Physical Characteristics of Sharks
An article from Wikipedia
Sharks (superorder Selachimorpha) are a type of fish with a full cartilaginous skeleton and a streamlined body. They respire with the use of five to seven gilldermal denticles that protect their skin from damage and parasites and improve fluid dynamics; they also have replaceable teeth. Shark teeth are prized by collectors for their beauty, and although greatly feared most shark populations have fallen by 90%. Sharks range in size from the small pygmy shark, Euprotomicrus bispinatus, a deep sea species of only 22 centimetres (9 in) in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres (39 ft) and which, like baleen whales, feeds only on plankton, squid, and small fish through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt and fresh water and in deltas.
The physical characteristics of sharks are different from those of bony fish, but the large number of species and the diversity of shark habitats means that there are many variations on the "typical" shark body.
The skeleton of a shark is mainly cartilage. The sharks have at least 230 bones.
Shark teeth are relics of shark evolution and biology. Shark skeletons are composed entirely of cartilage. Often the only parts of the shark to survive as fossils are teeth. Fossil shark teeth have been dated back hundreds of millions of years. The most ancient types of sharks date back to 450 million years ago, and they are mostly known from their fossilized teeth. The most common, however, are from the Cenozoic Era (65 million years ago).
The teeth of sharks are not attached to the jaw, but embedded in the flesh, and in many species are constantly replaced throughout the shark's life. When they lose a tooth they will replace then within 7 to 8 days. All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. In some sharks rows of teeth are replaced every 8-10 days, while in other species they could last several months. The lower teeth are primarily used for holding prey, while the upper ones are used for cutting into it. The teeth range from thin, needle-like teeth for gripping fish to large, flat teeth adapted for crushing shellfish.
A shark can have hundreds of teeth in its jaw. Sharks, as well as other Chondrichthyes, have the ability to replace their teeth if they become damaged during feeding or fall out due to natural causes. Many icthyologists have suggested that sharks can lose tens of thousands of teeth within the span of a few years.
Megalodon Teeth.Carcharodon megalodon teeth are among the most sought after types of shark teeth in the world. These teeth are in extremely high demand by collectors and private investors, and they can fetch steep prices. This shark lived during the Miocene and Pliocene eras, roughly about 16 to 1.5 million years ago. Its teeth on average range between 1.5 to 6.5 inch in length. But the largest teeth of this shark are more than 7 inch long. These huge teeth indicate that the megalodon could grow up to more than 16 m (52.5 ft) long, growing bigger than the largest fish alive in the world today, the whale shark.
Large numbers of megalodon teeth have been discovered across both coasts of the United States. The most plentiful locations within the U.S. are the Carolinas, Georgia, and parts of Florida and Virginia.
These large and magnificent sharks disappeared relatively close to the rise of modern man. However, there have been many unconfirmed reports of large animals resembling large, carnivorous, sharks.
Megalodon teeth have been discovered that some argue date as recently as 10,000 to 15,000 years ago. This claim is based on the discovery of two teeth by the HMS Challenger scientific expedition. These teeth were dated by estimating the amount of time it took for manganese to accumulate on them, although it is quite possible the teeth were fossilized before being encrusted.
Transitional teeth.Identifying shark teeth is tedious work, especially with all of the damaged, worn, and different teeth. It is even more difficult because of the so-called "In-Between Teeth". These are teeth that are from a shark species that was evolving into another, different species. An example of this can be seen with Carcharocles auriculatus teeth that were in the state of evolving into Carcharocles angustidens. These are teeth that are no longer auriculatus, but not yet angustidens either.
Great White/Mako Transitional Teeth.The most common and most referred to transitional shark teeth are the ones coming from what is believed by some to be an unusual form of great white shark. Great white shark transitional teeth are often characterized for their wide crowns. These teeth can also be identified by the way the serrations fade, being more pronounced near the root, and disappearing close to the tip of the tooth.
Many paleontologists now believe that these transitional teeth represent the evolutionary path between Isurus hastalis and the Great white shark. The evolutionary history of the great white shark and its relation to megalodon are hotly debated.
Because of their transitional state, these teeth are rare. These teeth are prized by collectors, hobbyists, and museums.
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Some sharks have a modified slit called a spiracle located just behind the eye, which is used in respiration. Due to their size and the nature of their metabolism, sharks have a higher demand for oxygen than most fish and they can not rely on ambient water currents to provide an adequate supply of oxygenated water. If a shark were to stop swimming, the water circulation would drop below the level necessary for respiration and the animal could suffocate. The process of ensuring an adequate flow of the gills by forward movement is known as ram ventilation. Some sharks, such as the blacktip reef shark, Carcharhinus melanopterus, and nurse shark, Ginglymostoma cirratum, can pump water over their gills as they rest. There are also recorded instances, as in certain caves along the Yucatan coast, where sharks rest on the cave floors and allow the fresh water outflow to pass over them. The outflow is strong enough to still allow for respiration; it is believed that the reason for this behaviour is that the fresh water helps remove parasites. The grey nurse shark, Carcharias taurus, is known to gulp air at the surface and store it in its stomach to provide additional buoyancy. Some sharks, if inverted, enter a natural state of tonic immobility - researchers use this condition for handling sharks safely.
In contrast to bony fishes, sharks do not drink seawater, instead they retain high concentrations of waste chemicals in their body to change the diffusion gradient so that they can absorb water direct from the sea. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to a marine environment. A few exceptions to this occur, as with the bull shark, which has developed a way to change its kidney function to excrete large amounts of dilute urine.
Other highly specialist feeders include the cookiecutter sharks, which feed on the flesh sliced out of other larger fish and marine mammals. The teeth in these sharks are enormous, compared to their size, with the teeth of the lower jaw being particularly sharp. Although they have never been observed feeding they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rasp off flesh.
Some seabed dwelling species are highly effective as ambush predators. Angel sharks and wobbegongs are perfectly camouflaged for lying in wait in order to suck prey into their mouths. Many benthic sharks feed solely on crustaceans which they crush up with their flat molariform teeth.
Other sharks feed on squid or fishes, which are swallowed whole. The viper shark has teeth which can be pointed outwards to strike at and capture prey that is then swallowed intact. The great white and other large predators can either swallow small prey whole or take huge bites out of large animals. Thresher sharks use their long tails to stun shoaling fishes, and sawsharks may either stir prey up from the seabed or slash at swimming prey with their tooth-studded rostra.
Many sharks, including the whitetip reef shark are cooperative feeders and hunt in packs in order to herd and capture elusive prey. These social sharks are often highly migratory, travelling huge distances around ocean basins in large schools. These migrations may be partly necessary to find new food sources.
Digestion of the food can take a long time in sharks. The food moves from the mouth to the 'J' shaped stomach, where it is stored and initial digestion occurs. Unwanted items may never get any further than the stomach, and are coughed up again. Many sharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of any unwanted contents.
One of the biggest differences in digestion in sharks when compared to mammals is the extremely short intestine. This short length is achieved by the spiral valve with multiple turns within a single short section instead of a very long tube-like intestine. The valve provides a very long surface area for the digestion of food, requiring it to pass around inside the apparently short gut until fully digested., when remaining waste products pass into the cloaca and vent.
The most obvious internal organ in sharks is the huge liver, which often fills most of the body cavity. In the basking shark, the liver makes up about a quarter of the body weight and may weigh up to a ton. In basking shark fisheries this was the major product as it contained up to 80% in weight of very high quality squalene oil.
The tails (caudal fins) of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration is dependent on tail shape. Some typical shark tail shapes are discussed below:
Tiger shark - The tiger sharks tail is pronouncedly epicercal (the upper lobe is longer and heavier than the lower lobe). Movement is controlled by swinging the body from side to side. The large upper lobe delivers the maximum amount of power for slow cruising or sudden bursts of speed. The tiger shark has a varied diet, and because of this it must be able to twist and turn in the water easily when hunting turtles, fish, stingrays, and other sharks.
Nurse shark - The nurse shark is common in shallow waters around the tropical coasts of America and Africa, and is nocturnal. Its prey consists mainly of invertebrates such as crabs, lobsters, sea urchins and octopuses. This causes the sharks to spend most of their time on the seabed. As a result high acceleration is not necessary and the lower lobe has almost completely disappeared. They swim with an eel-like motion, using broad sweeps of their elongated tails to cruise slowly.
Porbeagle - The porbeagle is a heavily built pelagic shark, closely related to the mako and great white, which hunts on schooling fishes such as mackerel and herring. The tail is used for propulsion rather than having to swing their body from side to side. The large lower lobe provides greater speed to help them keep pace with their fast swimming prey, and their lateral keels may reduce drag making for more efficient hunting.
Thresher shark - The thresher is found in tropical and temperate waters around the world and feed on fish and squid, which they are believed to herd, then stun with the powerful and elongated upper lobe. The three species of thresher are active and strong-swimming sharks - the evolution of the highly elongated tail (which may be half of their total length) has not been at the expense of speed or agility.
Great white shark - The great white is primarily a coastal and offshore species, but can be found far from land. Its body and tail have a shape similar to that of tuna. Its upper and lower lobes are of almost equal size. This provides for both slow cruising and fast chasing speeds.
Cookiecutter shark - The cookiecutter shark hunts squid and crustaceans, but will attach itself to larger fish or dolphins with its strong suction lips. Using its scoop-shaped lower jaw it will then cut out a conical plug of flesh. Its tail has broad lower and upper lobes of similar shape which are luminescent and may help to lure prey towards the shark.
In general, sharks swim ("cruise") at an average speed of 8 km/h (5 mph), but when feeding or attacking, the average shark can reach speeds upwards of 19 km/h (12 mph). The shortfin mako may range upwards of 50 km/h (31 mph). The shortfin mako shark is considered to be the fastest shark and one of the fastest fish. The great white shark is also capable of considerable bursts of speed. These exceptions may be due to the "warm-blooded", or homeothermic, nature of these sharks' physiology.
Unlike bony fish, the sharks have a complex dermal corset made of flexible collagenous fibres and arranged as a helical network surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy. A similar arrangement of collagen fibres has been discovered in dolphins and squid. Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.
A few of the larger species, such as the shortfin mako, Isurus oxyrinchus, and the great white, are mildly homeothermic - able to maintain their body temperature above the surrounding water temperature. This is possible because of the presence of the rete mirabile, a counter current exchange mechanism that reduces the loss of body heat.