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Shark
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Shark

Sharks are a group of fish with a full cartilaginous skeleton, a streamlined body plan with between 5 and 7 gill slits along the sides (most often) or side of the head (the first modified slit is behind the eye and called a spiracle), dermal denticles covering the body to protect from parasites, and rows of replaceable teeth in the mouth.

Table of contents
1 Characteristics
2 The name
3 Classification
4 Reproduction
5 Surviving shark attacks
6 Shark senses
7 Shark fishery
8 Sharks in mythology
9 Related articles
10 References
11 External links

Characteristics

Sharks have keen olfactory senses with abilities to smell one part blood in one million parts seawater. Some species have even external barbels (Nurse Shark) that aid even more in sensing prey. Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, though their eyesight is well adapted to their marine environment. Some sharks have stronger nocturnal adaptations, allowing them to see in dark environs. They have a nictitating membrane to protect the eye during predation. The sharks rely more on their superior sense of smell to find prey, however. Once the shark is in the general area of the prey, then the shark begins to use a combination of its smell coupled with the use of the lateral lines running along the side of the shark, used to sense electrical pulses sent out by wounded or dying fish. Their teeth are not attached to the jaw, but embedded in their flesh. The lower teeth are primarily used for holding prey, while the top are used for cutting into it. (Gilbertson, 7.3)

There are exceptions to the "large", "marine" and "predatory" portions of the characterization. Sharks include everything from the hand-sized pygmy shark , a deep sea species, to the whale shark, the largest fish (although sharks are not closely related to bony fish) which is believed to grow to a maximum length of 18m (59 feet) and which, like the great whales, feeds only on plankton. The bull shark is a unique species in that it can swim in both salt water ocean and fresh water rivers. A few of the larger species, the Mako and White shark, are mildly homeothermic, able to maintain their body temperature at a level above the ocean's temperature.

The name

Until the late 16th century sharks were usually referred to in the English language as 'sea-dogs\'. The name "Shark" first came into use around the late 1560s to refer to the large sharks of the Caribbean Sea, and later to all sharks in general. The name may derive from the Maya language word for shark, xoc, pronounced "shock" or "shawk".

Classification

Sharks are a member of Class Chondrichthyes which includes the rays, skates, and Chimaeras. There are 368 recognized species of sharks.

The first sharks appeared in the oceans 400 to 350 million years ago. Most of the species we know today are as old as the Jurassic period. There are eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:

The Lamniformes contains the extinct Megalodon (Carcharodon megalodon), which like all extinct sharks is only known from its teeth (the only bone found in these cartilaginous fishes, and therefore the only fossils produced). A reproduction of the jaw was based on some of the largest teeth (up to almost 17 cm (7 inches) in length) and suggested a fish that could grow 15 m (50 feet) long. The jaw was realized to be inaccurate, and estimates revised downwards to around 6 m (20 feet).

Reproduction

The sex of a shark can be easily determined. The males all have their pelvic fins modified into a pair of claspers. The name is somewhat misleading as they are not used to hold on to the female, but are the shark's version of the mammalian penis. (As a side note, Class Chondrichthyes has the distinction of having the animal with the largest intromittent organ - an organ used for transmitting sperm - in relation to body length. This animal is the clearnose skate (Raja eglanteria) which has claspers of 15 cm (6 in) in size on a fish that reaches 1 m (3 feet) in length.)

Mating has rarely been observed in sharks. The smaller catsharks often mate with the male curling around the female. In the less flexible species the two sharks swim parallel to each other while the male inserts the clasper into the female's oviduct. Many females in the larger species have bite marks that appear to be a result of a male grasping her to maintain position.

Sharks have a much different reproductive strategy than most fishes. Instead of producing huge numbers of eggs and larvae (99.9% of which never reach sexual maturity in fishes that use this strategy) sharks normally produce around a dozen pups, some species up to 70-80 and some as few as 2-3. These pups are either protected by egg cases or born live. No known sharks provide parental protection for their young, but females have a hormone that is released into their blood during the pupping season that apparently keeps them from feeding.

There are three ways in which shark pups are born:

Surviving shark attacks

If a shark is swimming near you, immediately move into shallow waters to limit its mobility. If a shark bites you, beat and claw its eyes, nose, and gills. If you damage these sensitive regions it will most likely release you and retreat. If available, surfboards or other flotation devices can be shoved into a shark’s mouth to confuse or distract it. If you are wounded, immediately get out of the water as the blood will attract more sharks quickly. If for some reason a shark has to be pulled ashore (perhaps to retrieve a severed limb) grasp it around the tail and pull it ashore backwards (most of the shark’s strength is derived from its tail motions, so grabbing it there makes it much weaker.) If you are forced to grapple with a shark in this manner, be wary of its skin, which has the texture of sandpaper and can tear flesh from your body as easily as its teeth.

The best way to survive a shark attack is to avoid one. Do not swim when you are bleeding or in areas that have high population densities of aggressive sharks.

Shark senses

Sharks have two senses that many animals do not have:

Shark fishery

Each year, 100 million sharks are killed by people because they are fished commercially and recreationally. In the past they were fished simply for the sport of landing a good fighting fish (mako sharks for instance), In the past, sharkskin (covered in effect with tiny teeth - dermal denticles) was used for the purposes that sandpaper currently is, others for food (Atlantic thresher, mako and others), and some species for other products. Though the number one killer is in the making of Shark Fin Soup millions of sharks are brutally murdered for their fins that are cut off with a hot metal blade then the live animal is tossed back into the water to perhaps die. Their have been cases where hundred of animals were swept up on local beaches live and kicking without anyway to pull themselves back into the sea to rot. Conservationists have been doing their best to pass laws to make finning illegal in the U.S., but the people just don't care. Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fishes that are harvested. This has caused concern among biologists regarding the increase in effort applied to catching sharks over time, and many species are considered to be threatened.

Sharks in mythology

Sharks figure prominently in the Hawaiian mythology. There are stories of shark men who have shark jaws on their back. They could change form between shark and human at any time desired, and for any length. A common theme in the stories was that the shark men would warn beach goers that sharks were in the waters. The beach goers would laugh and ignore the warnings and go swimming, subsequently being eaten by the same shark man who warned them not to enter the water.

Hawaiian mythology also contained many shark gods. They believed that sharks were guardians of the sea, and called them Aumakua. A listing of them follows:

In other Pacific Ocean cultures, Dakuwanga was a shark god who was the eater of lost souls.

In ancient Greece, shark flesh was forbidden to be eaten at women's festivals.

In Greek mythology, Cerberus saved Delia from the stomach of a shark, fell in love with her and became her protector.

Related articles

References

External links