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by Marlowe Starling
Contributing Writer
September 16, 2026
from
QuantaMagazine Website
The ribbonlike Cestum veneris, or
Venus' girdle,
is one of the
largest ctenophore species
and grows up to
1.5 meters in length.
Comb jellies are helping answer
fundamental questions in biology,
from how
the earliest nervous systems evolved
to how
bioluminescence works...
Around
700 million years ago, a group of
organisms resembling little more than glowing, gelatinous blobs
split off from the rest of the animals, forming possibly the
earliest branching animal lineage.
Nearly 200 species of ctenophores, commonly known
as comb jellies (but unrelated to jellyfish), live today in
environments ranging from the cold depths of the sea to warm coastal
surface waters.
Their magic isn't just in their persistence or
iridescence; it's in their DNA.
Over the past decade, ctenophores have helped
answer long-standing questions about fundamental biology, from how
early nervous systems evolved to the origins of the
mesmerizing phenomenon of bioluminescence.
Having access to closely related species across
such variable environments "lets you ask questions about how certain
things evolved," such as adaptation to high pressure or
light-sensing genes, said
Steven Haddock, a
marine biologist who studies ctenophores at the Monterey Bay
Aquarium Research Institute.
"That's one of the reasons why we work with
ctenophores," said
Pawel Burkhardt,
an evolutionary biologist at the University of Bergen who
studies the origins and evolution of neurons and nervous
systems.
"They're very exciting to work with, and
they're also extremely beautiful organisms."
For more than a century, scientists thought that
sponges, or porifera, were the first to branch off - the sister
group to all other animals. But over the past two decades, evidence
has emerged that ctenophores were earlier.
In 2023 - after years of a "ping-pong game"
between labs debating which group came first, Burkhardt said -
a landmark paper
analyzing chromosome organization found that ctenophores, not
sponges, are the sister group, though this is yet to be fully
settled.
How the phylogenetic
tree
looks under the
Ctenophora-sister hypothesis.
Kristina Armitage/Quanta Magazine
What makes this all the more
surprising is that sponges lack muscles and neurons,
while ctenophores have muscles and exhibit evidence
of a simple nervous system.
"If you think about the
earliest branching animal lineage, you would
expect less complexity," Burkhardt said.
"That changes a lot of the
assumptions [about] how the very first animal
may have looked."
The more researchers investigate
comb jellies, the more complex they appear and the
more we learn about the origins of animal life.
Some species have recently been
observed reversing their development from adult to
larval stages. Others have special types of lipids
that help them
withstand extreme pressure in the deep sea.
They hold clues to the evolution
of more and more complex body shapes.
It's really important to study
organisms that might seem strange or weird because
they can tell us a lot about the physical, chemical,
and biological principles of life, which can then be
applied to ourselves, said
Itay Budin,
a biophysicist who studies cell membranes at the
University of California, San Diego:
"We are as distantly related
to a ctenophore as a ctenophore is to a
jellyfish."
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Comb jellies are ancient marine predators
whose hairlike extensions known as cilia refract light as they
swim, giving them an iridescent shimmer.
They are the largest
animals known to use cilia for movement, and use sticky cells
called colloblasts to catch prey as they move through water.
Marine Biological Laboratory Grass Lab |
Using
volume electron microscopy, Burkhardt's lab identified
17 unique cell types in Mnemiopsis leidyi - 11
of which were previously unknown - in an all-important
structure called the aboral organ (center).
Together,
these cells help ctenophores sense light, pressure, and
gravity, and the aboral organ is tightly integrated with
a continuous network of fused neurons that forms the
nervous system.
"Having more neurons condensed at a
certain place, that's one theory [for] how the very
first brains evolved," Burkhardt said.
The research was
published in March 2026
in Science Advances.
Alexandre Jan, Michael Sars
Centre/University of Bergen |
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Nervous
systems are typically defined as networks of neurons
that communicate across synapses.
But Burkhardt's lab
found something unique in ctenophores:
Beneath the
animal's outer surface is a nerve net (pink mesh in this
3D reconstruction) whose neurons are connected by
continuous cytoplasm - but with no synapses between
them.
"I don't think any other animal has a nervous
system like that," Burkhardt said.
Because ctenophores
are one of the earliest branches of the animal family
tree, the finding indicates that evolution may have
crafted nervous systems twice: in ctenophores, and
separately in jellyfish and all other animals.
Pawel Burkhardt |
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Genome
regulation is fundamental to determining when and
where genes are turned on and off.
But
over long stretches of the genome, this process
becomes more complicated, requiring DNA and proteins
to
physically fold into loops.
Researchers think this was critical to enabling
multicellular organisms to develop specialized cells
without needing to evolve new genes.
"These are key for cell-type specialization and
building complex tissues," said
Arnau Sebé-Pedrós,
an evolutionary biologist who studies genome
regulation at the Center for Genomic Regulation
in Barcelona.
The
ctenophore M. leidyi (pictured) has more
than 4,000 loops in its genome of just 100 million
base pairs (compared to 3 billion in the human
genome), which suggests that looped DNA may have
evolved 150 million years earlier than scientists
thought.
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Many
species of marine life can produce their own light
through bioluminescence.
As one of the most distant
relatives of other glowing multicellular organisms,
ctenophores harbor clues in their genomes that point to
how and why this evolved.
Haddock's team found that a
few non-glowing ctenophore species lack genes that they
suspect help synthesize a light-emitting chemical called coelenterazine.
Subsequent research
identified the full-length gene in bioluminescent
ctenophores.
"It's the most abundant, widespread
light-emitting molecule in the ocean," Haddock said.
It
appears in copepods, shrimp, squid, mollusks, and more.
"It is hard to explain how these things crop up
independently across the tree of life," Haddock said.
"They're maybe co-opting some kind of precursor gene
that they [all] have, and then they're modifying it to
use it for a different purpose."
S. Haddock/MBARI/The Bioluminescence
Web Page |
Lipids are
cone-shaped molecules that make up all cell membranes.
Unlike most lipids, a special type called the plasmalogen only has one oxygen molecule instead of the
usual two. Human brains have them, and so do deep-sea
comb jellies.
They flex in a way that makes it possible
for the ctenophores to withstand extreme hydrostatic
pressure.
When the ctenophore
Bathocyroe aff.
fosteri was brought to the surface and released
from that pressure (left), the plasmalogens expanded
(center), cell membranes split apart (right), and the
comb jelly disintegrated.
"We went into this asking a
very fundamental question about life in the deep sea," Budin said, "but it involved a biomolecule that's also
very important in our bodies [and] in human health."
Fewer plasmalogens in human brains can be associated
with neurogenerative diseases such as dementia, Budin
said.
These lipids may help cell membranes fuse and
break fast enough for neurons to fire.
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Two of the comb jelly species studied under
pressure by Budin and colleagues show the
animals' remarkable variety and complexity.
On
the left is the deep-sea lobate comb jelly
Bathocyroe aff. fosteri.
"Bathocyroe"
means "master of the deep," and the species is
found at depths ranging from 200 to more than
3,000 meters.
The red pigmentation in the animal's stomach is
thought to block light emitted when it engulfs
bioluminescent organisms, so its prey doesn't
give away its location to other predators.
The shallow-water comb jelly Leucothea
pulchra (right) is named after the Greek
goddess of sea-foam.
This species is rarely found deeper than 20
meters. The orange knobs are prehensile, sticky,
and used for prey capture.
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During
embryonic development, cells arrange themselves into
distinct morphologies due to a blastopore:
an indentation
that ultimately becomes the anus or mouth
(central area pointing upward in this image
of M. leidyi).
The same developmental process for embryos in
ctenophores is also found in bilaterians, the
large group of animals that share our bilateral
symmetry.
This indicates that the blastopore organizer -
which turns a mere ball of cells into a complex,
multicellular embryo - is conserved across
species.
"Understanding
where the organizer came from tells us which
parts of our development are ancient and
robust, and which are recent inventions,"
said
Andreas Hejnol,
an evolutionary biologist at Friedrich
Schiller University Jena in Germany.
Human embryos use the same signaling pathways
that Hejnol and his colleagues found in
ctenophores.
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