Imagine what a fish looks like right now.- Have you ever noticed that their eyes are constantly bulging and they never blink? The thing is that thanks to the aquatic lifestyle, their eyes never dry out and they do not need eyelids to moisten them. It may seem that due to the inability to close their eyes, the fish never sleep, but this is not so. Just unlike people, fish do not sleep in the literal sense of the word, but simply are stationary. Their body is resting, but they are always on the alert - in case of danger, the fish always throw in pieces. And the poses and places in which they sleep depend on their class, because fish are divided into bone and cartilage. But what is the difference between them and why do scientists believe that urban lighting can kill both of them?
Bone fish (Osteichthyes) - This is the well-known cod, herring and similar aquatic inhabitants, whose bodies consist of hard bones. Cartilaginous Fish (Chondrichthyes) - These are sharks and stingrays, possessing a skeleton of flexible cartilage.
What kind of fish are there?
The main distinguishing feature between the usual fish and sharks is the presence and absence of them swim bladder. This organ is a bag inside the fish with air and only bone fish have it. When a fish draws oxygen into it, itrises up and can simply “freeze” at the same depth. And when the bag of air is empty - the fish gets heavier and sinks to the bottom. Thanks to this organ, most fish can still be in the water and rest, but with their eyes open, because they can always be in danger and need to be alert. During sleep, fish actively move their gills and absorb water in themselves to extract oxygen particles from it and thereby “breathe”.
The position of the body during sleep is different for all fish. According to biologists, cod during sleep releases its swim bladder and lies on its side, sinking to the bottom. The herring is able to sleep with its belly up, and the flounder buries itself completely in the sand - apparently, they hope that they will not be noticed by predators. It is noteworthy that almost all fish prefer to sleep among the vegetation, because there the probability of being detected by predators is very small.
Sharks and stingrays that are cartilaginousfish sleep completely differently. Due to the lack of a swimming bladder, they cannot just stop and “freeze” at the same depth - so they simply sink to the bottom and suffocate. And they can suffocate due to the fact that they do not have mobile gills, with the help of which they can absorb water and extract oxygen from it. They have only motionless gill slits, into which water enters only during the movement of the shark. So, it turns out that during sleep, sharks need to stay in motion?
Not always, but there are sleeping asleepsharks. For example, the Katran shark during sleep leaves only the spinal cord turned on, which makes the swimming muscles move. At the same time, the brain of the shark is resting, so we can assume that it is sleeping. Tiger sharks have a good replacement for gills in the form of small holes in the back of the eyes through which they suck in oxygen-enriched air. Species of sharks and stingrays, which cannot sleep on the go and are not endowed with an analogue of gills, simply lie at the bottom where there is a current - water independently penetrates their body, although not in such a large amount than when absorbed. In general, they have enough air for life.
On the topic of marine predators, I have another interesting material: 10 of the most dangerous sharks that kill people
What harm does urban lighting do?
As you can see, many fish find it quite difficult to rest.“They constantly need to be on the lookout and look for secluded places to sleep.” According to German professor Werner Kloas, fish are even harder because of people. The fact is that city lighting increases the brightness of water and fish can not relax enough. Moreover, under the influence of bright light, they produce little melatonin - a hormone that is responsible for the functioning of the immune system and the proper growth and development of the body. This was recently reported by New Atlas.
Melatonin also known as "sleep hormone" becauseproduced by the human brain in the dark and helps us sleep soundly. When changing time zones, some people drink melatonin specifically to restore their sleep patterns. And they’re doing it right, because violating their regime, a person risks his health - I wrote about this earlier
The professor found out about this by conducting a smallexperiment. He placed four groups of perch in different aquariums. For ten days, the first group of fish kept in complete darkness, while the others were exposed to light with a brightness of 0.01, 0.1 and 1 lux. To understand the brightness of the fish, imagine a night with a full moon - according to scientists, in such conditions there is a brightness of 0.3 lux. But cities are able to radiate brightness up to 150 lux, that is, almost dazzle the fish.
After 10 days, the levels of all fish groups were checked.melatonin. It turned out that intense lighting greatly slowed down the production of melatonin, and in the dark, the hormone was produced in the optimal mode. According to scientists, violations in the production of this hormone can lead to the fact that the fish will cease to properly develop and multiply. If cities create even more light than before, this could lead to the extinction of fish.
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In general, scientists have long been worried thatcities emit too much light. This problem even has an official name - light pollution. Since city lights are so bright that they are visible from space, they prevent scientists from exploring distant planets and galaxies. In one of the articles, my colleague Lyubov Sokovikova even discussed whether a strong glow of cities could attract the attention of aliens. It turned out quite interesting, so I highly recommend reading it!