These Adorable Jellyfish Show Learning Doesn’t Even Require a Brain
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These
Adorable Jellyfish Show Learning Doesn’t Even Require
a Brain
Human
scientists—used to the benefits of a centralized,
complex brain—have been underestimating what a simple
nerve network can do
Tiny, brainless
jellyfish just did
something that on the surface may seem
impossible: the adorable creatures showed
evidence of learning.
Even
with just 1,000 neurons active at a time and
no central brain, Caribbean
box jellyfish (Tripedalia cystophora) can
learn from experience, researchers
argue in a new paper published September 22
in the journal Current Biology. The results
aren’t surprising, say several scientists
not involved in the project, but are a
reminder for people to think more broadly
about learning.
“If
you’re an animal and have to navigate the
world, you have to learn cues and
consequences. Otherwise you’re dead, and you
can’t reproduce,” says Christie Sahley, a
neuroscientist at Purdue University who was
not involved in the new research. “It’s just
a fundamental process, and it doesn’t take a
higher brain.”
Scientists
categorize learning into two types.
Nonassociative learning includes phenomena
such as habituation: if you gently poke an
animal several times, it will eventually
stop recoiling or shying away. Associative
learning is more complex because it requires
an animal to connect cues in its
environment; the classic example is Ivan
Pavlov’s experiment, which showed that
dogs repeatedly fed after hearing a bell
ring will eventually salivate solely at the
sound of the bell.
But
not many experiments have demonstrated
associative learning in simple animals such
as jellyfish, says Ken Cheng, an animal
behaviorist at Macquarie University in
Australia who was not involved in the new
research but wrote a commentary on it for
the same issue of Current Biology. In 2021
Cheng published a review of learning
in Cnidaria—a group that includes
jellyfish, corals, sea anemones, and
more—and found only a handful of studies
that tested for associative learning, all of
which were on sea anemones.
That’s
in part because scientists bring human
assumptions and priorities to the
experiments they design, says Jan Bielecki,
a neurobiologist at Kiel University in
Germany and co-author of the new research.
He sees that as a mistake.
“You
can’t judge a fish by its ability to climb
trees,” Bielecki says. “The parameters that
you use have to make sense to the animal,”
he adds. “You kind of have to meet them
where they are at.”
Bielecki
and his colleagues looked for associative
learning in small jellyfish that
sport four eye structures called rhopalia
that each contain six eyes and about 1,000
neurons, he says. (Each rhopalium takes
turns acting as the jellyfish’s
noncentralized nervous system.) Then the
team designed an experiment that made use of
the animal’s instinct to protect its bell,
the main structure from which its tentacles
sprout. In their native, sometimes cloudy,
waters, these jellyfish must use their
vision to navigate around tree roots.
So
scientists put the jellyfish in tanks that
were painted with three different levels of
contrast: high-contrast black-and-white
vertical stripes that represented nearby
tree roots; medium-contrast gray-and-white
vertical stripes that presented an optical
illusion of tree roots far beyond the tank’s
walls; or solid gray with no contrast. The
jellyfish navigated the black and white
stripes without issue—the contrast was stark
enough that they never actually hit the
tank’s walls. But without the experience of
hitting the tank, they didn’t learn to avoid
it. The jellyfish in the plain gray tanks
also didn’t learn; they bumped into the
walls throughout their time in the tank.
Only
the jellyfish in the gray-and-white striped
tanks learned to associate the décor with
the risk of collisions, Bielecki and his
co-authors found. Early in the 7.5-minute
trial period, these jellyfish bumped into
the tank walls, but by the end of the trial,
they were successfully staying clear of the
wall.
Impressively,
the jellyfish were successfully associating
the stripes with wall after just three to
five bumps. “What was surprising was how
fast they would learn this,” Bielecki says.
Although
it’s a clever experiment, says Catharine Rankin, a
behavioral neuroscientist at the University of
British Columbia who was not involved in the new
research, she’d like to see additional tests to
better understand what precisely the jellyfish are
doing and how
advanced the learning is.
“Show
me extinction. Show me if you present that same
visual cue over and over again, and the animals
never bump into anything—will they stop avoiding
it?” Rankin says. Sahley, who has studied learning
in a host of other simpler species, also notes
that she’d want to test how long the jellyfish can
remember the association between the gray stripes
and the impact risk.
Still,
scientists say the new study provides valuable
information about how learning works across the
diversity of animal life. Simple animals such as
jellyfish can better show the basic processes of
neurons than a human
or mouse brain can—with
hundreds of thousands of times more neurons, their
interactions are harder to unravel.
ABOUT
THE AUTHOR(S)
Meghan Bartels is a science journalist and news reporter for Scientific American who is based in New York City.