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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