Sharks are fish that live in groups. They have a special characteristic of having skeletons made up of cartilages. It is believed that they have been roaming the seas for the past 420 million years. There are around 400 different species of shark today. The smallest one is around 6.7 inches in length. The largest one, the whale shark could grow up to be 39 ft long. They are found in almost all seas and their habitat is between the surface and up to 2000 feet under water.
Sharks have a covering on their skins that protect their skins from parasites and from damage. Most of them are hunters and they have very good hunting skills. If surrounded by a school of sharks in the sea, even a human is not safe. However, due to various human activities including fishing, the number of fish belonging to some species is coming down.
Sharks have very good sense of smell. They are able to detect one part per million of blood in sea water. Also, they are able to detect the direction of the smell. It is done by the timing of the nostrils getting the smell. Even the eyesight of this fish is similar to other vertebrates. Therefore, they are able to see things very well.
Sharks keep them afloat due to their low density skeleton made of cartilages. Also they have a liver that is able to keep them afloat as it is large and is filled with oil. The liver of one of these fish is so large that it constitutes 30% of its body mass. However, they cannot sleep for long as they will sink to the bottom of the sea as they don't have the gas bladders like other fish to keep them afloat.
Sharks have brain to body mass ratios similar to those of mammals. Sometimes they show curiosity. This may be due to their intelligence.
These fish normally swim at speeds of around 8 kilometers per hour. However, when they attack their prey they might achieve speeds up to 30 kilometers per hour.
Sharks are mostly carnivorous. They are well adopted to catch their prey. However, there are some of them that eat plankton. Different species use different ways to feed. The whale shark sucks water in to its mouth in order to catch plankton as well as the small fish. There are some species of these fish that lay ambush in the seabed in order to catch other fish.