Summer hibernation, winter hibernation...are there such lazy fish?

Here’s some fun fact for you: 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.

Fish hibernation

It turns out that the fish are not sleeping in, but they are in hibernation. Hibernation refers to a state of relative stillness of organisms, accompanied by a significant reduction in metabolic activity. There are two types of hibernation, hibernation and aestivation, but it does not include the physiological sleep of the daily cycle of animals.

(Photo source: veer photo gallery)

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.

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.

Summer hibernation

Because conditions in the waters are relatively stable, most fish lack a typical dormant state, but in the equatorial swamps, dryness often lasts for weeks or months, prompting the fish living there to hibernate. Many fish of the genus Mystophila, such as climbing perch, thread-foot fish, and snakehead fish, as well as 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?

(Photo source: veer photo gallery)

The African lungfish has given its answer: before the dry season approaches, each fish will burrow into the mud, secrete a large amount of mucus from the skin, and mix with the mud to form a hard mud shell. The fish will sleep 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 and left for half a year, the lungfish will still be safe and sound. If the mud shell is placed in lukewarm water, after the mud dissolves, the fish will wake up in a few minutes and swim actively.

(Photo source: veer photo gallery)

When South American lungfish hibernate in the mud, they also create caves similar to those of African lungfish. The difference is that South American lungfish rely on a porous mud cover to seal their caves like the outside world. Loaches living in the waters along the Danube River, when the river dries up in the summer, they burrow into the mud and stop eating and drinking, entering a state of hibernation, relying only on their special intestines to breathe air and maintain their lives. There is also a peculiar tree fish in South Africa. In the summer, it climbs to the shade of a tree 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!

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 poikilothermic 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 breathing, heart rate, and respiratory oxygen consumption of animals are greatly reduced. The changes in the physiological functions of dormant animals are mainly controlled by the nervous center. During the dormant period, all endocrine glands stop their activities and various metabolic levels are generally reduced. However, when awakening, 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.

(Photo source: veer photo gallery)

Most fish do not have a hibernation period. A few fish, in order to cope with the difficulty of severe cold, begin to hibernate when the temperature drops in winter, and do not hibernate 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 drill into the mud bottom and caves, and enter a state of paralysis to a greater or lesser extent. For example, carp often hibernate in groups in the deep water. They form a circle with dozens to hundreds of them forming a puddle at the bottom of the water, leaning their heads against each other, and do not disperse until the spring of the following year. It really gives people a sense of déjà vu of "Fish team, come to a meeting". Fish breathe very slowly when they are dormant, and even the movement of the gill cover seems to be very difficult.

(Photo source: veer photo gallery)

Ding 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. In the winter, snakeheads migrate to deep waters and bury 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 in marine fish is rare, but flounder fry are known to live in shallow waters and bury themselves in the sand in the winter, becoming dormant.

Conclusion

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.

(Editor: Sun Chenyu)

Produced by: Science Popularization China

Author: Life Pulse Team

Producer: China Science Expo