© 1995 Oceanic Research Group, Inc.
All Rights Reserved
The shark. Perhaps no other sea creature is as well known, or as feared, as the shark. Sharks conjure images of horror in many people's minds. Much has been written of the shark's appetite for human flesh. There are those who will not set foot in the ocean, for fear of being eaten alive by this fearsome creature of myth and folklore. But are these myths true? Are sharks really the bloodthirsty man-killer we have labeled them, or have we created this myth through centuries of exaggerated stories? Should we fear the shark or admire it? Join us as we search for the truth about these magnificent but greatly misunderstood animals.
Sharks have lived in the seas for at least 300 million years. Before there were any dinosaurs roaming the Earth, there were sharks. Today, sharks inhabit all of the world's oceans at all latitudes and ocean temperatures. Some live in shallow water, while others live in water so deep that the sun never reaches them. No matter where you go, there are sharks to be found.
Sharks also come in many sizes. For example these Spiny dogfish sharks, common small sharks which prefer temperate waters, are only about 3 feet long, and will not grow over 5 feet. On the other end of the spectrum, this whale shark, which is about 30 feet long, can reach 60 feet when fully grown, making it not just the largest of all sharks, but the largest fish in the world. Fortunately, this behemoth has tiny teeth, and eats only plankton and small fish.
Biologically, sharks are fish, having both gills and fins. The sharks are different from ordinary fish in that they have a cartilaginous skeleton as opposed to a bony one. This makes their bodies very flexible. Fish with cartilaginous skeletons are grouped together in a special class which also includes the skates and rays, close relatives of the sharks. All of these cartilaginous fishes have another trait marking their difference from the bony fishes: the absence of a swim bladder.
A swim bladder provides a bony fish with a means of controlling its buoyancy by allowing it to add or remove air from this organ. Sharks, skates and rays have no swim bladders and thus have different means of dealing with the fact that they are slightly heavier than water, and tend to sink. The skates simply stay on the bottom, feeding on worms, crustaceans and mollusks which live in the sand.
The rays use their large wing-like pectoral fins to actually fly through the water. Just as birds use their wings to fly through the air, rays actually do need their wings to keep from sinking! Here, a manta ray uses its 12 foot wingspan to swim in a strong current. The Manta feeds like the whale shark, by filtering plankton from the water. A shark works more like an airplane than a bird. It uses its tail fin in a back and forth motion to provide forward thrust, while its pectoral fins, which are shaped just like airplane wings, provide lift to keep the shark from sinking. Just like an airplane, if the shark stops moving forward, it will sink and crash into the bottom! Because the tail is designed for producing power, while the pectoral fins are designed to produce lift, sharks are fast, but not as agile as most bony fishes. For this reason, many animals which could otherwise serve as food for sharks can get away from them using their superior maneuverability. In order to capture prey, sharks rely both upon the surprise attack and their superior senses.
Sharks have several interesting senses which are well adapted to the their lives as roving predators. First of all, they have an extremely good sense of smell. Some species of sharks can detect a single drop of blood dissolved in as much as a million gallons of water! This allows the shark to locate injured prey from quite a distance. It takes only seconds for a reef shark to find and devour this small tuna.
Sharks can also sense minute electrical disturbances in the water which are generated by the swimming muscles of fish. Sharks are among the most sensitive to electric currents of all ocean creatures.
Sharks have an extensive network of pores on their bodies which are connected to specialized sensory organs for sensing electric pulses.
This electro-sensory system is so sensitive, that it enables sharks to detect the small bioelectric currents produced by the muscles of prey fish at a distance of up to one meter or so.
This allows many types of sharks to feed quite readily in complete darkness. It is a popular misconception that sharks have such good senses to make up for poor eyesight. In fact, this is completely untrue. Sharks have excellent eyesight, not only in daylight, but also in very low light.
Sharks have a unique eye with a mirror behind the retina. This mirror reflects light back through the retina a second time, increasing sensitivity in low light. For this reason, it is believed that sharks can see reasonably well in nearly complete darkness. Starlight is plenty of light for a shark.
Because their eyesight is so important to them, sharks are very careful about protecting their eyes. Many species of sharks have a special protective eyelid, called a nictitating membrane, which is used to cover the eye when engaged in aggressive behavior such as fighting or feeding.
The membrane rolls over the eye from below, and minimizes the chance that a struggling fish or other shark could poke the animal's vulnerable eye.
Because sharks enjoy life at the top of the food chain, they don't have to worry much about being eaten (except perhaps by bigger sharks). Still, they do have camouflage. This camouflage is intended to help them sneak up on their prey. Shark camouflage is called countershading, and it is found on most sharks. Countershading means that the dorsal surface of the animal is dark in color while the ventral surface is white. When looking down on a shark, the dorsal surface blends in better with the darker water below, while the white belly of the shark blends in better with the lighter water above.
Sharks are voracious carnivores, meaning that they eat meat, usually in the form of fish or invertebrates. But they do not always have to kill what they eat. Sharks are opportunistic scavengers, and will frequently eat whatever they can find which is already dead. This does not mean that sharks are indiscriminate eaters: sharks will sometimes refuse food that they find objectionable. Here, a Gray Reef Shark tastes a grouper, and decides to pass!
If it is necessary for the shark to hunt for food, it will usually search for something which is easy to catch. Generally, sharks seek out sick or injured prey, as do wolves, tigers, and other large predators. Not only does this make capturing a meal easier for the shark, but it serves the purpose of keeping the gene pool free of weak animals. This is a very important way in which nature keeps natural populations healthy. Therefore, the shark is an essential part of the ecosystem, serving the purpose of "cleansing" natural populations. In addition, the role of top predators in the food chain is very important for maintaining balance in the chain. Without the top predators feeding on the smaller fish, the fish populations would grow unchecked and could soon outnumber their food resources. For these and other reasons, sharks are absolutely essential in the world's oceans.
Sharks are frequently seen with traveling companions. One such companion is the remora, a fish with a suction cup which it uses to attach itself to sharks, whales, turtles or other large marine animals. The suction cup on the top of the remora's head is in fact a highly evolved dorsal fin which forms a disk. Contrary to popular belief, the remora does not harm its host. It gets a free ride and feeds from its host's leftover scraps. Although remoras may sometimes annoy the shark, they perform an important service by removing parasites from the sharks skin. Other traveling companions are the so-called pilot fish. Pilot fish are so named because people used to believe that they guided, or "piloted" the shark to food. We now know that these fish are, in fact, looking for a free meal. But if they aren't careful, the pilot fish themselves may end up being the meal!
Any discussion of sharks usually ends up with the question of how dangerous they are to people. In terms of statistics, sharks are not dangerous at all. The number of people who have been injured by sharks is minuscule compared with the number of people who have been injured by dogs, or injured in car accidents. Even lightning has killed more people than sharks. Yet we still call them maneaters! Certainly sharks are capable of killing people, but in general, people simply are not their preferred food!
Some areas of the world are known for their large numbers of shark attacks. It is a fact that surfers are more frequently attacked by sharks in both California and Australia than are swimmers. In both of these places, seals are regular food for sharks. The sharks patrol the coastlines waiting for unsuspecting seals to fall prey. Strangely, seals are actually more agile in the water than sharks, and are able to outmaneuver them when engaged in a chase. This means that the shark must either sneak attack, or find an injured or sick seal.
Unfortunately, humans on surfboards have an uncanny resemblance to a seal as seen from underneath. And, the added fact that a human on a surfboard is by no means as graceful as a healthy seal, makes the shark think it has found a very easy meal.
The jaws of the shark are its most formidable weapon. Without their jaws and teeth, sharks would be practically harmless. The teeth of most sharks are extremely sharp and numerous. As teeth are worn out or broken, new ones rotate into place from behind the old ones giving the animal a constant supply of new, sharp teeth.
The shark's jaws have powerful muscles, which can generate well over 100 pounds of force in larger sharks. When this much force is applied to those teeth, with their fine cutting edges, it translates to a pressure of over 20 tons per square inch! Needless to say, with that much power, it takes very little effort for a shark to bite through bone and tissue.
Here, in the blue tropical waters of the south pacific, our film team will be attempting to produce a so-called "feeding frenzy," where a large number of sharks are motivated to attack at once due to the scent of blood in the water.
The process begins by pouring a mixture of water and blood into the ocean for a few minutes to attract the sharks. Their keen sense of smell will lure sharks to the boat within minutes.
We head to the bottom with a small tuna to be secured to the reef. It takes very little time for us to get the attention of the sharks. We are working with a medium sized shark common in these waters, called the Gray Reef Shark.
The first sharks arrive to investigate the fish. They are cautious as they smell the bait and contemplate their attack. Our divers attempt to keep the first sharks from attacking the fish until more sharks have arrived. A single shark could eat this fish in only a few bites. A feeding frenzy occurs when many sharks are competing for the meal, so we wait for more sharks.
Most sharks are discerning about what they attack and prefer fish to people. But the presence of fish blood in the water could possibly create an unpredictable situation, and accidents could happen in the confusion. Therefore, safety divers are assigned the task of protecting the cameramen from attack. Because the cameramen must pay close attention to their work, they cannot keep an eye on the animals around them.
Tom Krasuski attempts to keep the sharks away
from the bait until enough sharks have arrived.
When we can hold the sharks back no longer, the attack begins! After one shark has attacked successfully, taking a large bite out of the bait, they all try to get a piece of the action. There is fierce competition to get some of the meal. Sharks steal fish from each other, and may even bite each other in the confusion. The water fills with blood, appearing green under the effects of deep blue water. Bits of fish spread out from the scene, obscuring the frenzy in a haze of particles and blood. The vibrations and smells created by the frenzy attracts still more sharks, until there are more than 20 sharks in the water around us!
The frenzy begins when the sharks
begin to take bites out of the bait.
In the excitement and confusion, a shark accidentally attacks one of our cameramen! It is quickly driven off by our safety diver with a harmless whack on the head. The tuna is completely consumed in under a minute. After the fish is gone, the sharks are frenzied by the excitement, and continue to swim about, looking for more scraps. Our divers must be very careful to keep an eye out in all directions.
In a feeding frenzy, competition for food between
sharks keeps them moving at a rapid pace.
Although sometimes we tend to think of sharks as cold-booded killers who are completely indiscriminate about what they eat, this doesn't seem to be the case. In general, sharks are afraid of people and will not approach divers. In order to observe the behavior of sharks at close range, we found it necessary to draw them in using bait. The likelihood of a person even seeing a shark, nevermind being attacked by one, is very low under ordinary circumstances.
Sharks spend much of their time slowly cruising, keeping an eye out for a good meal. A common misconception is that they have to keep swimming at all times in order to maintain a flow of water through their gills. This is entirely untrue. Many species of sharks periodically rest on the bottom. Here, a White Tip reef shark rests in a protected cave. It is able to move water through its gills quite well in this position by swallowing water like most other fish. Perhaps the most well known bottom dwelling shark is the nurse shark. This lethargic nocturnal animal spends the daytime hours sleeping under coral ledges or in caves. At night it will go out in search of invertebrates to eat. Every year, more and more species of sharks are "caught in the act" of doing the impossible: resting on the bottom.
Recent interest in shark research has made another fascinating discovery about these fish. Sharks seem to have a natural immunity to cancer. While cancer is common in humans, it is unheard of in sharks. Scientists are hoping that some day soon, a cure for cancer in humans may come from that animal we love to hate...the shark.
Like all life in the sea, the shark has a place in the food chain, and without the shark, the world's oceans wouldn't be balanced. Already, many species of sharks are endangered because they are being ruthlessly killed out of ignorance. Additionally, sharks are killed for their fins, which, in many Asian countries, are used to make Shark's Fin soup. Frequently the animal is killed just for its fins, and the rest of the animal is discarded, or thrown back into the sea alive, to die a slow and painful death. Not only are these fishing practices cruel, but they are extremely wasteful of the resources of the sea.
It's a shame that this beautiful and elegant creature is so misunderstood by so many people. Like many other predators in the world, the shark is feared as a threat to man based more on myth than on fact. When we understand that sharks are designed by nature as predators, we begin to see the importance of the role they play in the ocean ecosystem and the true beauty of these remarkable animals. We need to remember that sharks, like all animals, have a place in the ecosystem and perform a function. Sharks are not bloodthirsty maneaters, but simply animals trying to survive. As visitors to the shark's world, it is our responsibility to treat the shark with the respect that a wild animal deserves. Only then can we truly begin to understand sharks and how they live.
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