Shark Tale

Sharks (superorder Selachimorpha) are fish with a full cartilaginous skeleton and a streamlined body. They respire with the use of five to seven gill slits. more...

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Sharks have a covering of dermal denticles to protect their skin from damage and parasites and to improve fluid dynamics. They have replaceable teeth. They are some of the world's most misunderstood predators, as they very rarely attack humans unless intimidated.

Sharks include species from the hand-sized pygmy shark, Euprotomicrus bispinatus, a deep sea species of only 22 cm in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres (41 feet) and which, like the great whales, feeds only on plankton through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt and fresh water (it is found in Lake Nicaragua, in Central America) and in deltas.

Physical characteristics

Skeleton

The skeleton of a shark is very different from that of bony fishes 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.

Respiration

Like other fishes, 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, many sharks have a higher demand for oxygen than bony fishes and cannot rely on ambient water currents to provide an adequate supply of oxygenated water. If these sharks were to stop swimming, the water circulation would drop below the level necessary for respiration and the animal would suffocate. The process of ensuring an adequate flow of the gills by forward movement is known as ram ventilation. Some sharks, such as the nurse shark, Ginglymostoma cirratum, can pump water over their gills as they rest.

The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the blood is pumped to the shark's gills via the ventral aorta artery where it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the ventricle of the heart and the cycle repeats. This way of respiration is highly inefficient and if the shark were to stop moving blood would not be able to move through the body due to the lack of strength of the heart's atrium.

Read more at Wikipedia.org