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Line 117: − − == Behavior == − − Many species of sharks give birth to a few dozen live young, others lay a similarly small number of eggs; these are often washed ashore and are known as [[mermaid]]'s purses. − − Sharks are reputed to be able to detect minute amounts of blood in water, and to be able to follow blood trails for miles. In fact, although sharks can detect blood at concentrations of as little as a few parts per billion, a very considerable quantity must be spilt before it is detectable amongst billions of gallons of seawater. Sharks actually use sounds and vibrations to locate most of their prey. − − ==Classification== − Sharks belong to the [[holobaramin]] that also includes skates and rays.
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'''Sharks''' are approximately 350 species of cartilaginous fish of the order Selachimorpha, inhabiting the world's oceans and several freshwater lakes and rivers.
'''Sharks''' are approximately 350 species of cartilaginous fish of the superorder Selachimorpha, inhabiting the world's oceans and several freshwater lakes and rivers.
==Description==
==Description==
Sharks vary in appearance, shape, and size, so there is no absolute description of a typical shark. In general, all sharks with several exceptions share the following characteristics:
Sharks vary in appearance, shape, and size, so there is no absolute description of a typical shark. In general, all sharks with several exceptions share the following characteristics:
====Cartilaginous skeleton====
All sharks possess a skeletal system made entirely of cartilage; there is no solid bone anywhere. The vertebrate extends to and forms the upper lobe of the claudal (tail) fin. The skull has no sutures, and in nearly all species the mouth is ventrally-located, i.e. under and behind the snout. The upper jaw is unattached to the skull, enabling the animal to extend both jaws forward to take in prey.
====Placoid scales====
Shark skin is covered in thousands of tiny placoid or dentical scales related to teeth; these scales are pointed backward and sharp.
====Lifetime of teeth====
The only hard part of a shark's body, their teeth are arranged in a "conveyor belt"-fashion, enabling continual replacement over the lifetime of the animal.
====Sensory organs====
Sharks possess several sensory organs which enable them to bear down on their prey or scavenge, often from miles away. Their eyesight is extremely sensitive to light and shadow. Their sense of smell enables them to detect a drop of blood in water a quarter-mile away, while their lateral lines (a nerve line extending along their flanks) enable them to detect the vibrations from distressed fish several miles away. The ''ampullae of Lorenzini'' are small pits on the snout, enabling sharks to detect weak electrical signals produced by the muscle contractions of bony fish.
==Diet==
With the exception of the two largest sharks - the plankton-feeding [[Whale shark|whale]] and [[Basking shark|basking]]) sharks - all sharks are predatory. Most species eat fish; some, such as the bullhead sharks, eat shellfish. One species ([[Great white shark|great white]]) routinely eats warm-blooded prey. And nearly all sharks scavenge dead remains found anywhere, in effect cleaning up the area quickly.
==Behavior==
Many species of sharks give birth to a few dozen live young, others lay a similarly small number of eggs; these are often washed ashore and are known as [[mermaid]]'s purses.
Sharks are reputed to be able to detect minute amounts of blood in water, and to be able to follow blood trails for miles. In fact, although sharks can detect blood at concentrations of as little as a few parts per billion, a very considerable quantity must be spilt before it is detectable amongst billions of gallons of seawater. Sharks actually use sounds and vibrations to locate most of their prey.
==Classification==
Sharks belong to the [[holobaramin]] that also includes skates and rays.
===Orders and Families===
===Orders and Families===
*Nurse sharks (Ginglymostomatidae, 3 species)
*Nurse sharks (Ginglymostomatidae, 3 species)
*Whale shark (Rhiniodontidae, 1 species)
*Whale shark (Rhiniodontidae, 1 species)
====[[Lamniformes]]====
====[[Lamniformes]]====
Mackerel sharks. Most species identified by a robust appearance; all lack a nictating membrane over the eyes.
Mackerel sharks. Most species identified by a robust appearance; all lack a nictating membrane over the eyes.
*Requiem sharks (Carcharhinidae, 52 species)
*Requiem sharks (Carcharhinidae, 52 species)
*Hammerhead sharks (Sphyrnidae, 9 species)
*Hammerhead sharks (Sphyrnidae, 9 species)
==Environmental Concerns==
==Environmental Concerns==