How do fish sleep in an aquarium?

Having an aquarium with fish at home, people sometimes admire these cute creatures for hours, but rarely wonder how they sleep, and whether they sleep at all. Perhaps many aquarium fish owners believe - and they are right - that fish know how to sleep. But when and how this happens with them, probably very few people know.

Let's take a closer look at this rather interesting topic in more detail, so as not to look like such adult "dunno" when some curious kid, having come to our house and having seen enough of the inhabitants of the home aquarium, suddenly inquires about where the goldfish sleeps here. The main thing is that we can answer truthfully, and not come up with different fables on the go.

Any living organism needs at least a short-term periodic rest, without which it is impossible to do without damage to health for a long time. Terrestrial living things - people, animals, birds, even reptiles and mollusks - sleep practically according to the same principle: the eyes are closed (or half-closed) for centuries, the vital processes of the body slow down, the muscles relax, the consciousness becomes dull (sometimes it turns off completely).

Only the postures taken in a dream differ, as well as the degree of adequacy of the sense organs in individual representatives of terrestrial living beings. A person is accustomed to lying down to sleep, although, if necessary, he can fall asleep in other positions of his body: sitting and even standing in special - extreme - cases.

Sleep in fish has its own characteristics, different from our usual understanding of this useful and vital phenomenon. In other words, a sleeping fish is not an unconscious individual, as could be characterized by sleeping animals or humans, since her brain activity remains, according to scientific research, practically at the same level.

Any change in an external factor that affects, at least indirectly, a sleeping fish, immediately brings it to its normal state. Deep sleep is a physiological state absolutely unknown to them.

The maximum that fish can afford during their rest is this is a slight weakening of the perception of the surrounding reality, while this environment does not touch it in any way, as well as an almost complete inaction... At the same time, they see and hear everything, ready at any moment to rush into the attack or, conversely, to hide from the predator. It probably looks like a person waiting for a train at the station, who cannot sleep for fear of missing the departure, and everything that happens around is tired of the long hours of waiting.

We know that we need to close our eyes to sleep, as it is unlikely that we will be able to fall asleep with our eyes open. But we also understand that closed eyes are not proof that a person or an animal is really sleeping, although this is most often the case. In any case, a dream can be assumed. In addition to closed eyes, there are other circumstances by which a sleeping person or animal can be identified, for example, by breathing, posture, sounds made, and so on.

But how to identify a sleeping fish, know only professionals and those few amateurs who can watch life for a long time in the underwater kingdom, surrounded by the glass walls of a personal aquarium. Fish, except sharks, have no eyelids - they were reborn into transparent intergrown plates covering the eyes. Thanks to them, fish see better in the water column due to the refraction of light on the surface of these plates. But the fact remains - the eyes of the fish do not close, and therefore it is impossible to determine by them whether the fish is sleeping or not. But there are other signs that will be discussed now.

So, let's list the differences in behavior that confirm that the fish is sleeping:

• lies on its side for a long time in some secluded place (in thickets, at the bottom, under a snag or other element of aquarium decor);
• hovered without movement in the middle or bottom layer of the aquarium water;
• drifts with the flow without being distracted by anything.

Someone probably has their own ideas about the problem described, but the main signs are still named. It remains to add that aquarium fish mostly sleep at night - too many annoying factors during the day in the face of universal attention from households. Only maybe, predatory fish, not in the power of which to resist nature, stay awake at night, trapping possible prey.

Now we will reveal all the hidden places where the aquarium animals go when the overwhelming feelings of fatigue and satiety require immediate rest. Each species of fish has its own characteristics and habits, laid down by nature and transmitted through genes to descendants from generation to generation. So, the specificity of sleep depends on what kind of information was transmitted from ancient times to each particular fish from its progenitors.

These are the places that, perhaps, for more than one hundred thousand years, have served faithfully as a reliable overnight stay for various species of the class of fish.

• There are fish that bury themselves in sand or silt to rest. It is very difficult to find them in this way.For example, the ocellated macrognathus can bury itself in the sand in a couple of seconds. In nature, the flounder also buries itself in the sand to take a nap.
• Often, fish, which have no one to be afraid of, sleep right at the bottom, without hiding anywhere. Such a fish, for example, is a catfish. And since he is by nature a predator, he naturally sleeps during the day. In the wild, in the same way - lying on the bottom - cod sleeps, but not in plain sight, but hiding behind stones or other objects. Astronotuses are also lovers of nap at the bottom, if the other option is to hang upside down in a dream - for some reason he does not like it this time.
• There are many species of fish that need to hide somewhere for sleep, for example, in an underwater cave, in dense thickets of aquatic plants, among stones or corals.
• Separately, it must be said about this, perhaps not quite usual way of sleeping, like wrapping oneself in a cocoon of secreted mucus. This is how a tropical fish called a parrot sleeps. This mucus protects it from predators who cannot detect it by smell - the cocoon interferes.

The latter method is also used by some other fish when they leave, for example, in hibernation.... A small school of fish, finding a quiet place somewhere in the bottom depression, gather in this pit and begin to secrete mucus, which envelops the entire group. Having thus arranged a cozy corner, they fall asleep for a long time, periodically moving along with their protective curtain from one edge of the pit to the other, which also ensures the location of individual individuals of the dormant community (equalization of conditions).

Watching the fish frolicking in the aquarium, you might think that they never rest. But this is unreal. Any living organism requires periodic rest. Although there are other types of underwater creatures for which rest in the way we are accustomed to is categorically contraindicated in the sense. Examples of such fish are shark and tuna. They are designed in such a way that they need to continuously pump water to their gills, otherwise they will live no more than an hour - they will suffocate from lack of oxygen.

Sharks and tunas must swim continuously with their mouths open so that water is constantly circulating through their gills. They can only breathe on the move.... But they also allow themselves a little rest. To do this, they find relative shallow water or narrowed places in sections of boulders or reefs, where a current is artificially created due to ebb or flow, wind and other natural processes. In these places, they lie down, fixing the body between two boulders with their muzzle against the current, and calmly rest, while not even moving.

Tuna and shark are cartilaginous fish. And nature also deprived all representatives of this class of fish with a swim bladder, which is found in fish of the bone class. This bubble is filled with air and helps bony fish stay calm in the water column - wherever they please. When cartilaginous fish stop moving, they immediately sink to the bottom. If, for example, a shark falls asleep on the move and stops, it will begin to dive until it is crushed by the pressure of the water column at a depth unacceptable for a shark.

But not all shark species wash their gills only when they move. For example, shark species such as whitetip reef, leopard and wobbegongs can chill on the sand of the shallow seabed for a long time. They have more developed gill muscles, so they can circulate water through them with simple movements of opening and closing their mouth.

An interesting hypothesis is that pelagic sharks sleep, most likely in the likeness of dolphins (dolphins are mammals, not fish, as others think), that is sleep, turning off the left and right hemispheres of the brain in turn, thereby giving them the same time to rest.

Bony fish, which are mainly found in home aquariums, often rest, simply hovering at any convenient depth, without performing any active actions. And they keep only due to the swim bladder. If an individual wants to go down a little deeper, then it will just need to bleed the air out of the bubble a little, and if it floats up, then, on the contrary, it will have to fill it up again.

Another way of resting some species of fish is hibernation in winter and summer. During hibernation, physiological processes in the body of hibernating fish are noticeably weakened, and to a greater extent than during normal rest. Aquarium fish do not have hibernation periods.

How fish sleep, see below.