Shark

For the NHL team, see San Jose Sharks

Sharks
Gray Reef Shark (Carcharhinus amblyrhynchos)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Superorder:	Selachimorpha

Sharks are in a class of fish called Chondrichthyes, with skeletons made of cartilage instead of bone. Cartilage is tough, rubbery material which is less rigid than bone. Cartilaginous fish also 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 420 million years, since the early Silurian.

Most sharks are predators, meaning they hunt and eat fish, marine mammals, and other sea creatures, but the largest shark eats krill, like whales. This is the whale shark, the largest fish in the world. Some common kinds of shark include the hammerhead shark, the great white shark, the tiger shark, and the mako shark.

Characteristics

Sharks come in many different shapes and sizes, but most are long and thin (also called streamlined), with powerful jaws. Their 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 in which they are attached to. In its lifetime, a shark can lose and regrow as many as 30,000 teeth.

Even with all those teeth, though, sharks can not chew. So they bite their prey and jerk it around so they can pull of a chunk to swallow. The chunks of food that a shark swallows ends up in its stomach, where they are digested. This is pretty slow, 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 10ft of shark length: so if a shark tooth is 2 inches long, the tooth came from a shark that was 20 ft long! Even more terrifying is that some of the Megalodon teeth are 6 inches long so that suggets a shark 60 feet long.

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 would feel smooth, but if you rub the other way, it would be rough. Sharks' teeth can be 20 times as big as human teeth and they can grow back if they are lost.

Senses

Smell

The shape of the hammerhead shark's head may enhance olfaction by spacing the nostrils further apart.

Sharks have keen olfactory 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 have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril. This is similar to the method mammals use to determine direction of sound.

They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.

Sight

Shark eyes are similar to the eyes of other vertebrates, including 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

Electromagnetic field receptors (Ampullae of Lorenzini) and motion detecting canals in the head of a shark

Sharks have tiny holes all over the shark's snout, especially between the eye and the tip of the snout. In them are which are nerve receptors called the ampullae of Lorenzini. ^p23 They can sense electricity in the water. Animal in the water give off electricity: every time an animal's heart beats or it moves, tiny currents of electricity are made. These tiny electric currents make signals that travel through water and get sensed. Sharks may use this sense when they catch their prey, even more than they use their sight.

Hearing

Although it is hard to test sharks' hearing, 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.

Prehistoric sharks

Because sharks have skeletons made of cartilage, they do not fossilize easily. That's because cartilage is softer than bone and falls apart before fossilizing. However, teeth are harder, and if that is not enough, they are shed throughout a shark's lifetime. Therefore, shark teeth are one of the most common fossils.

Sharks existed for at least two hundred million years before the dinosaurs. The earliest shark fossils were scales, so could it be that the earliest sharks were toothless? We are not sure. Early sharks did not look like sharks of today. For example, the upper snout of modern sharks is longer than the lower jaw. But in early sharks, they were the same length.

The first modern-looking sharks appeared in the Age of Dinosaurs. Sharks back then were preyed on by giant sea reptiles.

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.

Shark pups

Most sharks are ovoviviparous. This means shark eggs usually develop and hatch inside the female's body. As a result, sharks are born fully developed and can even swim and catch prey as soon as they are born. A female shark is pregnant for a short time: it will only take two months for a shark to develop and come out from its mother.

Some shark pups attack and eat the other pups growing inside the mother, so a few of these baby predators do not make it to their birthdays.

Not all sharks give birth to live young. Some sharks will lay egg cases, which are often called "mermaid's purses." They come in lots of shapes and sizes, depending on the kind of shark that laid them. The baby shark develops inside the egg case (outside its mother's body), and when it is fully developed, the egg case splits and the shark swims out. Most egg cases are rectangular, and others are spiral-shaped.

Avoiding sharks

Some good ways to be less noticeable to sharks are

• Do not carry dead fish into the water. Dead fish can attract sharks.
• Do not be in the water at dawn, or late at night. They do not like to hunt when it is hot, so come out when the sun's shining bright.
• Stay out of dark, muddy water.
• Do not wear bright colors - especially if they contrast.
• Do not swim in places with a steep drop off nearby. Sharks like to stay in those drop offs.
• Never make a shark angry by poking it or trying to play with it.
• If you see a shark, be calm and slow.
• Stare right at the shark if you see one. Sharks usually attack when they are not noticed.
• If all else fails, fight bravely with fists and legs, kicking it in the head and nostrils.

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 estimated that 100 million sharks are killed by commercial and recreational fishing. 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 language, Telugu language) are sold in local markets. Since the flesh is not developed, cooking the flesh breaks it into powder, which 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.