SHARKS+RASHAWN,DELON+AND+SERGIO


 * IS347 RASHAWN WILLIAMS,DELON RUFFIN**


 * CLASS 613 3/5/12**

=SHARK=

= =

Teeth
Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward as in a "conveyor belt"; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months. In most species teeth are replaced one at a time, except in cookiecutter sharks the entire row of teeth is replaced simultaneously. [|[] Tooth shape depends on diet: sharks that feed on [|mollusks] and [|crustaceans] have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the [|basking shark] are smaller and non-functional.

Jaws
Like its relatives, [|rays] and [|skates], the shark's [|jaw] is not attached to the [|cranium]. The jaw's surface, like the shark's [|vertebrae] and gill arches, needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny [|hexagonal] plates called " [|tesserae] ", which are [|crystal] blocks of [|calcium] [|salts] arranged as a [|mosaic]. [|[12]] This gives these areas much of the same strength found in the bony tissue found in other animals. Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the [|bull shark], [|tiger shark] , and the [|great white shark] , have two to three layers or more, depending on body size. The jaws of a large great white sharks may have up to five layers. In the [|rostrum] (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

Tails
Varying tail shapes have evolved in sharks adapted for different environments. [|Tail] ( [|caudal fins] ) vary considerably between species. The tail provides thrust, making speed and acceleration dependent on tail shape. Sharks possess a [|heterocercal] caudal fin in which the [|dorsal] portion is usually noticeably larger than the [|ventral] portion. This is because the shark's [|vertebral column] extends into that dorsal portion, providing a greater surface area for [|muscle] attachment. This allows more efficient [|locomotion] among these negatively boulant cartilaginous fishes. By contrast, most bony fishes possess a homocercal caudal fin. [|[15]] The tiger shark's tail has a large upper lobe which delivers maximum power for slow cruising or sudden bursts of speed. The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the [|porbeagle], which hunts schooling fish such as [|mackerel] and [|herring] has a large lower lobe to help it keep pace with its fast-swimming prey. [|[16]] Some tail adaptations have other purposes. The therhsher feeds on fish and sqiud, which it herds and stuns with its powerful and elongated upper lobboy

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 guts 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] , though their eyesight is well adapted to the [|marine] environment with the help of a tissue called [|tapetum lucidum]. This means that sharks can contract and dilate their [|pupils], like humans, something no [|teleost fish] can do. This tissue is behind the [|retina] and reflects light back to it, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger [|nocturnal] adaptations. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have [|nictitating membranes]. This membrane covers the eyes while hunting and when the shark is being attacked. However, some species, including the [|great white shark] (//Carcharodon carcharias//), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey. The importance of sight in shark hunting behavior is debated. Some believe that [|electro-] and [|chemoreception] are more significant, while others point to the nictating membrane as evidence that sight is important. Presumably, the shark would not protect its eyes were they unimportant. The use of sight probably varies with species and water conditions. The shark's field of vision can swap between [|monocular] and [|stereoscopic] at any time. [|[29]] A [|micro-spectrophotometry] study of 17 species of shark found 10 had only [|rod photoreceptors] and no cone cells in their [|retinas] giving them good night vision while making them [|colorblind]. The remaining seven species had in addition to rods a single type of [|cone photoreceptor] sensitive to green and, seeing only in shades of grey and green, are believed to be effectively colorblind. The study indicates that an object's contrast against the background, rather than colour, may be more important for object detection.

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. [|[33]] A small opening on each side of their heads (not the [|spiracle] ) leads directly into the [|inner ear] through a thin channel. The [|lateral line] shows a similar arrangement, and is open to the environment via a series of openings called lateral line [|pores]. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and [|tetrapods] the external opening into the inner ear has been lost.