The world's first coral
reefs occurred about 500 million years ago, and the first close
relatives of modern corals developed in southern Europe about 230
million years ago. By comparison, the Great Barrier Reef is relatively
young at just 500,000 years old. The current reef's structure is much
younger at less than around 8,000 years old.
Most modern reefs have formed on hard surfaces in the ocean, such as a
base of an old reef that died during a period when sea level was lower,
or the edge of a rocky island. Depending on how they start out, several
types of reefs can form. Some coral reefs form in the deep ocean and are
called atolls. The theories on how coral reefs form were first put
forward by Charles Darwin (of The Origin of Species fame) who proposed
that atolls form around the edges of high volcanic islands that
gradually submerge beneath the sea with changes in sea level or
subsidence of the land. Thus an atoll starts life as a fringing reef,
then becomes more of a ring growing on the shrinking land-mass, until
the land disappears and just the coral circle remains. In some cases,
the coral growth is unable to keep pace with the sinking island, and
sunken dead reefs have been found.
Class: Anthozoa [includes corals, anemones and sea pens]
Habitat: Coral reefs are found in shallow water, ranging to
depths of 60 m. Some species prefer either cooler temperate water while
others are found along tropical reefs, such as the Great Barrier Reef,
with waters ranging in temperatures from 18 - 33 °C.
Living in colonies: They generally occur in large numbers as colonies of
individual polyps linked by tissue. Resources, such as food, are then
shared amongst the individuals in the colony.
Coral Size: Individual polyps range from 3 - 56 mm in diameter or
height; while colony size varies from 75 mm -1500 mm (1.5 m) in width,
height or length.
Some corals have a
mutualistic relationship with algae called zooxanthellae. A mutualistic
relationship is one where both parties benefit from their partnership.
The algae use sunlight and the polyp’s waste products to make oxygen and
food. These substances leak into the surrounding tissues of the polyp
and can provide up to 98% of the polyp’s dietary requirements. These
corals are found in shallow water, as they require sunlight to survive.
They are generally fawn, brown or green in colour, due to the
yellow-brown colour of the zooxanthellae.
Stinging cells: All Cnidarians have characteristic stinging cells called
nematocysts in the tentacles and body wall. Each nematocyst cell
contains a coiled thread under pressure, which is ejected from the cell
when triggered by touch. These stinging cells are used for catching prey
and for defense, some having barbed ends connected to poison sacs, while
others are sticky.
Coral polyps: Coral reefs consist of hundreds and thousands of
soft-bodied, invertebrate animals, having no backbone. These animals are
called coral polyps. The individual polyp is radially symmetrical and
has a tubular body with tentacles surrounding the mouth at the upper
end. Each polyp’s body wall consists of two layers of cells, an outer
layer called the ectoderm and an inner endoderm layer. A
gelatinous material called mesogloea is found in between these two
Hard corals: Hard corals build reefs by growing atop the stony
skeletons of previous coral colonies. They consist of limestone cases
made by coral polyps extracting calcium from seawater. These limestone
cases form a ‘house’ for the coral polyp, consisting of a floor, outer
walls and a number of internal partitions. Inside corals’ clear outer
tissues live microscopic algae, which transform sunlight into sugars
through Photosynthesis The hosts help themselves to some of the sugars
and gain some colour through the process.
Reproduction: Corals reproduce in two ways: asexually and
Some corals divide to form new individuals. This is known as asexual
reproduction. Sexual reproduction takes place as mass spawning, where
polyps release millions of eggs and sperm. Polyps are either male or
female or both male and female. After the eggs and sperm are released,
they float to the surface. The fertilised eggs that escape predation by
other animals hatch into larvae and drift with the plankton. The tiny
percent that survive and settle on the reef then begin new coral
Major Natural Predator:
The Crown-of-Thorns Seastar was once an animal of great controversy
causing terrible damage to the coral reefs of the Great Barrier Reef.
Many believed that this seastar was a pest species invading the coral
reef habitat predating on and killing corals in great numbers. Research
has indicated that the Crown-of-Thorns Seastar is only found in
and a native species to Australian waters. The role that the sea star
plays by eating coral polyps forms a population control, making more
room for new coral reefs to form. Plague proportions are thought to
coincide with rainfall and increases in nutrients from rivers during
floods, and often occur cyclically every 17 years. Further scientific
studies are still in progress to determine whether these plaques can be
Scuba diving & snorkeling
From Cairns and Port Douglas are the most sought after outing onto the reef. With so
much to see like ship wrecks, coral gardens, the thousands of species of
marine animals and under water canyons. Cairns Great Barrier Reef has many qualified scuba
diving courses available and are amongst the safest in the world, with
strict guidelines that trainers must follow. You can easily become a
certified scuba diver and enjoy all the underwater attractions that the
reef has to offer.
Australian Marine Life, by G.J.Edgar
Steve Parish: Amazing Facts about Australian Marine Life Encyclopedia
Britannica and http://www.reef.crc.org.au/sitemap.html