Long portrayed in pop culture as remorseless people-killers, sharks in reality are no Hollywood monsters. Sharks are a diverse group of mostly predatory fish, including the largest living fish, with skeletons made of cartilage. They have plied the seas since before the dinosaurs lived and play vital ecosystem roles.
Sharks' long evolutionary history includes reaching tremendous size, when one of Earth's biggest apex predators, the gigantic megalodon (Otodus megalodon), hunted the oceans.
At high risk of extinction, these marine creatures as a group face far more dangers from humans than any surfer does from a great white shark.
How did sharks evolve?
All sharks belong to the subclass of fish called elasmobranchs, along with skates and rays. Lacking bones, elasmobranch skeletons consist of the hard but flexible material cartilage, which is also found in your ears and nose, according to the Smithsonian.
Because cartilage skeletons rarely fossilize, establishing the natural history of sharks can pose a challenge, according to the American Museum of Natural History in New York City. But sharks' impressive teeth, along with scales, leave enough of a record to estimate that the animals first evolved more 450 million years ago, according to the Natural History Museum in London.
That timeline means sharks prowled the primordial seas around 200 million years before the first dinosaurs, even predating the first forests, which didn't appear until 385 million years ago, Live Science previously reported. Over those eons, sharks survived "the big five mass extinctions." They even enjoyed a "Golden Age" beginning around 360 million years ago, when a tremendous global die-off claimed many fish, leaving sharks to dominate — and proliferate into new species, according to London's Natural History Museum.
How many shark species are there?
Sharks have evolved into roughly 500 species alive today, according to the Shark Foundation, and they range widely in size, shape and even color. You could fit the smallest living shark in your pocket; the dwarf lantern shark (Etmopterus perryi) spans just 3 inches (8 centimeters), according to the Smithsonian. Sharks also hold the titles of both largest fish and largest predatory fish. The "gentle giant" whale shark (Rhincodon typus) can stretch to nearly 40 feet (12 meters) long, eclipsing a full-size school bus, the World Wildlife Fund says.
You likely know the largest predatory fish, great whites, from their extensive film credits, most prominently 1975's "Jaws." Female great whites grow bigger, at an average length of 15 to 16 feet (4.6 to 4.9 m), while males reach 11 to 13 feet (3.4 to 4 m). The largest great white ever, nicknamed Deep Blue, spanned 20 feet, Live Science previously reported. Using computer modeling techniques, a 2008 study in the Journal of Zoology estimated that great whites exert a bite force of up to 2 tons (1.8 metric tons), or more than three times that of a large African lion (Panthera leo leo).
Prehistory's biggest shark dwarfed even Steven Spielberg's killer fish. Megalodon, which swam the ancient seas from about 23 million to 4 million years ago, grew up to 65 feet (20 m) long, according to 2021 re-estimates based on the creature's teeth published in the journal Palaeontologia Electronica. Megalodon has also gotten the pop culture treatment, starring in a fake documentary that suggested the behemoth fish survived today. But rest assured: Scientists insist there's no evidence for that.
Shark species also vary widely in shape, from the built-for-speed torpedo design of great whites to the long, tooth-lined snouts of saw sharks. The odd-looking goblin shark makes a distinct impression with its flabby body and shovel-like snout set above a retractable mouth, according to the Australian Museum. Shark skin paints a rainbow, too. Those little lantern sharks dress in a conservative dark brown but glam things up with light-emitting organs, according to the Smithsonian. One deep-sea goblin shark found by the Australian Museum in 2015 sported a bright pink hue, while carpet sharks get their name from ornate skin patterns, according to the Smithsonian.
Scientists uncover newfound species yearly, according to the World Wildlife Fund. In 2021, the so-called Godzilla shark — an ancient, 6.7-foot-long (2 m) beast discovered in 2013 and so nicknamed because of its reptilian spines — got a new name: Hoffman's dragon shark (Dracopristis hoffmanorum). Also in 2021, researchers discovered the 6-foot (1.5 m) kitefin shark (Dalatias licha), the largest glow-in-the dark shark — in fact, the largest glowing vertebrate — ever found, as reported in the journal Frontiers in Marine Science.
Where do sharks live?
Sharks are found in "just about every kind of ocean habitat," according to Smithsonian, including in the "open ocean, coral reefs and under the Arctic ice," as well as in "the deepest parts of the ocean." Deep-sea dwellers include glowing creatures, like that kitefin, that can plumb the ocean's "twilight zone," or 650 to 3,300 feet (200 to 1,000 m) deep.
Sharks make their homes in waters across the planet, with some migrating vast distances for food and mates. Seasonal temperature changes and the need to find a birthing spot also motivate these movements, according to the Dutch Shark Society. Using GPS tracking, scientists found that great whites can journey some 2,500 miles (4,000 km) from the California coast seeking feeding grounds, researchers reported in 2013 in the journal Proceedings of the Royal Society B.
Many sharks also migrate up and down, riding the oceanic elevator daily in search of food and amenable temperatures. These vertical journeys usually range from 30 to 300 feet (9 to 90 m), the Smithsonian says. However, blue sharks (Prionace glauca) can make daily drops as far as 1,900 feet (600 m) from the surface, likely to hunt deep-sea prey.
Biologists have found sharks in some unexpected spots. In 2015, Kavachi, an undersea volcano in the Solomon Islands, earned the moniker "Sharkano" when scientists realized two species called the active volcano home. These creatures, hammerheads and silky sharks (Carcharhinus falciformis), manage to thrive in the hot, acidic, sulfur-infused waters of the volcano, which erupted again in 2022. A 2022 study found that bull, nurse and hammerhead sharks split time between Miami's noisy, polluted environs and more natural environments, behaving more as "urban adapters" than urban avoiders, like wolves.
What do sharks eat?
While most sharks are carnivores, "you can find a shark that eats just about anything," according to the Smithsonian. Many of these creatures swallow their food whole, while others use specialized teeth. Port Jackson sharks (Heterodontus portusjacksoni) may use flat chompers to grind up meals, according to the Australian Museum in Sydney.
Unexpected shark meals include seagrass, which can make up more than half the diet of bonnethead sharks (Sphyrna tiburo). In 2018, scientists fed these small hammerheads "little sushi rolls" of seagrass and found that the animals could digest the vegetables about as well as sea turtles could, as reported in the journal Proceedings of the Royal Society B.
The usual, meat-centric diet of sharks can range from tiny plankton to much larger prey, including crabs and other crustaceans, fish, and hefty marine mammals, the Smithsonian says. While young great whites survive on smaller squids and sting rays, their grown cousins graduate to seals, sea lions, dolphins and dead whales, according to the Australian Museum. In 2010, researchers even found the beaks of two giant squids in the belly of a blue shark.
Like many of its namesake mammals, the whale shark filter-feeds, straining teeny plankton from up to 400,000 gallons (1.5 million liters) of filtered water every hour, according to the World Wildlife Fund. Sawsharks use their namesake snouts to scrape up critters from the seafloor, while cookiecutter sharks (Isistius brasiliensis) earned their name by extracting circular bites from whales and other large creatures, according to the Smithsonian. Bottom-feeding sharks, like carpet and nurse sharks, use their powerful lips and mouths to suck prey out of hiding places.
Even the fearsome great white may engage in bottom feeding. Bottom-dwellers made up more than one-fifth of the stomach contents of a group of 40 young great whites, Live Science reported in 2020. "The hunting of bigger prey … is not likely to happen until the sharks reach about 2.2 meters [7.2 feet] in length," the scientists said at the time.
Sharks differ from bony fish not just at the skeletal level, but also in their reproductive patterns, growing and maturing more slowly. They also invest more in a few offspring, putting "a lot of effort into producing a small number of highly developed young at birth,” the Smithsonian says, "rather than releasing a large number of eggs that have a high probability of not surviving."
The sex lives of sharks present a number of oddities to human observers, too. Instead of penises, male sharks use two claspers to get sperm into the female. About 15 shark species even engage in parthenogenesis, or "virgin birth," with females self-fertilizing eggs.
Newfound features of shark behavior have surprised scientists. A 2022 report in the journal Biology Letters found that some sharks sleep, though they do it rather creepily, with their eyes open. Often caricatured as lone wolves, sharks revealed a social life in a 2020 study in the journal Proceedings of the Royal Society B. By attaching small cameras and location transmitters to gray reef sharks (Carcharhinus amblyrhynchos), researchers found that groups of up to 20 sharks kept returning to the same areas, likely gaining hunting benefits from group behavior.
Such socializing just provides more evidence of sharks' underappreciated intelligence, marine conservation biologist David Shiffman told NPR, noting that some sharks learn to solve puzzles by watching other sharks do so. "They are a lot smarter than most people think, and they have more complex social behavior and more complex ability to process their environment," Shiffman, a postdoctoral scholar at Arizona State University in Scottsdale, told NPR.
Are sharks endangered?
Human activity has placed sharks in grave danger. In particular, overfishing has put one-third of sharks and related creatures, such as rays, at risk of extinction, according to a 2021 study published in the journal Current Biology.
Fishing kills an estimated 100 million sharks per year, according to a 2013 study, with fisheries killing 1 in 15 sharks yearly. Other research has found declines of as much as 90% in Pacific reef sharks near human populations, researchers reported in Current Biology in 2012.
People hunt sharks for various reasons — including meat, oil and cartilage — but the animals' fins motivate much of the harvesting. Shark fin soup, considered a delicacy in Asian and Western countries, garners up to $100 per bowl.
Because of the value of shark fins, some fishers engage in "finning," cutting off fins and discarding the rest of the shark to free up boat space. The mutilated animals are left to bleed to death or suffocate, according to the The Humane Society of the United States. (Many sharks need to keep swimming to pass water over their gills so they can breathe.)
Beyond such targeted fishing, many sharks die every year as bycatch, trapped by fishing operations targeting other catches. The onset of industrial fishing of tuna and other big fish, for instance, led to a 21% drop in abundance of large predators such as sharks, researchers reported in 2005 in the journal Ecology.
These threats can have outsize impacts on sharks. Because of sharks' investment in few offspring and long growth and reproduction times, each death hits shark populations harder than other fish, according to the Smithsonian. Losses of shark populations also hit ecosystems hard, as these apex predators keep food webs in balance. Sharks help to maintain fish population health by weeding out sick and weak individuals and may help regulate oxygen production, hunting down fish that eat oxygen-producing plankton, Live Science previously reported.
These ecological ripple effects then come back to bite humans, so to speak, as sharks maintain "marine and coastal ecosystems that billions of humans depend on for their livelihoods and food security," Shiffman wrote in Scientific American.
Because of these threats, horror filmmaker Eli Roth released a documentary alerting the world to the plight of sharks. "Fifty years ago the world came together to save the whales; then we did it for dolphins, and recently for orcas," Roth said in a statement about the 2021 film, called "Fin." "It's time to do the same for sharks, and time is running out."
Some governments and international groups have taken heed of such pleas. In 2009, the International Union for Conservation of Nature called on world governments to protect sharks through catch limits, protections on endangered species, the banning of shark finning and improved monitoring of fisheries. The U.S. banned finning in 2000, and several states have banned the sale and possession of shark fins, according to the Smithsonian. Internationally, agreements have prohibited and regulated shark finning in 17 regions, and individual countries have outlawed the practice. Palau created the first shark sanctuary, in 2009.
The Chinese government stopped serving shark fin soup at state banquets in 2014, while some restaurants now use alternatives, such as dried sea cucumber. Some hotels, like the Marriott chain, have banned shark fin dishes.
A largely irrational fear of these creatures continues to stand in the way of adequate protections, according to a book by Shiffman, "Why Sharks Matter" (Johns Hopkins University Press, 2022). The book argues that the world needs to do more to protect these animals, pointing out that "a fear of sharks is no more rational than a fear of cars, lawnmowers or toasters, all of which outperform the animals in lethality," according to a New York Times review. Shark attacks have "killed fewer humans within a year than such risks as accidents taking scenic selfies and encounters with vending machines," Shiffman wrote in Scientific American.
Watch director Eli Roth's "Fin" documentary on Discovery+. See the low risk of shark attacks put in terms of other rare risks, like getting struck by lightning, from the Florida Museum. Follow great white shark activity online with the Atlantic White Shark Conservancy Sharktivity app. Learn how shark teeth were mistaken for dragon tongues in the Middle Ages, along with other odd shark facts, from the Smithsonian.
Original article on Live Science.
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Michael Dhar is a science editor and writer based in Chicago. He has an MS in bioinformatics from NYU Tandon School of Engineering, an MA in English literature from Columbia University and a BA in English from the University of Iowa. He has written about health and science for Live Science, Scientific American, Space.com, The Fix, Earth.com and others and has edited for the American Medical Association and other organizations.