updated Mar 13
learn only 3 things about them ...
Many cnidarians are small and easily overlooked. Don't
step on it.
Most cnidrians have stingers. Don't touch them!
are important to the habitat. Don't kill or remove them.
seen? Cnidarians are commonly encountered on our shores.
Many cnidarians are often confused for plants or non-living things.
Some are so bizarre, we don't immediately link them with their more
Cnidarians belong to the Phylum Cnidaria that includes the more familiar
sea anemones, hard corals and jellyfishes. There are about 10,000
known species of cnidarians.
Cnidaria is divided into four main Classes.
Order (box jellyfish and sea wasps)
Milleporina (fire corals)
Subclass Alcyonaria (Octocorallia)
Features: Pronounced "ni-dare-ee-ya'
(the 'c' is silent), 'Cnidaria' means 'stinging thread' in Greek.
Indeed, one of the defining characteristic of cnidarians are their
stingers (more about these below).
Same all around: All cnidarians
have radial symmetry. That is if a cnidarian was sliced up like a
round birthday 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.
Lacking specialised respiratory organs, gases simply pass through
Cnidarians are not anal: Cnidarians
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'. Most cnidarians
have some sort of tentacles surrounding this opening. This situation
means cnidarians must digest a meal and spit out the indigestible
bits before eating again. This opening is also where cnidarians expel
eggs and sperm.
Stingers! All cnidarians have
some sort of stinger. These are used to gather food, paralyse prey
and defend against predators and rivals.
The stinger comprises a capsule with a long thread often barbed at
the base. The thread lies coiled in the hollow capsule (called a cnida)
closed with a tiny cap, and a trigger, ready to fire (the entire structure
is called a cnidocyte). 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 these stingers and
a video of a firing stinger, see the Tropical
Australian Stinger Research Unit website. This explosion is belived
to be one of the fastest cellular actions in nature, and the force
as powerful as a bullet.
There are three types of cnidae. Harpoon-like stingers that penetrate
the victim and inject toxins are called nematocysts. First, the barbed
portion penetrates the victim. Some are so powerfully released that
this portion can pierce the shells of crustaceans. The thread then
uncoils explosively inside the victim. The thread is hollow and toxins
are pumped through it. Most toxins only affect small creatures like
plankton, shrimp and fish. Some cnidarians, however, have toxins powerful
enough to cause extreme pain in or even kill people.
Another kind of cnida called spirocysts merely produce sticky threads
that adhere to surfaces or barbed threads, or long threads which entangle
around small things. These may help collect small food particles or
trap hard-bodied prey like crabs. The third kind called ptychocyts
are found only in peacock
anemones and fire off into sticky threads that mat together to
form the felt-like tube of these animals. There are more than 30 types
of cnidarian stingers! Stingers are one of the features used to identify
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 cnidarian
can have thousands of such stingers. Stingers are found all over the
body of a cnidarian, but the tentacles usually have a greater concentration
Besides being used to collect food, stingers are also used to ward
off other encrusting animals nearby. On study observed sea
anemones waging fierce war over territory. The peacock anemone
has special stingers that are used to make its tube.
What do they eat? 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. They may use mucus to trap small particles, or their stinging
tentacles to capture bigger prey.
Many cnidarians, however, supplement their meals. Many harbour microscopic,
single-celled symbiotic algae (zooxanthellae) within their bodies.
The algae undergo photosynthesis to produce food from sunlight. The
food produced is shared with the host, which in return provides the
algae with shelter and minerals. The zooxanthellae are believed to
contribute to the colour of cnidarians. Thus when a hard coral colony
loses its zooxanthellae, it turns white: a phenomenon called coral
What eats them? Despite their
stingers, many animals happily munch on cnidarians. Sea turtles eat
jellyfish, and often mistake floating plastic bags for jellyfish.
This can eventually kill the sea turtle. Nudibranchs
prey on anemones, corals and sea pens, each nudibranch usually specialising
on a particular prey. Some snails and fishes specialise in eating
corals. The Crown-of-Thorn sea star (Acanthaster planci) is
a well-known predator of hard corals and a population explosion of
these sea stars can decimate a reef. Fortunately, these sea stars
have not been encountered on our shores.
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 are
connected to another with living tissue that coats the hard skeleton.
In soft corals, the polyps are embedded in a thick common tissue instead
of a hard skeleton. 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; just like the way a balloon
half-filled with air can be re-shaped by pushing squeezing the balloon
to move the air 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 forms. Some go through both forms in their lifecycle,
others stay in one shape all their lives.
One form is the medusa. This 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.
Another form is the polyp. This 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. More about polyps of the Anthozoans
(Class Anthozoa), which do not undergo a medusa phase in their life
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 fertilised egg is the planula larva: a free-swimming
oval blob covered with cilia (tiny hairs) that is typical of cnidarians.
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
Role in the habitat: Among the
most significant cnidarians are hard corals, which are important reef
builders. Coral reefs thrive in clear nutrient-poor tropical waters.
Hard corals can grow here because of their symbiotic relationship
with zooxanthellae, single-celled algae that photosynthesise and share
the nutrients produced with the host coral. Thus corals are like the
trees of the rainforest, providing 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.
Neighbourly cnidarians: Many creatures
have adapted to deal with the stingers of cnidarians. Anemonefish,
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. Many
cnidaria also have a symbiotic relationship with zooxanthellae (see
above section on cnidarian food).
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
Their toxins and other features of cnidarians are increasingly being
studied for medical and other applications. A sea
anemone toxin is being studied for treating multiple sclerosis,
Nobel Prize was awarded to the scientists who developed a flourescent
protein from a jellyfish for biomedical use.
Status and threats: Many cnidarians
are listed among the threatened animals of Singapore. Globally, coral
reefs are seriously threatened by overcollection, habitat destruction,
pollution as well as global climate changes. Coral
bleaching may also threaten reefs.
Harvesting of marine wildlife such as corals 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 our shores, cnidarians are affected by human
activities such as reclamation and pollution. Trampling by careless
visitors, and poaching by hobbyists also have an impact on local populations.
Same all around, the peacock
Pulau Sekudu, Jul 05
Sequence of firing stinging cell
A carpet anemone
swallowing a dead fish.
Chek Jawa, Mar 02
clown anemonefish can live happily
in an anemone that would eat any other fish.
Kusu Island, Jun 04
Coral bleaching: When hard coral lose their zooxanthellae, they
Sisters Island, Nov 05
A sea anemone is a solitary
Sentosa, Apr 04
The mushroom hard coral
is a solitary polyp.
Sisters Island, Jul 04
Most hard corals are a colony
of many polyps.
Labrador, Jul 05
The medusa is the typical jellyfish
Tuas Jun 05
The polyp is the typical 'sea anemone'
shape of this Button zoanthid
Pulau Sekudu, Aug 04
Colonial polyps of a sea
Beting Bronok, May 03
Colonial polyps of a sea
Changi, Jun 05
Colonial polyps of a flowery
Pulau Semakau, Aug 11
cnidarians of Singapore
G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red
Data Book: Threatened plants and animals of Singapore.
Sclerectinia (hard corals)
Seriatopora hystrix (CR: Critically Endangered)
Stylophora pistillata (CR: Critically Endangered)
Gorgonacea (sea fans)
Junceella gemmacea (EN: Endangered)
spongiosa (EN: Endangered)
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.