• The warty comb jelly, Mnemiopsis leidyi, is a fascinating sea creature with a litany of weird biological traits, including regeneration.
• They can also de-age themselves physically into their larval stage after dealing with stress or trauma, according to a study.
• Starvation and physical disfigurement can prompt the jellies’ bodies to shrink and redevelop both physical and dietary traits of the species’ larval stage.

The warty comb jelly, Mnemiopsis leidyi, a fascinatingly weird creature that can regenerate parts of its body, reproduce from a larval stage, and even fuse its body with other comb jellies in order to survive when injured. It also doesn’t really have a visible butthole, only forming an opening to, uh, deposit waste when it really needs to.

It also, according to a 2024 study, can revert from an adult back to its biologically juvenile state after it heals from a traumatic injury or bout of starvation. It’s not the only weird sea dweller that can manipulate its age, as the similarly goopy ocean blob Turritopsis dohrnii (or “immortal jellyfish”) can restart its life from the beginning when it reaches maturity. But instead of the oddity being a sort of default life cycle quirk, M. Ledidyi’s de-aging process is actively used for survival.

Joan J. Soto-Angel, a marine biologist from Norway’s University of Bergan, and his colleague Pawel Burkhard conducted the study—published in the journal “Proceedings of the National Academy of Sciences”—that discovered this truly strange ability. The researchers tracked the morphological changes of M. leidyi over the course of about two weeks after putting specimens through the ringer.

Prior to this experiment, previous studies had observed adult comb jellies reducing their sizes and body masses to cope with a lack of food and nutrients. However, these older studies had ruled out that M. leidyi was actually reversing its age, so to speak.

But one day, the adult specimen of the jelly that Soto-Angel kept in a laboratory tank disappeared, and in its place was a comb jelly in its larval state. Unsurprisingly, this led Soto-Angel to the conclusion that there was more to dig into here, and the experiment then commenced in earnest.

Sixty-five adult warty comb jellies were isolated in tanks without food for 15 days, after which Soto-Angel and Burkhardt fed them a leaner diet once a week. The gelatinous lobes that M. leidyi develop to signify adulthood began to be reabsorb into the specimens’ bodies, and after several weeks, 13 of the 65 specimens had reverted back to both the physical appearance and dietary habits of a typical larva.

“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 from the university.

Soto-Angel and Burkhardt put 15 of those 65 specimens through even more extensive physical trauma for the sake of the experiment, surgically removing those lobes grown in adulthood to add another stressor by which to measure the jellies’ reaction.

Of those 15 (which were still part of the 65 total jellies), six of them reverted their age all the way backwards in just 15 days, compared to the roughly six weeks it took seven of the 50 that didn’t undergo the lobectomy to do the same. Clearly, the worse things get for these sea creatures, the more likely they are to retreat back into their childhood.

“This fascinating finding will open the door for many important discoveries,” Burkhardt said in the release. “It will be interesting to reveal the molecular mechanism driving reverse development, and what happens to the animal's nerve net during this process.”

Though unfortunate for the jellies, this study is a fascinating look at the plasticity of developmental (and un-developmental?) processes of these mysterious creatures. And, as the duo’s paper suggests, it opens the question about what other species in the animal kingdom could have some internal de-aging technology to work with.