Sharks, great raiders.. raided (by Marco Angelozzi) 

The sharks, above all after the diffusion of films that have given a distorted image of their behaviors, have been considered as murderous raiders, in the continuous search of food, activity that would bring them to feed of anything they see behind, without distinction.. 

Reality is very different;  these elegant cartilagineous fishes are only great raiders, inserted to the vertexes of the food chain of the environment in which they live and as such they have some notable offensive potentialities. Every kind of shark uses to feed itsels of determine preys, which are outcast with techniques that nothing leave to the case or to the fatality and the search of food, as in every living organism, spends important times within the life of these animals. It is not plundered everything the shark see, but in the 99% of the known kinds there is a real discrimination, based on instincts and preferences.

Great white shark (Carcharodon carcharias)
 

The man in fact, not doing part of aquatic fauna and probably because of the little adipose fabric present in his  bodily structure, doesn't reenter in the preys of any shark! Only some kinds that reach great dimensions can represent a danger for the man, as all the great raiders that live on the earth.  (The 50% of the sharks doesn't overcome the meter of length, the 82% doesn't overcome the 2 meters, and only 4% reach superior dimensions to the 4 meters, such to represent a serious danger). 

Every year, in all the seas of the globe, there are about 60 attacks, about 0-6 deadly. In almost all these cases the death is caused by the gravity of the first and only bite that the shark uses, together with the other senses, to become account of that that is found in front, because it really uses to select its preys and to discriminate what doesn't reenter in its diet. The shark in fact usually doesn't continue the attack and it doesn't swallow the prey but it gets further.. 

There are very dangerous situations that can attract the sharks and to increase the risk of an attack, as during a food frenzy, in presence of issue of sounds to low frequency or with blood and other bodily liquids in water. 

Also the outline of a surfer that swims with the hands toward the outside above its table can be exchanged for that a sea mammal, prey aspired by the great sharks.. 

Some attacks are known to underwater fishermen that swam in waters where great quantity of blood had been poured because of their fishing and where the dying preys were fixed dangerously to the belt.. 

All these considerations, united to the datum of fact what the man capture and kill every year around 100 million sharks, to feed the food and commercial market, can let us think that currently the shark is transformed by great raider to.. great prey.

       

Mako shark ((Isurus oxyrinchus) and white fin shark (Triaenodon obesus)
 

Many aspects of the sharks biology and ethology are not very well known, above all because of the difficulty to study these fishes in their natural environment and for their bashful and solitary behavior Here you can read many informations about their evolution, biology and morphology.

There are about 450 kinds of Sharks, an about fifty of which present in the Mediterranean, with dimensions that go from the only 24 cm of the Pygmy Shark from the ribbon tail (Eridacnis radcliffei) to the 18 meters of the Whale Shark (Rhiniodon typus), that represents with these measures also the greatest fish actually known.
The most ancient fossils of these animals go up to around 400 million years, and the biologist thinks that their evolution has even already arrived at the most level 100 million years ago.
The skeleton of the sharks is not bony, as that of the common fishes but cartilaginous, formed by cartilage, similar to that that constitutes the ear or the human trachea.
Contrarily of the common fishes, the sharks don't possess the swimming bladder (an inside afloat body that can fill him with gas) whose function of support to the floating is replaced partly from the big liver, that can reach the 25% of the animal weight. Sharks are predators and in their diet it can be present fish, smaller shark, shellfish, mollusc and sea mammal.

The biologist knows three kinds of sharks that eat plankton, filtering the water that enters from their jaws. They are the already quoted Whale Shark (Rhiniodon typus, up to 18 ms.) Elephant Shark (Cetorhinus maximus, up to 13 ms.) and the Great mouth Shark (Megachasma pelagos, up to 5 ms.).  Sharks form is generally tapered and hydrodynamic, with the lengthened face and pointed, to favor the advancement in water.

Sharks possess the same senses of the man, more other two to us strangers, that are the ability to perceive the electric fields and the waves of pressure spreaded in water.

Taste: the sense of the taste in the sharks is insured from the presence, in the mouth, but also on the surface of their skin, of gustatory papille similar to those human. The shark therefore, can also use this sense with the simple contact of the bodily surface.

Touch: along sharks body there are sensorial cells that are in deep relationship with the gustatory papilles and let the shark realizes when a physical contact happens.

Hearing: in the sharks it is present an inside ear, with the ability to perceive above all sounds of low frequency as those sent forth by wounded animals, and therefore potential preys for this superb animal.

Sense of smell: the sharks nostrils, always well visible on the inferior part of the face, are constituted by two channels to blind fund, at the end there are cells that analyze the presence of fragrant substances dissolved in water. The sensibilitynof the sharks is very developed, it is thought that a shark can individualize 1 part of blood in 100 million parts of water.

Sight: the sight of the sharks, contrarily to the popular thought, is very developed. The pupil can tighten or to widen in base to the light quantity, and in many sharks it is present, behind the retina, the tapetum lucidum, a series of reflecting plates that amplify the light andallow the sight also during the night. Under conditions of strong illumination the tapetum lucidum is darkened, not to dazzle the retina and to also provoke irreversible damages.

Side lines: the side lines, one for every side of the shark, are constituted by sensorial cells that give the ability to perceive the waves of pressure owed to the movements of the water.

Ampullae Lorenzinii: they are of the organs that take contact with the outside through small and numerous holes, present especially in the region of the head, full of a gelatinous conductor substance, in communication with nervous terminations. In this way the shark has the ability to perceive the electric fields produced by the animals (therefore individualize preys under the sand) and probably to recognize their position in comparison to the magnetic earth fiels.

Sharks body is covered with placoid scales, also called like small teeth. They have the same structure of those of greater dimensions, that are found in the jaws.
The placoid scales, besides constituting an effectiveness protection of the shark body, reduces also the resistance to the sea water, improving animal capacity to swin.
The teeth that are found in the superior and inferior jaws are modified placoid scales and they are very great.
Their form and disposition point out of what preys the sharks are usual to feed in fact long and sharp teeth as those of the bull shark (Carcharias taurus) are proper to pierce and to hold back small fishes and cefalopods, while those more breadths and pieces as those of the shark tiger (Galeocerdo cuvier) develop a greater resistance and they serve for shattering hulls and to shear pieces of food.

    

Teeth of tiger shark (Galeocerdo cuvier) and bull shark (Carcharias taurus)

 

In the sharks jaws the teeth are prepared in more file, six and the last toward the inside of the mouth are generally tilted, covered by a fold of fabric and not yet completely developed. During the growth the tooth is subject to a move in ahead for the continuous formation of the gengival fabric to which is connected. In their advancement they progressively straighten, because of the simple mechanical tension, until they don't become entirely functional. After a few times the teeth of the anterior lines are destined to fall, following traumatic breakups or spontaneously and the biologist think they are singly replaced every 8-15 days, more frequently in the youngest samples. The biologist think that some kinds of sharks renew a whole line of teeth to the time.