Page 88 - Present Day Wildlife_Float

Jellyfish are invertebrates; marine animals, with no backbone, heart, teeth or blood. There are
hundreds of different species, they inhabit all the worlds seas, oceans and lakes and are the oldest mul-
ti-organ animal on the planet, believed to have first appeared around 500 to 700 million years ago. Jel-
lyfish belong to a group called the Cnidaria. The largest known species is the Lion's mane jellyfish
(Cyanea capillata ), which reaches from its domed
top to the tip of its tentacles around 36.5 meters,
and the smallest, the Irukandji box jellyfish (Malo
spp.) being 1 cm

The most distinctive features of jellyfish are
their curved dome-shaped bodies and long dangling
tentacles trailing from their underside. They do not
have any bones; instead their bodies are filled and
supported by a jelly-like substance known as Meso-
glea. Although they live in water very few swim,
many simply drift, while others use a pulsating ac-
tion to move through the water. Jellyfish do not
have a brain and are therefore incapable of support-
ing a central nervous system. What they do have is
known as a nerve-net. This is a complex intertwin-
ing nerve system running through their bodies.
From the stimuli received from this nerve-net the
jellyfish are able to orientate themselves in the wa-
ter, test temperature, detect changes and, with no
ears, detect vibratory ‘sounds.’ Its nerve system al-
so supports its ‘eyes’. These are small coloured spots on the skin known as, ocel-
li, that are believed to help jellyfish distinguish between light and darkness.
Ocelli on most jellyfish are very simple and are thought incapable of distin-
guishing images. However, the Box Jellyfish’s ocelli are more advanced, support-
ing a retina, lens, and cornea, and many believe it can see. This could account for
its fast movement through water, confidently moving at six metres per minute.
The jellyfish’s mouth, which is also its anus, is on the tip of a small cone pro-
truding from the centre of its underside. They don’t however, have a nose and
lack any kind of respiratory system. Their oxygen intake is done through their
epidermis, or the top layer of skin on their body.

Jellyfish exist on a variety of aquatic life. They hunt and eat small fish, fish
larvae, shrimp, tiny crustaceans such as krill and copepods, tiny plants such as
algae, even other jellyfish. Their main hunting weapon is their tentacles. They
are lined with thousands of stinging cells with many containing small harpoons
filled with toxins. To disable or paralyse its prey it simply fires the harpoons into
its victim, injecting them with the toxin. Then a special set of tentacles around
its mouth captures the victim and feeds it to its mouth. Even touching the tenta-
cles can prove fatal not only for aquatic life but also for humans. Although, not
all jellyfish have toxins powerful enough to affect humans, many have toxins that
can cause discomfort and severe pain. Surprisingly, the deadliest toxin is carried
by the smallest of the species, the rukandji box jellyfish. They are the world’s
most venomous jellyfish. If not treated quickly, touching one of these will kill
you in a very short period of time. Poisonous or not, jellyfish are still eaten by
turtles, ocean sunfish, some seabirds, crabs, whales, even humans. The Chinese
have considered them a delicacy for over a thousand years.

Jellyfish are a remarkable species they have survived almost unchanged for
millions of years and survived every major extinction event nature can throw at
it. Over the years scientists have come to know its physical makeup intimately,
yet know very little about how it functions. Recent studies have shown it can learn and will not make
the same mistake twice. They are also sure, some of the species can actually see, yet both these func-
tions — remembering things and seeing things — require an active brain. However, the jellyfish does
not have a brain. So how does it do it?