The seemingly
simple jellyfish
By Nutan
Shukla
JELLYFISH is a fascinating creature
of the sea.It looks like a beautifully coloured,
semi-transparent, blob which can be seen floating
aimlessly on the surface of water. However, all the
jellyfish have 95 per cent water in their body and they
seem harmless because they never attack anybody. Some
species are, however, quite poisonous and their stinging
cells can even kill human beings.
These pulsating, inverted saucer like
creatures have bushes of tentacles around the rim. These
tentacles are capable of stretching downward for 50 feet
or more. If they come in contact with the skin, they
produce a fiery pain and raise great weals wherever they
have touched, for in their tissues they carry great
numbers of tiny structures known as nematocysts or
stinging cells.
There are many different
kinds of nematocysts to be found among jellyfish. These
perform different functions, therefore it is not easy to
describe these cells. Among these one kind of cells are
the ones that sting, but all of them appear to work on
the same principle. As far as a description of these
cells is concerned, it is often found in the books. If
one takes a rubber glove, pushes in the fingers so that
they are inside-out, and then blows sharply into the
glove, the fingers will pop out again. This is the sort
of explanation we often get about nematocysts. But a
nematocyst is not like a glove: it has only one very
long, thread-like finger in a flask-like capsule with no
opening through which air can be blown.
Under an appropriate
stimulus (touching some living body), a fluid containing
a poisonous substance is forced through the wall of the
capsule. It rapidly builds up pressure and fills the
coiled up, inverted, hollow thread so that it is thrust
out violently. It has a dart at its tip so that it can
pierce the skin and inject the venom. The thread usually
has little spines arranged around it in a spiral,
enabling it to attach itself or to wind itself around the
prey and draw it up to the mouth. Although frightening,
it is an intriguing mechanism. What is surprising is the
complexity of this lowly, simple creature.
Jellyfish are, of course,
not fish. They are properly called medusae and they
belong to a rather lowly group of animals usually called
coelenterates, meaning literally "hollow
inside", but used to indicate that the body cavity
is also the digestive organ. Roughly speaking, a
coelenterate is just a stomach surmounted by a mouth
surrounded by tentacles.
A nematocyst or stinging
cell is tiny and scarcely visible without a microscope.
It does not operate alone. The lightest touch against a
tentacle will trigger off a battery of hundreds or may be
thousands of these venomous darts, as though an area is
being seared with a hot iron.
More widely recognised as
a menace is the so-called Portuguese Man-of-War, a
relative of jellyfish, of the tropical Atlantic and the
Mediterranean. Despite the float in the body (which
appears more like a flask) this fish is not always seen
by the swimmer. By adjusting the amount of gas, it can
sink below the surface. Even if it is seen, it may
already be too close. To brush against the tentacles is
to be stung over a considerable area of skin by millions
of venomous threads. According to one expert, 55,000,000
nematocysts weigh only one gm and over 62 gm of them are
to be found in one gallon of tentacle tissue; how many
gallons there are of the jelly-like mass removed from the
water depends on the size of the creature.
Nematocysts are not under
any nervous or muscular control: each one fires off its
dart under a direct stimulus from outside. This stimulus
is not tactile but chemical, for if a relatively inert
substance like glass is brushed against a tentacle the
nematocysts do not respond. A food substance
generally protein or fatty matter is what turns
them on. It seems there may be a sort of control through
the cnidoblast, that is, the mother cell. When the animal
is fully fed, the stinging apparatus does not work so
well, it seems that if food substance is plentiful inside
the tissues, the flow of fluid is reduced and this
prevents the nematocysts from functioning. Without this
inhibition, the presence of food on the outside causes a
rapid intake of fluid that builds up considerable
pressure.
The thread, coiled
inside-out in the capsule, is often pleated to reduce its
volume and the barbs are crowded inside with their tips
towards the centre. The capsular fluid now fills the
thread, thrusting it rapidly outward; the pleats unfold
so that the length of the thread is greatly increased as
it exerts and twists, spacing the barbs out in the
characteristic spiral whorl round the outside; the long,
hollow tube springs out straight, strengthened by the
twist, and drives the dart at its tip into the source of
food.
Because nematocysts often
behave as though they are independent creatures lodged in
the tissues of the jellyfish, it is imprudent to handle a
speciman that appears to be dead (unless it is known to
be a relatively harmless kind) for the nematocysts may
still be capable of stinging. Some kinds of molluscs feed
on these animals and can actually pass the nematocysts
into special spaces in their skin, where they function on
behalf of their predator.
Sometimes after a storm
when many jellyfish have been broken against the stones,
fragments of tentacles may litter the beaches and it can
be dangerous to walk with bare feet. This is particularly
true in tropical regions.
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