Ctenophores, or comb jellies, are strange jelly-like animals that ghost through the sea propelled by tiny hairs called cilia. They're an enigmatic bunch, with origins that stretch back approximately 540 million years, and no one is sure exactly when they diverged from the rest of the tree of life.
Now, researchers have discovered that these alien-like creatures are even weirder than we thought: Their nervous system is like nothing ever seen before. Instead of relying on gaps between nerve cells called synapses for communication, at least part of the ctenophore nervous system is fused.
"We haven't actually seen this in any other animal before," study co-author Maike Kittelmann, a cell and developmental biologist at Oxford Brookes University in the U.K., told Live Science. "It means that there are other ways that neurons can connect to each other."
Nervous system evolution
The discovery raises questions about how all nervous systems evolved and adds fuel to a long-standing debate about how comb jellies are related to the rest of the animal kingdom. Many scientists thought that the nervous system in animals evolved only once, at some point after sponges broke off from the rest of the animal kingdom, as sponges do not have a nervous system. But some scientists think ctenophores diverged from other animals early and evolved their own nervous system separately.
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Comb jellies don't have brains, but have a weblike system of neurons known as the nerve net. It's within this nerve net that researchers found the fused neurons. The strange fused arrangement could hint that these systems evolved independently, Kittlemann said. But it's still an open question.
"We don’t really know for sure," she said.
The new research, published April 20 in the journal Science, looks at ctenophores in an early developmental stage, when they're just a few days old. At this stage, ctenophores can move around freely and even reproduce, but they're not full adults. (Depending on species, ctenophores have life spans between about a month and several years.)
The vast majority of nerve cells in animals communicate via synapses, which are gaps between cells. To "talk," neurons release chemicals called neurotransmitters across these gaps. But the new study found that within the ctenophore nerve net, the cells are fused and their membranes connected so that the path from cell body to cell body is continuous. This structure is called a syncytium.
"There are some other animals which show fused neurons but not to that extreme, where you have a whole nerve net," study co-author Pawel Burkhardt, who studies the evolutionary origin of neurons and synapses at Norway's University of Bergen, told Live Science.
Fused neural networks
The discovery raises a whole bevy of new questions, Burkhardt said, from how this fused network develops to how it functions. The same cells that are fused together also make connections to other nerve cells via synapses, and other parts of the ctenophore nervous system use synapses, too. It's not clear, Burkhardt said, why comb jellies use two different methods of communication between their nerve cells.
One possibility is that the fused nervous system has some advantage for tissue repair and healing, Leslie Babonis, an evolutionary biologist at Cornell University who was not involved in the new study, told Live Science. Ctenophores are capable of regenerating an entirely new animal from a small chunk of flesh.
"Maybe this is one of the secrets to their incredible ability for regeneration," Babonis said.
The research team only looked at one species of ctenophore — Mnemiopsis leidyi — in one developmental stage, so they now plan to find out whether other species have fused neural networks and whether this fusion persists through the animal's whole lifespan.
This could help answer questions about the evolution of the nervous system and whether it arose once, twice or more times. If many ctenophores have unique fused nervous systems, this could lend credence to the hypothesis that ctenophores evolved their nervous system separately from other animals. But it's also possible that all animal nervous systems still share a common origin, and ctenophores evolved the fusion later, the researchers said.
Only a handful of lineages in the animal kingdom have had their nervous systems closely studied, Leonid Moroz, a biologist at the Whitney Laboratory for Marine Biosciences at the University of Florida, told Live Science. Moroz was not involved in the current study but led a 2014 study of ctenophores, which found that the genetic and chemical basis of the ctenophore neural system is quite different from that seen in other animals.
If the nervous system is a poem, Moroz said, ctenophores use a different alphabet from the rest of the animal kingdom to write theirs. He argues that these jellies evolved their nervous system independently, and that other understudied animals may have done the same. Unraveling this diversity could lead to a deeper understanding of how neurological disorders arise.
"We need to understand syntax, we need to understand grammar," Moroz said. "But we cannot do it with only one or few species."
