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.





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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.

Above Images Come From The NOAA















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