Sharks respire with the use of five to seven gill slits. They have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics. They have several sets of replaceable teeth. Sharks range in size from the small dwarf lantern shark, a deep sea species of only 17cms in length, to the whale shark, the largest fish, which grows to a length of approximately 12 metres (39 ft).  The bull shark swims in both seawater and freshwater, as well as in deltas.

Until the 16th century, sharks were known to mariners as “sea dogs”. The name “shark” first came into use after Sir John Hawkins’ sailors exhibited one in London in 1569 and used the word to refer to the large sharks of the Caribbean Sea, and later as a general term for all sharks.

Physical characteristics:- The skeleton of a shark is very different from that of bony fish and terrestrial vertebrates. Sharks have skeletons made from cartilage, which is a flexible and dense connective tissue, but they are still considered bones. They function in the same way as human bones do. The teeth of sharks are embedded in the gums and are constantly replaced throughout the shark’s life. Some sharks can lose some 30,000 teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months. The shape of a shark’s tooth depends on its diet: those that feed on mollusks and crustaceans have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers 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. The tails of sharks provides thrust and so speed and acceleration are dependent on tail shape.

Like other fish, sharks extract oxygen from seawater as it passes over their gills.  The respiration and circulation process begins when deoxygenated blood travels to the shark’s two-chambered heart. Sharks have keen olfactory senses, able to detect as little as one part per million of blood in seawater. They are more attracted to the chemicals found in the guts of many species.  Shark’s eyesight is well adapted to the marine environment. The tissue behind its retina reflects light back to the retina, thereby increasing visibility in the dark waters.  Sharks have a sharp sense of hearing and can possibly hear prey many miles away.  The shark has the greatest electrical sensitivity known in all animals. The electroreceptor organs of the shark are used to detect the electromagnetic fields that all living things produce.   The lateral line is used by the shark to detect the movements of other organisms, especially wounded fish. The shark can sense frequencies in the range of 25 to 50 Hz.

Some sharks like the horn shark, cat shark, Port Jackson shark, and swell shark lay eggs. The bull and blue sharks and smoothhounds fall give birth to young ones. The maximum lifespan of a shark varies from species to species. Most sharks live for 20 to 30 years, while the spiny dogfish lives a record lifespan of more than 100 years. Whale sharks have been hypothesized to also live over 100 years. All sharks are carnivorous. In general, sharks swim at an average speed of 8 km/h, but when feeding or attacking, the average shark can reach speeds upwards of 19 km/h. The shortfin mako shark is considered to be the fastest and it may range upwards of 50 km/h.

Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 400 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, oceanic whitetip, tiger, and bull sharks. These sharks, being large, powerful predators, may sometimes attack and kill people.

An estimate states that, every year, 26 to 73 million sharks are killed by people in commercial and recreational fishing.  In the past, sharks were killed simply for the sport of landing a good fighting fish (such as the shortfin mako sharks). Shark skin is covered with dermal denticles, which are similar to tiny teeth, and was used for purposes similar to sandpaper. Other sharks are hunted for food (Atlantic thresher, shortfin mako and others), and some species for other products. Sharks are common seafood in many places around the world, including Japan and Australia. In India small sharks or baby sharks are caught by fishermen routinely and are sold in the local markets. Sharks are often killed for shark fin soup.  Shark cartilage has been advocated as effective against cancer and for treatment of osteoarthritis.