Sharks are a group of fish which have a cartilaginous skeleton, five to seven gill slits on the sides of the head and pectoral fins that are not fused to the head. There are over 470 species of them which range in size from the small dwarf lantern sharks of only 17 centimeters (6.7 inches) in length to the whale sharks, the largest fish in the world which reaches approximately 12 meters (39 feet) in length. They are found in all seas and are common to depths of 2,000 meters. They typically don’t live in fresh water, although there are a few known exceptions, such as the bull shark and river shark, which can survive seawater and freshwater. They have a covering of dermal denticles which protects their skin from damage and parasites in addition to making them more hydrodynamic. They have several sets of replaceable teeth. The great white, tiger, blue, mako and hammerhead sharks are apex predators – and at the top of the underwater food chain. Most of their populations are threatened by human activities.