Fishes that have a cartilaginous skeleton may be
classified as sharks. They belong to the class Chondrichthyes.
They may be found in all seas, but are generally more abundantly
found in tropical and subtropical waters. There are about three
hundred and forty varieties of sharks, with diverse behavior. They
are diverse in size too. The whale shark (rhincodon typus) is
the largest fish in the sea and measures up to forty-nine feet in
length. Whereas, the cookie-cutter shark (squaliolus laticaudus)
measures less than fifty centimeters in length.
Sharks have a
leathery skin covered with small, sharp, pointed scales. The
skin is generally grey in color. The physical structure of a
shark is interesting. Its vertebral column extends to the
upper lobe and its tail is asymmetrical. Most species of
sharks have rows of sharp teeth that are embedded in fibrous
membranes and not in jawbones.
feature about sharks is that they sink when they are not in
motion. This is because they don’t have swim bladders.
Sharks are keen-sensed
fishes that eat nearly all kinds of large marine animals. They are
primarily scavengers, and feed on injured fishes, carrion, and
garbage, as well as animals such as seals, turtles, birds and crabs.
Sharks have an acute sense of smell and easily detect minute
substances such as blood in water. They are also keenly sensitive to
sounds of low frequency and have fine directional hearing.
Sharks move in groups
called schools. When hunting in schools, sharks incite one another
into a feeding frenzy. They usually circle their prey and make
sudden crisscrosses. They strike the prey from below. Sharks attack
human beings very rarely. Ninety per cent of the shark varieties do
not harm human beings. However, some varieties such as the great
white (carcharodon carcharias) shark and some other
species are considered dangerous.
The great white shark
grows to a length of twenty feet or more. It is considered the most
dangerous among the species. Also known as the white pointer, the
tooth of the white shark is about two inches in length, which can
tear off a human limb in a single bite. Other varieties of sharks
considered dangerous for human beings are the mako or blue pointer,
the tiger and the hammerhead shark.
The pattern of
reproduction in sharks is yet another fascinating study. Sharks are
ovoviviparous, that is to say, the eggs are hatched within the
female and then live young ones are born. Of course, as with many
other things, there are exceptions here too and there are some
varieties that lay eggs externally. Many sharks produce large,
well-developed offspring. Sometimes, there are hundred young ones
that are there in a litter. The young ones of the larger varieties
could measure one meter in length at birth.
Dolphins and sharks
share a strange relationship. Sharks prey on solitary dolphins, but
when the dolphins are in a group, they easily outwit and may even
kill the shark. Dolphins have been known to kill sharks in some
aquariums. They butt the sharks with their pointed noses.