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Sharks
Sharks are a group (superorder Selachimorpha) of fish, with a full
that cartilaginous skeleton, a streamlined body plan, with normally
5, but up to 7 (depending on species) gill slits along the side
of, or beginning slightly behind, the head (in some species, a modified
slit called a spiracle, is located just behind the eye), dermal
denticles covering the body to protect from damage, parasites and
improve fluid dynamics, and rows of replaceable teeth in the mouth.
Sharks have keen olfactory senses, with some species able to detect
as little as one part per million of blood in seawater. They are
even more attracted to the chemicals found in the gut of many species,
and often linger near or in sewage outfalls. Some species, such
as Nurse sharks, have external barbels that greatly increase their
ability to sense prey. The short duct between the anterior and posterior
nasal openings are not fused like in bony fish.
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 with the help of a tissue called
tapetum lucidum. This tissue is behind the retina and reflects light
back to the retina, 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 have nictitating membranes. This membrane covers
the eyes during predation, and when it is being attacked. However,
some species, including the Great White, do not have this membrane,
but instead roll their eyes backwards to protect them. Sharks generally
rely on their superior sense of smell to find prey, but once they
are close they also use the lateral lines running along their sides
to sense movement in the water and also employ special sensory pores
on their heads to detect electrical fields created by prey and the
ambient electric fields of the ocean. Their teeth are not attached
to the jaw, but embedded in their flesh, and in many species are
constantly replaced throughout the shark's life. The lower teeth
are primarily used for holding prey, while the top are used for
cutting into it.
Sharks also have a sharp sense of hearing and can hear prey many
miles away. A small opening on each side of their head (not to be
confused with the spiracle) leads directly into the inner ear through
a thin channel. The lateral line shows a similar arrangement as
it 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 is grouped together as
the acoustico-lateralis system. In bony fish and tetrapods the external
opening into the inner ear has been lost.
There are exceptions to the "large", "marine"
(as in 'ocean-going') 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 known to grow to a maximum length of approximately 15 metres
(49 feet) and which, like the great whales, feeds only on plankton.
Although not unique among sharks, the Bull Shark is the better known
of several species to regularly swim in both salt and fresh water
environments (most famously in Lake Nicaragua, in Central America)
and in most deltas. 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. This is possible because
of the presence of the rete mirabile, a counter current exchange
mechanism that reduces the loss of body heat.
Like other fish, sharks extract their oxygen from seawater as it
passes over their gills. Due to their size and the nature of their
metabolism, sharks have a higher demand for oxygen than most fish
and they cannot rely on ambient water current to provide an adequate
supply of oxygenated water. If a shark were to stop swimming, the
necessary water circulation for respiration would become too low
and the animal could suffocate, although some sharks have been known
to "nap" on the bottom. Some sharks, like the Blacktip
Reef Shark and Nurse Shark, can pump water over their gills as they
rest. There are also known instances, such as in certain caves along
the Yucatan coast, where sharks of varying species rest on the cave
floors and allow the fresh water outflow to pass over them. The
outflow is strong enough to allow for respiration, and it is believed
that the reason for this behaviour is that the fresh water helps
remove certain parasites. Also, unlike other fish, sharks do not
have gas-filled swim bladders, but rather rely on an oil-filled
liver for (limited) buoyancy, so they sink when they stop swimming;
a resting shark always sinks to the sea bed. Sand tiger sharks are
known to gulp air at the surface and store it in their stomach to
provide buoyancy.
Unlike bony fish, the sharks have a complex dermal corset arranged
as a helical network and made of flexible collagenous fibres surrounding
their body. This works as an outer skeleton, providing attachment
for their swimming muscles. The sharks saves more energy while swimming
this way than if they didn't have their collagenous corset. A similar
arrangement of collagen fibres has been discovered in dolphins and
squids.
Their dermal teeth gives them hydrodynamic advantages as they are
reducing the turbulence when swimming.
A shark, if inverted, enters a natural state of paralysis. Researchers
use this condition for handling sharks safely.
A popular myth is that sharks are immune to disease and cancer,
however, this is untrue and there are both diseases and parasites
that affect sharks. The evidence that sharks are at least resistant
to cancer and disease is mostly anecdotal and there have been few,
if any, scientific or statistical studies that have shown sharks
to have heightened immunity to disease.
Urine in sharks accumulates in the blood and is then secreted through
the skin.
Their upper jaws are not attached to the skull
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