Sharks are some of the world´s most misunderstood predatorsSharks have much more to fear from humans than humans have to fear from sharksSharks are fish with a full cartilaginous skeleton and a streamlined body They respire with the use of five
to seven gill slits. 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, a deep sea species of only 22 cm in length, to the
whale shark, 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 skeleton of the shark is very different from that of bony fishes such as cod; it is made from cartilage,
which is very light and flexible. 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. Shark attacks in history
The first representation of an attack by
what must have been a shark was discovered on a vase unearthed at Ischia, Italy, an
island just west of modem day Naples. The vase shows a man seized by a fish reminiscent
perhaps of a shark, and has been dated c. 725 BC. The first account of an attack by a
marine monster dates back to Greek history, with Herodotus in 492 BC. He was not talking
specifically of the shark, even though the latter was probably involved, for the word
did not yet exist and no really lifelike graphic representation was to appear before
the I5th century. Again in Greek history, the poet Leonidas of Tarentum evokes the
tragic end of the sponge-fisher Tharsys, when he was being hoisted aboard his boat by
his two companions and was attacked by a sea monster which tore away the lower sections
of his body. Tharsys' companions brought ashore his remains and thus, the poet
elegantly noted, Tharsys was buried both on land and at sea. Shark behaviour - The natural history of sharks A shark is equipped with an extraordinary
array of sensors, probably the most diverse of any known predator. Low-frequency sounds
tend to travel great distances underwater, so a shark's auditory system could be the first
sense to pick up an interesting target. Rapid, irregularly-pulsed, broad-band sounds at
frequencies below 600 hertz, similar to those made by injured prey or a group of spawning
fish can alert a hunter such as a bull shark from over 1.6 km (1 mile) away.
At a distance of 0.5 km (1/3 mile), it is able to smell blood in the water and follow a
trail back to the source. It can detect one part of fish extract in 25 million parts of
seawater, the equivalent of ten drops of blood in an average-sized municipal swimming
pool. At 100 m (330 ft), the shark's lateral line system kicks in. This row of fluid-filled
sensory canals on either side of its body responds to pressure changes and movements,
so a shark can almost feel the presence and location of something moving in
the water - a kind of "touch-at-a-distance". Sharks strike fear into the minds of humans This fear is really a fear of the unknown, for little
evidence exists that indicates sharks are really very dangerous. Sharks have much more
to fear from humans than humans have to fear from sharks. Each year thousands of tons
of sharks are killed by fishermen for food and other products while very few humans are
ever bitten, and fewer still die from shark bites.
Sharks belong to the class Chondrichthyes, a group which also includes rays, skates
and chimeras. Unlike other vertebrates, sharks have a skeleton made of cartilage rather
than bone. Only about 800 species of cartilaginous fish are known to exist compared to
over 20,000 known species of bony fishes.
Sharks differ from the more common bony fishes in several ways. Besides having no bones,
sharks and their relatives do not have the overlapping scales that cover many bony fish.
Nor do they have a swim bladder, the air-filled balloon-like organ that keeps most fish
upright. Sharks have gill slits but no gill-cover, which is common to the
bony fish. Sharks eat a variety of foods The largest of sharks, the Whale and Basking
(Cetorhinus maximus) sharks, eat plankton, while the Port Jackson shark often feeds on
sea urchins that are crushed by flat teeth and account for the purple stain found around
the mouth. Many of the larger, fast-swimming sharks are true carnivores and an astonishing
variety and amount of food have been found in some. For example, a 3.5 m whaler shark
taken near Sydney was reported to contain eight legs of mutton, half a ham, the hind
quarters of a dog, 135 kg of horseflesh, a ship's scraper and a piece of sacking.
This was in the days when abattoirs dumped remains in the water, a practice
unthinkable today. The numerous accounts of indigestible items taken from sharks'
stomachs are probably the result of the feeding frenzy which occurs when a group
of sharks indiscriminately attack anything that comes their way. Books about sharks 
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