The Earth's only immortal species is a tiny transparent jellyfish that travels the world in the ballast tanks of cargo ships. It's the only known animal capable of reverting completely to a sexually immature stage after having reached maturity, as Dr Karl Kruszelnicki explains.

Down through the ages, there have always been myths about immortality, that godlike ability to live forever. Well, sometimes myths can have a nugget of truth. Indeed, it was our scientists—more specifically, the marine biologists—who found a creature that comes closest to immortality: a tiny transparent jellyfish.

Jellyfish are special in many ways. For starters, they have neither a brain, nor a heart. They have only a single opening through which food comes in, and waste comes out. So jellyfish eat via their anus.

Jellyfish are also the most efficient swimmers in the oceans. They use less energy to cover a given distance than any other ocean creature. How do they gain that extra efficiency?

Well, first, they push water away from the direction they want to travel in. That motion is more efficient than pushing water to the side, as fish do.

Second, when their umbrella-shaped bell contracts, it creates two vortex rings, or rotating rings of water. The first vortex pushes away from the jellyfish. The second vortex begins to spin, which sucks in water—for free—and gives an extra push to the jellyfish. So, jellyfish can travel an extra 30 per cent of distance for no extra energy.

But so far, there's only one jellyfish with the unofficial name of 'immortal jellyfish'. Its official name is Turritopsis dohrnii (it used to be called Turritopsis nutricula).

Besides being eternal, they seem to be everywhere. Yes, immortal jellyfish are spreading through the oceans of the world. It seems as though they are getting free rides in ships. When in port, after unloading their cargo, ships suck in water to fill their ballast tanks. Ships do this so that they can ride better at sea. This is how they accidentally pick up some immortal jellyfish. This gives the jellyfish more chances to meet with immortal jellyfish of the other sex.

After an adult male immortal jellyfish squirts his sperm out into the ocean waters, some of them end up inside the female. The egg and sperm join, which then creates the fertilised eggs. After a little while, these then turn into tiny free-swimming larvae called planula. After a little more time, the planula give up swimming, dive down to the sea floor, and attach themselves to a rock.

They then change shape entirely, turning into columns of highly branched polyps.

After a few days, another change of shape happens. Tiny jellyfish (about one millimetre across) split off from the tips of the polyp and, like miniature umbrellas with tentacles, float through the ocean. After two to found weeks, they become sexually mature males or females. They're now about five millimetres across, and their bright red stomach is visible through their transparent body. They eat plankton, tiny molluscs, larvae and fish eggs.

They might be 'kind of immortal', but the immortal jellyfish are not impervious to all threats. They can be eaten by bigger creatures, or get killed by being sucked into a vent of a nuclear power plant, so they are not un-killable.

But the 'immortality' steps in when T. dohrnii suffers a attack, or starvation, or some kind of environmental stress. Instead of dying, they change firstly into a tiny blob, and then shift back to the polyp stage within three days. They regroup as a polyp colony sitting on a rock. This new polyp is genetically identical to the original jellyfish, but is packaged differently.

Did that original jellyfish die? Not really.

Did that original jellyfish continue to live in the same body? No. It's kind of like a butterfly that instead of dying changes back to a caterpillar, or an aged chicken turning back into an egg. It's not a blueprint for humans to use so that we could potentially cycle indefinitely between a baby and an aged adult, and then back to baby, and so on—forever.

Technically it's more like 'regeneration', but it's the closest that we have to immortality. Once we learn how T. dohrnii does it, we could apply this knowledge to medical science for humans.

According to