Nature Feature - Great Barrier Reef

The Great Barrier Reef

The Great Barrier Reef was named by Captain James Cook when he was trying to land on the Queensland coast in 1770. He was greatly frustrated by what he called 'a great barrier reef' that ran from just north of Brisbane, up to and beyond Cairns in the far north. While Captain Cook considered the reef a major annoyance, it has proved a boon to the people of Queensland. It is rich in an incredible variety of tropical marine life, acts as a natural barrier against the ferocious tropical storms of the South Pacific, and has become an internationally recognised tourist attraction, drawing people from all over the world.

Built by tiny marine animals, the Great Barrier Reef is so vast that it was visible to the astronauts who walked on the moon. Within it lies a colourful variety of life as rich as that found in a tropical rainforest. The Great Barrier Reef is actually a system of coral reefs extending for more than 2000 km, parallel to the Queensland coast in north-eastern Australia, from Lady Elliott Island in the south, to the Gulf of Papua New Guinea in the north.

The Great Barrier Reef began to grow about 18 million years ago. The current growth phase has been occurring for the last 8,000 years, building up new coral layers on the old platforms. The outer reefs are located between 15km and 200km from the coast and enclose a total area of approximately 230,000 sq. km. The main barrier comprises over 2,100 individual reefs, protecting some 540 islands closer to shore, most of which have fringing reefs. The islands range in size from tiny pinnacles to over 100 sq. km.

The barrier reefs are close together in the north and south, becoming more widely scattered in the central section. Mangrove swamps are common in the north, sheltered by the reefs. Between the reefs and the coast, the sea is relatively shallow, with a broad silt-covered bottom, seldom much deeper than 100m. On the seaward side, the reef drops away steeply to the seabed thousands of meters below.

The outer reef is the area of most active coral growth. Exposed to the full force of waves and storms, the coral is continually dislodged. Much of this settles to be cemented into place as rock. On a grand scale, the reef is continually being destroyed and rebuilt. Coral reefs are essentially living rocks, made up of colonies of billions of tiny anemone-like creatures called polyps. The polyps embed themselves in protective layers of limestone secreted by their outer cells. Every year, the reef expands upwards and outwards as the polyps reproduce and the new creatures grow.

Each individual polyp is connected to its neighbours by strands of living tissue, tying the reef together and producing fantastic, colouful formations. Some seaweeds (calcareous algae), splashed in pink, add limestone to the reef structure. Other red algae produce a 'mortar' that cements areas of sediment together. In time, the whole community forms a reef with a thin veneer of living coral on top of layers of limestone skeletons.

Each species of coral has its own, often distinctive growth pattern. Coral shapes in the reefs vary from mounds, sheets and large table-like structures, to fans, branches and delicate antlers. Much of the coral is white. This is the skeletal matter of dead polyps.

By contrast, most of the living polyps are brightly coloured. A profusion of colour is added by other creatures of the reef - sea fans, sea whips and sponges show brilliant hues of red, yellow, orange, purple, pink and green.

Stunning blue and yellow damselfish, bright pink, purple and yellow wrasses, vivid red and white sea slugs, emerald green brittle stars and bright blue starfish all add to the spectacle. The brilliant colours, as well as serving to confuse predators, clearly identify the different species, helping them to find a mate and avoid rivals.

The reef is inhabited by around 400 different species of coral, 200 species of cowrie with their distinctive shells, and in excess of 2,000 species of fish - 10 percent of the world's known fish species.

A huge variety of marine animals live on and around the Great Barrier Reef. Tiny, drifting jellyfish live in the same waters as huge, predatory sharks. Sea turtles breed in numbers greater than anywhere else on the planet. Gentle dugongs, marine relatives of the elephant, swim languidly through the sea grasses. Groupers, predatory fish which can grow to massive sizes up to half a tonne, have been known to menace skin-divers. Birds, such as the reef heron, spear and catch fish in the shallows.

A number of animals burrow into or eat the reef itself. These incluse boring sponges, worms, bivalve molluscs, sea urchins, snails and sea slugs. Brightly coloured parrotfish chip off small chunks of coral with their beak-like front teeth, grind it down and extract the fleshy polyps. Starfish secrete digestive juices onto the polyps and then ingest the coral soup. A single crown-of-thorns starfish can strip the living coral polyps from one square metre of reef in a single day.

Sources:
'IMP Wildlife Fact File', Card 3 Group 9, 1999.
Information sheet accompanying the Australian Themes/Landmarks Privy Mark Series - Perth Mint, 1999.

The Great Barrier Reef is featured on the following Australian coin:

2000 One Ounce Kookaburra Bullion Silver Privy.