PHYLUM: CNIDARIA (COELENTERATA)
Cnida = nettle (stinging)
CHARACTERISTICS

Multicellular animals.

Body is essentially a sac with two cell layers in the body wall and a single opening, the
mouth.

The mouth is surrounded by tentacles, which contain stinging cells (nematocysts)
that assist in capturing the prey.

They are built on a circular body plan and are radially symmetrical.

There are two common forms: (also see next section figure 180 A)
o the polyp which attaches itself to the rock for example the sea anemone and
o the medusa which is bell shaped and floats mouth downwards in the water
for example the jellyfish.
Living Shores: Figure 180 A
Detail of the body wall of a cnidarian
Living Shores: Figure 180 B
Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES
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PHYLUM CNIDARIA
CLASS
ORDER
HYDROZOA
HYDROIDA
SIPHONOPHORA
Hydroids
Bluebottles
Polyps solitary or bud to form
colonies
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Floating colonies of several polyplike or jelly-like individuals that
are attached to a stem or disc.
Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES
SCYPHOZOA
ANTHOZOA
Jellyfish or jellies
Sea anemones, zoanthids, sea fans, soft
coral, hard coral
Umbrella shaped jellies
These animals have a barrel shaped body
with a mouth at one end. The mouth is
surrounded by a set of tentacles used for
catching food.
Medusoid stage completely lacking.
CLASS: ANTHOZOA

ANEMONES
Habitat
Some anemones, such as the plum anemones live in small holes and cracks in intertidal zone in
rock pools. Others live in caves and on reefs below the low tide. Sandy anemones crowd into
sandy gullies.
Description
Anemones are simple solitary polyps. They look like
brightly coloured flowers and show a great variety of
sizes and colours. Their bodies’ look like thin sacs filled with water – like water balloons.
Its mouth is surrounded by tentacles and a curtainlike pharynx that hangs down inside the body and acts
like a valve. This allows food to enter without water
loss.
When exposed to air they withdraw their tentacles,
preventing them from drying out or being damaged.
Feeding
They feed on small animals such as shrimps and fish,
which they catch with the aid of the stinging cells in
their tentacles. Many stinging cells (nematocysts)
contain venomous liquid that subdues the prey. Some
types of stinging cells are barbed and sticky, some
actually wrap around the prey.
Predators
They are eaten by some fish e.g. Butterfly fish and by
some nudibranchs.
Did you know?
Nemo and his dad Marlin – two clownfish- are
exceptions to this, they live symbiotically with the
giant anemone. The clownfish – also known as
anemone fish - shelter amongst the tentacles of the
sea anemone and are protected from the stings by a
special coating of slime.
Clownfish living amongst the
tentacles of a sea anemone
Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES
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SOFT CORAL
Habitat
Soft coral is abundant in sub-tidal zones with a wide depth range off both the west and East
Coast.
Description
They form colonies of polyps with eight feathery tentacles and are sometimes brilliantly coloured
to warn of their stinging capabilities. No solid limestone skeleton.
Feeding
They catch zooplankton (small microscopic animals) with the stinging cells in their tentacles.
Predators
Soft corals cannot be very nutritious because much of their bulk is made up of watery mesoglea
(jelly) and their energy content is among the lowest of all animals.
SEA FANS (Gorgonians)
Habitat
Sea fans occur in deeper water growing in caves, on rocky reefs and in crevices where they are
protected from the force of waves. They are common around the Cape Peninsula and along the
south Coast.
Description
The branched tree-like, colony has a stiff central
rod made up of a flexible material known as
gorgonia. They are usually coloured white, pink,
orange or red. Tiny cup-like polyps are visible on
the branches when they are feeding.
Feeding
The tentacles catch minute food particles drifting
in the water.
Predators
Free-living anemones.
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Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES

HARD CORAL
Habitat
Hard or stony corals grow best in clear, warm waters and their occurrence is limited to the depth
to which light can penetrate the water for the algae that are found associated with the hard coral.
Coral growth is seldom found deeper than 60 metres.
Description
The polyps of corals produce a limestone skeleton, secreted by the outer layer of the body wall. A
few hard corals occur around the Cape for example Noble and Cup coral. Hard corals function as
the main builders of coral reefs. As hard corals lay down more of their underlying limestone or
calcium carbonate skeleton the reef grows. They are also important in providing both food and
shelter to a large number of reef organisms.
Feeding
Colonial corals cannot always obtain enough food from the water and in order to survive they
have great numbers of microscopic algae (zooxanthellae) in their body walls. The algae use
sunlight to make food for themselves and their hosts by means of photosynthesis. Through
digestion, coral polyps release these nutrients to the algae. Coral and algae then apparently cycle
these nutrients between them, reducing nutrient loss to the water.
The coral also protects the algae. This relationship is called mutualism as both these organisms
benefit from this arrangement.
CO2 + H2O  Food (Carbohydrates) + O2 + H2O

Predators
Some species of fish such as parrotfish and butterfly fish, as well as some sea snails and sea slugs,
feed on corals.
Living Shores: Figure 198
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

Did you know?
The activity of the zooxanthellae also assists in the release of lime for the
construction of a coral skeleton.
The biggest threat to corals, apart from humans who collect them and pollute the
sea, is coral bleaching.
Coral bleaching
Coral reefs have recently been affected by bleaching i.e. the discoloration or loss of its symbiotic
algae.
The cause of these widespread bleaching incidents is uncertain but recent research indicates that
the cause may be unusually warm waters. The optimum temperature for coral growth is between
26°C and 27°C. Temperatures above 29°C cause the stressed coral polyps to actively expel the
algae giving the coral a bleached appearance.
Bleached corals have difficulty recovering; a reef can take years to recover, and subsequent
bleaching incidents may make it impossible. Without their symbiotic algae, corals are unable to
deposit the calcium carbonate skeleton that makes up the foundation of a coral reef. Not only
corals, but also all reef organisms could potentially lose their habitat because of bleaching
incidents, as the calcium carbonate structure of the reef erodes away.
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Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES
JELLYFISH
Habitat
Ocean
found
the open
often wash
drifters
mainly in
ocean, but
ashore.
Description
Jellies have rounded bell-shaped bodies (medusae), which contain a layer of jelly that consists of
approximately 95% water. This layer varies in thickness in different jellies. They have tentacles at
the open end of the bell that is armed with stinging cells. There is a square mouth with oral arms
called the manubrium.
Feeding
They feed on small fish, shrimps and plankton e.g. fish
larvae.
Predators
They have very few predators of their stinging tentacles.
However, they are an important source of food for some
species of turtle and the ocean sunfish.
Plastic bags drifting in the ocean are often mistaken for
jellyfish by these animals resulting in them eating it by
mistake.


Did you know?
Some species are considered a delicacy in Japan.
That the box jellyfish can give a painful sting and
that it’s Australian relative can kill a human being in 3 minutes.
Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES
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
HYDROIDS
Polyps 2mm to hydroid colonies of 25cm
Habitat
Many of the tufts growing on rocks are hydroids and not seaweeds at all. Some hydroids are
found growing on shells of snails. The hydroids protect the snail by deterring predators while the
hydroids eat the scraps or waste products from the snail. This relationship is commensalism as
the one organism benefits while the other organism is not harmed.
Description
Hydroids form colonies, which often look like delicate trees or feathers. Other species of hydroids
are branched in an orderly and elegant fashion. The tiny polyps are borne on the branches and
may be housed in small cups. The polyps can extend for feeding and can retract into the cups
after feeding. Some hydroids have an external sheath or perisarc made of chitin and protein.
Divers avoid hydroids as they have a fairly powerful sting.
Feeding
Hydroids are carnivorous and use their
tentacles to catch minute shrimps, crabs,
fish and eggs floating in the water. The
digestive enzymes of hydroids are able to
handle large prey.
Predators
Predators are nudibranchs (sea slugs), flat
worms and butterfly fish.
Did you know?
Hydroids are named after Hydra, the multiheaded goddess of Greek mythology
because they multiply by budding.
The Goddess grew two heads in place of
every one Hercules cut off.
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Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES
BLUEBOTTLES
Habitat
Bluebottles live on the surface of the open ocean, but are often blown ashore by onshore winds.
Description
They are unusual hydroid colonies, which consist of
highly specialised individuals that are all
interdependent.
Feeding
They prey on small fishes that they catch with their
tentacles.
Predators
Marine turtles and sea swallows prey on bluebottles.
Plough snails and sea slugs (nudibranchs) may eat
bluebottles washed ashore.
Structure of the bluebottle or
Portuguese man-o’-war
Living Shores: Figure 186
The smaller raft hydroid with a circular raft and
the by-the-wind sailor with an oval raft and
vertical sail are smaller colonies similar to the
bluebottle. They set sail to the wind as a colony,
producing a thin bright blue membrane of stiff
material, orientated diagonally across the colony’s body.
Did you know?
The painful stings of a bluebottle
should be washed off with salt
water and are best treated with sap
from the leaf of a sour fig plant or
vinegar. A medical practitioner
using antihistamines should treat
severe cases.
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Quick review
1.
Identify the animals shown in the table below by giving the Phylum and the common
name.
Phylum:
Phylum:
________________________
Common name:
______________________
Common name:
2.
List three characteristics that all Cnidarians have in common.
3. Label the following parts on the animal below.
b
a
4. Name one interesting thing you would share with a visitor at the touch pool
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Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES