Sharks are cartilaginous fish with highly streamlined bodies. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs. There are about 440 species (about 1% of all living fish species) of shark, ranging in size from the small dwarf lanternshark, a deep sea species, which reaches 17 centimetres in length, to the whale shark , the largest fish, which reaches about 12 metres and which filter feeds on plankton, squid, and small fish.
Most species of shark live 20 to 30 years while whale sharks (Rhincodon typus) may live over 100 years.
Sharks are found in all seas and are common down to depths of 2,000 metres . Only a few species such as the bull shark and the river shark live in both seawater and freshwater. They breathe through five to seven gill slits. Sharks' skin is covered with tooth-like projections (dermal denticles) that protect their skin from damage and parasites and allows them to move faster. Their skin resembles sandpaper.
They have several sets of replaceable teeth which are pointed and serrated (notched like the edge of a saw) for sawing through flesh. Shark teeth are embedded in the gums rather than directly attached to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward as in a "conveyor belt"; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to10 days to several months.
Sharks are predators and occupy the top levels of many aquatic food webs. Many species become active after dusk and hunt during the night. Most sharks feed on other fishes. Large sharks, such as the Great White Shark (Carcharodon carcharias) and Tiger Shark (Galeocerdo cuvier), prey on large marine mammals such as seals, sea-lions, dolphins as well as large fishes, turtles and even sea birds. Several species of shark are known to be dangerous to humans: the Great White Shark, Tiger Shark, Bull Shark (Carcharhinus leucas) and other whaler sharks (Carcharhinus sp.).
Unlike bony fish, sharks do not have gas-filled swim bladders to maintainbuoyancy. Instead, sharks rely on cartilage which is about half as dense as bone and on a large liver (up to 30% of their body mass) filled with oil . The liver can make .Most sharks sink when they stop swimming and need to constantly swim in order to breathe and cannot sleep very long, if at all, without sinking. .
Most sharks are "cold-blooded", meaning that their internal body temperature is the same as that of the surrounding environment. Some sharks, such as the great white shark, are homeothermic and maintain a higher body temperature than the surrounding water.
Sharks swim at an average speed of 8 kilometres per hour but when feeding or attacking, the average shark can reach speeds upwards of 19 kilometres per hour.