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Feeding the Beast
Everything about him was beautiful except his jaws. . . . When the old man saw him coming he knew that this was a shark that had no fear at all and would do exactly what he wished.
—Ernest Hemingway, The Old Man and the Sea
Like most living beings, humans included, sharks are opportunists. Their biological goal is to stay well fed, safe, and physically strong long enough to reproduce and to produce enough offspring to maintain the long-term survival of their species.
Sharks eat every chance they get. Their ability to survive and thrive depends on how well they can use the food they snap up. It helps that they have a digestive tract wide enough to accommodate whole fish, big chunks of mammal meat, and whatever else they eat. A big stomach in which to store food helps too—as does an intestine that quickly digests the food for energy.
Try Googling these keywords, “things found in shark stomachs.” You’ll find lists of hard-to-believe items, including everything from a can of peas to a whole polar bear (in the stomach of a Greenland shark). Sharks have another superpower that makes them the survivors they are. They can evert their stomachs, turning them inside out when they eat something indigestible or that just rubs them the wrong way. Sharks can get rid of things they literally can’t stomach.
Great white sharks, such as this one off the coast of South Africa, are known to breach, or leap out of the water, to catch fast-moving prey such as seals. The sharks do not breach often because doing so requires a lot of energy. But when they do, they may reach speeds of 40 miles (64 km) per hour and leap 10 feet (3 m) into the air.
“Gangsters of the Deep”
When it comes to finding food, nobody does it better than sharks. What do sharks think as they find food, which takes up most of their time? Eugenie Clark’s work put to rest the idea that sharks are like Bruce, the robotic shark star in Jaws. She said, “People generally thought that sharks are dumb eating machines. After some study, I began to realize that these ‘gangsters’ of the deep had gotten a bad rap.” Clark trained sharks to push a button for food. From that experience, she concluded that sharks are intelligent, since they can be trained.
To find their food and to survive in the ocean, sharks have evolved shark sense—superpower observation and detection skills. Alison Kock, who studies great white sharks, finds sharks to be inquisitive. And curiosity is another measurement of intelligence. What’s more, Kock’s colleague Leonard Compagno, director of the Shark Research Institute in Princeton, New Jersey, says that great whites have a social order. Each shark takes a certain role in the pecking order. Sharks sometimes hunt in groups, a measure of cooperation and another sign of shark smarts. “Once, when there were several people on the boat, the great white looked each person in the eye, one by one, checking us out,” Compagno told Smithsonian magazine. “They feed on large-brained social animals such as seals and dolphins, and to do this you have to operate on a level higher than [the] simple machine mentality of an ordinary fish.”
Shark Sense
Humans have five senses: vision, hearing, smell, taste, and touch. Sharks have four senses. But don’t be fooled into thinking that means they are clueless about their environment. For each sense, they have highly developed supersensitive sensors. Most sharks are nocturnal, swimming and hunting in the dark and resting during the daytime. The four shark-sensing superpowers help them track down food and detect prey whenever it’s around.
Photoreception (Light Sense and Vision combined)
Sharks see well. Their retinas, the light receptor in their eyes, have adapted so the animals can see well in whatever light is available—dark or bright. Some sharks see colors.
Many sharks, including tiger sharks, have a nictitating membrane (a third, translucent eyelid) in their eyes. This defense mechanism covers the eyeball and protects it from the sharp claws or teeth of struggling prey. The membrane is an extra fold of eyelid that slides up to expose only a slit of the shark’s eye. When this happens, it is a sure sign that the shark is starting a bite. Great whites do not have nictitating membranes, so they roll their eyes backward to protect them while attacking prey.
Some sharks, such as the blue shark (above), have a nictitating eyelid membrane. The shark can close this membrane to protect the eye when attacking prey. Sharks such as the great white, which do not have this third eyelid, roll back their pupils for protection instead.
Chemoreception (Smell and Taste combined)
This sense helps sharks detect chemical clues in the water. Sharks have nostrils in their snouts that lead to the olfactory bulb, an internal smell processor. Nasal barbels (flaps) hang from some sharks’ snouts. (Look for them on bottom-dwellers.) The barbels touch or drag along the bottom of the seafloor to taste for and sense the vibrations of prey that might be hiding under sand or sediment. Taste receptors in a shark’s mouth and pharynx (throat) offer more information when the shark takes a test bite.
This nurse shark rests on the sand in the shallow waters of the Bahamas National Shark Sanctuary in the West Atlantic Ocean. Its long nasal barbels hang from its top lip and help the shark sense prey under the sand.
Mechanoreception (Motion Detectors)
Sharks have vibration sensors along their sides, in their ears and snouts, and all over their bodies. The lateral line is a row of pressure sensors along the shark’s sides and head. The sensors pick up motion through changes in pressure waves and water currents that come from whatever is moving in the water. Sharks process the information from different types of vibrations (such as the distressed thrashing of a hurt fish) to determine what to check out and what to ignore. Endolymphatic ducts, narrow channels in the shark’s inner ear, also pick up motion. Touch receptors over the shark’s whole body offer more clues about nearby movements. Certain sharks with poor vision, such as bull sharks, will even head-butt something to find out more about it before taking a bite.
Electroreception (Electrical Charges)
Can sharks really smell blood a mile (1.6 km) away? Can they really sense fear? Ichthyologists have shown that both of these popular ideas are myths. But sharks can sense a pounding heart, through electroreception sensing. To humans, electroreception seems like extrasensory perception (ESP), extra information we can’t logically or physically sense. But sharks actually do have an extra sense.
Sharks can detect electricity from miles away using ampullae of Lorenzini. These tiny jelly-filled pores in the snout detect very weak electrical pulses, about one billionth of a volt per meter. But what kind of electricity are they homing in on? The pores pick up electricity generated by moving muscles or firing neurons (nerve cells) of fish or other prey. Seawater conducts this electricity quite nicely, sending it straight to the shark’s ampullae of Lorenzini and to the brain. Hammerhead sharks are especially good at finding prey because the ventral (lower) surface of their bodies has an abundance of ampullae of Lorenzini. Besides locating prey, ampullae of Lorenzini may help sharks navigate the ocean by figuring out where they are. Earth’s electromagnetic field is a skin of energy that circles the planet, with a different level of energy at every spot. Many animals with a magnetic sense, including sea turtles and birds, use this field to navigate. With their ampullae, sharks have this ability too.
Pores known as ampullae of Lorenzini help sharks such as this lemon shark sense electrical impulses associated with muscle movements in prey animals. The pores are on the underside of a shark’s snout and near its nostrils. They are connected to long, jelly-filled tubes that lead to cells below the skin that sense electrical activity. The pores are named for a seventeenth-century Italian doctor, Stefano Lorenzini. In Latin, ampullae refers to “amphora,” a type of oval, two-handled pot. In anatomy, ampullae refers to widened tubes, such as ear canals or the shark pore system.
Light: Another Super Sense?
Do sharks glow in the dark? David Gruber, a scientist at Baruch College in New York City, dives with deep-dwelling sharks to learn more about them. He discovered that two species of catshark—the chain catshark (Scyliorhinus rotifer) and the swell shark (Cephaloscyllium ventriosum)—biofluoresce, or glow in the dark. They may use this ability to locate or communicate with other sharks that live in deep, dark waters.
In biofluorescence, molecules in an animal’s body take in blue (high-energy) light and give it back as green or red (lower-energy) light. Humans can’t see biofluorescence. Only fish and some other sea creatures, including certain stingrays (a relative of sharks), fish, and sea turtles, can see the glow. Gruber and his team realized that catsharks probably can see the biofluorescent green light as well as the blue light.
Why do sharks biofluoresce? To find out, Gruber’s team built a shark’s-eye camera, adding filters to let in green and blue light. Wearing scuba gear, the scientists dove in Scripps Canyon, a large underwater gorge near San Diego, California. Sure enough, glowing green sharks loomed out of the deep.
The camera found that different catshark species each shine with their own patterns. And the patterns also vary between males and females. “This work forces us to take a step out of the human perspective and start imagining the world through a shark’s perspective,” Gruber told National Geographic. His future work will involve a closer look at the messages catsharks send out through biofluorescence. He thinks the green glow helps them locate and attract mates and to lure food prey.
Eating
Sharks are carnivores (meat eaters). The meat they eat includes everything from the tiniest plankton and krill to clams and crabs, squid, octopus, seabirds, sea snakes, small fish, big fish, jellyfish, marine mammals such as dolphins and manatees, sea turtles, and even other sharks and their relatives, rays. Most sharks—but not all—prey on animals that are smaller than they are. Only a few, including tiger sharks and great whites, will attack live whales and other marine mammals. Most sharks will feed only on dead ones.
Each species of shark eats a particular type of food prey and has the physiology (body features) for stalking and catching it. This includes teeth, jaws, skin color, and attacking behaviors best suited to catching the prey animal. For example, a great white shark charges up to attack seals and sea lions from below. It stuns the prey with a shocking chomp, retreats, and waits for the prey to die of blood loss before returning to eat it. Some scientists say that if the first bite isn’t tasty enough, the shark may not return to finish the meal.
A shortfin mako will swim in a figure eight, signaling that it is getting ready to attack. Other sharks bump and circle their prey. A speedy blue shark will swim through a school of squid or fish with its mouth wide open, eating whatever swims in.
Dogfish attack in groups like a pack of sharp-toothed dogs. (That’s how they got their name.) Packs of dogfish sharks often number in the hundreds as they pursue mackerel, capelin, squid, jellyfish, and other prey. Even newborn spiny dogfish—at most 49 inches (124 cm) long when fully grown—will attack prey more than twice their size. Adults will go after orca and seals. The spiny dogfish gets its name from two sharp and poisonous spines in front of each dorsal fin. The shark may stab its prey with the spines to inject venom. Threshers use their big swooping tail fins to stun and confuse prey fish. The fish become confused, and the threshers can easily corral them, or round them up.
Bottom-feeders such as the horn shark hunt alone at night. They sometimes slurp down so many sea urchins that their teeth turn purple. Surface skimmers such as the basking shark open wide to let plankton swim in. And others, including the velvet belly shark, live in the deeps during the day and migrate up to the surface to feed on squid and fish at night.
Sharks don’t eat that much—3 to 5 percent of their body weight per meal. They make the most of their opportunities, eating whatever is available in the waters and every day if they can. And, if necessary, many shark species can lay off the chow for a few weeks, living off the oil reserves in their large livers.
Shark Teeth
As meat eaters, sharks are well equipped with mouths and teeth. The basking shark has the biggest mouth among sharks. With its 3-foot-wide (1 m) mouth, this shark is a sort of swimming vacuum cleaner, eating tiny surface-dwelling plankton. Like its fellow plankton eaters, the whale shark and megamouth shark, basking sharks are mammoth. Their multitude of tiny teeth (up to one hundred per row) aren’t for chomping prey. They function as a filter to sift out prey from each mouthful of water. The water flows out through the gills, and food stays in.
Megamouth sharks were first discovered in 1976, off the coast of Hawaii, and are rarely seen by humans. The deep-sea sharks have giant, bathtub-sized mouths, though they are the smallest of the filter-feeding sharks. Because they do not actively pursue prey, they are slow swimmers and their muscles are not well developed.
Like the basking shark, the great white shark has jaws that open extra wide. But the great white isn’t after tiny plankton. It is after huge prey—usually a seal. The zebra bullhead shark’s jaw is adapted to feed from the seabed. Other sharks, such as lemon sharks, have teeth shaped like daggers to hold onto fast-moving, thrashing prey. Sharks such as nurse sharks, which prey on smaller fish, have smaller mouths and teeth.
Over millions of years and many generations, sharks became efficient predators. For example, ancient sharks had mouths in front of their eyes and at the end of their snouts, like dogs, eagles, and other carnivorous animals. Modern sharks mainly have ventral mouths that are on the underside of their body. As a shark attacks, its jaws shift and jut, its snout turns, and it throws its whole head and powerful body into biting and ripping at its food prey.
Shark teeth are different from other meat-eater teeth. Instead of growing from sockets, like human or wolf teeth, shark teeth are set in rows of grooves. Sharks have two sets of teeth, functional teeth for the kill and replacement teeth to fill in for lost functional teeth. And sharks lose teeth while feeding, as often as every two days. They replace any lost tooth with a tooth from the next row. It’s something like the way a vending machine works. Buy a granola bar, and as it drops into the tray below, another bar slides forward to fill the empty slot. Depending on species, a shark may have as many as fifteen replacement rows.
The shape and size of shark teeth help researchers and beachcombers identify the species of shark and the type of prey they eat. Big, triangular teeth come from sharks such as great whites that feed on seals and other marine mammals as well as on big fish. The shortfin mako’s narrow, curved teeth are perfect for nabbing speedy, darting schooling fish. The houndshark’s teeth line up like small white stones along the shark’s jaw to form what looks like a pebbled lip. These teeth are just right for crushing crunchy crustaceans (for example, crabs and lobsters). Horn sharks have smooth, flat teeth that resemble human molars to break the exoskeletons (exterior skeletons) of crabs, mussels, and other shellfish. A sand tiger has spiky teeth for piercing and tearing the flesh of its prey.
Some sharks have teeth that change shape as they grow larger and shift from hunting smaller prey to hunting something bigger. For example, a small shortfin mako will have little, thin teeth for eating small fish when it is young. As the shortfin mako grows to adult size, its teeth become bigger and stronger so it can graduate to hunting swordfish and dolphins. Every shark has just the right set of teeth for the job!
Shortfin mako have long, smooth-edged teeth. Those at the front of the jaw are curved for grasping fish prey, which they swallow whole.