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
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
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
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..
Mako shark ((Isurus
oxyrinchus) and white fin shark
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.
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
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
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
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.
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.
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riproduzione, anche parziale, del testo e delle
foto presenti in questo articolo, senza il consenso dell'autore