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Thanks to experts of zoology we can also know, that different animals experience “dream” in different ways. And as for fish, it goes on in utterly different way. Though, their perceiving of the environment reduces considerably as well.
Fish do not sink into absolutely unconscious state in which they would wholly stop realizing the world around. Brain activity of the majority of fish remains almost the same, and it means, that they are not able to fall into deep sleep. During its “dream” fish usually drifts without active movement on the tank’s bottom and promptly returns to its usual state under the influence of any external factor.
I have noticed many times, that my tank fish could stay almost motionless for very long time just slightly wagging their caudal fins. But if only somebody approaches the glass, even not touching it and not switching the light on abruptly or something else – they become animated and start swimming all over the tank as if it “was that exact direction it wanted to swim”.
Some fish kinds have special periods in cold seasons, when they “hibernate”, but of course in a way which differs from the customary comprehending of this term. But nevertheless. This “dream” cycle is accompanied by fish inertia, reduction of metabolism processes in its organism and slowing of all physical functions down.
Though it differs from the inertia, which is inherent to animals or humans, of course. During this period fish either hide, or stay temporarily in lower layers of their habitat – on the bottom of a tank or a pond, if this happens in wild nature.
There are some kinds of fish, which “hibernate” in summer, in hot weather period. It helps them to save from dehydration. This is a very interesting phenomenon, because it actually enables fish to survive outside water during drought or extraordinary high temperatures.