Sharks

2 Pages 423 Words

Shark, any of several fast-swimming, generally large fish known for their many sharp teeth, distinctive dorsal fin, and skill in locating underwater prey. Like other fishes, sharks are generally cold-blooded and breathe with gills. But unlike the more common bony fishes, which have skeletons made of rigid bone, sharks—and their relatives skates, rays, and chimaeras—have flexible internal skeletons made of cartilage.

Among the most ancient vertebrates in the sea, ancestors of modern sharks originated almost 400 million years ago. Today sharks live in essentially the same way they did more than 200 million years ago, before the rise of the dinosaurs. Scientists have identified nearly 375 species of sharks living today, ranging in size from the dwarf dogfish, less than 20 cm (8 in) in length, to the massive whale shark, which reaches lengths of more than 15 m (50 ft) . Most sharks inhabit tropic or temperate marine waters, but some species have been found in polar seas. The bull shark leaves ocean waters to enter freshwater rivers and lakes, including the Zambezi River in southeastern Africa, the Mississippi River in the United States, and Lake Nicaragua in southwestern Nicaragua. Depending on the species, sharks inhabit either shallow coastal waters or the open ocean. Some species, such as the sixgill shark, live at depths of more than 1,800 m (6,000 ft).

No matter where they live, sharks play a crucial role in their ecosystems. As highly efficient predators, sharks keep ecosystem populations in check by hunting and killing other animals, particularly those that are weak or diseased. Biologists fear that without sharks, populations of other animals would swell to unsustainable levels. These animals would eliminate the available food in their ecosystems, leading to massive die-offs from starvation. It might take years, or even decades, for the organisms in the ecosystem to again achieve the delicate balance that sharks help maint...