdifferentiation in Developmental Biology, Disease, and in Therapy,” Hongbao Ma’s article on “Turritopsis nutricula,” and Wikipedia’s article on “Turritopsis nutricula,”
Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at firstname.lastname@example.org.