Physical characteristics of sharks
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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.
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[edit] Skeleton
The skeleton of the shark is very different from that of bony fish such as cod; it is made from cartilage, which is very light and flexible, although the cartilage in older sharks can sometimes be partly calcified, making it harder and more bone-like. The shark's jaw is variable and is thought to have evolved from the first gill arch. It is not attached to the cranium and has extra mineral deposits to give it greater strength. The most common type of jaw can swing down and forward in order to catch comparatively large prey. The skeleton of a shark is split into two main parts: the axial skeleton (the head and trunk of the body), and the appendicular skeleton (supporting the fins). [1]
[edit] Respiration
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.[2]
[edit] Buoyancy
Unlike bony fishes, sharks do not have gas-filled swim bladders, but instead rely on a large oil-filled liver (which may constitute up to 25% of their body mass) for buoyancy. The effectiveness of this is limited, so sharks sink when they stop swimming 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.
[edit] Osmoregulation
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 urea.[3]
[edit] Teeth
The teeth of carnivorous sharks are not attached to the jaw, but embedded in their flesh, and in many species are constantly replaced throughout the shark's life. All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth are continuously growing 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.[4] Carnivorous sharks' jaws are not attached to the skull, so when a shark bites, the lower jaw "catches" the prey and springs upwards and then the upper jaw springs downward allowing both sets of teeth to sink into the prey. Different bite patterns occur depending on the species. There are many variations in the teeth between shark species and information about a shark’s diet can be deduced by the study of its teeth. The teeth range from thin, needle-like teeth for gripping fish to large, flat teeth adapted for crushing shellfish.
[edit] Feeding and digestion
All sharks are carnivorous and many people believe that sharks will eat just about anything; for a few species, such as the tiger shark, this is true. The vast majority of sharks, however, are far more specialised for particular prey items, and rarely stray from these. Some of the most specialised sharks have developed a filter feeding technique, which is employed by the whale, basking and megamouth sharks. These three shark species have evolved plankton feeding independently and use different strategies. Whale sharks feed using suction to take in large concentrations of plankton and small fishes. Basking sharks are ram-feeders, swimming steadily, with their mouth wide open, through plankton blooms. Megamouth sharks may make their suction feeding extra efficient with the use of luminescent tissue inside the mouth the attract prey in the deep ocean. This type of feeding was only possible through the evolution of gill rakers, long slender filaments that form a very efficient sieve, analogous to the baleen plates of the great whales. Plankton is trapped in these filaments and swallowed from time to time in huge mouthfuls. Teeth in these species are very small compared to the size of the animal, because they are not needed for feeding.
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 of flesh. [3]
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. [5]
Digestion of the food can take a long time in sharks, particularly in the cold-blooded species. 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. Int eh basking shark, the liver makes up about a quarter of the body weight and may weigh up to a tonne. In basking shark fisheries this was the major product as it contained up to 80% in weight of very high quality squalene oil. [6]
[edit] Tails
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.
[edit] Speed
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).[7] 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.
[edit] Dermal denticles
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.
[edit] Body temperature
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.
[edit] References
- ^ Hamlett, W. C. (1999). Sharks, Skates and Rays: The Biology of Elasmobranch Fishes. Johns Hopkins University Press.
- ^ The Undersea World of Jacques Cousteau Special, "The Sleeping Sharks of the Yucatan," NBC.
- ^ a b Compagno, Leonard; Dando, Marc & Fowler, Sarah (2005). Sharks of the World. Collins Field Guides.
- ^ Gilbertson, Lance (1999). Zoology Laboratory Manual. New York: McGraw-Hill Companies, Inc..
- ^ Stevens, John D. (1989). Sharks. Sydney: Weldon Owen Pty Ltd..
- ^ Pratt, H. L. Jr; Gruber, S. H.; & Taniuchi, T. (1990). Elasmobranchs as living resources: Advances in the biology, ecology, systematics, and the status of the fisheries. NOAA Tech Rept..
- ^ Reefquest Center for Shark Research. What's the Speediest Marine Creature?
