Comb jellies, or ctenophores, are unique among animals, having been the first group of animals to branch off from the rest of animals.

This evolutionary uniqueness makes them ideal for studying the evolution of neural development and neural pathways. While ctenophores don’t have what we would think of as a brain, they do have nerve cells that are connected together throughout their bodies in a web-like “neural net.” Understanding these strange nervous systems will give scientists clues about the vast range of possible nervous systems in evolution and perhaps even about how our own brains evolved. Allen Distinguished Investigator Joseph F. Ryan, Ph.D. is part of a team studying these ancient creatures’ unique nervous systems.

Easy, breezy, mucusy comb jellies are a modern-day wonder. At a glance, they might resemble what we call jellyfish, but they’re not the same thing. Jellyfish and comb jellies are part of two different phyla, Cnidaria and Ctenophora, with two very different evolutionary histories.

Comb jellies are a diverse group of species categorized by their distinct physical and genetic differences. Residents of every ocean, they can be found in glowing rainbow hues, peculiar shapes, or sometimes in the mouths of other jellies. Unlike many other animals, climate change is making them destructively bloom in some areas. When competitors slowly disappear from an area or fertilizer run off leads to an abundance of plankton, ctenophores are not shy to establish dominance.

“Mucus always messes everything up. If you’re trying to pipette, it’s going to be a challenge to get the cell you want,” said Ryan, an associate professor at the University of Florida who studies marine invertebrates to better understand general principles of all life forms. “For example, when we need to isolate DNA from them we’ll often take an animal and roll it on a paper towel. The mucus will come off and then you end up with less mucus but you also lose some cells.”

Sliminess aside, jellies are notoriously fragile. When removed from water, they can just fall apart.

“They’re marine animals. If you’re in the middle of the country, and have to make sea water, that can be a challenge if there’s only certain places that have access to running seawater into buildings,” said Ryan. “They’re also pretty delicate. You can’t just kind of put them in a bucket.”

How do jelly cells communicate?

The foundational makeup of mammalian and comb jelly nervous systems show similarities and differences that can teach us something about nervous systems overall. For example, ctenophores cannot sense some neurotransmitters — molecules that send chemical messages between neurons — like serotonin or GABA but they can sense glutamate, which is widely found across all animals.

Message transmission from cell to cell is a hallmark of nervous systems — our neurons primarily connect and pass information through synapses, specialized connections between two neurons that allow the transmission of neurotransmitter molecules. But synapses aren’t the only way cells can communicate. Ryan and his colleagues have been studying a form of cell-to-cell communication known as gap junctions, tiny bridges that form between two cells and can open and close to allow the passage of certain molecules.

Earlier this year, Ryan and his colleagues at the University of Florida published a study about the proteins that form gap junctions, which in invertebrates are known as innexins and in mammals are called connexins. Because comb jellies are so evolutionarily distant from the rest of animals, it was an open question whether their gap junctions are like other animals. Ryan and his colleagues found that like other animals, comb jellies also have innexins — implying that this form of cellular communication is as ancient as all animals. Comb jellies have several different kinds of innexins that appear to have evolved from a single ancestral protein, the researchers found.

Open jelly science

The University of Florida team is making a big splash. On the surface, comb jellies aren’t the most sought-after study partners, but Ryan believes there is a big reward in the future of his team’s work.

The ctenophore research community is small but devoted, Ryan said, with less competition to publish on the same topic as you might find in some more crowded fields of research. “There are so many questions that that need to be asked and answered. And if you work on an animal that’s hard to work on, it’s easier to be open. It’s easier to talk about your science with other people that are working on those animals as well,” he said.

A close-knit community leads to collaboration, shared momentum, and transparency in research. The mystery of the comb jelly’s nervous system is a love letter to the philosophy of open science. Visibility and impact of the lessons learned from the comb jelly will tell a story for all animal life.

The Allen Distinguished Investigator awards are funded by the Paul G. Allen Family Foundation. The Paul G. Allen Frontiers Group, a division of the Allen Institute, recommends funding and supports the administration of the awards.