The total water reserves on Earth are about 1.36×1018 cubic meters, and although it seems like a lot, only about 2.53% of the freshwater resources account for it. So as early as the last century, the World Meteorological Organization pointed out that mankind will face a freshwater crisis in the future.
Water resources on the earth are distributed, and fresh water is very scarce
Compared to humans, marine life is not bothered by water shortage, because seawater, which seems salty and undrinkable to us, seems to be their "daily drinking water" itself.
So why do marine animals drink seawater? Do they have a need to drink fresh water? If so, how do they drink fresh water?
How do marine organisms drink fresh water?
Although fresh water on Earth is relatively scarce, it is relatively easy for organisms living on land to ingest it. For marine organisms, if fresh water is needed, it may be more troublesome.
Access to fresh water for marine organisms is difficult
In fact, although marine life lives in the ocean and the surrounding environment is high-salt seawater, most of them also need fresh water, even marine fish that everyone thinks are salty.
Therefore, most of the marine organisms that we see that can drink seawater "tons and tons" are not really able to drink seawater, but they have passed some method and desalinated the water resources they need.
First of all, let's look at vertebrates that rely on gills to breathe, that is, various "fish". There is a special type of cell on the gills called "chlorine cells", which are generally located on the surface of the gill filaments of the gills.
Fish have special chlorine cells on their gill filament
Under the microscope, we can see that there are round or oval openings about 12 to 23 microns in diameter, and within these openings are a large number of irregularly arranged chlorine cells.
These chlorine cells are the fish's "desalination", which help the fish filter as much salt as possible to maintain a stable osmotic pressure in their bodies.
It has been determined that seawater bony fish can swallow 7% to 35% of their body weight every day, most of the swallowed seawater is absorbed through the intestine and penetrates into the blood, and the excess salt (mainly Na+, K+) inhaled with seawater is excreted by salt-secreting cells (chlorine cells) on the gills.
Chlorine cells help bony fish flush out excess salt from seawater
It is worth mentioning that those who obtain fresh water through chlorine cells are basically bony fish in the ocean. Other cartilaginous fish use other methods, using urea in the body to expel excess salt from seawater. This means that they get fresh water by "ingesting it and then filtering it out".
Take the typical shark among cartilaginous fish as an example, this guy's kidneys are very developed, which can reasonably discharge the water that penetrates into the body, and as for salt, the shark will excrete it through the rectal glands. In this case, the shark excretes the substance more frequently than ingested.
Sharks have a unique salt drainage technique
Secondly, let's look at amphibians and reptiles in the ocean, mainly represented by turtles and sea snakes.
Let's start with sea turtles, they still have a great need for fresh water. Unlike the chlorine cells of fish, turtles have filters on either side of their eyes, which are their "salt glands." When ingested in seawater, it uses salt glands to expel excess salt, thereby desalination.
Turtles have salt glands that drain salt on either side of their eyes
Of course, turtles would be happy to drink fresh water more conveniently. For example, when it goes ashore to lay eggs, it will properly drink fresh water such as rainwater.
In addition, sea snakes rarely come ashore compared to turtles, because the land environment is quite dangerous for them. But they need to drink fresh water, so what to do?
It was once thought that sea snakes relied on salt glands under their tongues to drain salt, but when Harvey Leyeshire, a biologist at the University of Florida, studied black-backed sea snakes, he found that these guys would rush to the beach to find fresh water left after rain.
Blackback sea snakes venture to the beach to get fresh water
The results showed that black-backed sea snakes do not rely solely on drinking seawater directly to replenish fresh water, because their salt glands are too small and the rate of excretion of salt is relatively slow. So, in order to get more fresh water, black-backed sea snakes will go to the beach after the rain to collect rainwater to drink.
Finally, there are mammals living in the ocean, which also have a demand for fresh water, and they are actually similar to mammals on land, and it is difficult to adapt to the salinity of seawater.
Since there are no special chlorine cells like teleost fish, and the kidneys are not as strong as cartilaginous fish, such animals basically choose to "not drink seawater".
Marine mammals generally choose not to drink seawater
The problem is that there is only seawater in the ocean, if you don't drink seawater, will you die of thirst?
In fact, not drinking seawater does not mean that you do not consume water. Because these organisms can obtain fresh water through "dry rice", when food enters their bodies, they can be fully metabolized and decomposed, and eventually become the fresh water their bodies need.
It should be noted that there are also some leaders in mammals who "filter seawater", such as the familiar whale. This guy is not only large, but also has strong kidney function, so he can filter it after swallowing seawater. Under the influence of their own filtration and dietary intake, whales are not bothered by the problem of insufficient fresh water.
The whale's kidney function allows it to drain salt smoothly
It is worth mentioning that in addition to the marine organisms introduced above, there is another organism that is more special, that is, it is called "broad salt species" by scientists. Because these guys, who can live in fresh water and enter brackish waters, are very adaptable.
The Amazing "Broad Salinity Species"
Most of the broad salt species are fish, which are highly adaptable to changes in salinity in waters. For example, the familiar Yangtze River endemic species Chinese sturgeon, this guy belongs to the broad salt species.
The Chinese sturgeon is a broad salt species
You think it's a freshwater fish, but you don't think that when it grows into a juvenile sturgeon, it will run towards the sea, because the nutrients in the freshwater can't make it enter another stage of growth.
And this organism enters seawater from fresh water and from seawater to freshwater, and will be regulated using different methods. From fresh water to seawater, their physical state will shift from drainage salt retention to salt drainage and water retention. In this case, the osmotic pressure regulation mechanism applicable to the ocean is turned on.
There are three main regulatory pathways in this process, the first is to swallow seawater to balance the water metabolism in the body. The second is to reduce urine output, because at this time the permeability of their renal tubules will be enhanced. The third is to cytologically change the chlorine cells on your gill filament, in short, to use cortisol to proliferate and better excrete salts.
Broad-salinity species use two regulatory mechanisms to adapt to freshwater and seawater
It can be seen that the adaptability of animals is very strong, especially broad salt species, and this magical regulatory mechanism in their bodies may be the result of years of evolutionary accumulation.
It's not hard to see that many organisms in nature have "good ways to filter seawater and then ingest fresh water." Didn't humans evolve any advantages? We really can't drink any seawater?
Can people really not drink seawater?
Perhaps most people have heard a saying called "drinking seawater is better than dying of thirst", mainly because the salinity in seawater is about 3 to 4 times the salinity of human blood, and our kidneys are not so strong, in this case, the salt produced by seawater intake will change the water required by cells and blood, and even affect human brain function.
Humans can't drink seawater because the salt in it changes cellular moisture
However, a biologist named Alain Bombard has crossed the Atlantic Ocean and verified that if you drift at sea, you can try to drink moderate seawater.
Simply put, it is to scientifically allocate water resources under difficult circumstances. Mixing and matching scarce fresh water and seawater can also prolong life and improve your chances of survival!
Some can be drunk after properly mixing seawater and fresh water
It can be seen that the human body still has a certain adaptability to seawater. I don't know if in the future, when there is really no drop of fresh water left, people will also evolve the special "desalination device" of marine life.