Sharks are among Earth's most ancient animals. The fossil record dates ancestors of modern sharks to as far back as 400 million years ago. Shark species have changed relatively little during that time span and are sometimes called living fossils.
Sharks can replace lost teeth in as little as 24 hours and may use thousands of teeth over the course of a lifetime. Ancient teeth are the source of most known shark fossils. (Shark skeletons, which are composed of cartilage, decompose quickly.)
Sharks are diverse reproducers, and their mating has been observed only on rare occasions. Some species are egg-laying (oviparous), while others bear live young (viviparous). Adult sharks do not care for their newborn pups, which are born or hatched as smaller, juvenile versions of their parents.
Two shark species can survive long periods in fresh water: the bull shark and the speartooth shark (Glyphis glyphis). Both species can engage in river journeys of epic scale. Bull sharks, for example, have been caught 1,700 miles (2,800 kilometers) up the Mississippi River and 2,100 miles (3,480 kilometers) up the Amazon. Neither species, however, lives in landlocked fresh water without ocean access.
A major cause of shark mortality is "finning," a process in which fishers kill sharks solely to remove their fins. Fins can sell for U.S. $400 per kilogram (U.S. $880 per pound) or more.
Known in China as yu chi or "fish wings," shark fins are used to make the traditional delicacy shark-fin soup. The culturally celebrated but controversial soup is found widely in Asia and will even be on the menu at Hong Kong Disneyland when the park opens in September.
Sharks are apex predators, and as such, they affect the entire ocean food chain from their position at the top. Because of their dominant role, sharks have long life spans and don't reach sexual maturity until they are 12 to 20 years old. Even then, sharks have low reproductive rates. Such predator populations, once diminished, have a hard time bouncing back.
As elasmobranches, sharks have skeletons made not of bone but of cartilage, tissue similar to that found in human noses and ears. Cartilaginous skeletons are lighter than bone and help sharks to remain neutrally buoyant (able to float without sinking or rising).
The media can have a voracious appetite for "shark bites man" stories. The summer of 2001, for example, saw an explosion of shark-attack hype and was even heralded on the cover of Time magazine as the "Summer of the Shark." Yet 2001 was statistically average: The year saw 76 shark attacks and 5 fatalities worldwide, compared to 85 attacks and 12 fatalities in 2000.
Thirty years ago the blockbuster Jaws brought the terror of shark attack to movie theaters. The record-breaking film, directed by Steven Spielberg and based on a best-selling novel by Peter Benchley, grossed nearly 130 million dollars (U.S.) in the United States alone. The movie arguably made sharks public enemy number one.
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