Two thousand years ago, the first emperor of China became obsessed with acquiring immortality, using his empire’s vast resources toward this endless quest. Unfortunately, Qin Shi Huang died at the age of 49 from ingesting mercury, which he mistakenly believed to be the elixir of life.
No one likes the thought of growing old. Despite our many human endeavours to escape or delay the process of aging, it seems to be an inevitable part of life. But … Why? Why do living things gradually fall apart when they grow older? There is a word for it: senescence. Senescence is the state of the gradual decline of normal functioning. At the cellular level, it means cells stop dividing and eventually die. It can also apply to an entire organism—when a living thing can no longer respond adequately to outside sources of stress—or to specific organs or tissues, like leaves dying and falling from trees in autumn.
While there are ways we can slow down or speed up the rate at which senescence occurs, it is still going to happen one way or another. However, there are a few species that can escape the aging process completely.
The “Immortal” Jellyfish, Turritopsis Dohrnii
To date, there’s only one species that has been called “biologically immortal”: the jellyfish Turritopsis dohrnii. Its body is bell-shaped and transparent, measuring just 0.18 inches tall and 0.18 to 0.4 inches across, making it smaller than a pinky nail. These small, transparent animals hang out in oceans around the world and can turn back time by reverting to an earlier stage of their life cycle.
A new jellyfish life begins with gametes, fertilized eggs, which grow into planula larvae. After a quick swim, the planula larva latches onto a surface (such as a rock, the ocean floor, or a boat’s frame), where it develops into a polyp: a tube-shaped structure with a mouth at one end and a kind of “foot” at the other. It remains stuck in place for some time, growing into a little colony of polyps that share feeding tubes with each other.
Eventually, depending on the jellyfish species, one of these polyps will form a “bud” that sticks out, or it may produce separate segments stacked on top of one another that can then break away from the rest of the colony. This process is responsible for the next stages of the jellyfish life cycle: the ephyra (a small jellyfish) and the medusa, which is the fully-formed adult stage capable of sexual reproduction.
For most other jellyfish, this stage is the end of the line. But Turritopsis dohrnii has a neat party trick: when it faces some kind of environmental stress, like starvation or injury, it can revert back to being a tiny ball of cells, which then changes back into a sexually immature polyp. It is a bit like a butterfly turning back into a caterpillar, or a frog becoming a tadpole again. This phenomenon is called transdifferentiation, the process of converting one cell type to another.
Where are immortal jellyfish found?
Immortal jellyfish are thought to have originated in the Mediterranean Sea; however, they are now found in oceans all around the world. It is thought this recently noticed invasion may have been predominantly caused by humans.
A prevailing theory is that ships are responsible for spreading the creatures throughout Earth’s oceans. Ballast water is pumped in and out of vessels like cargo and cruise ships to maintain stability. It is highly possible that immortal jellyfish get drawn in with this water and are able to survive ocean crossings thanks to their ability to reverse their life cycle when they experience stresses, such as a lack of food.
The inconspicuous nature of immortal jellyfish aids their global spread. These tiny, transparent creatures vary in appearance by location; Turritopsis dohrnii in hot regions like Panama have around eight tentacles, compared to 24 or more in milder waters like the Mediterranean and Japan. The reasons for these variations remain unclear. Their adaptability allows them to thrive in diverse environments and survive under stressful conditions such as food scarcity, contributing to their successful colonization of oceans worldwide.