These sea creatures can fuse their bodies

In a Frankenstein-y feat, a small, gelatinous sea creature can merge its body with a neighbor’s. The animals — called sea walnuts, a type of comb jelly — can then sync up their bodily functions, researchers report October 7 in Current Biology.

Biologist Kei Jokura first discovered the mashup when he found an unusually large sea walnut (Mnemiopsis leidyi) floating in a collection tank. Turned out, it was actually two jellies joined together.

Jokura, who did this work while at the Marine Biological Laboratory in Woods Hole, Mass., suspected they had become injured in the collection process and, due to tight quarters, fused their bodies while healing. Comb jellies are known for their ability to regenerate parts of their own body, but the sight of two attached sparked Jokura’s curiosity.

“At first, I was very surprised,” says Jokura, now at the National Institutes of Natural Sciences in Okazaki, Japan. Then he thought, “Let’s try to reproduce this under the microscope.”

Jokura and colleagues took pairs of comb jellies, trimmed off a piece from each of them and pinned them to dissection dishes with their cut ends abutting. In nine out of the 10 pairings, the injury sites fused overnight, creating a continuous stretch of tissue.

They didn’t just look like one organism — they acted like one also. When prodded on one side of the dual jelly, both bodies responded by contracting. That suggests the jellies’ nervous systems might also have fused, the authors say (SN: 4/20/23). And both bodies circulated food between them, despite the team feeding only one mouth of the duo.

Because comb jellies are spread out in the open ocean, fusion probably happens only in a lab setting, says Steven Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. In his own work with comb jellies, he’s seen another species, the sea gooseberry (Pleurobrachia pileus), fuse after collection, but not in a synchronized way. “Nothing like this, where they’re coordinating their reactions,” he says. “It’s pretty remarkable.”

The sea walnuts’ fusion prowess suggests that the animals lack the ability to distinguish between their body and another body, a trait called allorecognition. In humans, allorecognition is what can cause the body to reject organ transplants. Comb jellies are one of the oldest lineages of animals, which means the absence of allorecognition in sea walnuts might hold clues for how the trait evolved (SN: 12/12/13). But to understand that, Jokura says, more research needs to be done on their neurology and the genes responsible.