Massive jellies’ odd shape stumps researchers. Then they discover two rear ends

For a creature that doesn’t have a brain , how do jellyfish-like animals know what is “self” and what isn’t?

The idea of “self” is far from a philosophical phenomenon, and species across the animal kingdom use their nervous systems to understand the world around them and their own needs .

For sea walnuts, this is a bit harder.

Mnemiopsis leidyi, also known as sea walnuts , comb jellies or ctenophores, are a type of animal similar to jellyfish that eat plankton and have translucent, bell-shaped bodies , according to a case published Oct. 7 in the peer-reviewed journal Current Biology.

Researchers were studying the Atlantic animals when they collected a few of them and held them in a saltwater tank, according to the case.

The next day, they “noticed an atypically large individual with two aboral ends and two apical organs,” researchers said.

Aboral ends, or rear ends, are the tops of the jellies opposite the mouths, and apical organs are sensory organs, and ctenophores only have one of each.

“We hypothesized that this unique specimen arose from the fusion of two damaged individuals that were in close contact in the holding tank after being collected the prior day,” researchers said.

To test this theory, the team excised, or cut, one side of the lobes of two jellyfish and then placed them together in a tank with their cut sides facing one another, according to the case. They set up multiple tanks, then left them all overnight.

When they came back in the morning, most of the individual ctenophores had fused together.

“Nine out of ten (90%) independent grafting experiments were successful, and all fused ctenophores survived for the full holding time of about three weeks in the holding tank,” researchers said.

Time-lapse videos of the animals showed their lobes contracting as two became one.

“We conducted time-lapse imaging at one-second intervals, starting immediately after grafting. During the first hour, the lobes of the two ctenophores showed spontaneous movements consisting of relaxation and contraction. Movements of the grafted lobes were independent and asynchronous,” researchers said. “Surprisingly, after one hour, the timing of the contraction of the two lobes started to synchronize. After two hours, 95% of the lobe contractions were completely synchronous.”

When the researchers looked at the fused jellies’ internal structure, they noticed that not only did the two animals share an outer body, but their nervous systems and digestive systems combined as well, creating one fully functioning animal, according to the case.

The animals were fed small crustaceans that were filled with fluorescent material, researchers said, and they were able to watch the material move from one side of the combined body to the other and get digested, eventually being excreted from both rear ends.

This showed that the bodies were not only physically connected, but their internal processes were working together as well, according to the case.

The ability for animals to understand what is part of themselves and what isn’t is called allorecognition, researchers said, and based on this case “we conclude that M. leidyi lacks an allorecognition mechanism, which may be a feature that arose later in animal evolution.”

Because the animals spend most of their lives completely alone, there is rarely the opportunity to use allorecognition, so sea walnuts may have never evolved to have that skill, according to the case.

“The mechanisms by which organisms recognize ‘self’ from the ‘non-self’ remain poorly understood,” researchers said.

The international research team includes Kei Jokura, Tommi Anttonen, Mariana Rodriguez-Santiago and Oscar M. Arenas.

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