A reddish-orange stomach, encased in a transparent bell, floats in the middle of the water column. On the edge of the bell, up 80-90 pale white tentacles float in the water, searching for tiny zooplankton which constitutes most of its diet. When these ethereal tentacles find a victim, microscopic stinging cells called nematocysts fire harpoon-like coiled threads that poison and ensnare its victim. The captured plankton is brought to the jellyfish’s mouth at the base of the bell (which also serves as its anus) where it is then passed on to the orange stomach. Only about .2 inches across when fully grown, Turritopsis dohrnii appears to be more or less like any hydrozoan, a family of marine invertebrates.
But this small hydrozoan has one feature unlike any other animal: it is theoretically capable of living forever, giving it the moniker: The Immortal Jellyfish.
To understand Turritopsis dohrnii’s immortality, we must first explore the jellyfish life cycle. The classic image of a jellyfish, a floating bell trailing tentacles, represents only the medusa stage of its lifecycle. When a medusa has reached sexual maturity, it releases sperm or eggs into the water. Male and female sex cells find one another and develop into larvae that settle on the seafloor.
The second stage of the jelly life cycle is called the polyp. The polyp develops from the larvae that settled onto the seafloor, turning into an upright stalk with upward-facing tentacles, resembling a sea anemone. The developed polyps then begin a process called budding: the polyp begins segmenting from the top, turning into a stack of tiny pseudo-medusa. Called ephyrae, these tiny creatures are genetically identical to the polyp which creates them, making this a form of asexual reproduction. Only a millimeter across, ephyrae will eventually develop into a full-fledged medusa, completing the jellyfish reproductive cycle. To learn more about jellyfish and their life cycle, check out Critterfacts profiles on a collection of jellyfish!
This two-staged form of reproduction is called alternation of generations. This reproductive cycle is characterized by two distinct forms. The two forms are physically very different and reproduce in different manners. One form produces sexually and the other asexually. This incredible life cycle is prevalent in plants and algae, but jellyfish are one of the few animals who reproduce in this manner.
Just because it is immortal does not mean it cannot die. Here a jellyfish is eaten by sea turtles, one of their common predators.
So how does this lifecycle lead to immortality in Turritopsis dohrnii? The immortal jellyfish is unique among its family for the ability to revert to an earlier stage of its lifecycle.
When placed under stress: physical damage, temperature change, salinity change, and starvation, Turritopsis dohrnii medusa can rewind the developmental clock and change the direction of its lifecycle. The medusa ages backward, reverting to its polyp stage and re-settling on the ocean floor. After returning to its juvenile stage, the jellyfish is again able to bud, creating more medusa. In practice, a stressed medusa can revert to an early stage of its life cycle and reset its aging, granting Turritopsis dohrnii a character trait unique to the animal kingdom: immortality.
Extensive investigation has been done into aging in humans. Millions are spent researching age-related illnesses like alzheimers, cardiovascular disease, and cancer. Humans have always been fascinated by the idea of immortality. In the Epic of Gilgamesh humanity’s oldest known story of fiction, the hermit Utnapishtim is man granted immortality. Myths like the Philosopher’s Stone and Fountain of Youth show that we have remained intrigued by immortality. Our obsession with aging and death, and extensive biomedical research, have brought us no closer to eternal life. Perhaps Turritopsis dohrnii holds the key to better understanding immortality.
