The jellyfish that wouldn’t die

“Nothing can be said to be certain,” sighed Benjamin Franklin, “except death and taxes.”  But there is one creature inconvenienced by neither.

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The Epic of Gilgamesh was written in 2100 BC, and in it, Utnapishtim tells Gilgamesh, the King of Uruk, that he will find at the bottom of the sea “a plant that looks like a box-thorn” that will make him young again. So, Gilgamesh ties rocks to his feet to enable him to walk on the seabed, takes a deep breath and wades in. He takes a piece of the “plant” and resolves to test its restorative powers on an old man upon his return to Uruk.

To cut an epic poem short, a serpent steals Gilgamesh’s hard-won sample, so the experiment is never conducted. But 4000 years later, in 1988, a German marine biology student reopened the case. Christian Sommer was studying hydrozoans, small invertebrates which, at certain points in their lives, resemble either a jellyfish or the coral that Utnapishtim might well have compared to box-thorn.

Snorkelling off the cliffs of Portofino in Italy, Sommer collected a species of hydroid. Hydroids are the sessile life stage of certain hydrozoans—a colony, fixed to the substrate, of individuals that bud off polyps. These in turn mature into free-swimming, jellyfish-like medusae. Ordinarily, medusae spawn both sperm and eggs, which, once fertilised, grow into small larvae called planulae. Wherever these planulae settle, a new hydroid colony grows once again.

Back at the lab a few days later, Sommer noted that his captive hydroids had begun to release medusae. Ordinarily, medusae have to mature sexually before they can release planulae, but when he checked the jar the next day, he found it populated with newly settled polyps.

He discovered that his medusae, when stressed, fell to the bottom of the jar, where they proceeded to defy biology. Sommer watched the adults regress back into baby polyps, short-circuiting their own life cycle. “This was,” noted Sommer’s supervisor, Ferdinando Boero, “like a butterfly transforming back into a caterpillar.”

Normal life cycle: Typically, adult medusae release gametes, which become larvae and settle on the seafloor to form a colony of hydroids. The hydroids elongate as they mature, and ‘bud off’ into free-swimming juvenile medusae, which grow into adult jellyfish.
Alternative life cycle: When stressed, Turritopsis fall to the seafloor, regress, and transdifferentiate into a hydrorhiza, a trick no less radical than an adult human turning into an egg and being reborn as a baby.

In the jar was the world’s only known undying creature, a bell-shaped medusa called Turritopsis dohrnii. About the size of your little toenail, Turritopsis can nevertheless bear up to 90 tentacles around a transparent bell. At its centre glows a bright-red stomach. It has since hitchhiked in ships’ ballast water from the Mediterranean to most of the major seas.

When Sommer and Boero presented their findings to a workshop of the Hydrozoan Society, some authorities, such as Volker Schmid, expressed disbelief, so, recalled Boero, “I went diving and found some colonies of Turritopsis with medusae. Under the eyes of Volker, the mildly stressed medusae (a little pinch with a tweezer is enough) first became a ball of tissue, and fell to the bottom of the jar. Then the ball of tissue produced a hydrorhiza (the basal stolon of a new hydroid) and, from it, a new polyp came out. Volker was amazed.”

A 1996 paper, Reversing the Life Cycle, related how Turritopsis “from newly liberated to fully mature individuals, can transform back into colonial hydroids, either directly or through a resting period, thus escaping death and achieving potential immortality”.

Turritopsis was duly dubbed the Benjamin Button jellyfish. But is it truly immortal? The answer depends on how the animal does its trick.

Many creatures can change cells from one type to another. Your pancreas can do it, and newts have shown that they can use other cells to regrow lost eye-lens cells. But first, those cells have to de-differentiate—become generic stem cells—and then re-differentiate into a specific cell type. Turritopsis, however, has a way of changing one type of mature cell directly into another, without having to first revert to an intermediate or progenital state, a phenomenon called transdifferentiation.

It’s the only animal so far known to do this, and it can do it without being prompted by mutilation: Japanese marine biologist Shin Kubota kept a colony undisturbed in his lab between 2009 and 2011, and observed that it rebirthed itself 10 times, sometimes within a month.

It’s impossible to determine an average lifespan for the jellyfish Turritopsis dohrnii, because it does not die of natural causes. By continuously reverting to its childhood, it outlasts the planet’s longest-lived species, which are primarily trees, shellfish, and giant reptiles.

So when Boero squeezed that medusa with the tweezers, did it actually die? Not really. Did it continue to live in its original body? No. It changed its adult cells into juvenile ones, which assembled into its former youth. So if its cells have all been altered, is it still the same individual? Not really. Its genes are the same, of course, and that might imply a kind of perpetuity, but plenty of other organisms clone themselves as well. Technically it’s not immortality, but it’s the nearest example we know of.

What’s more interesting is just how closely related you are to Turritopsis. The notion sounds mad—the thing has no brain, no heart, and its bum doubles as its mouth. But in 2003, the Human Genome Project took us down a few pegs. We thought our genome held around 100,000 protein-coding genes; turns out we have just 21,000, lumping us in with chickens, roundworms and fruit flies. At the same time, mapping has shown that cnidarians—jellyfish, corals, hydras, sea anemones and the like—are more genetically elaborate than we gave them credit for.

Which means we might be able to put Turritopsis trick to use ourselves, coaxing cells to skip the ‘stem cell’ step and transdifferentiate directly. As cells—or people—age, cell processes become unbalanced, and cell death ramps up, which in turn speeds up ageing and leads to a number of neurological diseases.

But because the tiny medusa has found a way to cheat the ageing process, it could help us find a way to treat or prevent such diseases.

Transdifferentiation could also help us fight cancer, if it can show us how to bypass a problem with microRNAs (miRNA). These are short (around two dozen nucleotides) non-coding strands of RNA that control how and when genes get expressed—they’re an on-off switch. When that switch is off, a stem cell waits, unassigned, undifferentiated. When the switch turns on, the cell morphs into its destiny: it might become a skin cell, or a muscle cell, or a skeletal cell.

But studies in mice have found that misfiring miRNA can cause cancers—sometimes, when a cell’s miRNA gets switched off, it loses its identity and becomes dysfunctional, allowing runaway cell division or stifling cell cycle arrest. In short, it becomes cancerous.

Turritopsis might eat out of its own anus, but it doesn’t get cancer. This tiny gelatinous blob holds secrets that may change all our lives.

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