In new research, marine biologists trained the Caribbean box jellyfish (Tripedalia cystophora) to learn to spot and dodge obstacles; their findings challenge previous notions that advanced learning requires a centralized brain and sheds light on the evolutionary roots of learning and memory.
The Caribbean box jellyfish (Tripedalia cystophora). Image credit: Jan Bielecki.
Tripedalia cystophora is a small species of box jellyfish in the family Tripedaliidae.
This jellyfish is about 1 cm in diameter and is native to the Caribbean Sea and the Central Indo-Pacific.
Tripedalia cystophora has a complex visual system with 24 eyes. Living in mangrove swamps, the jellyfish uses its vision to steer through murky waters and swerve around underwater tree roots to snare prey.
“Learning is the pinnacle performance for nervous systems,” said Dr. Jan Bielecki, a researcher at Kiel University.
“To successfully teach jellyfish a new trick, it’s best to leverage its natural behaviors, something that makes sense to the animal, so it reaches its full potential.”
Dr. Bielecki and colleagues dressed a round tank with gray and white stripes to simulate Tripedalia cystophora’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, Tripedalia cystophora 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 Tripedalia cystophora 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,” said Dr. Anders Garm, a researcher at the University of Copenhagen.
“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 scientists 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.
“It’s surprising how fast these animals learn; it’s about the same pace as advanced animals are doing,” Dr. Garm said.
“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.”
The study was published in the journal Current Biology.
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Jan Bielecki et al. Associative learning in the box jellyfish Tripedalia cystophora. Current Biology, published online September 22, 2023; doi: 10.1016/j.cub.2023.08.056
