Cnidarians
Phylum Cnidaria
| The
most obvious cnidarians on Chek Jawa are the large carpet anemones
that often amaze first-time visitors. |
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'Cnidaria'
means 'stinging nettle' in Greek. There are about 9,000
species of cnidarians.
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Members of the
Phylum Cnidaria include jellyfish, sea anemones, soft and hard corals,
peacock anemones, sea pens, zoanthids and gorgonians.
No-front, No-back: The body of
a cnidarian displays radial symmetry. That is if it was sliced up
like a cake, all slices would look the same! This means there is no
distinct front or rear end of a cnidarian. There is, however, a distinct
upper and lower surface. Usually these surfaces are identified as
oral (the surface with the mouth) and aboral (the opposite side).
Nerves but no brains! Cnidarians have cells arranged in
tissues. Most have nerve cells and muscles, but lack organs such as
brains, hearts, circulatory or excretory systems. They have simple
digestive systems comprising basically of a sac with one opening.
All cnidaria do not have an anus! Indigestible bits go out the same
way they first came in, through the mouth.
 Stingers!
Most cnidarians have tentacles. All cnidarians have some sort of stingers
(called cnidocytes) comprising a long, thread often barbed at the
base, in a hollow capsule closed with a tiny cap. The thread lies
coiled up in the capsule, ready to fire. When stimulated by touch,
chemical signals or triggered by the nerves of the animal, the pressure
within the capsule is instantly raised. This blows out the cap and
the thread everts and uncoils explosively.
For more about nematocysts and a video of a firing nematocyst, see
the Tropical
Australian Stinger Research Unit website.
Some types of stingers penetrate the victim and inject paralysing
toxins. These are called nematocysts. The thread of the stinging cell
is hollow and toxins are pumped through it. Most toxins only affect
small creatures like plankton, shrimp and fish. Some are released
powerfully enough to even penetrate shelled creatures like small crustaceans.
Some cnidarians, however, have toxins powerful enough to hurt or even
kill people.
Other types of stingers merely produce sticky threads, barbed threads,
or long threads which entangle. These may help collect small food
particles or trap hard-bodied prey like crabs. There are more than
30 types of cnidarian stingers! Each cnidarian will have a unique
set, so stingers are used in identifying the species.
Once the prey is stung, the tentacles then wrap around the subdued
or immobilised prey and bring it to the mouth.
Once fired, the stinger is not re-used. New stingers replace used
ones. Although tiny, these stingers are effective because a cnidaria
can have thousands of such stingers. Stingers are found all over the
body of a cnidarian, but the tentacles usually have a greater concentration
of stingers.
Farming their Food: Almost all
cnidarians are carnivorous. Many eat small creatures or trap detritus,
plankton and other microscopic titbits. But many also capture and
eat large prey. Many cnidarians, however, more supplement their meals
with food provided by photosynthetic zooxanthellae (symbiotic single-celled
algae) that they harbour in their tentacles and bodies. These algae
produce food from sunlight. In exchange, the cnidrian provides protection
and simple minerals.
All for One and One for All: Many
cnidarians are colonial, that is, many individual animals live together
as one animal. Corals are made up of countless tiny polyps that remain
connected to each other. In sea pens, several kinds of polyps are
interconnected, each having a different shape and function.
Skeleton of Water: All cnidarians
have a hydrostatic skeleton. That is, fluids maintain the shape of
their bodies, much like air in a balloon. Muscles push against this
fluid-filled body to change the body shape; the shape of a balloon
half-filled with air can be changed by pushing the air in it around.
This is how jellyfish pulse their umbrella-shaped bodies to swim,
how anemones extend and move their tentacles, and how stingers are
fired off. Some cnidarians, however, also produce a hard skeleton
that also provide protection. Hard corals, for example, have an external
skeleton while gorgonians have an internal skeleton.
Medusa and Polyp: Cnidarians come
in one of two typical shapes. Some go through both shapes in their
lifecycle, others stay in one shape all their lives. The medusa is
the typical jellyfish shape familiar to many of us: an umbrella-shaped
body with the mouth facing downwards and surrounded by tentacles.
This form is usually free-swimming, moving by contracting the umbrella-shaped
body to expell water and move off in the opposite direction.
The polyp is the flower-like shape that we are familiar with in sea
anemones. In this body form, the animal generally has a mouth facing
upwards and surrounded by tentacles. The other end is usually fixed
onto something or buried into the ground, so this form is usually
immobile.
Cnidarian Babies: Cnidarians typically
practice external fertilisation, with eggs and sperm released simultaneously
into the water. In some, the genders are separate, while others may
be hermaphrodites. Most cnidarians undergo metamorphosis and their
larvae look nothing like their adults. The form that first hatches
from the eggs are free-swimming oval blobs covered with cilia (tiny
hairs). These drift with the plankton. In some small jellyfish, these
larvae eventually settle down and develop into polyps that feed and
grow. These polyps may reproduce asexually by budding off more polyps.
Eventually, the polyp may reproduce asexually by budding off medusa
forms. These medusa swim off and develop into adults that may eventually
reproduce sexually. The original polyp may remain alive to produce
medusa forms again later on.
There are many variations of this development. Some large jellyfish
don't have a polyp stage. Sea anemones and corals don't develop the
medusa stage.
Role in the habitat: Hard corals
are important reef builders. Coral reefs provide homes for small animals
and are a haven and nursery for ocean-going creatures. Reefs also
protect the shoreline from strong waves, storms and erosion.
Many creatures have adapted to deal with the stinging cells of cnidarians.
Anemonefish, shrimps and crabs live in safety among their deadly tentacles.
Some nudibranchs not only eat the stingers, but are also able to transfer
these, unsprung, to the ends of their own 'tentacles', ready to protect
the nudibranch from disturbers. Hermit crabs and snails may also have
anemones on their shells as additional protection against disturbers.
Human uses: All kinds of corals
hard and soft, sea anemones and other cnidaria are extensively harvested
from the wild for the live aquarium trade. Hard coral are also mined
as building materials in some coastal areas. Living coral reefs, however,
are worth far more to humans when they left alone. Reefs bring in
tourists which generate business beyond the shore (e.g., hotels, restaurants
and travel-related industries). Reefs are also homes to a bewildering
variety of creatures, some of which protect themselves with toxins
or other chemicals that may have pharmaceutical applications.
Status and threats: Harvesting
of marine wildlife may involve the use of cyanide or blasting, which
damage the habitat and kill many other creatures. Like other creatures
harvested for the live aquarium trade, most die before they can reach
the retailers. Without professional care, most die soon after they
are sold. Those that do survive are unlikely to breed successfully.
Like other creatures of the intertidal zone, cnidarians are affected
by human activities such as reclamation, pollution and trampling.
They are also affected by global warming. It is believed that high
water temperatures may result in hard coral bleaching, where the polyps
and zooxanthallae part ways, affecting the growth of reefs.
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Carpet anemone
Peacock anemone
Sea pen
Zoanthid
Hard coral
Gorgonian
Hydroid
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