Sharks (lat. Selachii) are a superorder of cartilaginous fish (Chondrichthyes), belonging to the subclass of elasmobranchii and having the following distinctive features: an elongated body of a more or less torpedo-shaped shape, a large heterocercal caudal fin, usually many sharp teeth on each jaw.
The Russian word "shark" comes from the Old Norse "hákall". The most ancient representatives already existed about 450-420 million years ago.
More than 526 species of sharks are known: from the deep-sea shallow Etmopterus perryi, only 17 centimeters long, to the whale shark (Rhincodon typus) - the largest fish (its length reaches 20 meters). Representatives of the superorder are widely distributed in the seas and oceans, from the surface to a depth of more than 2000 meters. They mostly live in sea water, but some species can also live in fresh water. Most sharks are so-called real predators, but 3 species - whale, giant and largemouth sharks - filter feeders, they feed on plankton, squid and small fish.
