Cognition: Jellyfish learn without a brain
Despite their rudimentary nervous system, cnidarians are apparently capable of associative learning. They do not need a brain for this.
Although they have no central brain, jellyfish are able to learn from experiences. This is surprising, as they are among the animals with the simplest nervous systems known. A team led by Jan Bielecki from Kiel University reports on this in the scientific journal "Current Biology".
The research group has successfully trained mangrove cube jellyfish (Tripedalia cystophora) to recognise and avoid obstacles. The animals are no bigger than a fingernail and typically live in shallow, coastal mangrove swamps. There, they use their light-sensing organs to hunt for prey such as copepods in the murky water. In doing so, they have to constantly move between the roots of the mangrove trees - preferably without bumping into them.
Bielecki and his team placed the jellyfish in a round tank with grey and white coloured stripes on the inside wall. The grey stripes imitated plant roots, the white ones the gaps between them. If the grey had a light shade with a low contrast to white, the supposed roots appeared to be far away. The animals then often came very close to the stripes and collided with them. After a few minutes in the tank, however, they began to keep a much greater distance and only collided with the inner wall of the tank half as often.
Adapted behaviour
The experts concluded that the jellyfish had learnt to avoid obstacles, even if they appeared to be far away. The simultaneous exposure to visual stimuli (the sight of the stripes) and mechanical stimuli (the collisions with the inner wall of the tank) triggered a process of associative learning in the animals. This led them to change their swimming behaviour and avoid further collisions.
In order to find out more about how this works, the team isolated the animals' sensory organs. Jellyfish each have several sensory bulbs, so-called rhopalia, which are located on the edge of their umbrella. These are club-shaped protrusions of the body that contain organs for sensing gravity as well as eye-like light sensors. The rhopalia are connected to each other by nerves and generate electrical signals that control the animal's locomotion.
Bielecki & Co. presented individual sensory bulbs with grey coloured stripes that moved - as if the animal was swimming towards them. If the stripes were a light shade of grey, the rhopalia did not react to them: the supposed obstacles still appeared to be far away. However, when the research team administered additional electrical impulses, such as those produced when a jellyfish hits an obstacle, this changed. The sensory pistons then produced signals that caused the animal to take evasive action. According to this, the rhopalias play a central role when jellyfish process experiences and adapt their behaviour accordingly.
"It's amazing how quickly these animals learn," said Anders Garm, one of the researchers involved, commenting on the new findings in a press release. "Even the simplest nervous system appears to be capable of advanced learning, and this may prove to be a fundamental cellular mechanism that was 'invented' early in the evolution of the nervous system." Jellyfish are among the oldest animals in the world; they existed as long as 500 million years ago.
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