Sharks have diversified into over 470 species. They range in size from the small dwarf lantern shark, a deep-sea species of only 17 centimeters in length, to the whale shark, the largest fish in the world, which reaches approximately 12 meters.
Sharks are found in all seas and are common down to depths of 2,000 meters. They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive in both seawater and freshwater. They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They also have several sets of replaceable teeth.
Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain. Their predatory skill fascinates and frightens humans, even though their survival is greatly threatened by human-related activities.
Sharks have roamed our oceans since before the time of dinosaurs, but their long reign at the top of the ocean food chain may be ending. The onset of industrial fishing over the past 60 years has drastically depleted their populations. 30 percent are threatened or near threatened with extinction.
Shark finning–the practice of catching a shark, slicing off its fins and then discarding the body at sea–takes a tremendous toll on shark populations. Up to 100 million sharks are killed every year to primarily support the global shark fin industry, valued for the Asian delicacy shark fin soup.
In general, sharks grow slowly, mature late and produce few young over long lifetimes, leaving them exceptionally vulnerable to overexploitation and slow to recover from depletion. As key predators, their depletion also has risks for the health of entire ocean ecosystems. For example, tiger sharks have been linked to the quality of seagrass beds through their prey, dugongs and green sea turtles, which forage in these beds. Without tiger sharks to control their prey’s foraging, an important habitat is lost.
Despite the relative rarity of shark bites, the fear of sharks is a common phenomenon, horror fiction and films, such as the Jaws series created a misleading myth and the danger presented by sharks has been severely exaggerated. Statistics show that the chance of being killed by a shark in water in the entire human history is dramatically far less or even negligible than that of dying from a bee sting, a snakebite or even lightning in just one year.