Scientifically Speaking | The merging marvels of the deep: How sea walnuts fuse to survive

Sea walnuts, a type of comb jelly, display an ability to fuse into a single organism after injury, synchronising their nervous and digestive systems in hours

Sea walnuts, a type of comb jelly found in the waters of the Atlantic, have a remarkable ability that sets them apart from other marine creatures. When injured, two comb jellies can fuse into one, sharing their nervous and digestive systems. Though they resemble jellyfish, comb jellies belong to a different lineage, and this fusion behaviour has never been seen in jellyfish.

Sea walnut (Mnemiopsis leidyi)(Steven G. Johnson/Wikimedia Commons)

While working with these animals at the Marine Biological Laboratory in Woods Hole, Massachusetts, biologist Kei Jokura noticed an unusual specimen in a seawater tank. It wasn’t just the size that was strange. This comb jelly had two mouths and two rear ends, suggesting it wasn’t a single animal, but two fused together.

Intrigued by the possibility that the fusion might have resulted from injury, Jokura and his team set up an experiment to see if they could replicate the event in their lab. On October 7, their findings were published in the journal Current Biology.

The team removed a small part from each of the two sea walnuts and placed them side by side, with their wounded sides touching. Remarkably, in nine out of 10 cases, the jellies fused overnight. When researchers poked one side of the fused organism, the entire body reacted as one, signalling that their nervous systems had fully synchronised.

Other scientists had noticed similar events, but Jokura’s team was the first to document how these fused creatures act like a single organism. In their experiments, the researchers found that after just a few hours, the nervous systems, muscles, and digestive systems of the two jellies became seamlessly connected. The fusion was not just a physical connection, the two animals had become functionally one!

Even more surprising was the speed of this process. Within two hours, the muscle contractions of both jellies were in perfect sync, as if they had always been part of the same body. The digestive systems also merged: when one mouth ate, both jellies shared the meal, and the food was eventually expelled through both anuses, although not always at the same time.

This ability to fuse without rejecting the other body points to a lack of allorecognition. Allorecognition is a biological mechanism found in most animals, including humans, that helps distinguish between their cells and foreign cells. In humans, allorecognition causes our immune systems to reject tissues and organs from others as “foreign” during transplants.

The implications of this discovery are wide-ranging. Given that sea walnuts lack allorecognition, they could provide valuable clues about the early evolution of immune systems and how animals developed the ability to distinguish between their cells and foreign cells.

As Jokura points out, comb jellies might also help scientists understand the origins of nervous systems and regeneration. Their quick fusion of nervous systems could reveal basic principles that could be applied to more complex animals. Studying how two animals can merge their nervous systems might eventually lead to insights into how humans could improve nerve regeneration or organ transplants.

Why do these creatures fuse in the first place? The researchers speculate that it might be a survival strategy, allowing injured jellies to recover by blending with another individual. But whether this fusion provides any real advantage in the wild is still unknown. So far, it has only been observed in the lab. It’s entirely possible that the tank’s cramped, artificial setting encourages these fusions, while in the open ocean, such events may be rare.

One thing is clear, however. These simple animals hold a fascinating key to understanding how life forms regenerate and heal.

Anirban Mahapatra is a scientist and author, most recently of the popular science book, When The Drugs Don’t Work: The Hidden Pandemic That Could End Medicine. The views expressed are personal.