Sharks Pictures

     Sharks are a type of fish that have a dangerous reputation.  They have a streamlined body and a full cartilaginous skeleton.  The shark has 440 species, and they measure from only 17 centimeters or 7 inches in length to 12 meters, or 39 feet in length.  Sharks are found in all oceans and are common all the way to depths of 2,000 meters or 6,600 feet in the ocean.  Some will live in deeper waters.  Sharks do not usually live in freshwater, but a few exceptions to this are the river shark and the bull shark which are able to live in seawater as well as freshwater.  Sharks respire using five to seven gill slits.  They also have a covering of dermal denticles that will keep their skin protected from damage as well as parasites.  The dermal denticles also help improve the fluid dynamics of the shark so it is able to move faster.  Sharks also are known for their several sets of teeth that are replaceable. People are often frightened of the predatory skills of the great white and hammerhead sharks.

     Shark skeletons are unlike those of terrestrial vertebrates and bony fish.  Sharks and other carilaginous fish have skeletons that are made of cartilage and from connective tissue.  The benefit of cartilage is that it is both durable and flexible.  The problem is that it has only half of the density that bone has.  This will reduce the weight of the shark's skeleton and save its energy.  The sharks do not have a rib cage. 

      The shark has a tremendous jaw.  It's jaw is not attached to its cranium.  The surface of the jaw requires additional support because of its exposure to physical stress and because it needs power and strength.  A layer of hexagonal plates called tesserae are crystal blocks of calcium salts.  These give the shark similar strength as bony tissue that is apparent in other animals.  There is usually only one layer of tesserae found in sharks; however, the jaws of large specimens like the bull shark, great white shark, or tiger shark have two or three layers or more.  The white sharks' jaws may have as many as five layers.  The cartilage is able to be spongy and flexible in order to absorb the power of impacts. 

      The teeth of the sharks are in the gums instead of being directly fixed to the jaws.  This means that they are commonly replaced during the life of the shark.  Many rows of replacement teeth grow inside the jaw and then move forward to replace teeth that  have been lost.  Some sharks have the ability to lose up to 30,000 teeth in their lifetime.  Tooth replacement in sharks can be anywhere from 8-10 days to months.  In many species of sharks the teeth are replaced individually as they are lost, but in the cookiecutter sharks, the entire row of teeth are replaced at one time.  Sharks' teeth shape largely depends on the diet of the species.  Sharks that feed on crustaceans and mollusks usually have dense and flattened teeth used for crushing.  However, sharks that feed on fish usually have needle-like teeth used for gripping.  Fish that feed on larger prey like mammals often have pointed lower teeth that are utilized for gripping as well as having triangular upper teeth that have serrated edges that are used for cutting.  The teeth of plankton feeders like the basking shark are often greatly reduced and are non-functional. 

     The fins of the shark do not allow them to swim in a backwards direction.  When there are objects in front of them, sharks can only drift from those objects.  The dermal denticles of a shark help it to save energy because they have an outer skeleton that allows them to save energy.  They are able to reduce their turbulence when they are swimming, so their movement is much better.

     The tails of sharks are very unique.  The caudal fins are very different based on the species.  They are also adapted to the needs of that species of shark.  The tail helps the shark thrust, and as a result, speed and acceleration of the shark are dependent on the shape of the tail.  The different tail shapes have changed in sharks and are adapted for different environments of the sharks. 

     Sharks are unlike bony fish in that they do not have gas-filled swim bladders that are used for buoyancy.  Instead the sharks need their liver which is filled up with oil that has squalene.  Since the cartilage is half as heavy as bone, this is also helpful to the sharks.  The liver takes up to 30% of the body mass of the shark.  Most shark species will swim constantly so that they can continue to breathe and they are unable to sleep for very long or they will find themselves sinking.  Certain shark species, such as the nurse shark, can pump water across their gills.  This will enable them to rest at the bottom of the ocean. 

     Sharks breathe by extracting oxygen from the seawater as it passes over its gills.  Shark gills are in a row behind its head.  This is different from other fish.  Another slit is just behind the eye of the shark, and this is called a spiracle.  This spiracle will help with water intake for the shark during respiration and is especially important for bottom dwelling sharks.  While the shark moves, water will pass through its mouth and over its gills.  While resting, the majority of sharks will pump water over their gills to ensure that they have a supply of oxygenated water so they can breathe.  Some species no longer have the ability to pump water through their gills and have to swim constantly without resting.  These species would likely asphyxiate if they were not able to move.  Respiration and circulation occurs in sharks when the deoxygenated blood travels to the two-chambered heart of the shark.  The shark pumps blood to its gills through the ventral aorta artery, and it is here that it branches off into brachial arteries.  The reoxygenation occurs in the gills and this blood then flows into the efferent brachial arteries, which together form the dorsal aorta.  The blood then goes from the dorsal aorta to different parts of the body.  The deoxygenated blood that comes from the body will flow through the posterial cardinal veins and goes into the cardinal sinuses.  The blood then enters the heart ventricle and the cycle will repeat itself. 

     The majority of sharks are cold blooded.  This means that their internal body temperature is the same as their environment.  Some sharks are homeothermic and have a higher body temperature than the temperature of the surrounding water.

     Sharks have olfactory senses that are in the short duct located between the anterior and posterior nasal openings.  Some species can detect just one part per million of blood located in seawater.  The eyes of sharks are like the eyes of other vertebrates, and sharks are able to contract and dilate their pupils.  Sharks do have eyelids, but they do not blink because the water actually cleans their eyes.  Some of the species have what is called nictitating membranes.  These membranes cover the eyes of a shark while it is involved in predation as well as when the shark is being attacked.  Some species that do not have this membrane, including the great white shark, do not have the membrane, so instead they will roll their eyes back in order to protect them while they are striking at their prey.  The hearing of sharks is thought to be sharp.  They have a small opening on each side of their heads.  This opening leads to the inner ear by way of a thin channel. 

     Reproduction of sharks involves internal fertilization.  The male shark uses the posterior part of its pelvic fin, more specifically a pair of intromittent organs called claspers to deliver the sperm into the female.  Sharks produce a small number of young that are well developed.  Sharks may practice oviparity, viviparity, and ovoviviparity to reproduce young.  The majority of sharks are ovoviviparous.  This means that the young are born alive and able to function.  Other sharks use oviparity, in which they lay eggs.  Some of these sharks include the Port Jackson shark, swellshark, catshark, and horn shark.  Other sharks have a viviparity method, and here the fish are born alive and functional.  These include requiem sharks, smoothhounds, and hammerheads. 

     Sharks are often very social and live in large schools; sometimes these schools will contain over 100 individual sharks.  Most sharks are carnivorous.  The tiger shark will eat about anything, but the majority of sharks will go after a particular type of prey.  Megamouth, basking, and whale sharks utilize filter feeding.  These species evolved plankton feeding independently.  Whale sharks will use suction to take in large amounts of small fish and plankton.  Basking sharks will swim through blooms of plankton with their mouth open wide.  Megamouth sharks will use luminescent tissue that is inside their mouths to attract prey that is in the deep ocean.  Gill rakers are necessary for this form of feeding.  The shark will trap the plankton using filaments and will swallow large mouthfuls on occasions.  The teeth of these species of sharks are smaller because they do not need them for feeding.  `The cookiecutter sharks feed on flesh that they grab hold of and rip from other fish and marine mammals.  The teeth of cookiecutter sharks are huge in comparison to their relatively small size for sharks.  The teeth of the lower jaw are very sharp.  Angel sharks and wobbegongs will utilize camouflage in order to wait for prey and then suck them into their mouths.  Some sharks such as the viper dogfish will feed on squid or fish.  The great white shark and other large sharks have the ability to swallow their prey whole or else take rather large bites of their prey.  Thresher sharks can use their elongated tails in order to stun shoaling fish, and sawsharks can stir prey from the seabed or slash at swimming prey.  The digestion of food can take a long time for sharks.  The food will go into their stomachs which are shaped like a J.  Unwanted items that arrive in the stomach may never get beyond the stomach, but instead will be vomited or the stomach may turn inside out, sending unwanted items from the mouth.  The sharks also have a very short intestine. 

     On average, sharks will swim at an average speed of five miles per hours or eight kilometers per hour.  However, when a shark is attacking or feeding, it may reach speeds beyond twelve miles per hour or nineteen kilometers per hour.  The shortfin mako shark may reach speeds of thirty-one miles per hour.  It is one of the fastest sharks and fastest fish in all.  The great white shark is also a fast shark and is capable of very fast bursts of speed. 

     Sharks are found in all seas, but they generally don't live in freshwater.  There are several exceptions to this, which are the river shark and the bull shark which are able to live in either freshwater or seawater.  The sharks normally exist in waters that are at depths up to 7,000 feet or 2,000 meters, but are rarely found below 3,000 meters or 10,000 feet. 

    It is believed that there are around 100 million sharks killed by humans annually, and this is primarily due to recreational and commercial fishing.  Sharks are commonly eaten as seafood in a number of locations in the world, including Australia and Japan.  Some sharks are being killed for shark fin soup.  This involves the removal of the fin of the shark with a hot metal blade.  The sharks are captured, finned, and then the finless animal is released into the water, only to die from predators or from suffocation.  This is a big trade in black markets around the world, and shark fins may even be sold for around $300 per pound.  There are millions of sharks that are poached illegally every year for their fins.  Many sharks are also killed for their meat.  The shark is one type of fish that children and pregnant women are advised to stay away from in the United States.  Shark cartilage has been proposed as effective against cancer and for the treatment of osteoarthritis. 

    Sharks have a very dangerous reputation.  Even though there are more than 360 species of sharks, only four of these species have been involved in unprovoked attacks on humans, and they are the oceanic whitetip, tiger, bull sharks, and great white.  These sharks are powerful predators and can sometimes attack and kill humans.