When two sea aliens become one

Comb jellies, the delicate bells that pulse their iridescent bodies through the ocean, are some of the strangest creatures on Earth. “They are the aliens of the sea,” said Leonid Moroz, a neuroscientist at the Whitney Laboratory for Marine Bioscience in St. Augustine, Florida.

The aliens belong to the oldest branch of the animal family tree. They split from the ancestors of all other living animals about 700 million years ago and have traveled down their own odd evolutionary path ever since. Studies by Moroz and others suggest that comb jellies evolved their own nervous system, as well as their own muscles and digestive tract – complete with two anuses.

But a study published Monday makes clear that scientists have barely begun to understand the creatures’ bizarre biology. Researchers found that a pair of unrelated comb jellies can spontaneously fuse together into a single body. This surprising ability is not only raising more questions about these ancient animals, but also giving clues about the evolution of our own immune system.

“It opens a Pandora’s box – a good Pandora’s box,” said Moroz, who was not involved in the research.

The box was accidentally opened by Kei Jokura, a biologist who studies comb jellies, also known as ctenophores. Last summer, Jokura traveled to the Marine Biological Laboratory in Woods Hole, Massachusetts, to study how comb jellies use light to navigate.

Each day he strolled down to the seaside and netted golf-ball-size comb jellies belonging to a species known as Mnemiopsis leidyi, or sea walnuts.

One night, when Jokura was inspecting his latest haul, he noticed that one of the comb jellies was twice as big as the others. Then he noticed it had two mouths.

Jokura scooped the animal out of the tank with a beaker to show it off. “All of a sudden, late at night, Kei is saying, ‘There’s a weird ctenophore here that looks like two fused ones!’ ” recalled Mariana Rodriguez-Santiago, a postdoctoral researcher from Colorado State University who was spending the summer in Woods Hole studying frogs.

Rodriguez-Santiago set her frog research aside to help Jokura and two other scientists investigate the comb jelly monster. A close inspection confirmed it was actually two animals, but they seemed to behave as one. When the researchers poked one of the comb jellies, it contracted its muscles to escape – and so did its partner.

The researchers wondered if they could create more fused comb jellies. They cut off pieces of tissue from two animals and then held them in place next to each other overnight. By the next morning, the two comb jellies had fused together. The researchers carried out this Frankenstein-like experiment nine more times. In all but one of the trials, they succeeded in fusing the animals.

It took just two hours for the animals to fuse and behave like a single creature. When Jokura and his colleagues put food in the mouth of one comb jelly, the food particles eventually ended up in the gut of the other one.

Comb jellies can merge thanks to their extraordinary ability to recover from wounds, said Casey Dunn, a biologist at Yale University who was not involved in the research. When comb jellies are cut in half, the animals heal in two hours and regrow their missing halves in two days. “It doesn’t surprise me at all that they could fuse,” Dunn said.

Jokura, who now works at the National Institute for Basic Biology in Okazaki, Japan, said it was impossible to know how often comb jellies fused together in the wild. Putting two injured comb jellies in the same crowded tank may have created the right, rare conditions for them to join together.

Even if this turns out to be true, Dunn said, the finding is still significant. “The fact that it’s unlikely to occur in the wild does not diminish its importance,” he said. That’s because other animals don’t easily fuse. In fact, animals will typically attack foreign tissue as soon as their immune systems sense it.

This deadly response – known as allorecognition – is the reason patients have to take immune-suppressing drugs in order to receive organs from other people. The fact that allorecognition is so common in the animal kingdom suggests that it evolved hundreds of millions of years ago as some kind of defense mechanism.

Jokura’s new study suggests that comb jellies have no allorecognition. It’s possible that this defense evolved in animals only after comb jellies branched off on their own 700 million years ago.