Caribbean box jellyfish can learn to dodge obstacles even though they lack a central brain, according to new research that challenges our understanding of how learning works. These tiny jellyfish, no bigger than a fingernail, live in mangrove swamps where they must navigate around underwater tree roots to catch prey.
Scientists studied how these jellyfish learn through a process called associative learning, where animals form mental connections between what they sense and how they behave. The researchers placed jellyfish in a round tank decorated with gray and white stripes that looked like their natural habitat. The gray stripes mimicked distant mangrove roots. At first, the jellyfish swam into the stripes frequently, thinking they were far away. After just 7.5 minutes, the jellyfish learned to stay farther from the walls, successfully turned away four times more often, and bumped into walls half as much.
To understand how this learning happens, researchers examined structures called rhopalia, which are the jellyfish's visual sensory centers. Each jellyfish has several rhopalia containing eyes that control its swimming movements. When scientists showed isolated rhopalia gray bars and paired them with weak electric shocks that mimicked bumping into something, the rhopalia learned to respond to the bars alone. This proved that jellyfish combine visual and physical sensations to learn, with the rhopalia acting as learning centers.
The findings suggest that even the simplest nervous systems can perform advanced learning, which may be a fundamental ability that evolved very early in animal history.