(An artible by Phie Jackson read on science.com on 22nd September, 2023)
Despite
lacking a centralized brain, the translucent creatures can learn from past
experiences to avoid bumping into obstacles.
When
it comes to learned behavior, even the simplest minds are capable of advanced
thought. The Caribbean box jellyfish (Tripedalia cystophora), which doesn’t even have a brain, can alter
its behavior based on past experiences, new research reveals. The creature uses
this learning ability along with its astoundingly complex visual system to
navigate the murky mangrove swamps it calls home, suggests a study
today in Current Biology.
Scientists
have known for some time that animals in the phylum Cnidaria—which includes
jellyfish, corals, and sea anemones—are capable of basic forms of learning when
repeatedly presented with a stimulus in their environment. They will either
respond to it less (what’s known as habituation) or more (known as
sensitization). Humans do this all the time—if you hear a sound over and over
again, your ears might tune it out. In other instances, the sound may become
difficult to ignore.
Habituation
and sensitization are types of so-called nonassociative learning, as they don’t
require connections between two different stimulus types in order to generate a
response. In contrast, associative learning, which involves linking different
types of stimuli together to then modify behavior, is regarded as a more
advanced kind of learning.
Until
recently, scientists believed that only animals with advanced nervous
systems—such as humans and other mammals—could do it. Because a jellyfish’s
nervous system is dispersed throughout its body, with no centralized brainlike
structure, these animals have been viewed as incapable of associative learning.
Even
among other brainless animals, T.
cystophora stands
out. Whereas most Cnidarians can only vaguely detect light sources, this
fingernail-size, translucent creature has a whopping 24 eyes, arranged in
clusters around the body to allow it to perceive the world with impressive
visual detail and navigate around underwater mangrove
roots in the
Caribbean Sea and Central Indo-Pacific region.
Anders
Garm, a marine biologist at the University of Copenhagen, suspected T. cystophora might also be capable of
associative learning. Research from other labs had found some evidence of the
phenomenon in sea anemones, but Garm and his colleagues were skeptical. In one
of these previous studies, researchers subjected anemones to electric shocks
and bright flashes of light.
Over
time, the anemones shrank away from the flashes, but because neither stimulus
occurs in the animals’ natural habitat, it was difficult to know whether the
anemones were truly exhibiting an association between the light and the
electric shock, or they had just become highly sensitized to the stimuli. “It’s
like judging a fish by the ability to climb a tree,” says Jan Bielecki, a
neurobiologist at Kiel University and lead author of the Current Biology study.
For
their own test of the learning potential of Cnidarians, Garm and Bielecki
wanted to harness natural behaviors. T. Cystophora’s sophisticated eyes make it good at avoiding
obstacles with high visual contrast, but their murky habitat often makes
objects difficult to distinguish. “This is where learning comes in,” says
Michael Kingsford, a marine biologist at James Cook University who wasn’t
involved with the study.
The
researchers constructed a tank lined with visual stimuli in the form of gray
and white stripes to simulate the jellyfish’s mangrove habitat, with the gray
stripes mimicking the shapes of distant mangrove roots.
At
first, the jellyfish swam close to the stripes and frequently bumped into the
tank wall. But as time went on, they seemed to associate the gray stripes with
a mechanical stimulus—the pain of collision—and consistently swam away from
them. T. cystophora had connected two different
stimulus types to change its behavior—both hallmarks of associative learning,
the team reports. The jellyfish were also able to change their behavior rather
quickly, after just a few run-ins with the tank wall.
To
better understand the underlying neurobiology, the researchers isolated one of
the jellyfish’s visual centers, known as rhopalia. Each rhopalium contains six
eyes and generates electrical signals that modulate the jellyfish’s iconic
pulse movement. When T.
cystophora
encounters an obstacle, rhopalia send out more signals, causing the animal to
swerve.
The
scientists showed the rhopalium a projected image of a moving gray bar to mimic
a jellyfish approaching a mangrove root. At first, the visual center didn’t
respond, but when the researchers added in a weak electric shock to mimic the
mechanical stimulus of a collision, the rhopalium began to generate the type of
high-frequency electrical signals that promote obstacle dodging. These findings
suggest this structure, which functions as a type of miniature visual brain,
also serves as a learning center.
These
results are especially intriguing, Bielecki says, because they suggest learning
is an “integral part of neuronal function,” and possibly something that evolved
early on and conserved across different animal species. “It gets back to the
evolutionary roots of nervous system function and the acquisition of learning,”
Kingsford adds.
This
would also make sense evolutionarily, the researchers explain, because learning
is vital for survival. If an animal can’t change its behavior based on past
experiences, it’s far more likely to wind up in deadly situations, Bielecki
says. That jellyfish are one of the world’s oldest animals—they’ve been around
for more than 500 million years per some
estimates—is
evidence that they have indeed learned a thing or two along the way.
Etiquetas: Sobre plantas y bichos
