Physical characteristics of sharks

The physical characteristics of sharks are different from those of bony and many other kinds of fish, but the large number of species and the diversity of shark habitats means that there are many variations on the "typical" shark body.

Skeleton

The skeleton of a shark is mainly made of cartilage.

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.

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. All sharks have a dorsal fin

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. Sand tigers are fairly large, stocky sharks with blunt snouts and large, unserrated teeth. Their skin ranges from a sandy-brown to grey-brown colour and they have dark spots, which fade as they mature.

Also Known as: Carcharias taurus These sharks use their sharp teeth to catch small fishes, squid, crabs and lobster.

Size 4-10 feet long

Diet Fishes, squid, crabs and lobster

Lifespan 15 years or more

Range: Temperate and tropical oceans worldwide

Habitat: Sand tiger sharks frequent habitats from 6 to 600 feet deep, including the surf zone, coral and rocky reefs, and the ocean floor. The surface layer of the ocean is known as the Epipelagic Zone or The Sunlight Zone and extends from the surface to 200 meters (656 feet). It is also known as the sunlight zone because this is where most of the visible light exists. With the light come heat. This heat is responsible for the wide range of temperatures that occur in this zone. Sand tigers inhabit the Atlantic, Pacific and Indian Ocean and are sometimes encountered in the Mediterranean

Predators: Young sand tiger sharks may be preyed on by larger sharks, but adults have virtually no predators other than humans.

Relatives: There are more than 400 species of sharks, and only a few are dangerous to humans. Sharks are closely related to skates and rays.

Family life: Baby sand tiger sharks develop within eggs that are carried and hatch in their mother’s uterus. The largest hatchling in each uterus eats the unhatched eggs and smaller embryos. As a result, each female gives birth to just two offspring, one for each uterus. The pups are born after 8 to 9 months, and then take about 4 to 6 years to reach breeding age. Shark populations recover slowly from overfishing, partly due to this slow growth rate.

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.

Skin and dermal denticles

Main article: Dermal denticle

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.