The Species That Can Actually De-Age Itself

Aging is a fact of life for most organisms, although there are a few species for which the bell does not toll. The most famous of these is the immortal jellyfish (Turritopsis dohrnii), which in the 1980s researchers discovered could revert to a previous life stage. Now, researchers have witnessed the comb jelly (Mnemiopsis leidyi), a species of tentaculate ctenophores, slowly regressing in age under harmful conditions.

Findings published in the Proceedings of the National Academy of Sciences in October 2024 discuss the reverse development of the comb jelly (a type of zooplankton that lives in the photic zone), a process that the scientists discovered by accident. During laboratory observations, they noticed that an adult specimen was no longer in its tank, but a larva was there instead. That's when researchers Pawel Burkhardt and Joan J. Soto-Angel began trying to replicate the reversion on 65 other specimens.

The experiments involved removing some of the creatures' gelatinous tissues and sources of food. As a result, they reverted from the lobate adult stage to the cydippid larval stage, visually shrinking in size over several weeks. When they were given food again, 13 of the creatures started growing and eventually reproducing again, too. "Witnessing how they slowly transition to a typical cydippid larva as if they were going back in time, was simply fascinating," Soto-Angel said in a press release. "Over several weeks, they not only reshaped their morphological features, but also had a completely different feeding behavior, typical of a cydippid larva."

The reverse development process in the comb jelly, which is also called a sea walnut and lives in various types of saltwater ecosystems, makes it a valuable model for further aging and biological research, especially in terms of what drives the process and what happens to its neurological system. However, it's different from the age regression that immortal jellyfish, an animal that lives longer than tortoises, exhibit under life-threatening conditions.

When an adult (medusa) immortal jellyfish is starved or injured, it reabsorbs its tentacles and transforms into its post-larval, benthic polyp. After sinking, its cells merge, losing their identities and turning into a blob-like cyst that has no resemblance to either a jellyfish polyp or medusa. Within two days, the blob begins to grow polyp-typical characteristics and eventually returns to the usual developmental cycle of immortal jellyfish.

This process is considered transdifferentiation, a rare metamorphosis that makes the animals appear to never die of old age. As Joan J. Soto-Angel explained to Science, "It's not the exact same [creature] which transforms back to the preceding stage." Like with the reverse development in the comb jelly, though, scientists believe that research into the key molecular mechanisms behind the process can be useful for human health.