Although sharks are fish, they differ from other fishes in many ways. Most fish have skeletons of bone but sharks have skeletons made of cartilage. Cartilage is softer and more flexible than bone.
Tiger Shark / Galeocerdo cuvier / Family Carcharhinidae
Tawny nurse shark / Nebrius ferrugineus / Family Ginglymostomatidae
Sharks breathe through five to seven gill openings, along each side of the head whereas fishes only have one gill opening per side and this is covered by a protective plate. Sharks have tiny tooth-like scales called 'denticles' embedded in their tough leathery skin whereas fish are covered by large, thin, round scales. Sharkskin feels like rough sandpaper.
Another difference relates to buoyancy. Bony fishes have a swim bladder, a special sac of air inside them, to stop them sinking. Sharks don't have this sac but maintain their buoyancy by having an oil-filled liver and by balancing the lift created by their flattened pectoral fins (the paired fins just behind or below the gill openings) with the downward force created by their tail.
Sharks' bodies are slender and streamlined, enabling them to swim and turn quickly. Their paired pectoral and pelvic fins help in steering and for use as brakes. Many sharks are counter-shaded for camouflage: their dark-coloured top half makes them hard to distinguish from the background water when viewed from above, and their pale or white underside blends in with the lighter colour of the water and sky as seen from below. Unlike fish, sharks have a backbone that extends to the tip of the tail. This helps the shark to be a very powerful swimmer.
Sharks on the Great Barrier Reef
There are many different kinds or species of sharks to be found on the Great Barrier Reef. They come in all shapes and sizes and range from small bottom-feeding sharks, like the cat sharks and wobbegongs, to the larger more active species such as tiger sharks and hammerheads.
Hammerheads, one of the most distinctive sharks, have broad flat T-shaped heads, with eyes on the ends of the T. The largest hammerheads reach lengths of six metres and weigh well over a tonne, though most are only half this size. Hammerheads generally feed on fish and squid. One species, the great hammerhead, frequently attacks rays and other sharks and has occasionally bothered humans.
Most sharks are predators. Many are active at dusk and after dark and hunt and kill other animals. Their powerful jaws are filled with rows and rows of razor-sharp teeth. The first two rows are used for feeding. The others are new teeth that will replace the old ones when they become worn or drop out. Each type of shark has teeth shaped according to its diet. Sharks with spiky teeth seize their food (usually fish), sharks with saw-edged teeth can cut the toughest of things, even turtle shells. Some sharks have crushing plates rather than teeth in their mouths and feed on crabs and molluscs.
Sharks are always alert to changes in the sea around them. Any disturbance attracts their attention and they will cruise in slowly to take a closer look. Shark eyes are set on the sides of their heads giving sharks a wide range of vision.
While shark eyes may not be able to form clear images or detect colour as our eyes can, sharks are better than people at seeing in dim light. Even in the dark of night sharks can quickly pick up any flickering movements and sense its direction. They can do this because they have a keen sense of smell and sensitive areas on their skin that can pick up vibrations made by other creatures in the sea.
Sharks mostely live in the ocean, however both they are widely distributed throughout habitats along the coastline. They can be found more than 200km up rivers and in the ocean below 2000m deep.
Sharks reproduce either by laying eggs in tough, little cases, or by giving birth to fully formed young. Carpet sharks and cat sharks are examples of sharks that lay eggs. The egg cases are left to develop among weeds or coral and the young hatch after several months. Some sharks such as nurse sharks, wobbegongs and whale sharks also produce eggs in cases but these are retained within the body of the mother until the young hatch.
Other sharks, like the reef sharks and hammerheads, give birth to litters of shark pups after a pregnancy of about ten months. Birth or hatching usually occurs in shallow coastal waters and shark pups look just like their parents. Each shark pup must fend for itself and hunt for its own food.
Many species of shark are selective feeders. The food varies, some sharks feed mainly on small fish and squid while others feed on sea urchins, crustaceans and cephalopods.
Sharks are very sensitive to vibrations and pressure changes, and this is used mainly for detecting prey over middle distances. The sense of smell is very acute and can detect fish oil and blood more than 1.6kl away. Vision, taste and touch are senses used in prey detection at close range.
Sharks and humans
Sharks are far more threatened by humans than humans are by sharks. Attacks by sharks on swimmers and divers on the Reef are rare. Very few species of shark attack people and few attacks are fatal. Most sharks do no harm to humans at all.
Around the world, sharks are killed for profit. Their flesh is eaten, their tough skins used for leather goods, their livers for oils, and tourists buy their teeth and jaws as souvenirs. Many sharks are also killed accidentally. They are caught in nets or get tangled in ropes, plastic straps and other rubbish that is thrown into the sea.
Like all creatures, sharks play a vital part in the life of the sea. They are well adapted to their role as a top predator and are both elegant and fascinating to encounter on the Great Barrier Reef.
Banded wobbegong, Orectolobus ornatus. An ambush or 'lie and wait' predator, the banded wobbegong eats fish, crabs, lobsters, octopuses and other bottom-dwelling invertebrates. Although generally harmless, it will bite if harassed. (Up to 290 cm)
Epaulette shark, Hemiscyllium ocellatum. This shark is commonly found in shallow water, in tidal pools and on the reef flat. It feeds mainly on bottom-dwelling invertebrates and small fish that it hunts at night. This species is often observed 'walking' on its pectoral fins. (Up to 100 cm)
Leopard shark, Stegastoma fasciatum. This is a bottom-dwelling shark that feeds on molluscs, crustaceans and sometimes small fish. The teeth of this species are fused into plates for crushing the hard shells of its prey. It is not an aggressive shark. (Up to 250 cm)
Tawny nurse shark, Nebrius ferrugineus. A nocturnal hunter of cephalopods, crabs, prawns, lobsters and fish, this species is able to suck its prey from rocky crevices. If caught, it is able to reverse this sucking action and spit water at its captor. (Up to 320 cm)
Grey nurse shark, Carcharias taurus. This shark has specialised teeth designed to grasp and firmly hold fish. Although it is generally considered harmless if left alone, this species has caused fear among divers who, in the past, have systematically killed it. (Up to 318 cm)
Blacktip reef shark, Carcharhinus melanopterus. This is a common shark on coral reefs with black-tipped fins. It can be easily recognised by its rounded snout, lemon-brown to grey back and very distinct black fin tips. It is generally seen swimming slowly around the reef flats and shallow lagoons with its dorsal (back) and tail fins sticking out of the water. A timid species, it tends to remain within a home range of a few km2. Its small size allows it to access areas unavailable to other sharks. (Up to 140 cm)
Whitetip reef shark, Triaenodon obesus. This common coral reef shark spends most of the day resting in caves or below ledges, and is active at night. Its home range is similar to the blacktip reef shark and it feeds on small fish and cephalopods. It has distinct white tips on its first dorsal fin and upper tail fin. It is often seen around island reef edges and about the coral beds at depths to forty metres. (Up to 170 cm)
Lemon shark, Negaprion acutidens. This species is a member of the whaler family and is named for its pale yellow to brown colour. It is found on shallow reefs, in shallow lagoons and mangrove swamps and may be aggressive if provoked. (Up to 300 cm)
Grey reef shark, Carcharhinus amblyrhynchos. This shark is found in deeper water on outer reef slopes and in lagoons and feeds mostly on fish. A member of the whaler shark family (tiger, blacktip reef, lemon sharks), it is a strong swimmer and is one of the more dangerous of sharks. Easily recognised by the broad black trailing edge of its tail fin, it is one of the few species of shark on the Great Barrier Reef that can be aggressive to divers. If divers approach it too closely or startle it, the grey reef shark performs threatening behaviour. This includes hunching of the back, rapid opening and closing of the mouth and exaggerated swimming movements. After this, the shark will either attack or flee. (Usually around 180 cm)
Tiger shark, Galeocerdo cuvier. One of the most dangerous animals on the Reef, this species eats a wider variety of foods than other sharks including fish, crustaceans, birds, sharks, rays, turtles and mammals. It feeds at night in shallow reefs, retreating to deeper water during the day. (Up to 7.5 m)
Whale shark, Rhincodon typus. This species can grow to 12 m and is the largest living fish, feeding on squids, small crustaceans, and fish, especially sardines, anchovies, mackerel and tuna. Whale sharks are highly migratory and are rarely seen by visitors to the Reef. If found, they are generally swimming slowly just below the surface of the ocean off the ribbon reefs of the outer Barrier Reef. It is harmless to humans.