Jellyfish change their behaviour based on past experiences, researchers have revealed, in a study that suggests learning could be a fundamental property of the way nerve cells work.
Unlike humans, jellyfish do not have a central brain. However, box jellyfish have clusters of neurons associated with the creatures’ eye-like structures, known as rhopalia, with this system – known as rhopalia – acting as visual information processing centres.
Now researchers studying the tiny Caribbean box jellyfish say they have found these creatures are able to learn from past experiences in a process called associative learning, just as Pavlov’s dogs learned to salivate at the sound of a bell.
Such abilities, they add, had until now unequivocally been shown only in more complex animals such as vertebrates and molluscs.
The researchers say their results were not a surprise given the ability to learn is crucial for survival. But, as jellyfish belong to one of the earliest animal groups to have evolved, they say the findings suggest such learning could be an integral function of nerve cells.
“You don’t need a highly developed brain to learn; it’s something that’s integral in the nerve cell itself,” said Dr Jan Bielecki, first author of the study at Kiel University.
Writing in the journal Current Biology, Bielecki and colleagues report how they made their discovery by recording the behaviour of Caribbean box jellyfish when they were introduced to a water tank decked out with different patterns.
Twelve jellyfish were placed in a tank decorated with grey and white striped walls, seven were placed in black and white striped tank, and eight were placed in a tank with plain grey walls. The striped walls, the team say, mimic mangrove roots that the jellyfish would try to dodge in their natural habitat to avoid damage to their delicate bodies.
The researchers found that jellyfish placed in the black and white striped tanks never collided with the sides – probably because the stripes represented nearby obstacles – but the jellyfish in the plain grey tanks frequently bumped into the walls.
However, while jellyfish in the grey-striped tank initially collided with its walls, this behaviour decreased over a period of 7.5 minutes, with the animals increasing their distance to the walls by about 50% and halving the number of contacts with the walls.
The team say that suggests while the jellyfish initially perceived the grey stripes as distant obstacles, they soon learned the pattern was associated with an increased risk of collision, with the stripes closer than first perceived.
In one set of experiments, the researchers dissected Caribbean box jellyfish to isolate the rhopalia.
When the team presented these structures with moving grey stripes, they detected no response in the neurons. But after pairing the moving stripes with an electrical zap to mimic a collision, the team found the system subsequently reacted to the moving stripes alone, sending signals that would have resulted in a burst of swimming in the living animal – in other words, signals that would allow the jellyfish to dodge an obstacle.
The team say the findings show the learning centre is in the rhopalia, and that learning is based on a combination of visual and mechanical stimuli.
But Bielecki added they also point to another conclusion: “It supports this suggestion that very few, or maybe even just a single nerve cell, can learn,” he said.
