Science

Sep 24, 2023

September 23, 2023 Jellyfish can sleep – about this a few years ago in Current Biology said employees of the California Institute of Technology who observed jellyfish of the genus Cassiopeia. The sleep of jellyfish is all the more surprising because their nervous system is extremely simple: they have nothing that even remotely resembles a brain, and all they have is a nerve ring and clusters of nerve cells that are associated with the balance organs and not too complex eyes. However, even with such an unsightly nervous system, you can sleep. Moreover, you can learn with it. Learning jellyfish are discussed in an article that was just published in Current Biology researchers from the University of Copenhagen and the University of Keele. They experimented with a small box jellyfish Tripedalia cystophoragrowing to about one centimeter in diameter. T. cystophora swims in the waters of the Caribbean Sea, entering mangrove lagoons, where it looks for small crustaceans for food. The simple eyes that sit on special protrusions on the body help the jellyfish navigate the muddy waters of the mangrove lagoons. Each jellyfish has four of these projections, called rhopalia, and each ropalia has six ocelli and about a thousand neurons. The eyes do not see surrounding objects in all details, but they feel the contrast. The closer the object, the clearer and more contrasting it looks. And here a lot depends on the water: in clear water, only relatively distant objects will be low-contrast, but in muddy water, even what is nearby will appear very blurry. The water in lagoons, depending on the ebb and flow of the tides, the growth of algae, rain, etc., can become more transparent or more cloudy. The researchers were interested in whether jellyfish are able to understand that a fuzzy object may suddenly be very close. In the experiment, twelve jellyfish were placed in an aquarium with striped black and white walls. The stripes could be made sharper and less harsh. When the stripes became less sharp, the jellyfish initially swam too close to the walls of the aquarium and even banged their foreheads against them, so to speak. But literally after a few minutes they realized what was going on and stopped swimming close to the walls, and the number of collisions decreased on average from 1.8 per minute to 0.78 per minute. The researchers also conducted an experiment with individual rhopalia. This sensory organ, along with the neural ganglion, was shown blurred stripes and at the same time an electrical impulse was given through the electrode, which looked as if the jellyfish had collided with an obstacle. And if at first the neurons of the rhopalia did not react in any way to the blurred stripes, then after pseudo-collisions they began to react to them – they generated impulses that should force the jellyfish to change course and avoid the collision. It turns out that both the jellyfish itself and a separate part of its nervous system are quite capable of learning that circumstances have changed, capable of comparing different stimuli and drawing a simple conclusion from them. Even such a nervous system, which is not too complex, to put it mildly, is quite capable of learning. Here we should not forget that the jellyfish has ropalia T. cystophora not one, but four, and maybe all together they allow her to do more complex things.