What makes a shark a shark?
The smallest shark, a dwarf lantern shark, is rarely seen and little-known. (© Chip Clark/Smithsonian Institution)
No matter their size, all sharks have similar anatomy. Like other elasmobranchs (a subclass of animals that also includes rays and skates), sharks have skeletons made of cartilage—the hard but flexible material that makes up human noses and ears. This is a defining feature of elasmobranchs, as most fish have skeletons made of bone. Cartilage is much lighter than bone, which allows sharks to stay afloat and swim long distances while using less energy.
Every shark also has several rows of teeth lining its jaws. Unlike people, which have a limited number of teeth in their lifetime, sharks are constantly shedding their teeth and replacing them with new ones. A shark can lose and replace thousands of teeth in its lifetime! Not all shark teeth are the same, however. Some have pointed teeth for grabbing fish out of the water. Others have razor-sharp teeth for biting off chunks of prey, allowing them to attack and eat larger animals than bony fishes of the same size. Sharks that eat shellfish have flatter teeth for breaking shells. Filter-feeding sharks that sift tiny plankton from the water still have teeth, but they are very small and aren’t used for feeding.
Another defining feature of sharks is their array of gill slits. Unlike bony fishes, which have one gill slit on each side of their bodies, most sharks have five slits on both sides that open individually (and some shark species have six or seven). After water flows into a shark’s mouth as it swims, it closes its mouth, forcing the water over its internal gills. The gills extract oxygen from the seawater, after which the water is expelled through the gill slits behind its head. When they’re resting, many shark species pump water over their gills to make sure the oxygen never stops flowing. This is called buccal pumping and is used by many sharks that spend their time sitting still on the seafloor like nurse sharks (Ginglymostoma cirratum), angel sharks (Squatina sp.) and wobbegongs (Orectolobidae). But some sharks are unable to pump water this way and, if they stop pushing water into their mouths by swimming, will suffocate. These sharks include the great white shark (Carcharodon carcharias), mako shark (Isurus sp.) and whale shark (Rhincodon typus).
Extreme close-up of great white shark skin denticles. (© Trevor Sewell/Electron Microscope Unit, University of Cape Town)
Over many millions of years of evolution, sharks have become some of the speediest swimmers in the ocean thanks to several adaptations. The first is their unique skin, which is made up of millions of small v-shaped placoid scales, also called dermal denticles. The rows of denticles are smooth in one direction—if a shark is “pet” from head to tail—but in the opposite direction, they feel like sandpaper. The denticles look more like teeth than typical fish scales and allow water to flow smoothly past the skin, reducing friction and increasing their swimming efficiency. Swimsuit designers have even taken a page from the shark, creating a fabric that mimics the design of shark denticles to improve human swim times. (Whether or not that actually helps people swim faster is up for debate.)
Many shark species known for speed also have slim, torpedo-shaped heads, like the great white shark (Carcharodon carcharias) and the shortfin mako (Isurus oxyrinchus), which is the fastest known shark. It can swim 25 miles per hour at a regular pace and reach 46 miles per hour in quick bursts that allow it to fly into the air. Sharks gain additional speed by stiffening their tail while swinging it back and forth.
Bony fish maintain their position in the water column with the help of a swim bladder—a gas-filled organ in their body that allows them to stay neutrally buoyant. Sharks don’t have swim bladders, and instead get help from their very large livers full of oil and the fact that their cartilage is about half as dense as bone. A shark’s lightweight skeleton allows it to put more energy into swimming and use dynamic lift to maintain its place in the water.
Sharks have six highly refined senses: smell, hearing, touch, taste, sight, and electromagnetism. These finely honed senses, along with a sleek, torpedo-shaped body, make most sharks highly skilled hunters. (© Shark Foundation)
Sharks have six highly refined senses for both hunting and communication: vision, taste, smell, hearing, touch and electro-reception. These finely honed senses coupled with sleek, torpedo-shaped bodies make most sharks highly skilled hunters.
The structure of shark eyes is remarkably similarly to our own. Like ours, the pupils of many shark species change size in response to varying levels of light. They have rods, which sense light and darkness, and most have cones, which allow them to see color and details. (Some sharks have no or few cones, making them colorblind.) Like a human eye, a shark eye has a cornea, lens, pupil and iris. Unlike us and more like cats, sharks have a layer of mirrored crystals behind their retinas called the tapetum lucidum. This layer allows them to see better in dark and cloudy waters, in the deep sea or at night.
But within that basic plan, there is a wide range of seeing ability among shark species. Some have large eyes, such as the bigeye thresher shark (Alopias superciliosus), with eyes six centimeters in diameter. Other sharks have very small ones, like the one-centimeter diameter eyes of the brownbanded bamboo shark (Chiloscyllium punctatum).
Like cats, sharks have a layer of reflective cells behind their retina called the tapetum lucidum. This layer allows them to see better in dark and cloudy waters, in the deep sea or at night. (João Pedro Silva (Flickr))
A 2007 study found that shark eye size varied depending on the shark’s habitat. Many sharks that stay near the surface have evolved to hunt in the sunlight and rely on their vision more than other senses, so have large eyes. Some deep-sea sharks also have big eyes to pick up faint traces of light down in the darkness—but their eyes are loaded with light-sensing rods and have fewer color-sensing cones. Researchers also have found that bioluminescent deep-sea sharks have a higher density of rods in their eyes than their non-bioluminescent counterparts, allowing them to see more details in the dark water when bioluminescence is present. Sharks that live in shallow water on the seafloor often have the smallest eyes because floating sediment kicked up from the bottom blocks their vision. These animals instead rely on senses like smell and electroreception over vision. Lastly, sharks that hunt fast-moving prey like fish and squids have bigger eyes (and presumably better eyesight) than those that eat non-moving prey.
Sharks have eyelids, but they don’t blink; they close their eyelids to protect their eyes from damage when fighting or feeding. But their eyelids don’t close all the way. In addition, some species have a clear membrane (the nictitating membrane), which slides down to protect the eye in dicey situations. Shark species that don’t have the membrane, like the great white shark, will roll their eyes back in the socket when they are attacking prey for protection.
Cookie cutter sharks approach their victims from below, latch on with their suction cup style lips, bite and then twist. A nice, round hole is left as a souvenir. (Jennifer Strotman, Collections Program)
Sharks don’t have a very strong sense of taste. Taste buds that line the mouth and throat allow them to taste their food before they make the commitment to swallow. This helps them avoid dangerous prey items, which might have a bad taste. This could also be why many shark bite victims survive: the shark takes a bite, gets a bad taste in its mouth, and decides it doesn’t want to eat, releasing the person.
Sharks don’t have what we think of as a typical tongue. Instead they have a small piece of cartilage on the floor of their mouth called a basihyal that lacks taste buds. In most sharks, it doesn’t appear to serve any real function. But the cookie-cutter shark (Isistius brasiliensis) uses its basihyal to rip small chunks of flesh from fish and other animals.
Sharks have truly remarkable noses. As they swim, water passes into their nostrils and across sensory cells lining the skin inside. These sensory cells are able to detect relatively small amounts of a chemical signal in the water. A shark’s two nostrils can also detect smells separately to determine from which direction they originated, allowing them to smell in stereo. Just like we can tell where a sound is coming from depending on which ear the sound waves hit first, sharks can tell where a smell is coming from depending on which nostril the smell hits first. Now those are some impressive nostrils!
Sharks have two small openings on their head (behind and above their eyes) that lead to internal ears. There, sensitive cells allow sharks to hear low-frequency sounds and to pick up on possible prey swimming and splashing in their range.
Great white sharks – marvels of evolution. (© Michael Rutzen)
Sharks don’t have fingers that they can use to feel and touch. Instead, like other fish, a shark has a lateral line running along the middle of its body from head to tail. The lateral line system is a series of pores that lets water flow through the shark’s skin, where special cells called neuromasts can detect vibrations in the water. A fish swimming nearby displaces water as it goes along, creating ripples; when those ripples hit the lateral line system, the shark can detect both the direction and amount of movement made by prey, even from as far as 820 feet (250 meters) away. Because of this ability, they can sense prey in total darkness.
Not only can sharks detect vibrations through their lateral line system, but they also have a “sixth sense” of sorts that allows them to detect the small electric fields that all animals create when their muscles contract. Sharks detect the electrical fields through small pores on their head that are full of special cells called ampullae of Lorenzini. These cells are filled with a jelly-substance that conduct electric charges received from ions, like sodium and chlorine, which are found in salt water. When a fish moves its muscle to swim, the shark can feel it; when one is wounded and flopping around, it sends out a large electrical signal that will attract the shark.
Sharks also use electroreception to navigate. They can sense the Earth’s electromagnetic field, which likely allows them to migrate across large distances without getting lost. They can also sense objects in the water, allowing them to create a map of their immediate environment.