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Shark Bytes
Evolution | Life Cycle | Shark Superlatives | The Human Factor | Shark Tagging Evolution The first sharks on Earth: Ancient relatives of sharks first appeared in the world's oceans about 400 million years ago. Today, sharks are classified among the elasmobranchs, which also includes the rays and skates. Number of species: There are well over 350 known species of sharks, and new species are described every year. The difference between a shark and other fish: Unlike bony fish, sharks, rays, skates, and chimaeras are cartilaginous fish, meaning their skeletons are made of cartilage, not bone. Their skin is covered with denticles, tooth-like scales that differ from the scales of bony fish. And they have five, six, or seven gill slits per side, not one per side as in bony fish. Life Cycle Average life span of a shark: Less than 25 years Longest living shark species: Spiny dogfish, 70-100 years Birth: Sharks give birth to their young in one of three different ways:
Number of offspring: Varies from one to up to 100, as observed in the tiger shark (Galeocerdo cuvier) When sharks stop growing: Never Shark Superlatives Largest Shark: The whale shark (Rhincodon typus) is the largest fish in the world, measuring up to 50 feet long. Smallest Shark: Mature males of the dwarf dogshark (Etmopterus perryi) do not exceed seven inches in length. Average swimming speed of a shark: About a yard per second Fastest clocked speed: In sudden, brief bursts, the lemon shark (Negaprion brevirostrus) can attain speeds approaching 20 mph; mako sharks (Isurus sp.) are thought to be even faster. Youngest shark to bite a human: A marine biologist, while probing the uterus of a pregnant sand tiger shark, was bitten by an unborn pup. The strongest shark bite: The greatest force of a shark bite ever recorded measured 132 pounds of force between the jaws of a dusky shark (Carcharhinus obscurus). Largest egg in the world: An egg case of the whale shark found in the Gulf of Mexico measured 12 by 5.5 by 3.5 inches. Most travelled shark: A blue shark (Prionace glauca) tagged off New York was recaptured 16 months later off Brazil, 3,740 miles away. Freshwater shark: Alone among the sharks, the bull shark (Carcharhinus leucas) is frequently found far up rivers. Most exciting recent discovery of an unknown shark: In 1976 and 1984, respectively, two specimens of a 15-foot-long, plankton-feeding shark were caught in the Pacific. To date at least ten specimens of this new species, whose gaping mouth inspired the name "megamouth," have been found. Lowest note a shark can hear: 10 Hertz (or 1.5 octaves below the lowest key on the piano). The lowest note a human can hear is 25 Hertz, so we miss out on some of the very low frequencies that sharks can detect. Highest note a shark can hear: 800 Hertz (or G above High C on the piano), so humans can hear many high sounds that sharks cannot. The Human Factor Worldwide shark attack rate: Less than 100 a year, with only 25 to 30 fatalities. Given the number of people who spend time in the ocean, this is low. Greatest threat to sharks: Humans Number of sharks killed by fishers each year: 30 to 100 million Percentage of shark species threatened with extinction: Up to 80 Shark Tagging Longest time span between tagging and recapture of a shark: Two sandbar sharks were tagged in the same week in 1965 and were recovered within the same week 19.7 years later, 1,000 miles from the tagging site and only 160 miles apart. What to do if you catch a tagged shark: Retrieve the tag and record the date and the place of capture. Note any other information you can obtain, such as weight, measurements, and sex. Then send the tag and information to one of the following addresses: Gamefish Tagging Programme Cocos Island | Sharkmasters | World of Sharks | Dispatches E-mail | Resources | Site Map | Sharks Home Editor's Picks | Previous Sites | Join Us/E-mail | TV/Web Schedule | About NOVA Watch NOVAs online | Teachers | Site Map | Shop | Search | To Print PBS Online | NOVA Online | WGBH © | Updated June 2002 |