Sleeping for months at a time, what kind of "lazy" fish can sleep so much?

As the saying goes, "Spring makes you sleepy, Autumn makes you tired, Summer makes you take naps, and Winter makes you sleepy in March." As the weather is getting colder, it is becoming more and more difficult for you to get up, and you wish you could "stay in bed forever."

Let me share with you a little-known fact: All vertebrates sleep, and fish are no exception. The only difference is that when fish sleep, their eyes remain open and their gills move .

Maybe you are not surprised by this, after all, everyone can sleep! But did you know that there are fish that can sleep for a whole quarter? What kind of lazy fish can sleep so much? Let's take a look.

01

Fish hibernation

In fact, fish are not sleeping in, but they are in hibernation . Hibernation refers to a state of relative stillness of organisms, accompanied by a significant decrease in metabolic activity. Hibernation is divided into two types, hibernation and aestivation, but does not include the physiological sleep of the daily cycle of animals.

Dormancy is seasonal in most cases. Many animals hibernate to avoid severe cold during the winter, and animals in low-latitude arid areas hibernate to survive the hot summer . Seasonal climate change affects both the physiological activities of organisms and their food sources. Therefore, in the long-term evolution process, organisms have developed different ways of adaptation, using hibernation to resist adverse conditions.

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Seasonal dormancy is often controlled by the body's internal rhythm, and changes in external conditions only play a triggering role. In order to increase energy reserves in the body, fish, like other higher organisms, often need a long preparation phase before falling asleep.

02

Summer hibernation

Because conditions in the waters are relatively stable, most fish lack a typical dormant state, but in the equatorial swamps, they often dry up for weeks or months, which forces the fish living there to hibernate. Many fish of the genus Mystophila, such as climbing perch, silk-foot fish, snakehead, and other fish with secondary respiratory organs, spend the summer buried in the mud in a state of torpor, and resume their normal lives after the rains, when the ponds and rivers are filled with water.

In order to avoid death during the dry season, lungfish have a more complex method of hibernation. African lungfish , also known as mudfish, live in small rivers that dry up for a long period of time every year due to the high temperatures in the tropics. There is no water at all, and the sun will bake the mud at the bottom of the river into a very hard state. How do they survive in such a harsh environment?

West African lungfish, Image source: Wikipedia

The African lungfish has given its answer: before the dry season approaches, every fish will burrow into the mud, secrete a large amount of mucus from its skin, and mix it with the mud to form a hard mud shell. The fish sleeps in the mud shell until the next rain.

There is a small path inside the mud shell that connects to the outside world. During the period of dormancy in the shell, the lungfish relies on this path to breathe air. Even if the lungfish is dug out of the mud shell, as long as the mud shell is not peeled off, the lungfish will still be safe and sound for half a year. If the mud shell is placed in lukewarm water, the fish will wake up in a few minutes and swim actively after the mud dissolves.

Image source: georgialifetraces

When hibernating, South American lungfish also create caves in the mud similar to those of African lungfish. The difference is that South American lungfish's caves are sealed with a porous mud cover like the outside world.

The loach living in the waters along the Danube River burrows into the mud and stops eating or drinking when the river dries up in the summer, entering a state of hibernation, relying only on its special intestines to breathe air and sustain life. There is also a peculiar tree fish in South Africa, which climbs into the shade of a tree in the summer and sleeps for more than two months to survive the hot summer days.

It can be seen that these fish have put a lot of thought into avoiding the heat!

03

Hibernation

Hibernation refers to the hibernation of animals during the winter, which is an active process . Although the body temperature of animals during hibernation is close to the ambient temperature, especially for cold-blooded animals, when they encounter appropriate stimulation or reach the awakening period according to their own rhythm, their body temperature can rise in a short period of time and return to normal body temperature.

During the hibernation period, the number of breaths, heart rate, and oxygen consumption of animals are greatly reduced. The changes in the physiological functions of dormant animals are mainly controlled by the nerve center. During the dormant period, all endocrine glands stop functioning, and various metabolic levels generally decrease. However, when awakened, the physiological activities of various organs quickly return to normal in a certain order, and this process consumes a lot of energy. After the entire hibernation period, the weight of hibernating animals will be significantly reduced.

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The vast majority of fish do not have a hibernation period. A few fish, in order to cope with the difficulties of severe cold, begin to hibernate when the temperature drops near winter until the water temperature rises again in the spring of the following year.

Unlike amphibians, reptiles and mammals, they are not completely unconscious, but simply stop eating, hide among algae and rocks, or burrow into mud and caves, and enter a state of paralysis to a greater or lesser extent. For example, carp often overwinter in groups in the deep water. Dozens to hundreds of them form a puddle at the bottom of the water, forming a circle with their heads leaning against each other until the following spring, when they disperse. It really gives the impression of "fish team, holding a meeting". Fish breathe very slowly when they are dormant, and even the movement of their gill covers seems to be very difficult.

Tench mullets spend the winter buried in river mud. Even if you dig them out of the mud and put them on the shore, you can't even tell they're alive unless you hit them with a stick. Snakeheads migrate to deep water in the winter, burying their bodies in the mud to hibernate, stop eating, and hibernate. Eels have a similar hibernation pattern to snakeheads in the winter. They usually find deep water and bury themselves in the mud, becoming paralyzed.

Hibernation is a very rare phenomenon among marine fishes. It is known that young flounder live in shallow seas and bury themselves in the sand in winter, entering a state of rest.

Seeing this, everyone must have understood that summer hibernation or hibernation is not because these fish are too lazy, but they are adapting to the environment they are in. In nature, hundreds of millions of creatures on the earth have chosen to adapt at the same time. Only by adapting can they survive better.

Produced by | Science Popularization China

Author｜Life Pulse Team

Producer｜China Science Expo

Submitted by: Computer Information Network Center, Chinese Academy of Sciences

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