Sharks are found in all seas. They generally do not live in fresh water, with a few exceptions such as the bull shark and the river shark which can swim both in seawater and freshwater. Sharks are common down to depths of 2,000 meters (7,000 ft), and some live even deeper, but they are almost entirely absent below 3,000 meters (10,000 ft). Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics.
They have numerous sets of replaceable teeth. Well-known species such as the great white shark, tiger shark, blue shark, mako shark, thresher shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain. Many shark populations are threatened by human activities.
