Experiments Reveal That Jellyfish May Learn Without Brains

By Mark Miller

Researchers in Denmark and Germany have shown that the Caribbean box jellyfish—a grape-sized creature with no brain—may be able to learn to navigate complex root systems in mangrove forests.

The jellies swim through the mangroves to find crustaceans to eat, but they have to avoid bumping into roots and damaging their fragile bodies. According to the article “Brainless Jellyfish Are Capable of Learning, Study Suggests” published in Smithsonian Magazine, Anders Garm, a biologist at the University of Copenhagen, and other researchers wanted to discover how the invertebrates are able to swim safely through the mangrove maze.

Bucket Bumps

The team set up an experiment using buckets of water painted on the inside to simulate the jellyfish habitat. One was painted with clearly contrasted black and white stripes to replicate the mangrove roots in clear water. Another displayed gray and white stripes to mimic how the roots might look in cloudy conditions.

They placed the jellyfish in the buckets and recorded their activity. As anticipated, the jellyfish—likely relying on their 24 eyes—had no difficulty staying clear of the painted roots in the clear water. In the cloudy water, however, they did bump into the sides—at first. But in under eight minutes, the average number of collisions in the cloudy water decreased from 1.8 to 0.78 per minute, as reported in the Science News story “These brainless jellyfish use their eyes and bundles of nerves to learn.”

In another part of the experiment, the team placed the jellyfish in an unpainted bucket. They swam into the side again and again, not altering their movements. “It was only when they had a combination of visual stimulation and mechanical stimulation that they would actually learn something,” said Jan Bielecki, an electrophysiologist at Kiel University, in the journal Nature.

Look in the Eyes

The combination of feeling and seeing seemed to hold the key to the jellyfishes’ cognitive capabilities, but scientists thought that the eyes alone were where the learning might be occurring.

To find out, they performed an isolated test of the four vision centers or rhopalia (each containing six eyes) of a jellyfish. They applied an electrical pulse to these sensory cells to imitate contact with an obstacle. At the same time, they displayed striped images to the cells. In approximately five minutes, the cells associated the pulses with the stripes and sent signals to stay away from the stripes.

These results offer evidence that jellyfish are capable of associative learning. In other words, they can link two discreet stimuli—the electrical pulses and the stripes—and that connection may change their behavior.