Caribbean box jellyfish have demonstrated an unexpected ability to learn from experience, overturning previous mistakes that learning was limited to more advanced creatures.
The creation and storage of memories of experiences, as well as the alteration of our behavior based on these memories, is a fundamental function of the nervous system. This type of associative learning, known as operant conditioning, has been established in flies, mice, and humans. Having the ability to learn from past experiences and act in different ways can be particularly important for survival purposes.
Recently, researchers have discovered that box jellyfish also have the ability to learn from past negative experiences and change their behavior accordingly. This is the first time that this ability has been demonstrated in these creatures.
“The study is the first to convincingly demonstrate operant conditioning in the form of avoidance learning in any animal in the phylum Cnidaria — jellyfish, box jellyfish, sea anemones, corals, ]and] hydras,” Ken Cheng, a researcher at Macquarie University, Australia, who was not involved in the study, wrote in an email.
Cnidarians, a group of animals that includes jellyfish, are among the first creatures to have a primitive nervous system. This suggests that associative learning is common in animals with any type of nervous system. A type of box jellyfish, called Tripedalia cystophora, lives in sunlit surface waters in the Caribbean. It hunts for tiny crustaceans that live among the underwater roots of mangroves, but risks injury if it bumps into them.
The jellyfish has a visual system consisting of four neuron clusters called rhopalia, each with about a thousand neurons and six eyes. The rhopalial nervous system, in addition to assisting with visual cues, serves as a swim pacemaker and has other functions. Research has shown that when the box jellyfish visually senses the underwater roots of mangroves, its swim pacemaker quickly guides it away from the barrier in an avoidance behavior. The jellyfish uses visual contrast between the prop roots and water to determine its distance from the obstacle.
However, the changing water conditions and the presence of algal matter can affect the jellyfish’s ability to accurately assess contrast and distance, making it difficult to avoid collisions while foraging.
To see if jellyfish can learn from their past encounters with obstacles, Jan Bielecki and colleagues at Kiel University in Germany conducted a study. The researchers let the jellyfish decide how to conduct the tests and used stimuli that were familiar to the animals in their natural habitat. The study used the jellyfish’s obstacle avoidance behavior and the observation that the animals have a regulatory mechanism in place to protect their fragile bodies.
The researchers placed fingernail-sized jellyfish in an opaque round tank with a diameter of 16 cm and subjected them to one of three visual scenes: a plastic cylinder with alternating vertical black and white stripes, gray and white stripes, or a completely gray space. The black and gray stripes were meant to imitate prop roots.
When faced with gray roots, the jellyfish initially swam along the wall of the cylinder, but they learned to stay away from the walls over 7.5 minutes. The jellyfish increased their distance from the wall by 50% and halved the number of times they bumped into the barriers. The gray stripes and wall bumps acted as visual and mechanical stimuli, respectively, and both types of stimuli were necessary for associative learning.
On the other hand, the jellyfish almost entirely stayed away from the wall with black stripes, leading to barely any collisions and an absence of mechanical stimulation. In this scenario, the jellyfish did not increase the number of times they carried out an avoidance maneuver, but they slightly increased their distance from the wall.
When faced with a uniform gray wall, the jellyfish kept making contact, but with no visual stimuli available, no learning occurred, and the bumping continued through the trial.
To find the learning center of these jellyfish, Bielecki’s team isolated the rhopalia. The researchers placed an isolated rhopalium in front of a projecting screen to provide a visual stimulus to the lens, while an electric pulse acted as the mechanical stimulus. The swim pacemaker’s signal frequency, responsible for faster swimming contractions away from an obstacle, was used as a proxy for avoidance behavior. The swim pacemaker grew more responsive when exposed to both visual and mechanical stimuli. When the two stimuli were uncoupled, however, the pacemaker was no more responsive than normal. The researchers were surprised by the speed of learning in the jellyfish, as five training sessions were enough to achieve learning. This confirmed that the rhopalia was the learning center of the jellyfish.
“Box jellyfish exhibits associative learning with a very sparse nervous system,” said Bielecki. Even a primitive and decentralized group of neurons can foster learning, suggesting that “learning was an integrated part of neurons from the beginning of nervous system evolution.” In the future, the research team plans to understand learning at the cellular level in jellyfish.
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References:
1. Jan Bielecki, et al., Associative learning in the box jellyfish Tripedalia cystophora, Current Biology (2023). DOI: 10.1016/j.cub.2023.08.056
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