Shark



Sharks are a group (superorder Selachimorpha) 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.

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 conditions. Some sharks 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, the shark also uses the lateral lines running along the side of the shark 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 (and in lake Nicaragua). 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.

A shark if inverted enters a natural state of paralysis. This state is called 'tonic' and the shark usually becomes dull and unresponsive for a while. Researchers use this condition for handling sharks safely. The condition is termed tonic immobility.

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".

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.

Surviving shark attacks

Although shark attacks on humans are incredibly rare (just 55 attacks worldwide in 2003, of which four proved to be fatal - [1] (http://www.flmnh.ufl.edu/fish/sharks/statistics/2003attacksummary.htm)), people do run the risk of attack if swimming in waters around shark feeding areas. Sharks in general are not aggressive, nor are they man-eaters - most attacks involve the shark mistaking a human for its usual prey, taking a bite, realising its mistake and then leaving.

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 are known to be near shark feeding zones.

Shark fishery

Each year, 100 million sharks are killed by people in commercial and recreational fishing. In the past they were fished simply for the sport of landing a good fighting fish (mako sharks for instance). 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.

However, sharks are most often killed for shark fin soup, in which many sharks are hunted for their fins, which are cut off with a hot metal blade before the live animal is tossed back into the water. There have been cases where hundreds of de-finned animals were swept up on local beaches without any way to convey themselves back into the sea. Conservationists have campaigned for changes in the law to make finning illegal in the U.S.

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.

Organizations like the Shark Trust are trying to limit shark fishing.