Most of us react with fear at any reference to sharks. In our minds we immediately conjure up images from the movie Jaws or from sensationalised media reports of shark attacks. Unfortunately, these animals are greatly misunderstood and have an unfair reputation for being dangerous. For the most part they are usually quite harmless.
Sharks are in fact one of the most sophisticated and enduring creatures on the planet. They pre-date the dinosaurs by more than 300 million years. What is referred to as the modern shark first appeared over 200 million years ago and because of its fantastically well-constructed physiology, highly efficient hunting and survival skills remained relatively unchanged over all these years.
There are about 440 species of sharks world-wide. Of these, there are about 182 species of sharks in Australian waters. About 70 of these are fairly common alone the Australian coastline.
Sharks are similar to other fish. They swim underwater, extract oxygen from the water through a series of gills located on each side of their heads and have tails and fins like other fish. Unlike most fish, however, sharks do not have a bony skeleton or a swim bladder (an internal organ used to create buoyancy).
Sharks make up about 1% of the total species of fish in the world and come in all shapes and sizes. They can range in size from the massive Whale Shark, the biggest fish in the world, which can grow to an 18 metres in length and weigh up to 21 tonnes to the Spined Pygmy Shark which is only about 25 cm long. Highly active sharks tend to be sleek streamlined fish. Less active ones tend to be fairly flat. The larger and more active sharks cruise and hunt in the upper and middle depths of the oceans. Smaller ones tend to stay near the sea floor. The scientific name for sharks is Chondrichthyes elasmobranchs.
Sharks have no overlapping scales covering their bodies like other fish. Instead they have a very rough skin made up of thousands of tiny, tooth-like scales called 'dermal denticles'. These denticles feel very rough when rubbed in a forward direction but flatten and smoothen in a backward direction reducing hydrodynamic drag making swimming more efficient and quieter than bony fish. Dried shark skin was once used as sandpaper and for non-slip hand-grips for swords.
Sharks belong to a family of fish that do not have an internal skeleton. Instead they have a thick skin composed of a mesh-work of tough and flexible fibres made of collagen, a type of cartilage. (The same kind of stuff that our nose and ears are made of). This mesh-work forms a solid tube-like framework onto which all its muscles and organs are attached. You can think of its skeleton like the skin of an airplane.
It is not clearly understood why the shark has a cartilaginous skeleton. Some have suggested it is for speed and agility to hunt prey but then there are small sharks which don't swim fast and don't need flexibility to hunt prey. The most plausible explaining is parallel evolution. That is to say a line of fish evolved with skeletons and another without. From an evolutionary perspective they were both efficient so they both survived (another similarity in the animal kingdom and placental and marsupial mammals). One advantage of a cartilaginous skeleton is that it is a lot lighter than an equivalent bony one. This may explain why sharks don't have a swim bladder like most bony fish. As a result they will sink if they don't keep swimming. They counter this sinking tendency by constantly swimming - a bit like how an airplane stays aloft by getting dynamic lift by moving through air. Sharks cannot swim backwards. Some such as the epaulette shark can use its front fins to "walk" backwards.
Some sharks have multiple rows of teeth, which they loose and are replaced continuously. These multiple rows of teeth grown inside the jaw and are steadily moved forward to replace those in front which may be worn or lost. Some sharks can grow as many as 35,000 teeth in their lifetime.
The shape of the teeth are related to the shark's diet. Sharks that feed on molluscs and crustaceans have dense and flattened crushing teeth. Those that feed on fish have needle-like gripping teeth. Those that feed on large mammals have pointed lower teeth for grabbing and triangular serrated upper teeth for cutting. Plankton-feeders have small non-functional teeth.
Sharks have a very unusual dislocatable jaw structure. In most animals the upper jaw is firmly attached to the skull and doesn't move. The lower jaw is hinged onto the skull and moves in an up and down direction.
In sharks the upper and lower jaw form a special biting unit which sits beneath the skull. When a shark attacks its prey, it thrusts its mouth - consisting of this biting unit of both the upper and lower jaws forwards to grab its prey. All sharks are capable of this feat but it is most prominent in those such as the great white shark. It is this terrifying extended mouth with razor sharp teeth that most people envision when they think of a shark.
Eyes — Shark eyes are similar to most other animals. They have eyelids but do not blink as the surrounding water cleans their eyes. Some have a nictitating membrane sometimes referred to as a third eyelid (like an eagle) to protect the eyes while hunting. Some sharks, such as the great white shark, rolls their eyes backward to protect them when attacking prey. They have very good eyesight and identify an object by the contrast it makes to its background. Unlike bony fish their pupils can dilate and contract.
Smell — Sharks have an excellent sense of smell. Some specifies can detect blood in sea water in quantities as low as one part per million.
Hearing — Sharks have a small opening on the sides of their heads that lead to their inner ear. It is not certain exactly how well their can hear.
Electroreception — Sharks have numerous electroreceptors called ampullae of Lorenzini around its head. These receptors are the most sensitive of any animal and can detect the electromagnetic fields produced by animals in the sea. They also use these sensor to detect the earth's magnetic field and use this for orientation and navigation.
Lateral Line — Sharks can also detect vibrations in water with through their Lateral line which runs the length of their bodies.
Most sharks are bottom dwellers (live just above the seabed) and are found in deep water on the Australian continental shelf. Some are also found closer to shore and also live in river estuaries. A few such as the Northern River shark and spear-tooth shark (endangered) live in fresh water rivers.
Larger sharks can travel as much as 80 kilometers a day in search of food.
All sharks are carnivores. This does not mean, however, that they are all scary sea monsters of the deep. Some such as the huge Whale Shark is quite harmless and feeds on plankton. Sharks usually feed in the evenings and night. Each species has its own food preferences depending on its lifestyle and habitat.
On the top of the marine food chain are the apex predator sharks such as the great white and tiger shark. These sharks are fearless and will attack animals much larger than themselves. A great white shark's diet includes sea lions, dolphins, turtles, small whales, other sharks and even sea birds. The great white usually attacks its prey from below and strikes so fast that most of its victims have little time to react. They have been known to leap great heights out of the water in pursuit of prey. Smaller prey they bite and swallow whole. Larger prey their grasp with their sharp dagger-like lower teeth and triangular serrated upper teeth and rip them apart into smaller pieces so they can be swallowed.
These sharks usually feed on other fish including other sharks. They have long needle-like gripping teeth. These are well suited for catching small fish. The shake kills its prey by biting it and swallows the fish whole.
The largest sharks, such as the whale shark, are filter-feeders. These sharks strain small fishes, krill, larvae and other types of plankton out of the water. They eat huge quantities of tiny animals. Filter feeders have reduced, non-functional teeth.
Bottom-feeders, are usually rather small and scurry along the ocean floor. These sharks usually feed on feed on molluscs and crustaceans such as crabs. The Port Jackson shark has two types of teeth to process its food. The front teeth are pointed for grasping its prey and back teeth are flat and molar-like for crushing hard shells.
Bony fish have a special organ called a Swim Bladder which keeps them floating at a desired depth. It provides them with buoyancy underwater. Think of this bladder like a hot air balloon. In a hot air balloon the height of the balloon above ground is controlled by the hot air inside the balloon. More air means up. Less air means down. The same principle applied to the swim bladder in a fish.
Sharks do not have swim bladders!
Some sharks such as the great white shark would suffocate if they didn't keep swimming all the time. This is because they have no muscles to move water through their gills to breathe. They must move to force water through their gills to extract fresh oxygen from the water.
Sharks swim through water in a manner very similar to how airplanes fly through the air. As explained earlier the shark's body is built of a lightweight and flexible mesh-like framework of cartilage covered in hydrodynamic skin. An airplane too has a similar structure with a lightweight frame covered in an aerodynamic skin.
Both an airplane and a shark would fall to the ground (ocean floor in the case of a shark) if they did not keep moving through their medium. This is because both require dynamic lift to keep them afloat.
The shark's front pectoral fins act in a similar fashion to an airplanes wings, providing lift. Dynamic lift is only provided when an appropriately shaped object is moving through its medium. In the case of the case of an airplane this is provided by its engines which thrust the plane forward and the wings provide the necessary lift. In the case of the shark, its powerful tail provides it with forward motion and its front fins provide the necessary lift. When the shark tilts its fins upwards it can rise. When it tilts them downwards it can dive. An airplane accomplishes the same outcome using the flaps on its wings. A shark has to keep moving constantly to stay floating at a desired depth.
The vertical dorsal fins on its back act as rudders and together with adjustments its makes to its pectoral fins steer the shark through the water.
• In 2012-17 (5 years) there were an average of 23 shark attacks each year. Of these 2 each year resulted in death, 15 resulted in injures and 6 were injury free.
• 10% of all shark attacks were fatal.
• 90% of shark attacks are a consequence of mistaken identity where the shark mistakes a human for its usual prey.
• Most shark attacks occur less than 35 meters from the shore, mainly around popular beaches.
• Bull sharks have a higher attack rate because they lives in shallow saltwater and freshwater which increases their chances of coming into human contact.
• The tiger shark is responsible for a highest number of total unprovoked attacks. It has a voracious appetite and does not always swim away after one bite of a human, but stay to finish the meal.
• The great white shark is responsible of the least number of attacks but is the most fearsome. It usually retreats after a "sample bite" of a human.
Are shark dangerous? Some are – Absolutely!
However, of the 440 species of sharks, very few sharks are actually dangerous to humans.
Sharks usually receive very bad publicity in the media. Any shark attack is dramatised out of proportion to actual facts and many innocent sharks are killed as a consequence. On average only 2 person per year dies as a result of a shark attack.
The four species of sharks responsible for most unprovoked shark attacks in Australia are the White, Tiger, Bull and Whaler sharks.
These man-eating sharks are hyper-carnivorous apex predators and have little fear of other creatures, including humans. They are large, fast and deadly killing machines. Sharks aren't particularly fond of eating humans. We are not part of a shark's natural diet. Unfortunately humans venture into their territory and are often mistaken for prey. Sharks are naturally curious about any unusual animal in their environment. Most people are bitten only once and released. This suggests that the shark releases the person as soon as it realises that it doesn't taste like its usual prey and abandons the bite, but this could still cost you an arm or leg or death from bleeding.
You are four times more likely to die from bumping into another person than by bumping into a shark.
Contrary to popular believe sharks don't find humans tasty. They like blubbery meat and we don't have much of that. Once they have tasted us they usually spit us out and swim away.
There are a number of shark attack theories. These include mistaken identity, curiosity and humans invading the shark's personal space.
Most shark attacks appear to be a case of mistaken identity. Sharks eyesight is designed to pick up variations in light. They identify an object by the contrast it makes to its background. From below silhouetted against the sky the outline of a surfer or swimmer looks very similar to that of a seal or turtle on which some sharks feed.
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