It all started as a curious observation in a seawater tank. Two comb jellies, gelatinous marine creatures known as Mnemiopsis leidyi, did something no one expected: they fused. Not in the sense of clinging together or cooperating, but by truly merging—becoming one organism.

For Kei Jokura, a researcher at the University of Exeter and the National Institutes of Natural Sciences in Japan, it was as if nature had pulled off a magic trick right under his nose. “Our findings suggest that ctenophores may lack a system for allorecognition, which is the ability to distinguish between self and others,” Jokura said. The implications of this, it turned out, were staggering.

A Strange Discovery and A Surprising Union

Jokura and colleagues had a tank in their lab filled with M. leidyi, translucent, jellyfish-like creatures, typically found floating through the world’s oceans. These animals, known for their dazzling displays of bioluminescence, rarely get much attention. But one day, the scientists noticed something unusual: a larger-than-normal individual, with features that suggested it might have two separate bodies. It wasn’t long before they realized this wasn’t just one comb jelly. It was two jellies that had fused.

Intrigued by the sight, the scientists decided to test whether this phenomenon could be replicated under controlled conditions. They placed two comb jellies originating from different parts of the ocean in close contact and, after a night together, found that the animals had fused into a single organism.

Nine out of ten times, the creatures fused — and survived for weeks in this bizarre chimera-like state. This fusion effect was only observed in injured jellies, indicating that the healing process in these jellies plays a role in their ability to merge. It looks like a survival strategy although it’s not clear what the benefits are.

The two jellies, now physically one, began to share some physiological functions. For instance, when one side of the fused creature was stimulated, the opposite side responded, suggesting that their nervous systems had somehow connected.

The researchers also observed the merged digestive system in action by feeding one side of the fused animal. Food particles, tracked using fluorescent labeling, moved through the shared digestive canals, traveling from one half of the animal to the other.

Although the digestive systems of the two original jellies were now intertwined, some aspects of their bodily functions remained separate. The waste from the digested food, for instance, was expelled from the two different excretory openings at separate times, so not every aspect of their physiology was merged into one.

Lacking Self-Recognition

The fusion of the comb jellies raises fascinating questions about how animals recognize themselves as distinct individuals. Most animals possess some form of allorecognition—systems that allow them to differentiate between “self” and “non-self,” particularly when it comes to tissues. In humans, for example, this ability is what helps the immune system distinguish between the body’s own cells and potentially harmful invaders.

But in M. leidyi, such a system seems to be absent. Without allorecognition, the comb jellies could merge without rejecting each other’s tissues. This is particularly unusual because many related species in the animal kingdom, such as colonial invertebrates like corals and sponges, have evolved complex mechanisms to avoid unwanted tissue fusion with genetically different individuals.

While M. leidyi is not likely to encounter other members of its species in the open ocean long enough to naturally fuse, the researchers believe that the absence of allorecognition in this species might offer insight into how these systems evolved—or didn’t—in early animals.

The Evolutionary Puzzle

Ctenophores, or comb jellies, are among Earth’s most ancient animal lineages. Their evolutionary history stretches back hundreds of millions of years, long before most modern animal groups emerged. Although they lack brains, they have a nervous system that stretches across their translucent bodies like a web.

As one of the simplest creatures to possess a nervous system, they offer a unique window into the evolution of biological systems that allow animals to communicate, move, and respond to their environment.

By studying how comb jellies like M. leidyi can fuse and function as a single organism, scientists hope to understand better how more complex nervous systems, such as those in vertebrates, evolved. If these ancient animals can so quickly integrate parts of their nervous and digestive systems, what might this tell us about the flexibility and adaptability of biological systems in general?

“The allorecognition mechanisms are related to the immune system, and the fusion of nervous systems is closely linked to research on regeneration,” Jokura says. “Unraveling the molecular mechanisms underlying this fusion could advance these crucial research areas.”

The findings were reported in the journal Current Biology.