Jellyfish, with no central brain, shown to learn from past experience
Cell Press
They trained
Caribbean box jellyfish (Tripedalia cystophora) to learn to spot and
dodge obstacles. The study challenges previous notions that advanced learning
requires a centralized brain and sheds light on the evolutionary roots of
learning and memory.
No bigger than a fingernail, these seemingly simple jellies have a complex visual system with 24 eyes embedded in their bell-like body.
Living in mangrove swamps, the animal uses its vision to steer through murky waters and swerve around underwater tree roots to snare prey.
Scientists
demonstrated that the jellies could acquire the ability to avoid obstacles
through associative learning, a process through which organisms form mental connections
between sensory stimulations and behaviors.
"Learning is the pinnacle performance for nervous
systems," says first author Jan Bielecki of Kiel University, Germany. To
successfully teach jellyfish a new trick, he says "it's best to leverage
its natural behaviors, something that makes sense to the animal, so it reaches
its full potential."
The team dressed a round tank with gray and white stripes to simulate the jellyfish's natural habitat, with gray stripes mimicking mangrove roots that would appear distant. They observed the jellyfish in the tank for 7.5 minutes. Initially, the jelly swam close to these seemingly far stripes and bumped into them frequently.
But by the end of the experiment, the
jelly increased its average distance to the wall by about 50%, quadrupled the
number of successful pivots to avoid collision and cut its contact with the
wall by half. The findings suggest that jellyfish can learn from experience
through visual and mechanical stimuli.
"If you want to understand complex structures, it's
always good to start as simple as you can," says senior author Anders Garm
of the University of Copenhagen, Denmark. "Looking at these relatively
simple nervous systems in jellyfish, we have a much higher chance of
understanding all the details and how it comes together to perform
behaviors."
The researchers then sought to identify the underlying
process of jellyfish's associative learning by isolating the animal's visual
sensory centers called rhopalia. Each of these structures houses six eyes and
generates pacemaker signals that govern the jellyfish's pulsing motion, which
spikes in frequency when the animal swerves from obstacles.
The team showed the stationary rhopalium moving gray bars to mimic the animal's approach to objects. The structure did not respond to light gray bars, interpreting them as distant.
However, after the researchers trained the rhopalium with weak electric stimulation when the bars approach, it started generating obstacle-dodging signals in response to the light gray bars.
These electric stimulations mimicked the mechanical stimuli of a collision. The
findings further showed that combining visual and mechanical stimuli is
required for associative learning in jellyfish and that the rhopalium serves as
a learning center.
Next, the team plans to dive deeper into the cellular
interactions of jellyfish nervous systems to tease apart memory formation. They
also plan to further understand how the mechanical sensor in the bell works to
paint a complete picture of the animal's associative learning.
"It's surprising how fast these animals learn; it's about the same pace as advanced animals are doing," says Garm. "Even the simplest nervous system seems to be able to do advanced learning, and this might turn out to be an extremely fundamental cellular mechanism invented at the dawn of the evolution nervous system."
