Shark

Sharks are a superorder of fish. This superorder is also known by its scientific name Selachimorpha. 

Like other Chondrichthyes, sharks have skeletons made of cartilage instead of bone. Cartilage is tough, rubbery material which is less rigid than bone. Other cartilaginous fish include skates and rays. 

There are more than 350 different kinds of sharks, such as the great white and whale sharks. Fossils show that sharks have been around for 450 million years, since the late Ordovician period.This includes the early fossil sharks which are classified under Elasmobranchii. Elasmo-research.org - Biology of sharks and rays That is 200 million years before the dinosaurs existed. 

Most sharks are predators: they hunt and eat fish, marine mammals, and other sea creatures. However, the whale shark - the largest fish in the world - eats krill, like true whales.  

It is widely believed that sharks are "silent killers". However, a recent study shows that sharks emit a low growl from their throats which resonates through their scales. 

Common kinds of shark include the hammerhead shark, the great white shark, the tiger shark, and the mako shark. Most sharks are cold-blooded, but some, like the great white shark and the mako shark, are partially warm-blooded. 

Just a few million years ago, a giant shark called Megalodon swam in the seas. It was 18 meters long, twice as long as the closely-related great white shark, and it ate whales. Megalodon died out 1.6 million years ago.

Much of what we understand about prehistoric sharks comes from the study of their fossils. While sharks have skeletons made of soft cartilage that can fall apart before fossilizing, their teeth are harder and easily fossilized. Prehistoric sharks, like their modern descendants, would grow and shed many thousands of teeth over their lifetime. For this reason, shark teeth are one of the most common fossils.

 Characteristics 
Sharks come in many different shapes and sizes, but most are long and thin (also called streamlined), with very strong jaws.

Sharks have skin covered in millions of tiny teeth-like scales that point to the tail. If you rub along a shark towards the tail, it feels smooth, but if you rub the other way, it is rough.

 Teeth 
Sharks' teeth are constantly replaced throughout their lives. Sharks eat so violently they often break a few teeth, so new teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on "conveyor belts" formed by the skin which they are attached to. In its lifetime, a shark can lose and regrow as many as 30,000 teeth.thumb|A comparison of teeth from Florida shark species

Even with all those teeth, though, sharks cannot chew. So they bite their prey and jerk it around so they can pull off a chunk to swallow. The chunks of food that a shark swallows end up in its stomach, where they are digested. This is a slow process, however, so a meal might take several days to digest. This is why a shark does not eat every day.

Sharks have different-shaped teeth, depending on what they eat. For instance, some sharks have sharp, pointy teeth, while bottom-dwelling sharks have cone-shaped teeth for crushing shells. Because there are so many different kinds of sharks, and because each kind has its own kind of special teeth, many people enjoy collecting shark teeth. 

Shark teeth collectors can guess how large a shark was by measuring the shark tooth. First, they measure the length of the tooth in inches. Every inch of tooth equals 10 feet of shark length: so if a shark tooth is 2 inches long, the tooth came from a shark that was around 20 ft long. Some Megalodon teeth are 6 inches long - suggesting that this shark was 60 feet long.

 Diet and hunting 
Sharks come in various shapes and sizes, so their prey differs depending on the type of shark and where they live. Sharks typically eat up to 3% of their body weight every two to three days. Sharks' meals are spaced apart because they often have trouble chewing despite their large teeth; this means digestion can take anywhere from 68 to 82 hours. 

While most sharks are carnivores, a recent study shows that there are exceptions. The bonnethead shark is omnivorous, feeding on small animals and sea grass. It has been discovered that wild whale sharks may be omnivorous as well, though this may be due to scarcity in their usual diet. If this is the case, we may see more sharks broaden their diet to include plant life as food scarcity worsens.  

Sharks find their prey by using their sense of smell and vibrations as well as the movement of their prey. The lateral line is an organ found in the skin, which sharks use to sense the vibrations of their prey. Sharks have different hunting strategies to catch their prey depending on size and species. Larger sharks hunt down their meal and often swallow them whole or eat them in large chunks. Some will twist their prey into the sand or stun them with their tail, while others will hunt for small animals such as crustacean. 

Prey for different shark species varies greatly. Hammerhead sharks eat a lot of squid and octopus. Dolphins and small whales are often prey for large sharks like the great white or tiger sharks, while whale sharks mainly subsist on animals like plankton and krill.

 Fins 
Sharks use their fins for swimming, stabilizing, steering, and lift. Each fin is used in a different manner.  
There are one or two fins present along the dorsal midline called the first and second dorsal fin.  These fins help the shark from constantly rolling around. They may or may not have spines.  When spines are present, they are used for defense, and may also have skin glands with them that produce an irritating substance. 

The pectoral fins are behind the head and extend outwards. These fins are used for steering during swimming and help to provide the shark with lift. 

The pelvic fins are behind the pectoral fins, near the cloaca, and are also stabilizers.

Not all sharks have anal fins, but if they do have them, they are found between the pelvic and caudal fins.

The tail region itself consists of the caudal peduncle and the caudal fin. The caudal peduncle sometimes has notches known as "precaudal pits", which are found just ahead of the caudal fin. The peduncle may also be horizontally flattened into lateral keels. The caudal fin has both an upper lobe, and a lower lobe, which can be of different sizes; the shape depends on the shark species. 

The primary use of the caudal fin is to provide a "push" while the shark swims. The upper lobe of the caudal fin produces most of the push, and usually forces the shark downwards. The pectoral fins and the shape of the body (like an airfoil) work together to counter this force. 

The strong, non-lunate caudal fin in most benthic shark species allows the shark to swim close to the seabed (such as the nurse shark). However, the fastest swimming sharks (such as the mako sharks) tend to have lunate-shaped (crescent-shaped) caudal fins. 

 Senses 
Smell
thumb |240px |The shape of the hammerhead shark's head may enhance olfaction by spacing the nostrils further apart.|alt=Eyelevel photo of hammerhead from the front

Sharks have keen olfactory (smelling) sense organs in the short duct between the front and back nasal openings. They can detect blood from miles away: as little as one part per million of blood in sea water may be enough.

Sharks can determine the direction of a certain scent based on the timing of scent detection in each nostril.<ref>Jayne M. Gardiner, Jelle Atema 2010. The function of bilateral odor arrival time differences in olfactory orientation of sharks. Current Biology 20 (13),<rc-c2d-number> 1187-1191 </rc-c2d-number> </ref> This is similar to the method mammals use to determine direction of sound. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.

Sharks are attracted to the chemicals found in the intestines of many species. For this reason, they often linger near or in sewage outfalls. 

Sight
Shark eyes are similar to the eyes of other vertebrates; they have similar lenses, corneas and retinas. Their eyesight is well adapted to the marine environment. They can contract and dilate their pupils, like humans, something no teleost fish can do. A tissue behind the retina reflects light back, thereby increasing sight in darker waters.

 Sensing electric current 
right |300px |thumb |Electromagnetic field receptors (Ampullae of Lorenzini) and motion detecting canals in the head of a shark|alt=Drawing of shark head.

Sharks have tiny holes all over their snouts, especially between the eyes and the tip of the snout. Inside these holes are nerve receptors called the ampullae of Lorenzini. Long J.A. 1995. The rise of fishes: 500 million years of evolution. Johns Hopkins, Baltimore. Chapter 5, p100: Class Placodermip23 They can sense electricity in the water. 

Animals in the water give off electricity: every time an animal's heart beats or moves, tiny currents of electricity are created. These tiny electric currents make signals that travel through water. Sharks can sense these currents and may use this sense when they catch their prey, even more than they use their sight.

 Hearing 
It is hard to test sharks' hearing. However, they may have a sharp sense of hearing and can possibly hear prey many miles away. A small opening on each side of their heads (not the spiracle) leads directly into the inner ear through a thin channel.

 Lateral line 
The lateral line detects changes in water pressure. It is open to the environment by a line of pores. This and the sound-detecting organs are grouped together as the 'acoustico-lateralis system', because they have a common origin. In bony fish and tetrapods, the external opening into the inner ear has been lost.

This system is found in other fish as well. It detects motion or vibrations in water. The shark can sense frequencies in the range of 25 to 50 Hz.

 Reproduction
About 70% of all known shark species give birth to live young, with the gestation period lasting from 6 to 22 months.  

 Baby sharks 
Baby sharks (called pups) are born with a full set of teeth, and are capable of taking care of themselves. Once born, they quickly swim away from their mothers, who sometimes feed on the pups. Litters vary from one or two pups (as with great white sharks) to one hundred pups (blue sharks and whale sharks).   

 Oviparous sharks 
Some sharks are oviparous and lay their eggs in the water. Shark eggs (sometimes called "mermaid's purses") are covered by a tough, leathery membrane.

 Ovoviviparous sharks 
Most sharks are ovoviviparous, meaning the eggs hatch inside the female's body. The babies develop within the mother, but there is no placenta to nourish the pups. Instead the young feed on the egg's yolk. The pups eat any unfertilized eggs and sometimes each other. Very few pups in a litter survive until birth due to this form of sibling cannibalism. 

Great white sharks, mako sharks, nurse sharks, tiger sharks, and sand tiger sharks give birth this way.

 Viviparous sharks 
Some sharks are viviparous, meaning that the females give live birth: the eggs hatch inside the female's body, and the babies are fed by a placenta. The placenta helps transfer nutrients and oxygen from the mother's bloodstream, and transfers waste products from the baby to the mother for elimination. 

Examples of viviparous sharks include the bull sharks, the whitetip reef sharks, the lemon sharks, the blue sharks, the silvertip sharks, and the hammerhead sharks. Although long thought to be oviparous, whale sharks are viviparous, and pregnant females have been found containing hundreds of pups.

 New shark discoveries 
New sharks are still being found. Dave Ebert found ten new species in a Taiwan market. Over the past three decades he has named 24 new species. They include sharks, rays, sawfish and ghost sharks  these cartilaginous fish are all related.BBC News magazine. 

 Fishing 
Some sharks are not endangered, but some are hunted for food (like shark fin soup) or sport fishing.  In 2013 five species of shark, along with two species of manta ray, received international protection as part of the Convention on International Trade in Endangered Species.

It is thought that 100 million sharks are killed by commercial and recreational fishing.<ref>HowStuffWorks "How many sharks are killed recreationally each year - and why?". Animals.howstuffworks.com. Retrieved on<rc-c2d-number> 2010-09-16</rc-c2d-number>.</ref> Sharks are a common seafood in many places, including Japan and Australia. 

In the Australian state of Victoria, shark is the most commonly used fish in fish and chips, in which fillets are battered and deep-fried or crumbed and grilled. In fish and chip shops, shark is called "flake". 

In India, small sharks or baby sharks (called sora in Tamil and Telugu languages) are sold in local markets. Since the flesh is not matured (not adult), cooking the flesh breaks it into powder. The powder is then fried in oil and spices (called sora puttu/sora poratu). The soft bones can be easily chewed. They are considered a delicacy in coastal Tamil Nadu.

 Shagreen 
Shagreen is a fabric made of shark skin. It is often described as being an untanned leather, often dyed green.

 Related pages 

 Lateral line

 References 

 Other media 
BBC One: Blue Planet: The woman who dances with sharks. 
Sharks attack bait ball