Sharks are surely one of the most fearsome creatures on Earth. But they are also one of the most sophisticated and enduring. The oldest shark fossils date from more than 300 million years ago, before the age of the dinosaurs. And a few existing shark species, such as the hornshark, have kept the same basic physical characteristics for more than 150 million years.
Photo courtesy Carl Roessler A rare, tiger ragged-tooth shark, photographed off the coast of Columbia
Scientists attribute this remarkable longevity to the shark's superior physiological developments. Sharks are outfitted with several special characteristics that make them highly effective, both as hunters and survivors. In this article, we'll examine these unique qualities to see why the shark is top dog in the ocean.
When people think of sharks, they generally imagine an animal like the one in this picture. The tall dorsal fin, torpedo-shaped body and giant teeth of this great white shark are familiar to most everybody. But there are actually more than 400 different shark species alive today, and they vary considerably in size and appearance. In fact, roughly 50 percent of shark species are less than a meter long. So what makes a shark a shark?
Photo courtesy Carl Roessler One of the most well-known sharks, the great white
Photo courtesy Carl Roessler A cat shark, characterized by a stocky body and flowing fins
Sharks come in all shapes and sizes. One of the smallest sharks, the spined pigmy, is only 6 inches long when fully grown. The largest shark, the whale shark, can grow to more than 40 feet long. Highly active sharks tend to be torpedo-shaped, but many less active sharks are fairly flat, like rays.
Sharks are also very diverse in their habits and habitats. They live all over the world and swim at every ocean depth. Larger, more active sharks tend to hunt in the middle and upper depths of the ocean, while many smaller sharks stay near the ocean bottom. Some sharks swim long distances every day, while others live a relatively sedentary life, sticking to a small area.
Sharks, along with rays and chimeras, are distinguished from other fish primarily by their body composition. Most other fish have skeletons made of bone, just like mammals, reptiles, amphibians and birds. Sharks and rays, on the other hand, have skeletons made entirely of cartilage, the same flexible material in your nose and ears. Cartilage is sturdy like bone, but it has a much lower density. This material keeps sharks relatively lightweight, so they don't sink in the ocean and they don't need an air bladder like other fish.
Sharks also have a very unique skin texture. They don't have the large, prominent scales found in bony fish. Instead they're covered with smaller, tooth-like scales called denticles. These tough, protective denticles are aligned so that they channel water over the shark's body, minimizing drag due to friction.
Like bony fish, sharks breathe by extracting dissolved oxygen from water. The water enters the mouth, passes through the gills and is expelled through gill slits behind the head. In bony fish, these slits are covered, but in most sharks you can see them clearly. As the water flows through the gill opening, it passes tiny gill filaments. These filaments are covered with microscopic blood vessel capillaries, which have a lower oxygen content than the water around them. This imbalance causes oxygen in the water to diffuse into the shark's bloodstream, where it is distributed throughout the body.
Some sharks have a gill pump, a set of muscles that suck in water and push it past the gills. This works something like our lungs -- the shark can continuously gather oxygen while it is in a still position. Most sharks also extract oxygen using ram ventilation, passing water over the gills by moving forward. Some highly-active sharks depend on ram ventilation almost entirely, which means they stay in motion most of the time!
Photo courtesy Carl Roessler A nurse shark photographed off the coast of Australia: Nurse sharks, which hunt mostly at the ocean bottom, have a gill pump that lets them breathe without moving through the water.
Sharks also differ from most bony fish in the way they move. In the next section, we'll find out how sharks swim so quickly and gracefully through the ocean.
Most fish depend on the law of averages for reproduction. They produce a huge number of offspring, but only a small percentage will survive. Sharks, on the other hand, mate only occasionally and give birth to a small number of babies at a time. Female sharks are extremely particular about choosing a mate, and males may have to contend with tough competition. This helps ensure that the babies will be healthy.
The birthing process varies from species to species. Some sharks lay eggs in protected areas on the ocean floor. When the shark pup is fully developed, it breaks through the tough egg casing and heads out on its own. In most shark species, the pups develop inside the womb and are fully formed at birth. These shark pups typically look like adult sharks, except, of course, much smaller.
Most bony fish have a special swim bladder that helps them move around in the water. When the fish takes in oxygen, it can release some of the gas into the bladder. This increases the fish's buoyancy, so it rises through the water. To sink down to the bottom, the fish squeezes some of the gas out of the bladder, decreasing its buoyancy. In this way, a fish is something like a blimp or hot air balloon that uses the upward lift of atmospheric buoyancy to change altitude.
A shark is more like an airplane. It doesn't have a swim bladder, so it uses its forward movement to control vertical position. The tail is like the shark's propeller -- the shark swings it back and forth to move forward. In an airplane, this forward movement pushes air around the wings. In a shark, this forward movement pushes water around the fins. In both cases, this movement of matter creates lift -- the fluid is different, but the principle is exactly the same.
Sharks have two sets of paired fins on the sides of their body, in the same general position as the main wings and horizontal tail wings of a plane. The shark can position these fins at different angles, changing the path of the water moving around them. When the shark tilts a fin up, the water flows so there is greater pressure below the fin than above it. This creates upward lift. When the shark tilts the fin down, there is greater pressure above the fin than below it. This pushes the shark downward. (To learn more about lift, see How Airplanes Work.)
The shark also has one or two vertical dorsal fins on its back and sometimes a vertical anal fin on its underside. These fins work like the vertical stabilizer wing on an airplane. They help the shark keep its balance as it moves through the water and they can be moved from side to side to turn the shark left and right.
Photo courtesy Carl Roessler A silky shark photographed in the Red Sea: Silky sharks' long graceful bodies make them excellent swimmers.
This fin arrangement gives sharks amazing maneuverability. They can cruise at high speeds, stop suddenly and make sharp turns in every direction. This is one of the reasons they are such effective hunters. They move more quickly and with greater control than any of their prey -- most of the time, a shark's prey doesn't even know what hit it.
Photo courtesy Carl Roessler Two silvertip sharks off the coast of Papua New Guinea: Silvertip sharks can grow as large as 10 feet long.
Of course, before a shark can swoop in for the kill, it has to locate its prey. In the next couple sections, we'll examine the finely tuned senses that help sharks locate and track their food.
Basic Shark Senses
One of the main reasons sharks are such effective predators is their keenly attuned senses. Initially, scientists thought of sharks as giant swimming noses. When researchers plugged the nasal openings in captive sharks, the sharks had trouble locating their prey. This seemed to demonstrate that the shark's other senses weren't as developed as the sense of smell. Further research demonstrated that sharks actually have several acute senses, but that they depend on all of them working together. When you take one away, it significantly hinders the shark's hunting ability.
Photo courtesy Carl Roessler The blue shark is characterized not only by its distinctive color but also its long snout. Blue sharks were once the most common shark species, but they are now an endangered species, due to over-fishing.
The shark's nose is definitely one of its most impressive (and prominent) features. As the shark moves, water flows through two forward facing nostrils, positioned along the sides of the snout. The water enters the nasal passage and moves past folds of skin covered with sensory cells. In some sharks, these sensitive cells can detect even the slightest traces of blood in the water. A great white shark, for example, would be able to detect a single drop of blood in an Olympic-size pool. Most sharks can detect blood and animal odors from many miles away.
Another amazing thing about a shark's sense of smell is that it's directional. The twin nasal cavities act something like your two ears: Smell coming from the left of the shark will arrive at the left cavity just before it arrives at the right cavity. In this way, a shark can figure out where a smell is coming from and head in that direction.
Sharks also have a very acute sense of hearing. Research suggests they can hear low pitch sounds well below the range of human hearing. Sharks may track sounds over many miles, listening specifically for distress sounds from wounded prey.
In sharks, eyesight varies from species to species. Some less active sharks that stay near the water's surface don't have particularly acute eyesight, while sharks that stay at the bottom of the ocean have very large eyes that let them see in near darkness. Most all sharks have a fairly wide field of view, however, since their eyes are positioned on each side of the head. The most extreme example of this is the hammerhead, whose eyes actually protrude out from the head.
Photo courtesy Carl Roessler Hammerhead sharks are characterized by their wide head structure. The sharks eyes and nostrils are positioned at the ends of these protrusions.
Many shark species also rely heavily on their sense of taste. Before these sharks eat something, they will give it a "test bite" first. The sensitive taste buds clustered in the mouth analyze the potential meal to see if it's palatable. Sharks will often reject prey that is outside their ordinary diet (such as human beings), after this first bite.
In addition to these familiar senses, sharks also possess some senses we don't fully understand. In the next section, we'll look at these senses to see how they work and how they help sharks.
Extra Shark Senses
In the last section, we saw that sharks have a wider hearing range than human beings, as well as a much better sense of smell. As it turns out, they also use one sense we don't have at all.
The ampullae of Lorenzini give the shark electrosense. The ampullae consist of small clusters of electrically sensitive receptor cells positioned under the skin in the shark's head. These cells are connected to pores on the skin's surface via small jelly-filled tubes. Scientists still don't yet understand everything about these ampullary organs, but they do know the sensors let sharks "see" the weak electrical fields generated by living organisms. The range of electrosense seems to be fairly limited -- a few feet in front of the shark's nose -- but this is enough to seek out fish and other prey hiding on the ocean floor.
Water flows through the lateral line systems. Vibrations in the water stimulate sensory cells in the main tube, alerting the shark to prey and predators.
Another unique sense organ is the shark's lateral line. The lateral line is basically a set of tubes just under the shark's skin. The two main tubes run on both sides of the body, from the shark's head all the way to its tail. Water flows into these main tubes through pores on the skin's surface. The insides of the main tubes are lined with hair-like protrusions, which are connected to sensory cells. When something comes near the shark, the water running through the lateral line moves back and forth. This stimulates the sensory cells, alerting the shark to any potential prey or predators in the area.
By themselves, none of a shark's sense organs would be adequate for effective hunting. But the combination of all these senses make the shark an incomparable predator. The success of sharks is due largely to these physiological advancements -- they are superbly built to find food. They are also quite good at catching food, of course, as we'll see in the next section.
Sharks terrify people because some of them can, and sometimes will, feed on human beings. Sharks do attack humans from time to time, but the risk of attack is actually very slight. Only a small proportion of shark species have been known to attack, and even these species exhibit this behavior very rarely.
These sharks typically attack defensively, when a person has wandered into their environment, or accidentally, because they've mistaken a person for their standard prey. In both cases, chances are the shark will flee after only one bite. They don't have any real interest in humans as a food source.
Between 75 and 100 shark attacks are reported every year, and fewer than 20 are fatal. Statistically, this is in extremely low number. To put it in perspective, more people are killed by pigs or falling coconuts every year than by sharks.
A shark's only real weapon is its mouth. Like its fins and sense organs, the shark's mouth is a highly effective physiological adaptation, perfectly suited to its job. There are two elements that make the mouth so effective: the teeth and the jaws.
Shark teeth are something like the teeth of land predators. They have very sharp points that will cut into meat. Sharks are exclusively carnivores, so for the most part they don't need any grinders for chewing plant matter. Some bottom dwelling sharks do have special grinding teeth for cracking shells open, but more active sharks have teeth suited only for eating flesh. There are a wide variety of shark teeth, just as there are a wide variety of sharks. These teeth can be divided into two general categories.
Many shark species, such as the goblin shark and sand tiger, have very long, thin teeth. This structure is well suited for catching small fish. The shark kills the fish immediately, by piercing it with a single bite. Then it swallows the fish whole.
Sharks that go after larger prey need a different strategy and a different sort of tooth. They tear into their prey several times, biting off large pieces of flesh. Sharks in this group, which includes the infamous great white shark, have wide serrated teeth. These teeth act something like a hunting knife -- they cut easily through tough flesh and bone. Many sharks have combinations of long pointed teeth and wider serrated teeth, so they can hold their prey in place while they cut into it.
Photo courtesy Carl Roessler A 15-foot great white shark: Great whites attack their prey with a mouthful of razor sharp, serrated triangular teeth.
Shark teeth have the same basic consistency as our teeth, but they don't sit in the mouth in the same way. Our teeth rest in sockets, and aren't replaced after childhood. Shark teeth are attached to the jaw by soft tissue, and they fall out all the time. This is crucial to the shark's effectiveness -- worn or broken teeth are continually replaced by new, sharper teeth. In some sharks, such as the great white, these teeth are arranged in several rows.
Click the button to see how a shark bites its prey.
Sharks have a very unique jaw structure, which makes their mouths especially effective weapons. In most animals, the lower jaw moves freely but the upper jaw is firmly attached to the skull. In sharks, the upper jaw rests below the skull, but can be detached when the shark attacks its prey. This lets the shark thrust its entire mouth forward to grab onto its prey. Jaw mobility varies among different species, but all modern sharks have this ability to some degree.
Daily Life of a Shark
In recent years, scientists have uncovered a lot of new information about shark physiology, but the day-to-day life of sharks remains fairly mysterious. Most shark species are very difficult to study because they travel quickly over long distances, sometimes deep in the sea. They live in a world that is largely inaccessible to humans.
The largest species of shark is also one of the least aggressive. The whale shark can grow to 45 feet or more, but it feeds mainly on plankton. It doesn't have the sharp teeth and aggressive hunting strategy of other large sharks. It simply swims through the water with its mouth open collecting the small creatures in its path.
We do know that sharks are solitary animals, for the most part. They typically live and hunt by themselves, joining up with other sharks only in certain circumstances, such as mating. Some sharks will form schools on occasion, however. Researchers aren't really sure why this occurs because sharks don't really need protection from predators and they don't feed in schools. At this point, it's still unclear why sharks behave this way. In any case, the occurrence is very rare. Most of the time, sharks swim alone.
When they hunt, most sharks rely on the element of surprise in some way. In some camouflaged bottom-dwelling sharks, such as the various wobbegong species, this is a passive exercise. The shark blends in with the ocean floor, waiting for its prey. When a fish gets close enough, the shark opens its mouth wide and swallows the fish whole.
In active hunters, the element of surprise works a little bit differently. Great whites and other sharks that hunt bigger animals proceed very cautiously when approaching their prey. Once it has found a potential meal, the shark will circle at some distance, sizing up the situation. When it is ready, the shark moves in quickly, landing a good bite before the animal knows what's happening. Often, this first attack is sufficient to bring down the prey. Researchers have observed great whites behaving this way when hunting sea lions -- they will take one good bite, and then wait for the sea lion to die from blood loss. This sort of hunting takes a lot of energy out of a shark, so these species usually won't feed more than a couple times a week. Sharks that feed on smaller prey typically eat a few times every day.
Photo courtesy Carl Roessler A large school of hammerhead sharks, photographed off the coast of Malpelo Island, Columbia
On rare occasions, active sharks will cooperate in a hunt. Researchers have observed this phenomenon primarily in sevengill sharks. When these sharks hunt large fur seals, they rely on strength in numbers -- one large fur seal is too big for a single shark to take down itself. The sharks form a wide ring around a single seal, and slowly move in. When they get close enough, one shark will suddenly attack, and the rest will follow. This sort of behavior sometimes occurs in other shark species, but it is extremely rare.
Scientists also know that migration plays a big part in the lives of most shark species. The main reason most sharks migrate is that their food migrates. Different marine animals gather in certain areas throughout the year, for breeding, to lay eggs and other reasons. Sharks remember these annual patterns and return to these areas every year to take advantage of the population boom. Sharks will also remember human activity when it involves food supplies. Many species gather around fishing boats, for example, because they know fishermen might discard extra bait and small catches.
Threats to Sharks
Despite their superior physiology and hunting skills, many shark species are now threatened with extinction due to human activity in the ocean. The main threats to sharks are over-fishing and accidental bycatch. In many parts of the world, sharks are in very high demand, for their meat, skin and cartilage, which is used in several medicines. These shark products sell at very high prices, making them an attractive catch to fishermen.
Photo courtesy Carl Roessler A whitetip reef shark off the coast off the coast of Australia: People fish whitetip reef sharks for their meat, even though it has been known to cause severe food poisoning.
Sharks mate only rarely and have a relatively small number of babies at a time. Consequently, they can't replenish their population quickly. Sharks also have fairly long lifespans -- on average, sharks live 25 to 30 years, and some sharks live 100 years or more. If left alone, a female will mate many times in its life. With this reproductive pattern, the death of every single shark obviously has a significant effect on the shark population.
Over-fishing is actually a problem for both sharks and humans. If humans kill too many sharks in a given amount of time, the population will dwindle and they won't be able to catch many sharks in the future. The only way to maintain profitable shark fishing over time is to allow sharks to continue to reproduce, which means decreasing shark fishing significantly. Sharks are also killed accidentally, primarily by long lines used to catch other fish. Researchers suggest we must ban certain fishing methods, or some shark species will die out at some point in the near future.
One major obstacle to conservation efforts is our ignorance about sharks. We still don't fully understand their behavior, their breeding habits or their migration patterns. For most shark species, we don't even have an idea of their population size. This makes it very difficult to organize effective conservation methods since we can't accurately calculate safe fishing restrictions.
Sharks have persevered for hundreds of millions of years, while thousands of other animals have come and gone. When you consider this incredible history, and the unique physiological characteristics found in sharks, it's clear that it would be a great tragedy to lose any shark species. They are among the most remarkable animals on earth, and there is still so much we don't know about them.