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Caribbean Reef Shark
New Providence Island, Bahamas
Caribbean Reef Shark
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More About Sharks ...
Sharks are a group of fish with a full cartilaginous skeleton, a streamlined body plan with 5, up to 7gill 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.

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 have similar parts of their eyes that other vertebrates have like a cornea, lens, and retina. So their eyesight is well adapted to their marine environment. There are some sharks with green eyes that even adapted them so well they are able to see in dark. 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. The lateral lines are used to sense electrical pulses that are 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 the 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 a 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 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:

Hexanchiformes: Examples from this group include the cow sharks, frilled shark and even a shark that looks on first inspection to be a marine snake.
Squaliformes: Examples from this group include the bramble sharks, dogfishes and roughsharks.
Pristiophoriformes: These are the sawsharks, with an elongate, toothed snout that they use for slashing the fishes that they then eat.
Squatiniformes: Angel sharks.
Heterodontiformes: They are commonly referred to as the bullhead, or horn sharks.
Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the largest of all fishes, the whale shark.
Carcharhiniformes: They are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, reef and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
Lamniformes: They are commonly referred to as the mackerel sharks. They include the goblin shark, basking shark, megamouth, the threshers, mako shark and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction.
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).

There are three ways in which shark pups are born:

Oviparity - Some sharks lay eggs. In most of these species, the developing embryo is protected by an egg case with the consistency of leather. Some of these cases are corkscrewed into crevices for protection. Oviparous sharks include the horn shark and the swell shark.
Viviparity - These sharks actually maintain a placental link to the developing young, more analogous to mammals than other fishes. The young are born alive and fully functional. Hammerheads, the requiem sharks (like the bull and tiger sharks), the basking shark and the smooth dogfishes fall into this category. The blue shark produces the most young of sharks that have had the number of pups recorded, the maximum reported being 82.
Ovoviviparity - Most sharks utilize this method. The young are nourished by the yolk of their egg and by fluids secreted by glands in the walls of the oviduct. The eggs hatch within the oviduct, and the young continue to be nourished by the remnants of the yolk and the oviduct's fluids. As in viviparity, the young are born alive and fully functional. Sometimes they are functional even before being born, as some species practice oophagy, where the first to hatch eat the remaining eggs in the oviduct. Sand tigers, makos, threshers, porbeagles and possibly great whites have oophagous young. The survival strategy for the species that do this is that the young are able to grow to an even larger size before being born. The whale shark is now considered to be in this category after having been classified as oviparous for a long time. Whale shark eggs found are now thought to have been aborted.

Shark attacks
The danger of a shark attack has been sensationalized by the media. A person is more likely to be struck by lightning than attacked by a shark. [1] (http://www.worldandi.com/newhome/public/2004/january/nspub.asp) There are, on average, 100 shark attacks per year, with 5 to 15 of them being fatal. Many attacks are the result of the following factors:

Harassment by humans - Cases have occurred when individuals, teenagers in particular, sometimes show off to their peers by grabbing the tails of slow moving, generally placid nurse sharks. Usually the sharks will simply move away from the annoyance, but there have been cases of them turning and attacking the perpetrator.
Mistaken identity (see "Hit and Run" below) - The shark that can cause the most damage in an attack is the great white. While it has attacked swimmers, this usually occurs in murky waters. Most often, the attacks are made on bodysurfers. From below, the silhouette of a surfer on a board looks very much like the shark's preferred prey - a seal.

Sharks have two senses that many animals do not have:

Ampullae of Lorenzini - These small pits in the head detect electricity. The shark has the greatest electricity sensitivity known in all animals. This sense is used to find prey hidden in sand in bottom feeding sharks, by detecting the nerve impulses. It is this sense that sometimes confuses a shark into attacking a boat, when the metal interacts with the salt water.
Lateral line - This system is found in most fishes, including sharks. It is used to detect motion or 'sound' in the water. The shark uses this to detect other organisms moving, especially wounded fish. The shark can 'hear' frequencies in the range of 25 to 50 Hz using this sense.
Source: Wikipedia Read more about Sharks
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