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Watching the fish in the aquarium, you might think that they never rest or sleep. In the understanding of man, they are in constant motion.
However, like all representatives of the animal world, in fish, periods of active behavior are replaced by phases of slowing down of physical functions – this is the dream of fish.
Sleep fish is different from our understanding of sleep. Features of the structure and habitat do not allow the fish to fall into a state in which they would be completely disconnected from the surrounding reality.
In this state, most mammals are immersed during sleep. In fish, however, during sleep, the brain activity remains unchanged – they are not able to fall into a state of deep sleep.
Such a feature can cause the question: how do aquarium fishes sleep?
Studying the behavior of aquarium fish, you can see that at certain periods of time the fish are practically without movement, dying in the water. This is the sleeping fish. During sleep, the fish usually drift without active movement.
But the slightest impact of the external factor leads the fish to the active state.
Some fish can hide or be at the bottom of the aquarium. Many species of fish are fixed on the algae during sleep.
There are breeds of fish that fall into a kind of state, similar to hibernation: at this time all the physical processes in the body of the fish slow down, and the fish are inactive.
In the sleep state of the fish, different hemispheres of the brain continue to work. Therefore, despite the slowness of the processes, the fish remains conscious.
At the slightest danger, the fish can become active.
Answering the question whether the fish are sleeping, you need to take into account the difference in the understanding of sleep in fish and other animals. Most of the fish remain active, slightly slowed down, but conscious.
They quickly go out of sleep at the sight of danger or when approaching suitable prey. In fish, there are periods of activity and rest, but the fish are not unconscious, like other animals.
It hinders to see that the fish are sleeping, and that they cannot close their eyes. Fish have no eyelids, so their eyes are always open.
The eyelids are not needed by the fish, as the water itself cleans the surface of the eyes of aquatic creatures.
Each breed has its time to sleep. Some fish (mostly predators) sleep during the day and awake at night.
For example, catfish hiding during the day, and at night they are actively hunting.
During their holidays, many of these cold-blooded species become almost immobile. They, as a rule, have a rest in the bottom area.
This applies to many large river and lake species. But the statement that every fish sleeps at the bottom will not be entirely true. Some oceanic fish species must be constantly in motion, even while sleeping.
These include, for example, sharks and tuna. Water must constantly pass through their gills, otherwise they may suffocate. Therefore, tuna lay down on the water against the stream and, thus, swimming, rest.
And sharks – these contemporaries of dinosaurs – are completely missing the bubble that other species of fish have. This fact is another argument to be in motion, even during rest. After all, otherwise the shark will simply sink to the bottom and, in the end, no matter how funny it sounds, it will sink!
And also – stingrays and sharks do not have lids on the gills, like many bony fish, so the water goes there only when the fish is moving, and you can sleep relatively calmly, as the bony do, they don’t work – you need all the time somewhere to swim.
It is characteristic that some river predatory fish (for example, catfish or burbot) do not sleep at night. They prefer to hunt, and during the day – to rest.
However, among the fish, leading a daytime lifestyle, come across those who like to arrange a siesta in the middle of the day. But dolphins (mammals, which were previously classified as fish) never sleep at all!
The hemispheres – the left and the right – the dolphin’s brain rest alternately (each approximately 6 hours). The rest of the time both hemispheres are awake.
Fish sleep, but not all. In addition, they do not sleep in the literal sense of the word.
The fact is that they can not rest like people, because of the physiological structure of the eye and the lack of a swim bladder in some fish species.
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In order for anyone to fall asleep, they must close their eyelids. The fish have no eyelids that they could close, falling asleep with a sound and healthy sleep. However, this does not prevent them from having a rest, which is called “fishlike”.
Some of them generally lie on the side. In fact, they do not close their eyes.
It is impossible to say that the fish are unconscious, like a person during sleep. Yes, their consciousness is dulled, but not so much. Physical functions are suspended for a while, and the fish itself may not pay any attention to what is happening around.
However, with all this, the brain receptors are in combat readiness, which allows it to instantly get out of its torpor.
Great variety of fish species on Earth. Many of them have certain periods of activity and rest. “Sleeping”, remaining afloat, helps them swim bladder.
It is worth noting that not all fish have this organ, therefore some of them generally have to sleep in motion. These include all benthic and deep-sea fish species. Their “sleep” is a short respite during their continuous movement.
That is why the “dream” such a vacation can be called conditionally.
One of the brightest representatives of bottom fish that do not have a swim bladder are, of course, sharks. In order not to drown during sleep, these predators need to actively swim.
Unlike humans and many terrestrial animals, for which swimming is a rather short time, sharks are forced to involuntarily be in constant motion from the first day of their life to the last!
The lack of a swim bladder does not allow sharks to always remain in a “hanging” state in the depths of the depths, as many other species of fish can do. If a shark even for a moment stops its wavelike movements with muscular fins and an outstanding tail, then the gravity of its own body will inexorably pull it to the bottom!
It is curious that this is why dead sharks never float to the surface, but fall like a stone to the bottom.
As mentioned above, in the traditional understanding of sleep, these fish never sleep. They are in constant motion both day and night. However, some of them can still relax.
Special species of sharks living in coastal waters, swim in small underwater caves, located in the shallow depths of rivers or lakes. There they lie on rocky ledges or at the bottom.
It can be concluded that, in the literal sense of this understanding, not a single fish sleeps, but some lucky people, whom God has awarded with swimming bubbles, still can afford to rest somehow, which cannot be said about bottom and deep-water fish.