Sharks are ancient and unique animals that have managed to perfectly adapt to the surrounding reality. Here is a selection of facts that will help you look at predators from an unexpected angle. 
We are accustomed to certain types of sharks - usually those that can be dangerous to humans. But there are actually a lot of shark species.
Let's start with one of the most unusual sharks.
Pig shark
Pig sharks are notable for their appearance. Sailors are always amused by these fish when they get caught in their nets (this happens, however, infrequently). 
A curious species of sharks that live in the Mediterranean Sea and the Atlantic Ocean. Occasionally they swim into the Baltic Sea.
The side view is not so cute. More like a monster from a science fiction movie: 
They grow up to a maximum of one and a half meters. Not dangerous for humans. Not dangerous even for other fish. Because they feed mainly on crayfish, worms and mollusks.
These sharks themselves are of no value to fishermen. But nevertheless, their species is vulnerable due to human actions. People fish and actively destroy the food supply of these sharks.
And another interesting shark - this time a prehistoric one. After all, this particular shark was the largest fish in the history of our planet.
Megalodon
Everyone knows that the whale is the largest animal on Earth. But imagine, not so long ago there were fish swimming in the ocean that fed on whales!
Megalodon is the largest fish in the entire history of the Earth. It inhabited the waters of the world's oceans relatively recently - in the period 3-20 million years ago. 
Estimate the parameters: length 16 meters, weight – 40 tons. The length of one tooth is 18 cm! Do you want to understand how much it is? Here's the ruler in full size: 
Megalodon was at the top of the food chain among sea creatures. Megalodon fed on whales, sea cows and large fish.
Megalodon has, in principle, reached the limit of perfection - the existence of a larger fish in Earth’s conditions is physically impossible. 
These sharks hunted in an unusual way, not at all like they do today. Modern sharks use teeth to attack and try to bite through soft tissue. And they avoid hard bones - after all, broken teeth cannot be restored. The megalodon attacked like a battering ram - it tried to pierce the bones and internal organs of the victim. This is how Megalodon could cope with a huge whale.
Where did this storm of the seas go?
3 million years later, these sharks completely disappeared, losing out in competition to faster and more agile predators.
Why are only sharks among fish able to grow into such giants?
There are many predators in the water, but sharks are the most effective, as they were able to grow to their maximum size. Why do you think? 
Growing teeth is not an easy task. But try to find and catch food, which is very important for a predator of such dimensions. And in this, the shark is helped by another sense organ that land animals do not have.
Their sensory organ is the so-called ampullae of Lorenzini. This is an electroreception mechanism. The shark senses the slightest electrical impulses. And everything that moves produces such signals in water. The electroreception organs are located on the shark's snout. Ampullae of Lorenzini allow current to pass through, but are insulated by connective tissue, which acts as an insulator.
This helps them navigate even in muddy water.
The shark may not see the victim, may not smell it, but it will bite unmistakably and fatally!
Land animals lost this mechanism during evolution at an early stage. And this is logical - electrical impulses when interacting with air are an order of magnitude weaker.
"Fish Tornado" A clever way to fight small fish with sharks
Fish tornado is the name given to this clever way of protecting certain types of fish. When a predator approaches - a shark or tuna, they do not scatter in different directions, but, on the contrary, unite into a single structure. 
The goal is simple - to frighten the predator with its huge dimensions, because the shark can mistake it for a single giant organism 20 meters long. The “tornado” rotation is exactly what is needed to create the illusion of a single organism as much as possible. 
However, this does not work for every shark. Some begin to knock out small schools of fish from the tornado and eat them.
Why sharks don't like fresh water
Freshwater gray sharks are the only genus of sharks that live in fresh water. Found in Southeast Asia and Australia. They are so rare that scientists know almost nothing about them. 
But in general, sharks don't like fresh water.
It's all about osmoregulation. That is, the processes of water-salt balance that determine the internal pressure of a living organism.
To achieve osmotic balance with the surrounding seawater, the blood of most sharks is very salty. Their blood is saturated with chemicals such as urea and trimethylamine oxide. For this reason, dead sharks smell strongly of ammonia.
Sharks could become extinct by 2300
Sharks could become extinct in 2300, say scientists from the University of Sheffield. It turned out that shark teeth are extremely sensitive to acidity levels. And the level of carbon dioxide in the world's oceans continues to rise. 
The lucky seal escaped, dodging the scary teeth at the last moment
The acidity level of the world's oceans has increased by a third in 100 years. Carbon dioxide wears away a shark's teeth, and without teeth they quickly die, losing the opportunity to feed. It is curious that other fish, unlike sharks, turned out to be much more adaptive. Acidity has little effect on the condition of teeth.
Sharks can be... dwarfs
Imagine - a pelican ate a shark! What doesn't happen in the wild! 
Dwarf sharks are not at all dangerous. They grow to only 26 centimeters in length. Found throughout the Southern Hemisphere.
Cigar shark. Small but dangerous predator!
This is a rather small shark - on average they grow up to 50 cm in length. But it is very aggressive. The cigar shark attacks everything, even submarines. 
They live at great depths up to 4 km. At night they come to the surface in search of food. They have a specific hunting strategy. They suddenly attack their prey, bite off a piece of meat and immediately run back into the depths. Characteristic bite marks remain on the victim's body. 
They seem to be considered harmless to humans. Why, it seems, only one cigar shark attack has been recorded on a person who was swimming at night and far from the shore.
But the reason for the rare attacks is likely because people rarely swim in the open ocean at night. Yes, and still diving into the depths. And people simply don’t cross paths with these monsters.
California bull shark
Finally - a cutie shark! Yes, there are also types of sharks that instantly become heroes of social networks.
The bull shark is named for its distinctive appearance. 
The California bull shark is an extremely spoiled fish. Lives only in coastal waters off the West Coast of the United States, preferring California. It does not recognize temperatures below 20 degrees Celsius - when cold weather sets in, it begins to migrate to warmer regions.
And, like the pig shark, it loves shellfish. Because the teeth are not as powerful as those of its rugged cousins, and the bull shark prefers soft food. 
Bull sharks, unlike most other fish, care for their young. They do not spawn, but lay eggs. Moreover, before hatching, the sharks try to take the developed eggs to a safe place - for example, to a crevice.
Hammerhead sharks are very sociable 
“Hammerhead sharks are social animals,” says zoologist Gary Meaney. They unite in giant flocks. 
In the summer they migrate in large flocks in search of cooler waters. When night falls, they all go to the reefs and split up to hunt fish. And during the day they unite again into a giant company.
Schools of hammerhead sharks reach several thousand individuals!
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