Comb jellies fuse together when injured, study finds

It might not be what the Spice Girls envisaged when they sang 2 Become 1, but scientists have found comb jellies do actually fuse together if they are injured.

Researchers studying a species of the gelatinous marine invertebrates known as “sea walnuts” said they made the discovery after spotting an unusually shaped individual in the laboratory tank.

“I was very excited,” said Dr Oscar Arenas, co-author of the work, from the University of California, Berkeley.

Writing in the journal Current Biology , the team reports that, among other features, the creature appeared to have two “aboral ends”, or backsides.

In addition, Arenas said, the animal had two mouths – something the team had never seen before.

“This led us to wonder if it was the result of two independent animals fusing,” he said. “That same night, we began trying to replicate this observation.”

The team took pairs of sea walnuts – collected from different sites at different times – and, for each, removed part of the side of the body. Each pair was then pinned together overnight with their injuries touching.

The results revealed that, in nine out of 10 cases, the individuals had fused.

“Once we realised we could consistently reproduce the fusion, we shortened the time, and eventually we found the fusion occurred within a few hours in a petri dish,” Arenas said.

The team notes it is not the first time the grafting of comb jellies – or ctenophores – has been reported, but they say their experiments expand upon such observations.

Among other findings, the team found when they prodded one side of the fused creature both individuals jerked and contracted, a result that suggests the nervous systems of the pair might have merged, they say.

Arenas said the finding was exciting because very little is known about the ctenophore nervous system.

“Moreover, given that ctenophores are now considered descendants of the ancestors of all other animals, studying how their nervous system works is crucial for understanding the basic principles of neuronal function,” he said.

“Beyond that, our observations suggest that ctenophores might serve as an excellent model for investigating evolutionary processes of self-recognition systems and advancing our understanding of tissue grafting and regeneration in many tissues, including the nervous system.”

The idea that the nervous systems had merged was supported by the discovery that, one hour after the comb jellies were paired, their muscle contractions started to synchronise. An experiment involving six fused pairs suggested 95% of contractions within each pair were completely synchronous after two hours.

The researchers found that when they fed fluorescently labelled food to one of the comb jellies, particles passed into the digestive system of the other. The digested waste products, however, were expelled from both anuses in an unsynchronised manner.

Arenas said the study suggested comb jellies had few mechanisms for distinguishing their own tissues from those of others of the same species.

“I am convinced that it provides insight into the molecular mechanism of how single cells recognise themselves when they coalesce to become multicellular animals.”