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Swimming with Sharks
Our long-term satellite tracking data is showing that [individual shortfin mako sharks] can travel over 10,000 miles [16,093 km] in a single year, and they also are starting to show indications of round-trip, repeated and predictable migration patterns.
—Mahmood Shivji, Nova Southeastern University’s Guy Harvey Research Institute and the Save Our Seas Shark Research Center in Fort Lauderdale, Florida
Scuba divers have a universal signal for “there goes a shark”: a diver plants the heel of her hand on the top of her head and holds her fingers stiffly upward like a shark’s fin. But that’s where the similarity between human and shark swimming ends.
A shark masters the water the way a bird masters the air. As the shark swims, its caudal (tail) fin moves side to side, providing the main source of power to propel the shark forward. (Sharks can’t swim backward.) The motion of the larger upper caudal lobe creates a downward force offset by the lift of the shark’s head and chest. The pectoral fins on the sides of the shark’s chest are shaped to provide lift. Water flows more slowly over the rounded top edge and faster beneath the straighter bottom edge of the fins. This difference in speed of water flow raises the shark upward and creates thrust. The dorsal (back) fins provide stability by counterbalancing the lift and weight of the front of the shark’s body.
Sharks, such as this blue shark, move through water with a sinusoidal movement of the tail that is S-like, smooth, and repetitive. Sharks have two groups of muscles. Red muscle contracts slowly to provide stamina. It requires a lot of oxygen. White muscle contracts more rapidly for bursts of speed. It does not require as much oxygen.
Some sharks, such as the porbeagle, have caudal keels—ridges on the sides of their tails between the dorsal fins and caudal fin. The keels make the sharks more hydrodynamic (streamlined) so they can swim faster. Usually sharks swim slowly. The fastest shark may be the shortfin mako. Researchers recorded juvenile shortfin makos moving at 31 to 68 miles (50 to 110 km) per hour. Slower adults may swim in bursts of up to 46 miles (74 km) per hour.
Sharks have techniques for moving through the water to avoid predators and to catch prey. For example, the piked dogfish has a spike in front of each dorsal fin to defend against predators. For hunting, shark tails generally have a short lower lobe so they can press toward the seafloor to find food sources there. Shortfin makos, threshers, spinners, and great whites have what it takes to accelerate through the surface of the water to catch fast-moving prey. Researchers observed a 12-foot (3.5 m) shark that could jump as high as 8 feet (2.4 m) out of the water to pursue a leaping seal. (Check out videos of great whites lunging out of the water after skimming seals. They are not for the faint of heart!)
Shark Speed
When you make a beeline for something, you’re steering directly for it. When you make a shark line toward a target, the path is S-shaped, more snake than straight shot. This motion—called a sinusoidal (back-and-forth, wavelike) movement—comes from the way muscles that stretch the length of the shark’s body move in a repetitive, back-and-forth cycle. First, the muscles contract on the left and then the right, back and forth. The contractions pull the shark’s spine one way and then the other. The shark’s caudal fin powers this wave of motion, pushing the shark ahead. The fastest sharks start swimming with this wavy motion, then stiffen and straighten to cut down on energy use.
A shark’s skin feels like sandpaper under your fingers. You might think a rough surface would create friction and drag, slowing down the shark, but the opposite is true. The denticles on the shark’s skin align in a grooved pattern. The water easily flows through rather than around the shark’s skin, cutting down on resistance as the shark swims and helping it swim faster.
Olympic Skin
Swimmers were in an uproar as they prepared for the London Summer Olympics in 2012. At the previous Summer Olympics in Beijing, China, in 2008, many swimmers competed in Speedo LZR suits made from Fastskin FSII. This super-lightweight, thin, high-tech fabric compressed swimmers’ bodies into the suit, making them more hydrodynamic and therefore faster in the water. Critics of the fabric felt it gave swimmers who wore it an unfair advantage, so the London Olympics Organizing Committee banned the Fastskin suits. Speedo designers went back to the drawing board. And so did Harvard University biologist George Lauder.
Lauder knew of a “fabric” that was truly the fastest—the mako shark’s skin. To test his theory, he created a water treadmill to compare Fastskin to mako sharkskin. The water treadmill pumped water through a tank. Lauder then put two model fish, one made from strips of sharkskin, the other from strips of Fastskin, into the treadmill tank, where water flowed past each one. The result? As Lauder had suspected, the mako-skin fish allowed for faster flow of water than the Fastskin fish. When Lauder sanded the mako-skin fish to remove the denticles, the fish slowed down.
Lauder’s next experiment was to build model sharkskin. He used a 3D printer to make denticles from synthetic material. Then he embedded the 3D denticles into a thin, flexible layer of rubberlike material to mimic the sharkskin membrane that holds denticles in place in a real shark. But Lauder’s technology fell short. His 3D printer could only print denticles that were ten times the size of a mako’s actual denticles, which measure 150 microns, the thickness of a sheet of paper. About his experiment, Lauder said, “The essence of science is being able to control variables and manipulate things in interesting ways. With artificial sharkskin, we can experiment to see what surface structures mean. Maybe we could make a better sharkskin than sharkskin!”
Vacation Migration
But why do sharks swim? And where are they going? Sharks migrate mostly to breed and feed. Spiny dogfish, for example, migrate many miles to pursue their prey around the ocean. One tagged individual swam from Washington State to Japan—a 5,000-mile (8,047 km) one-way journey. Salmon sharks swim from the coast of Alaska to subtropical parts of the Pacific Ocean near Hawaii. One salmon shark traveled 11,321 miles (18,219 km) over 640 days—almost half the distance around Earth. Great white sharks wintering off San Francisco, California, also migrate seasonally 2,280 miles (3,669 km) to waters near the Hawaiian Islands. Many stop off at a feeding area halfway between Hawaii and Baja California Peninsula, Mexico, that is so popular with migrating sharks that scientists call it the Shark Cafe.
Some sharks follow predictable routes—just as people do when they flock to the same beach at certain times every year. For example, researchers in a small plane flying over the Atlantic Ocean see and take photographs every year of the annual blacktip shark migration off the coast of eastern Florida. Every winter this shark migration, the nation’s largest, creates headline news in Florida. Anyone who looks at the pictures of the sharks is bound to ask, “What are all those sharks doing at the beach?”
According to Florida Atlantic University shark researchers, about eleven thousand to fourteen thousand blacktips swarm this part of coastal Florida every February. From their plane, the researchers try to count every one. In the winter, the sharks are swimming southward from cold northern waters to warmer waters, following prey. In summer they’ll swim back north again, following the same types of prey animals.
Shark Migration
This infographic shows the annual spring migration territory of the great white shark. The Shark Cafe is a spot about halfway between western Mexico and Hawaii where the sharks stay from April to July. Scientists aren’t quite sure if they go there for mating or for food. In winter the sharks feed and mate along the coasts of central and Southern California.
Not all sharks migrate. Nurse sharks and bonnethead sharks are among the homebodies. Of the shark species that do migrate, coastal pelagic sharks such as blacktip, tiger, and sandbar sharks may swim about 1,000 miles (1,600 km) a year. They follow food, sticking with whatever water temperature is hospitable to their prey. Highly pelagic sharks sprint across the ocean. Among them are the fastest swimmers, such as blue and mako sharks. They ride currents to make better time as they cover great distances for food and reproduction. For example, researchers have tracked female blue sharks that mate in the spring and early summer off the coast of New England and swim across the Atlantic Ocean to pup off the western coast of Africa. They make their return trip up to fourteen months later by way of the North Equatorial Current, which moves westward across the Atlantic Ocean and toward the Caribbean Sea. From there the sharks catch the Gulf Stream current to move back up the eastern coast of the United States to their mating grounds. All told, the round-trip is 9,500 miles (15,289 km).
Sharks also migrate vertically, diving as deep as 2,900 feet (884 m) to live in colder waters during the winter. They slow down in these waters and need less food, so they eat less in winter. In summer, as the waters warm, the sharks return to the surface to feed on abundant food supplies there.
Sharks follow natural ocean currents and temperatures in their annual migratory treks. This infographic shows major currents in the Atlantic Ocean.
How Sharks Find Their Way
Scientists can’t pin down whether sharks navigate with their noses or with a sense of their position on Earth’s electromagnetic field. The answer is likely a combination of factors. Earth is surrounded by an invisible skin of electromagnetic charge that emerges from one pole, sheaths the planet, and enters again through the other pole. Every spot on Earth has a different and precise electromagnetic charge. Many animals instinctively sense the charge and use it to determine where they are in space and to orient themselves toward a target or destination.
Certain sharks instinctively swim along paths with waypoints along Earth’s electromagnetic field. Tiger sharks are one species that seem to have a mental map of the ocean in their heads. They migrate enormous distances (sometimes thousands of miles). Within smaller areas, they swim in straight lines—what scientists call directed walks—toward food sources. Scientists who observe the sharks say the animals seem to target the sources and know how to get there without wavering in their course.
Tiger sharks will travel long distances to find food. And they are not picky eaters. Researchers sometimes call them garbage cans of the sea because they literally eat trash that humans toss or leave behind—everything from boat cushions to bags of potatoes.
Older tiger sharks swim along straighter paths than younger ones do. This indicates to scientists that tiger sharks learn their way around, remembering destinations and previous routes. How are they doing it? Scientists aren’t precisely sure. Other wayfinding animals such as sea turtles, trout, and yellowfin tuna have magnetite crystals in their heads. The crystals respond to Earth’s electromagnetic signals, providing these animals with a natural Global Positioning System (GPS). But sharks don’t have magnetite, so how are they finding their way? One clue comes from when they are finding their way. Yannis Papastamatiou of the Florida Museum of Natural History led a study to learn more about how sharks orient themselves. In an interview about the study, he told the British Broadcasting Corporation that “the sharks’ ability to navigate is open to debate, but the fact that many of these [shark] journeys took place at night—you and I would think there’s nothing to orientate to, so orientating to magnetic fields is one possibility.”
Scientists do know that sharks rely on their ampullae of Lorenzini for navigation. These jelly-filled pores in their snouts respond to electrical signals. As sharks swim through Earth’s electromagnetic field, scientists say, their bodies generate current that the ampullae pick up. Magnetoreception—receiving the charges—is the key to the sharks’ mental map and helps them adjust their paths when they need to.
The Papastamatiou study also looked at other shark species to see if they too swim in straight lines. Threshers were a yes, but blacktip reef sharks were a no. Their walks were random, not direct. Scientists know that blacktips have a smaller range, staying near reefs. So they think their navigation system may not require as many sensory factors as sharks that move across much bigger territories.
Following Their Noses?
Andrew Nosal is a researcher at the Scripps Institution of Oceanography and Birch Aquarium, in La Jolla, California. For one study, Nosal caught, tagged, and transported twenty-six leopard sharks 6 miles (10 km) from their coastal home to an area much farther out to sea. To learn more about how they would navigate their way back home, Nosal stuffed the nasal passages of eleven of the leopard sharks with cotton to weaken their sense of smell. He left fifteen other sharks with fully functional noses. The discovery? The fifteen sharks sniffed out their location immediately, pulled a U-turn, and headed back home. No problem. The eleven sharks with plugged noses had a more difficult time of it. They lost their sense of direction at first and moved randomly and slowly. All the same, they eventually turned toward shore and made their way back home.
Leopard sharks have a distinctive pattern of leopardlike brown or black spots on their bodies. They are sometimes known as cat sharks. Researchers aren’t sure how they navigate.
As with much science, seeing what happens is sometimes easier than interpreting why it happens. Nosal figured that each of the twenty-six sharks was eventually able to sniff out chemicals in the ocean that they recognized as smelling like home. The closer the sharks were to shore, the denser the chemical scent and the more confident the animals were about their location. Other scientists who heard about the study disagreed. They pointed out that even the sharks with plugged noses turned toward the coast, so other factors for navigation might be at play. One scientist suggested that the sharks showed random movement because they simply didn’t like having something stuffed into their noses.
Another team of researchers, trying to figure out how blacktip reef sharks find their way without magnetoreception, did a similar experiment with young blacktips. Jayne Gardiner and a team from the New College of Florida in Tampa worked with five blacktips. The team plugged the sharks’ noses and found they got lost. “We have bits and pieces [about navigation] from various animals, but we don’t have the whole story yet,” Gardiner told the multimedia science website Seeker. “Most of us are in agreement that it’s a fairly similar story across different animals.”