Sharks have been on earth almost unchanged since before the time of the dinosaurs. Prehistoric sharks were in the seas 420 million years ago.

Today there are about 400 different kinds of shark. The largest shark is the whale shark at about thirteen and a half metres long. The smallest is the spined pygmy shark at less than 18 cm long.

Habitat and Distribution (where they are found)

Sharks are found all over the world in a wide range of marine habitats, from shallow coastal waters to deep oceans.

Body and Behaviours of sharks

Sharks' bodies are rounded, and taper at both ends, a shape which means they use little energy to swim.  

©Getty Images

©Getty Images

They have five different kinds of fins, which are stiff. One or two dorsal fins, a pair of pectoral fins, a pair of pelvic fins and a caudal fin (the tail) work together to lift, propel and stabilise the shark.  Some sharks have an anal fin.

  • The caudal fin, or tail, propels the shark through the water. It moves back and forth to propel the shark forward.  Its upward movement points the shark's head downward.
  • The dorsal fin is the fin on the top of the shark's back, the one that shows above the water when the shark swims near the surface. It keeps the shark's body stable as it swims.
  • The second dorsal fin is on the top of the shark's body, between the big dorsal fin and the tail. It helps keep the shark stable as it swims.
  • Pectoral fins, one on each side of the shark's underside towards the front of the body, help lift the shark as it swims, and offsets the downward motion given by the caudal fins (tail), so that the shark moves horizontally through the water. •Many sharks have a small second pair of pectoral fins close to the tail to help keep the shark's body stable in the water.
  • An anal fin is a small fin on the underside of the body near the tail. Not all sharks have this.
Whale shark, the biggest shark. Compare its size with that of the diver. ©Getty Images

Whale shark, the biggest shark. Compare its size with that of the diver. ©Getty Images

A shark's Senses

Sharks have excellent eyesight, especially adapted for seeing in dim light under the sea. They can see moving objects from a great distance. Sharks also have excellent senses of hearing, smell and touch. In many kinds of shark the eyes are on the side of the head so they can see forward, backward, up and down.

Breathing

As a shark swims it takes in mouthfuls of water. The water is pumped over the gills and leaves the body through gill slits at each side of the head. Inside the gills there are feathery fringes which trap the oxygen from the water before it leaves the shark's body.  In some kinds of shark, when it is not moving a body part called a spiracle is used to pump water over the gills. The spiracles are located just behind the eyes. The spiracle is either not there or poorly developed in fast swimming sharks.

Sharks have many teeth ©Getty Images

Sharks have many teeth ©Getty Images

Teeth

Sharks grow new teeth continuously as they break off. Some sharks can lose 30,000 teeth in their lifetime. New teeth grow in a groove inside the mouth and are replaced in rows. Lower teeth are mainly for holding prey while the upper teeth cut it.

Body differences between sharks and other fish

Sharks do not have a bony skeleton like other fish. Their skeletons are made of cartilage, which is more flexible than bone. Fish have a swim bladder inside their bodies that help their flotation in the water, and let them use less energy as they go up and down in the water. Sharks do not have a swim bladder.

Other differences between sharks and other fish are:

Sharks

Other fish

Have a cartilage skeleton Have a bone & cartilage skeleton
Can only swim forwards Can swim forwards & backwards
Have a large, oily liver Have a gas-filled swim bladder
Have gill slits with no gill cover Gill slits are covered
Eggs are fertilised in the female's body Eggs usually fertilised in the water, outside the body
Scales are rough, like sandpaper Scales are slippery & overlap
Upper part of tail fin is longer than lower part Upper & lower tail fins are same size

Diet and hunting

hammerhead shark ©Getty Images

hammerhead shark ©Getty Images

Sharks eat almost anything, including fish, crustaceans, molluscs, marine mammals and other sharks. They attack from beneath and behind, bite the victim and swim a short distance away to wait for the victim to bleed to death. Great white sharks rely on stealth and surprise to catch prey such as seals. Whale sharks eat krill and other tiny creatures. Sharks can sense the electric impulses given off by living things, and locate their prey in this way.

 

Life cycle of sharks

A shark developing in an egg case. Getty Images

A shark developing in an egg case. Getty Images

 

 

Sharks mate when the male deposits sperm into the female's body. Male sharks have claspers as part of their pelvic, or second pectoral, fins. These are placed into an opening in the female's body and sperm passes along grooves in the clasper.

 

 

There are three ways in which different sharks produce young:

Oviparous

Ovoviviparous

Viviparous

A shell or case is formed around the egg which protects it while it is developing. The female deposits the egg cases in the sea. Different sharks have differently shaped egg cases.

Horn and swell sharks develop this way.

A thin tissue covers an egg or group of eggs (called a candle) and this stays inside the mother's body. After a while, the tissue is shed and the young sharks continue to develop inside the mother's uterus (womb).

Mako sharks develop this way.

The young develop inside the mother's uterus and are born live. In some kinds of shark, the young eat each other before birth and the strongest one is the only one to be born.

Hammerhead sharks develop this way.

Conservation Status and Threats

©Getty Images

©Getty Images

There are 180 species of shark found in Australian waters, about 70 of them are endemic, or native species.

Of the sharks found in Australian waters:

  • The grey nurse (east coast population)  and speartooth sharks are classified as Critically Endangered.
  • The northern river shark is classified as Endangered.
  • The grey nurse shark (west coast population), whale shark, white shark, dwarf sawfish, green sawfish are all classified as Vulnerable.

Fishing, either commercially or for recreation, pose a threat to shark populations.  They also get tangled in commercial fishing nets.

Shark attacks

Fatal attacks on humans by sharks are actually not frequent, although it is a belief that it is. There have been about 53 fatal attacks in the last 50 years.

 There are ways to avoid risk when swimming, such as:

  • swimming at patrolled beaches;
  • avoiding places where sharks are known to gather;
  • swim, dive, surf where there are other people around;
  • avoid swimming far off the shore, in deep channels;
  • avoid areas where the water is murky;
  • if schools of fish begin to gather, leave the water;
  • do not swim with animals;
  • look carefully before jumping off a boat or wharf;
  • do not swim at dusk or night;
  • do not swim near people fishing or spear fishing;
  • if a shark alert is sounded, leave the water calmly and quickly.

 

Read about the great white shark:

http://animals.nationalgeographic.com.au/animals/fish/great-white-shark/

Read about what sharks eat:

http://www.whatdosharkseat.info/

Watch a video of hammerhead sharks:

http://thekidshouldseethis.com/post/76648411421