Thu 14 Jul 2016, 13:54
7 astonishing physical facts.
How sharks are built.
1. Sharks can replace thousands of teeth in a lifetime
When you’re rocketing from the depths to ambush prey, you’re going to lose a few teeth in the process. Luckily, sharks can replace their teeth quite quickly and painlessly.
Sharks can have anywhere from two to as many as 300 rows of teeth, depending on the species. The teeth don’t have roots, so they fall out easily. But sharks have a unique tooth replacement system. When a tooth falls out, another one moves up from the row behind to take its place in as little as a day or two. It’s estimated that a shark can grow and chomp through up to 50 000 teeth in its lifetime!
Sharks have a variety of teeth uniquely adapted to the way they live, hunt, and eat. Bulls, blue sharks, and wobbegongs have sharp, curvy teeth that are good for trapping fast-moving prey. Tiger sharks and great whites have big, pointy teeth to bite down on their prey and tear the flesh apart. When eating large prey, a shark uses its lower teeth to shred while its upper teeth clench down and hold pin place. Once it has a grip, the shark will shake its head violently from side to side to rip its food apart. No wonder they lose so many teeth!
Whale sharks and basking sharks eat tiny, soft foods such as plankton and krill. They have tiny teeth but don’t actually use them as adults. Instead they use gill rakers to filter all the seawater that comes into their mouths, separating the food (which is then swallowed) from other stuff.
2. How do sharks swim so fast?
Move over Chad Le Clos! Sharks are gold medalists in swimming. They’ve been swimming ceaselessly through our oceans for hundreds of millions of years, and have evolved a complicated set of muscles and fins to power their streamlined bodies.
A shark’s muscles and vertebrae are two parts of the same whole. Sharks have waves of muscles along their back and side flanks, arranged in zigzag blocks. Each block overlaps a vertebra in the shark’s spinal column. Sharks can move their spines very fluidly as they swim. Each muscle contraction moves the vertebrae closer or further apart, swinging the spine back and forth in an S-shaped motion. This helps the shark swim forward and push water back.
Shark fins vary in size and shape, but all sharks use their fins to lift and push the body forward. A shark’s pectoral fins, located on the sides of its body, provide lift, pushing the body up. One to two dorsal fins on a shark’s back help stabilize the shark, and a large caudal fin at the end of a shark’s tail propels the animal forward.
A shark’s speed can be determined by the force and shape of its tail. Slow-moving sharks have weaker tail muscles. Mako sharks have incredible speed, with strong muscles in their tails that act like propellers to help push them forward and zip through water at high speeds.
3. What keeps sharks from sinking to the bottom?
Ever wonder how sharks move around in the water without simply sinking to the bottom or floating to the top? Sharks experience both the upward push of buoyancy and the downward pull of gravity, but they have a few key adaptations that keep them exactly where they want to be in the water column.
Most fish have gas-filled swim bladders to keep them afloat, but not sharks. (Well actually, one shark, the sand tiger, stores air in its stomach like a makeshift swim bladder. But this is an exception.)
Instead, sharks produce an oily substance in their livers called squalene. Squalene helps reduce a shark’s natural density, which keeps the animal from sinking to the bottom. Another reason sharks don’t need a swim bladder is that, unlike fish, their skeleton is made of cartilage. Cartilage is about half the normal density of bone, so it also helps keep the shark afloat.
But even this isn’t quite enough to keep a shark from sinking. Their pectoral fins are the final ingredient. Sharks position their pectoral fins to create dynamic lift as they swim, much like a bird’s wings.
4. A shark can smell even a few molecules of blood
Sharks have an incredible sense of smell. Large volumes of water pass over the shark’s rostrum (its snout) as it swims through the water. As it does so, water passes through their nostrils and over their smell sensors.
A shark’s smell sensors (called olfactory receptors) are highly developed. It can smell even a tiny bit of fish blood or guts dissolved in water. As long as just a few molecules of scent pass over its smell sensors, a shark can smell prey from 330 feet to half a mile away.
Once a shark recognizes the scent of prey or food, it will turn and follow the scent until it gets to the prey. If the scent is picked up by the right nostril, they turn right, and if it’s picked up by the left nostril, they turn left. As the shark gets closer, the scent gets stronger and the shark knows it is going in the right direction.
5. Sharks can hear the low-frequency sounds of dying fish
Sharks don’t have ears on the outside of their bodies, but they do hear through an internal sound system. There is a tiny hole on each side of the top of a shark’s head that leads to ducts filled with fluids that carry sound waves. These holes are called endolymphatic pores.
A shark’s sense of hearing is, in fact, its strongest sense. It can hear sounds up to a kilometer away or more.
Sharks don’t talk, of course, but scientists believe that sharks may be able to communicate with each other by emitting and hearing very low-frequency sounds. And sharks hear things like changes in the water currents, which helps them stay balanced in the water and adjust their positions.
Their hearing system makes them very good at picking up the low-frequency sounds that travel through the ocean. One of the sounds sharks hear is called the Yummy Hum. Too low for humans to hear, the Yummy Hum is a sound that dying fish emit and it lets sharks know there’s something good to eat in the area.
6. Contrary to popular belief, sharks have good eyesight
Many people think sharks have poor eyesight, but scientists have determined that in most sharks their eyesight is actually quite good. Their eyes are similar to humans’; with irises and pupils that control the amount of light that is let in and lenses that help them focus.
Sharks don’t see their prey until it is about 100 feet from them. By that time, they are already aware of its presence because they have heard it, smelled its scent and felt its vibrations. At close range, vision takes over.
But how do sharks see in dark or murky waters? Most sharks have a mirror-like reflector at the back of their eyes called a tapetum lucidem. This magnifies light and lets sharks see better in low light and focus on what’s in front of them. Deep-sea sharks tend to have large, light-colored eyes that let in enough light for them to see in the murky depths. Sharks that swim nearer the surface have eyes that are darker and smaller to protect them from the bright light.
Many species of sharks have a thin layer of skin called a nictitating membrane that is lowered over their eyes for protection from the light or from injury during attack. Sharks that don’t have nictitating membranes, including the great white, can roll their eyes back in the socket to protect them.
7. A shark's skeleton is made of flexible cartilage, not bone
Shark skeletons are made only of cartilage; they don’t have any bones in their bodies. Cartilage is thick but flexible and allows sharks to twist and turn their bodies. This helps them to get into tight places when they are chasing prey. Since cartilage is lighter than bone, it takes less energy to push a shark’s body through the water, and is easier to keep afloat.
People have cartilage too. While the human skeleton is made mostly from bones, the connective tissue that connects different bones is actually cartilage. The cartilage is thick and flexible. The human hand wouldn’t be what it is without cartilage connecting arm to wrist and finger bones. Cartilage also allows people to bend their knees and elbows. The soft sturdy parts on the end of your nose and outer part of your ear is made out of cartilage too.
Sometimes, Calcium crystals mix with a shark’s cartilage and make it hard. This happens in certain parts of the sharks’ body, like jaws and fins. But calcification actually makes the cartilage stronger and tougher. An older shark, with calcified jaws, has a stronger bite than a young shark whose jaws are still soft.