Subtitles section Play video Print subtitles They glide through the water with unmistakable grace, remnants of an ancient past, they dive and they rise from the ocean's murky depths to it's sun-kissed shallows, rousing fear and awe like no other creature in the sea. The world's biggest living fish is a shark. Of the estimated 34,000 species of fish, the largest are whale sharks. These gentle giants usually grow to about 40 feet long and weigh an estimated 15 tons. Their mouths alone can span four feet wide. The gigantic whale shark, however, pales in comparison to the largest fish that ever existed, the megalodon. Dating to over 20 million years ago, it's thought that the prehistoric shark could've reached 80 feet long, weighing up to around 70 tons. Unlike whale sharks, the megalodon was carnivorous, and consumed any creature that fit into it's nearly 10 foot-wide mouth. Throughout their lives some species of shark can shed over 30,000 teeth. Unlike humans who are born with a set number of teeth in their jaws, sharks have a seemingly limitless supply. They can grow, lose, and replace their teeth as needed. Furthermore, most sharks have multiple rows of teeth in their jaws. The jaws of a great white shark, the largest predatory fish in the sea, can contain up to seven rows that hold up to 300 teeth at any one point. Most sharks, as they hunt their prey, end up losing their teeth individually. However, the cookiecutter shark loses and replaces the teeth in it's lower jaw all at once. Sharks are built for speed. The fastest known shark, the mako shark, can reach speeds of up to 46 miles per hour. This speed is largely due to their body's hydrodynamic design. Many sharks have torpedo shaped heads that allow them to cut through the water with little resistance. Plus, shark skin is covered with flat, v-shaped scales, called dermal denticles. The denticles help water flow smoothly over the skin, which reduces friction and helps sharks swim quickly and quietly. Sharks also have skeletons made of cartilage instead of bone. Cartilage is a much lighter material than bone so sharks have less weight to carry. Sharks may lay eggs, or bear live young. Egg laying sharks only lay a few large eggs. They may come in various forms, such as sacks called mermaid purses, or corkscrews. These eggs act as external wombs in which shark embryos complete their development. However, most sharks give birth to live young. Called pups, the young of most live-bearing species gestate for around one year. Some even begin practicing their predation skills while in the womb. Before they are born, the sand tiger shark pups compete with their siblings. In fact, the strongest pup in each of the two wombs devours its weaker brothers and sister. Some sharks are at risk of extinction. Every year an estimated 100 million sharks are killed worldwide, in large part for the shark fin trade. The sharks are caught and their dorsal fins are removed and sold at a hefty price, primarily in Asia. In traditional Chinese culture, serving and eating shark fin is a sign of status and wealth. Because of the high demand and value of shark fins, some shark populations have plummeted by up to 70 percent, causing a ripple effect in ecosystems and endangering at least 74 shark species. However, measures are being taken to protect sharks, with a number of countries and jurisdictions cracking down on unsustainable shark fishing. In China, shark fin soup is no longer allowed to be served at government banquets. A move hailed by shark conservationists. Through continued international conservation efforts, the loss of sharks may be curbed, allowing the creatures in all their power and grace to survive for many generations to come.