Brainless jellyfish are capable of grasping – 10/19/2023 – Science

In the sunny, mangrove-dotted waters of the Caribbean, tiny box-shaped jellyfish float in and out of the shade. These cubomedusae are distinguished from true jellyfish in part by their complex visual system — the grape-sized predators have 24 eyes. But like others, they are brainless, controlling their cube-shaped bodies with a distributed network of neurons.

This network, it is now known, is more sophisticated than previously imagined. In September, researchers published an article in the journal Current Biology indicating that the species Tripedalia cystophora has the ability to learn. Because cubomedusas strayed from our part of the animal kingdom long ago, understanding their cognitive abilities could help scientists track the evolution of learning.

The tricky part of studying learning in cubomedusas was finding an everyday behavior in them that scientists could train in the laboratory.

Anders Garm, a biologist at the University of Copenhagen and author of the new paper, said his team decided to focus on a rapid change of direction that cubomedusas perform when they are about to hit a mangrove root. These roots rise through the water like black towers, while the water around them appears pale. But that contrast can change from day to day as mud clouds the water and makes it harder to know how far away a root is. How do cubomedusas know when they are getting too close?

“The hypothesis was that they need to learn this,” Garm said. “When they go back to these habitats, they have to learn: How is the water quality today? How is the contrast changing today?”

In the lab, the researchers produced images of alternating dark and light stripes, representing mangrove roots and water, and used them to line the inside of buckets about six inches wide. When the stripes were a contrasting black and white, representing optimal water clarity, the cubomedusas never came close to the walls of the bucket. However, with less contrast between the stripes, the cubomedusas immediately began to collide with them. This was the scientists’ chance to see if they would learn.

After a few collisions, the cubomedusas changed their behavior. Less than eight minutes after reaching the bucket, they were swimming at 50% greater distance than the pattern on the walls and almost quadrupled the number of times they performed their U-turn maneuver. It seemed like they had made a connection between the stripes in front of them and the feeling of collision.

Additionally, the researchers removed visual neurons from the cubomedusae and studied them in a dish. The cells were exposed to striped images while receiving a small electrical pulse to represent the collision.

In about five minutes, the cells began sending the signal that would make an entire cubemedusa turn around.

“It’s amazing to see how quickly they learn,” said Jan Bielecki, a postdoctoral researcher at the Institute of Physiology at the University of Kiel in Germany and also an author on the paper.

Researchers who were not involved in the study consider the results a significant advance in understanding the origins of learning. “This is only the third time that associative learning has been convincingly demonstrated in cnidarians,” a group that includes sea anemones, hydras and jellyfish, said Ken Cheng, a professor at Macquarie University in Sydney, Australia, who study these animals. “And this is the coolest demonstration, packed with physiological data.”

The results also suggest that cubomedusae have some level of short-term memory, as they can change their behavior based on past experiences, said Michael Abrams, a postdoctoral researcher at the University of California, Berkeley who studies the neuroscience of jellyfish sleep. . He wonders how long the cubomedusas remember what they learned. If they are taken out of the tank for an hour and then returned, do they have to learn everything again?

In future work, the researchers hope to identify which specific cells control the cubomedusae’s ability to learn from experience. Garm and his colleagues are curious about the molecular changes that occur in these cells as animals incorporate new information into their behavior.

They also wonder whether the ability to learn is universal among nerve cells, regardless of whether they are part of a brain. This may explain its peculiar persistence on the tree of life. “There are organ systems coming and going all the time,” Garm said. “But nervous systems, once they’re there, rarely go away again.”

Perhaps the ability to learn is one of the reasons they are still here.