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Nature^103

CORAL

Tropical coral reefs are the ocean’s richest

habitats, but the creatures that create them

are surprisingly small. They look like tiny

sea anemones but have chalky skeletons.

1 SEA NETTLE

Jellyfish tentacles are armed

with stinging cells known as

cnidoblasts. The stings of most

jellyfish produce nothing more

than a painful welt in humans.

Cnidarians

Jellyfish, sea anemones, and. CORALS ,

along with tiny freshwater animals called

hydras, all belong to the same phylum

(group) – the cnidarians. All cnidarians

are simple, aquatic invertebrates with

stinging tentacles, which they use to

capture prey. Some are venomous and

have been known to kill people.

HOW DO JELLYFISH MOVE THROUGH THE WATER?

Jellyfish swim using a kind of jet propulsion. They

contract their hollow, saucer-shaped bodies (called

bells) to force water out, which propels them forward.

Their long tentacle-like arms, which trail out behind,

are used to sting and capture prey.

HOW DO SEA ANEMONES FEED?

Sea anemones capture food with their tentacles, then

pass it to the mouth in the middle of the tentacle

crown. Sea anemones spend their adult lives attached

to rocks on the seabed or in pools on the shore. Some

sea anemones have a muscular collar, which they can

pull over their tentacles if threatened.

WHAT IS A POLYP?

A polyp is an individual coral animal. It looks like

a miniature sea anemone, and feeds on the tiny plants

and animals, known as plankton, that float in

seawater. Most coral polyps live in communities that

slowly build up to form reefs. These reefs provide

homes for all sorts of other sea-living creatures.

HOW DO CORAL REEFS FORM?

Coral reefs are made from the skeletons of coral

polyps. Most coral polyps have a hard, chalky body

case that protects the soft parts inside. When they die,

the chalky skeletons remain. Over thousands of years,

they build up on top of one another to form a reef

that may stretch for hundreds of kilometres.

The phylum (major group) Cnidaria contains about 10,000 species, divided

into three classes:

• Hydrozoans (including hydras, the Portuguese man-of-war, and fire corals)


• Anthozoans (including all other corals and sea anemones)


• Scyphozoans (jellyfish)

Bell
contains
digestive cavity
and provides
propulsion Fringing
tentacles
trail behind
the bell but
do not contain
stinging cells

Arms

filled with

stinging cells

that paralyse

or kill prey

SEA ANEMONE 3

Sea anemones have soft,

cylindrical bodies topped with

a crown of stinging tentacles.

Most sea anemones are no

bigger than a man’s hand,

although the largest may grow

up to 90 cm (3 ft) across.

STAGHORN CORAL 3

Hard coral polyps emerge to feed at night, when

most of the fish and other creatures that might eat

them are inactive or asleep. If they are threatened,

the polyps quickly withdraw into their stony shells.

They re-emerge once the danger has passed.

FIND OUT MORE. Australasia and Oceania 272–273 • Islands 42

CNIDARIAN CLASSIFICATION

cnidarians