Jellyfish, despite lacking a centralized brain, exhibit advanced learning abilities similar to humans and other complex organisms, challenging traditional beliefs about neural learning processes.
Jellyfish Learning Skills: Challenging Neurological Concepts
Even without a central brain, jellyfish can learn from past experiences like humans, mice and flies, scientists report for the first time in the journal Sept. 22. Current Biology. They trained the Caribbean box jellyfish (Tripedalia cystophora) learning to detect and dodge obstacles. The study challenges previous ideas that enhanced learning requires a centralized brain and sheds light on the evolutionary roots of learning and memory.
Complex vision in a simple organism
No bigger than a fingernail, these seemingly simple jellies have a complex visual system, with 24 eyes embedded in their bell-like bodies. Living in mangrove swamps, this animal uses its vision to navigate through murky water and twist around underwater tree roots to catch prey. Scientists have demonstrated that jellies can acquire the ability to avoid obstacles through associative learning, whereby the organisms form mental associations between sensory stimuli and behaviors.
A Caribbean box jellyfish. Credit: John Bielecki
“Learning is the peak performance of nervous systems,” says first author Jan Pielecki of Kiel University in Germany. To successfully teach a jellyfish a new trick, he says, “it’s best to use its natural behaviors, something the animal understands, so it reaches its full potential.”
A simulated learning environment
In an attempt to recreate the jellyfish’s natural environment, the researchers decorated a circular tank with gray and white stripes, the gray stripes imitating distant mangrove roots. They observed the jellyfish in the tank for 7.5 minutes. Initially, the jelly swam close to these seemingly distant lines and collided frequently. But by the end of the experiment, the jelly had increased its average distance by about 50%, quadrupled the number of successful pivots to avoid a collision, and cut its contact with the wall in half. The findings suggest that jellyfish can learn from experience through visual and mechanical stimuli.
Caribbean box jellyfish live and feed among underwater mangrove roots. Credit: Anders Gram
“If you want to understand complex structures, it’s best to start as simple as you can,” says senior author Anders Karm of the University of Copenhagen, Denmark. “When we look at these relatively simple nervous systems in jellyfish, we have a greater opportunity to understand all the details and understand how it all comes together to perform behaviors.”
Understanding Learning Center
The scientists then sought to identify the process underlying the jellyfish’s associative learning by isolating the animal’s visual sensory centers, known as rhopalia. Each of these structures has six eyes and produces pacemaker signals that govern the jellyfish’s pulsating movement, increasing in frequency as the animal moves away from obstacles.
The researchers showed that the fixed rhombus moved gray rods to mimic the animal’s approach to objects. The system does not respond to light gray bands, interpreting them as distant. However, after the researchers trained the robalium with weak electrical stimulation as the bars approached, it began to generate barrier-pushing signals in response to the light gray bars. These electrical impulses mimic the mechanical impulses of collision. The findings further show that combining visual and mechanical stimuli is essential for associative learning in jellyfish and that the rhopalium functions as a learning hub.
Future directions
Next, the research team plans to dive deeper into the cellular interactions of jellyfish nervous systems to tease out memory formation. They also plan to further understand how the mechanical sensor in the bell works to paint a more complete picture of the animal’s associative learning.
“It’s amazing how fast these animals learn; it’s the same speed that advanced animals do,” says Garm. “It appears that even the simplest nervous system can perform advanced learning, and this may turn out to be the most fundamental cellular mechanism discovered at the dawn of the evolutionary nervous system.”
To learn more about this study, the surprising knowledge of jellyfish is changing our fundamental understanding of the brain.
Reference: “Associative learning in the jellyfish Tripedalia cystophora” 22 Sept. 2023, Current Biology.
DOI: 10.1016/j.cub.2023.08.056
This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), the Danish Research Council (DFF) and the Willam Foundation.
