A pair of comb jellies have fused together into one organism, demonstrating a bizarre new ability

Imagine being horribly maimed and the only way to survive was to merge your damaged flesh with another injured human. Scientists have discovered this startling ability in comb jellies, which can fuse together to share a nervous and digestive system.

Comb jellies, or ctenophores, are marine invertebrates with squishy bodies propelled by hair-like cilia. They're relatively simple creatures, although they do have muscles, nervous systems and sensory organs. Because of their fragile nature, study has been tricky, so there's still plenty we don't know about them.

That's why scientists were surprised to discover their new superpower - and did so by accident. In a tank of comb jellies in the lab, the team one day noticed a particularly big and misshapen individual. But weirdly, it seemed to have two apical organs - a kind of sensory structure - instead of the usual one, as well as two anuses.

The scientists suspected that this one was actually two, that had somehow fused together after being injured. To investigate, they removed part of the lobes from multiple comb jellies, and placed them in close pairs. And sure enough, 90% of the time the injured individuals merged to form one bigger comb jelly within just one night.

The fusion appeared to be seamless, and when the researchers poked one lobe the whole body responded. This seems to suggest that they'd merged all the way down to their nervous system.

"We were astonished to observe that mechanical stimulation applied to one side of the fused ctenophore resulted in a synchronized muscle contraction on the other side," said Kei Jokura, corresponding author of the study.

On closer inspection, the researchers found that the two halves of the Franken-jelly showed spontaneous movements for the first hour or so after fusion. But after that, they started to figure out how to coordinate their movements, so after about two hours 95% of their muscle contractions were in sync.

The team also checked the digestive tracts of these combined comb jellies. They fed one mouth brine shrimp with fluorescent tags, and watched the particles work their way through. Eventually, it became clear that both butts were fully functional, although it seems they took turns.

It's definitely an intriguing ability for the animal kingdom, and may have evolved as a survival strategy to save two injured jellies that might not otherwise make it on their own. But it also could be a by-product of them lacking a feature that appeared farther down an evolutionary branch.

Later life forms have what's called an allorecognition system, which specializes in figuring out which cells are our own and which ones are foreign. This is an important distinction, allowing our immune system to attack invading viruses, bacteria and the like - although it does mean organ transplants often end in rejection.

But comb jellies don't have this ability, so when two of them smush together, they just accept the others' cells as their own. Investigating how they do this could also unlock new insights into regenerative medicine, the team says.

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