Scientists have revealed that the comb jelly Mnemiopsis leidyi, also known as the sea walnut, can undergo reverse development, effectively “aging backward” to an earlier life stage. The comb jelly naturally does this when faced with unfavorable circumstances. This incredible ability — only the third-known instance in the animal kingdom — could redefine our understanding of animal development and evolutionary biology.

A Surprising Turn in the Life Cycle

The ability to reverse development, often dubbed as “reverse aging,” is an extremely rare biological phenomenon. It was first documented in the jellyfish Turritopsis dohrnii, famously known as the “immortal jellyfish” because it seemingly escapes death due to its ability to roll back its biological clock when injured or on the verge of starvation.

Now, researchers at the University of Bergen have shown that the comb jelly Mnemiopsis leidyi also possesses this remarkable ability. Beyond cnidarians, there is one reported case within the Platyhelminthes phylum, specifically in the parasitic worm Echinococcus granulosus.

However, while the immortal jellyfish achieves rejuvenation by cellular disintegration and redifferentiation, the comb jelly’s process seems to be a direct reversal. The same individual regenerates without breaking down into a different stage.

Unlike most animals that follow a one-way path from youth to maturity, these comb jellies can regress from a mature lobate form back to a cydippid, an earlier stage in their life cycle. This process is triggered by environmental stressors such as prolonged starvation or physical injury. Once the stress subsides and food becomes available, the jellies continue to develop into mature, reproductive adults again.

Implications for Evolutionary Biology

This discovery challenges long-standing assumptions about animal development, particularly among early-diverging animal lineages like ctenophores. Ctenophores, or comb jellies, are among the oldest extant animal groups. Their lineage potentially predates the split between animals with and without backbones, thought to have occurred sometime 480 million years ago.

The findings raise intriguing questions about the molecular and cellular mechanisms driving this reverse development. By studying how the comb jelly accomplishes this feat, researchers may uncover new insights into the mechanisms that govern aging and regeneration across different species. It also raises questions about how many other organisms might possess similar abilities that have yet to be discovered.

Moreover, this ability might explain some of the success of M. leidyi as an invasive species. The capacity to revert to a juvenile form allows these comb jellies to adapt to varying environmental conditions, potentially giving them an edge in survival and colonization of new habitats.

As research progresses, Mnemiopsis leidyi could become a new model organism for studying reverse development, aging, and cellular regeneration. While the idea of reversing aging in humans remains in the realm of science fiction, understanding the biological mechanisms at play in simpler organisms could one day inform medical research in unexpected ways.

The findings were reported in the preprint server bioRxiv.