25 Things You Didn’t Know About Sharks

A co-author of a fascinating new guide to sharks reveals some amazing facts about shark biology and behavior, as well as human interaction with them, in this exclusive gallery.
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Borrowing heavily from his very cool new book, Sharks: The Animal Answer Guide (co-authored with internationally respected shark expert George H. Burgess), Dr. Gene Helfman — recently retired from the University of Georgia — has come up with 25 factoids regarding sharks, in an exclusive gallery for Helfman also has added some new information. He describes Sharks: The Animal Answer Guide as “a mix of current science, history, anthropology, conservation, intriguing facts and dazzling photographs, assembled in readable and sometime humorous text, with technical information simplified but never dumbed-down.” You can order it from Amazon or Johns Hopkins Press. Also, check out this interactive companion website that expands on the book, answers questions and offers more info. — Doug Olander
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White sharks grow more slowly and live much longer than previous conventional wisdom — that whites lived no more than 23 years. But that’s all changed with recent analyses of hard-to-study growth rings in vertebrae of four male and four female white sharks from the North Atlantic. Using radiocarbon dating methods, including “time stamps” laid down during the A-bomb testing period of the 1950s, researchers found that the largest female was 40 years old and the largest male was 70 years old! Given that female sharks grow bigger than males (see Fact #13), and that the largest female in the study was “only” 17 feet long, there’s good reason to suspect white sharks live even longer. Knowledge of longevity, growth rates, and ages at sexual maturity are critical to designing conservation programs for slow-growing, long-lived vulnerable species such as white sharks. I snapped a shot of this 10- to 11-foot white shark as it cruised by a shark cage near Dyer Island, South Africa. The fish bears obvious scars from encounters with other sharks as well as lasting impressions from jamming its head forcefully inside shark cages. (Photo by Gene Helfman) Gene Helfman
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Contrary to the conventional wisdom portraying sharks as mindless killing machines, sharks actually have some of the biggest brains in the fish world. Using brain-to-body-weight ratios, dogfishes have relatively small brains and white sharks are intermediate, while reef sharks, hammerheads, sand tigers, and manta rays have the largest brains. Overall sharks have brains relatively bigger than: most bony fishes, one third of birds, and in some cases even some mammals. So actually, sharks are quite mindful killing machines. What do sharks do with all this brain power? They learn. For example, it’s popular for photographers in South Africa to tow plywood cutout dummies of seals behind a boat, hoping to get a large white shark to propel itself out of the water in an attack known as a Polaris breach. This worked for a while, but experienced photographers now report that the sharks go for the fakes less and less often. In this amazing image of a Polaris breach, a large white cartwheels through the air after launching from the surface to feed on a cape fur seal (in this case a decoy — some sharks are still fooled). (Photo by Austin Gallagher) Austin Gallagher (
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Hammerheads, such as the scalloped hammerhead shown here, are especially fond of stingrays; one individual had 96 stingray barbs embedded in its mouth, throat, and tongue. Researchers in the Bahamas witnessed how a shark manages to catch a stingray. The divers chanced upon a 10-foot hammerhead chasing a southern stingray. The shark thrust its head down against the back of the ray, knocking it to the bottom. Then, pinning it against the bottom with its head, the hammerhead pivoted around on the back of the ray, taking a bite from the front edge of the ray’s left “wing.” The ray limped off, and repeated the process, taking a bite from the front of the right wing. The crippled ray was then eaten by the shark. The shark had used its hammer to both knock down and immobilize the ray while taking bites. A pointy-nosed shark probably would have slid off the ray’s back, but the hammerhead was able to apply pressure to both pectoral fins at the same time. (Photo by David Hall, Courtesy David Hall,
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Offshore anglers have known for decades that thresher sharks use their long tail to catch prey. Why else would they so often get caught by the tail on both baited hooks and trolled lures? But only recently have we obtained video evidence of this behavior. California researchers towed cameras ahead of lures and found that common threshers strike baits repeatedly with their tails, as many as seven times in less than a minute. The strike occurs about half-way down the tail and hits the bait 65 percent of the time. None of the sharks tried to bite the bait first. Other researchers filmed pelagic threshers feeding on sardine schools in the Philippines by entering the school, braking with their pectoral fins, and slinging their tail tips over their heads. The tail tip was whipped at the sardines at speeds calculated to reach 80 mph. The sardines were crippled, some with broken backs and some even cut into pieces, making easy meals for the sharks. (Photo by Jason Arnold) Jason Arnold (
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Bull sharks are known to travel up rivers regularly and have attacked people in rivers in Iran, India, and South Africa. Bull sharks regularly move more than 100 miles up Nicaragua’s Rio San Juan and into Lake Nicaragua, and have been caught as far as 2,610 miles up the Amazon. One was even captured by a pair of fishermen near Alton, Illinois — 1,160 miles up the Mississippi, 15 miles north of St. Louis! That fish, pictured here, was caught by fishermen Herbert Cope and Dudge Collins in 1937. Recently, bull sharks in Australia gave new meaning to the term “water hazard.” Heavy rains caused the Logan River near Brisbane to overflow its banks and flood portions of the Carbrook Golf Club (motto, “Our hazards are deadly”). The flooding brought with it as many as six bull sharks in the 8- to 10- foot range. The sharks are seen regularly, so it’s prudent to just take a penalty if you find yourself in the lake on the 14th fairway at Carbrook. (Photo courtesy Ralph Manns) Courtesy Ralph Manns
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Many sharks give birth to live young after carrying them for two and even three years (e.g., spiny dogfish have a two-year “gestation” period). Some sharks have a placenta and umbilical cord, and some baby sharks are even born with a belly button (OK — the umbilical scar is actually located up between their pectoral fins, so it’s more of a chest button). This image shows a free-swimming, newborn spiny dogfish with yolk sack still attached. (Normally, shark pups use up their yolk supply and absorb the sack before birth, but this newborn was ejected prematurely when its mother was caught in a net.) (Photo by Jeff Rotman / Seapics) Jeff Rotman / Seapics
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The world’s most abundant shark is the spiny dogfish, also called Spurdog, Piked Dogfish and North Pacific Dogfish. These sharks occur along both coasts of North America. Recent estimates put the abundance of dogfish off the Atlantic coast of Canada at 535 million sharks. The Pacific coasts of Canada and the United States are home to almost 2 billion individuals, which won’t come as a surprise to West Coast anglers who often find them a great nuisance. (Photo courtesy Doug Costa, NOAA) Courtesy Doug Costa, NOAA
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Despite their abundance, the dogfish fishery in the North Atlantic that supplies the fish-and-chips market in England has been depleted by as much as 95 percent, prompting a Do Right By Dogfish campaign to reduce over-exploitation. New catch limits have been subsequently established for the fishery, from Maine to Florida. (Gene Helfman photo) Gene Helfman
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A mountain of dogfish — Bottom trawls targeting other species along the U.S. and Canada Pacific coasts can wind up with more spiny dogfish than any other species, as in this catch off Southern California. (Photo courtesy John Wallace/NOAA.) Courtesy John Wallace / NOAA
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Many fish — including sturgeons, paddlefish and catfishes — are sensitive to electric fields. But sharks are especially good at detecting the electric field generated by their prey, homing in on prey via electrical detection (including zeroing in on humans from as far away as 5 feet). We now know that the final strike of a shark is a response not to sound, vision, smell or hearing, but the prey’s electric field. The small holes between eye and nose tip visible on this blue shark (photo by Austin Gallagher) are ampullae of Lorenzini, the input structures for a shark’s electrical sense. The fishing-gear industry has capitalized on the response of salmon to electric fields and sells a generating system that can be hooked to downrigger lines. A friend and avid salmon fisherman in Puget Sound bought one and wired up his boat; he caught a lot of spiny dogfish. Austin Gallagher (
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And scarlet tanagers, brown thrashers, meadowlarks, catbirds, kingbirds, and swallows. As opportunistic as tiger sharks are known to be, they don’t frequent backyard bird feeders. But tigers in the Gulf of Mexico have learned to take advantage of exhausted songbirds that migrate every spring from South to North America. Those flying across the Gulf of Mexico pass over a string of oil and natural gas platforms, brightly lit up at night. Many birds, confused by the lights, circle the platforms by the thousands until they become exhausted and fall into the sea. Tiger sharks capitalize on this bonanza. Nearly half of 50 tiger sharks caught and examined over a two-year period near these oil rigs contained a birdwatcher’s list of small songbird species. (Photo by Austin Gallagher.) Austin Gallagher (
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Tiger sharks stalk their prey, very much in the way of terrestrial tigers, relying on the element of surprise. Video footage taken via “crittercams” attached to tiger sharks in western Australia showed that the sharks approached green sea turtles that were feeding on sea grasses from behind. If a turtle turned its head and looked at the shark, the shark veered off. But if the turtle was caught unawares, the shark attacked. Studies of stream-bank-dwelling Bengal tigers in India that attacked fishermen in canoes revealed that these predators also pounced from behind. When fishermen wore Halloween masks on the back of their heads, attacks from the shore declined significantly. Fishermen who were given masks but didn’t wear them were attacked at the same rate as fishermen without masks. It’s sort of like seatbelts and bike helmets: having one doesn’t help much if you don’t use it properly (in case you’re wondering, no one has tried painting faces on the backs of sea turtles as a test). Whatever their method of attack, tigers are formidable predators. The young chap in the photo (by Richard Pyle and Lisa Privitera) inspects a large tiger’s jaws, full of angular teeth of a type especially effective at sawing and tearing. Courtesy Richard Pyle and Lisa Privitera
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The question of whether sharks can see color has intrigued scientists for decades. Recent advances in microscopy have answered the question. Sharks are color-blind (though rays can see colors). Tests on 17 shark species, including tigers and bulls, indicate they lack the necessary cell types in their eyes. The sharks tested either possessed only rod cells in their eyes — good for black-and-white vision in low light — or had only one cone type that also would enhance brightness and contrast perception but not detect color differences. Shark coloration, or the lack of it, supports this conclusion, especially compared to rays. Sharks are pretty boring colorwise. They tend toward blacks, whites, browns, and grays in combinations that offer offer good camouflage (the blue backs of makos — like this shortfin (photo by David Hall, — and blue sharks are a different form of camouflage called countershading that is especially effective in open ocean conditions). Some rays, in contrast, sport blues and yellows, and their eyes contain the visual cells that would allow them to see these colors. What about yum-yum yellow? Early research suggested that sharks preferred to attack baits attached to floats painted international orange, the color that makes life jackets so visible to us. A literate Navy experimenter dubbed the color “yum-yum yellow” (and probably lost his commission). Mythbusters retested the idea in their episode, “Sharks Prefer Yellow?” and gave the idea “plausible” status. But the attraction could have been to brightness rather than the color itself. Bottom line: a shark may be able to smell minute quantities of blood in the water but doesn’t know that it’s red. David Hall,
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Fly-fishers in the Bahamas often report being followed by large lemon sharks while wading shallow water. It turns out that the sharks have learned that anglers release bonefish. But an exhausted bonefish may be too tired to escape its predators. One study estimated that as many as 40 percent of released bonefish are eaten by attendant lemon sharks. (Photo by David Hall, David Hall,
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The largest sharks are females, a fact that probably disappoints many 12-year-old boys. The size difference holds for most of the more notorious species, including white sharks. So references in Jaws to the Amity shark as “he” were undoubtedly wrong. (Photo by Chris & Monique Fallows) Chris & Monique Fallows (
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NOT WHAT YOU’D EXPECT. Florida is the shark attack capital of the United States, and Volusia County is ground zero. But most unprovoked attacks aren’t launched by white sharks, tigers, hammerheads or bulls. The most likely culprits are blacktip (shown here) and spinner sharks, both abundant in shallow water where people swim, surf, fish and dive. Tooth fragments from those attacked have been linked to these two species. Otherwise, identification at the time of the attack is unreliable because these are not easy species to identify, and during attacks, things happen fast and chaotically. (Photo by Doug Olander) Doug Olander
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Boys and men of a certain daring age can’t resist pulling the tail of a nurse shark sleeping on the bottom or cruising slowly over it, like the 5-footer shown here at Molasses Reef in Florida (photo by Gene Helfman). But a nurse shark is flexible enough to stick its tail in its mouth and, once it clamps onto some part of harasser, is hesitant to let go. (One diver suffered lacerated lips when he attempted to kiss a nurse shark). Such incidents are seldom fatal, except for the shark, which usually has to be killed before it will release its grip. Gene Helfman
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At least three species of sharks have been observed feeding cooperatively. Best known is the broadnose sevengill shark. Sevengills feed solitarily on fishes and small sharks, but when attacking large prey capable of defending themselves, such as cape fur seals, they attack in groups. A typical attack sequence can involve a dozen sevengills ganging up on a seal, surrounding it at the surface, thwarting any escape attempts. The sharks tighten the circle until one of them bites the seal, precipitating a feeding frenzy. All of the elements of cooperative feeding occur, including coordinated movement, division of labor and initial restraint. (Photo by Austin Gallagher) Courtesy Austin Gallagher (
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1) Sharks are cold-blooded killers. Nope. Whites, makos, porbeagles and salmon sharks have unique circulatory systems that help them keep a higher internal temperature than the outside water. White sharks may be as warm as 57 degrees F above ambient waters, and salmon sharks as much as 70 degrees F higher than the sub-arctic waters where they live. (2) Sharks must keep swimming or they will sink. Only partially true. Sharks are very close to neutrally buoyant. They achieve this thanks to their large, oily livers that provide flotation. A Basking Shark that tips the scales at 2,200 pounds on land actually weighs only 7 pounds in the water. The fearsome (but non-aggressive) sand tiger shark like that shown here (photo courtesy David Hall, may be the only shark that can increase its buoyancy by gulping air. (3) Most sharks swim at surface with dorsal fin exposed. Nope. Most species live on or near bottom. Why some sharks swim with their dorsal fin sticking out is discussed in the book. (4) Sharks must swim or they will suffocate. Nope. Many species can lie on bottom and pump water over their gills just fine. (Photo by David Hall) David Hall,
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The Discovery Channel announced Shark Week each summer by decorating its headquarters in Silver Spring, Maryland, with a giant inflated white shark. Head, tail and pectoral fins stuck out from the building’s back, front and sides. The head was about three stories high. Despite its size, it was apparently modeled after a juvenile white shark because the upper as well as the lower jaws (see inset) contained the slender teeth characteristic of fish-eating juveniles. White Sharks don’t develop their iconic triangular upper-jaw teeth until they mature at around 12 feet and start feeding on tougher marine mammals. (Photo by Gene Helfman.) Gene Helfman
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We can make a difference; the international grassroots campaign to reduce shark finning is working. The gruesome practice is illustrated by this scalloped hammerhead, sinking to the bottom off Cocos Island, Costa Rica, after commercial fishermen have cut its fins off and tossed it overboard (still alive). (Photo by Jeff Rotman.) Before 2011, about 22 million pounds of shark fins were imported annually into Hong Kong. This dropped to 7.4 million pounds in 2012, and fell another 35 percent in 2013! Young Chinese couples are increasingly refusing to serve shark-fin soup at their wedding banquets. The Shark Fin Trade Merchants Association has complained that sales of dried fins are down 60 percent. Tough. Jeff Rotman
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A factory in southeastern China is processing 600 whale sharks — like this 1,440-pounder being sliced up with a handsaw — a year to make shark oil for health supplements and for fins for the shark-fin-soup market. The products are exported to the United States and Canada, despite an international ban on all such trade. And the Chinese have begun killing manta rays to make gill-plate soup. The “logic” is that mantas filter food from water with their gills, so gill-plate soup can filter and detoxify diseases from someone’s body (cheaper than cleaning up the air in Beijing). (Photo by Wenn Photography
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Mating in sharks and rays can be an elaborate and sometimes violent affair, at least by human standards. Actual mating begins when a male grabs onto a female the only way he can, with his teeth. As this photo (courtesy Jeff Carrier) shows, a male may bite down on a female’s pectoral fin, gill area, or body, holding her while he inserts one of his modified pelvic fin claspers into the female’s cloaca. Females often bear bite marks and ragged scars on their body from these mating encounters. The skin of a female blue shark is thicker than that of a male, in fact just thicker than his teeth are long. Females don’t always appreciate this attention and have many ways of discouraging a male’s interest. Female nurse sharks avoid males by moving into shallower water and even burying their pectoral fins in the sand, making it harder for a male to grab her. When a male does succeed in grasping her fin, she may quickly roll over on her back and above the male, trying to twist loose and even pushing her ventral surface into the air, waiting for him to give up. Some female stingrays avoid copulation by burying in the sand, slapping their pectoral fins on the surface, and even stabbing a male with their tail barb. As a result, less than 10 percent of mating attempts actually end in successful clasper insertion. (Photo copyright Jeffrey C. Carrier. All rights reserved. Used with permission.) Copyright Jeffrey C. Carrier. All rights reserved. Used with permission.
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Unprovoked attacks by stingrays are rare, and fatalities even rarer. Most stingray encounters result from someone (such as a wade fisherman) stepping on a ray. The ray responds by flexing its tail upward and forward, driving its serrated, venomous spine into the wader’s foot or lower leg. The pain is described as excruciating. “Crocodile hunter” Steve Irwin was killed when a large stingray stabbed him in the chest, puncturing his heart. Whether the attack was provoked or not is a matter of conjecture. A scuba diver in Indonesia was also fatally stabbed in the chest when he mistook a large stingray for a (barbless) manta ray and attempted to ride it by grabbing on the front edges of the ray’s disk. (Photo of 4-foot-wide southern stingray by Gene Helfman.) Totally unprovoked ray encounters do however happen. Eagle and manta rays frequently leap entirely out of the water for reasons that aren’t altogether clear. A woman riding in a powerboat in the Florida Keys was killed when a 75-pound eagle ray landed on her after making such a soaring leap. She struck her head against the boat and died from blunt-force trauma. Another Florida boater was struck by a jumping eagle ray that landed in his boat, stabbing him in the chest. He fortunately survived. Gene Helfman
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Hollywood, tabloid newspapers and independent makers of sharksploitation movies would have us think there’s a slaughter going on. There is, but it’s the sharks being slaughtered. Sharks kill on average about six people a year worldwide; humans kill 30 to 70 million sharks a year (and some sources estimate much greater numbers.) By way of a comparison, Canada and Australia combined house fewer than 70 million people. U.S. fatalities from shark attack average fewer than one death per year. You’re more likely to be killed by a falling vending machine or a toaster than by a shark — Sharknado notwithstanding!
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We like to think of sharks as apex predators, but even large sharks have their enemies, big and small. Orcas (killer whales) are known to attack and eat white sharks, especially around the Farallon Islands off San Francisco. When this happens, other white sharks leave the area. Farther north off British Columbia, packs of orcas gang up and tear apart Pacific sleeper sharks, one of the world’s largest sharks at over 20 feet. One feeding frenzy produced a large slick of floating shark meat and liver, allowing the victimized shark to be identified using DNA analysis. At the other end of the size scale, each shark is a tasty, swimming platform for a zoofull of small, parasitic animals that feast on its blood and body tissues. About 1500 different parasites are known from sharks, skates, and rays, with each shark species home to between three and 14 unique parasitic species. Case in point: This photo (by George Benz) shows the eyeball of a Greenland shark infected with a parasitic copepod. A female copepod is anchored in the shark’s eyeball via a pair of modified legs. Her egg cases are the white bulb-like structures protruding from the eyeball to the right. Both eyes of some sharks may be infected, essentially blinding the shark, but they still manage to feed successfully. George Benz
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An industry developed around the misconception that sharks don’t get cancer and therefore products made from shark tissues, especially cartilage, can prevent the ailment. Millions of sharks have been killed to make cartilage extracts. But sharks do get cancer, including cartilage cancers as well as melanomas and cutaneous fibrosarcomas (skin and connective tissue cancers). The U.S. Food and Drug Administration and Federal Trade Commission filed injunctions against companies that made anti-cancer claims for shark cartilage products because clinical trials indicated that cartilage pills have little effect on cancerous cells. Products like that shown here require sharks to be killed to extract chondroitin from their cartilage. Never mind that the “vegetarian” capsules contain animal matter (and apparently the entire process is eco-friendly because it’s wind powered?) (Photo by Gene Helfman.) Shark products do have valid medical applications, including an anticancer drug (not from cartilage). Squalamine, originally isolated from shark livers, stomachs and gall bladders, has proven effective in treating lung and ovarian cancers and possibly some viruses. Squalamine is patented and produced by reputable drug manufacturers synthetically. There’s no need to kill a shark to obtain it! Gene Helfman