The Great Barrier Reef
The Great Barrier Reef is the world’s largest World Heritage area and the most extensive tropical marine reserve. Only the Heard Island and McDonald Islands Marine Reserves in the Australian sub-Antarctic are larger. It is approximately 347,800 square kilometres in area and about 2,300 kilometres long, running from just north of Fraser Island near Bundaberg through to the tip of the Cape York Peninsula. When it comes to length, the Belize Barrier Reef (off the Caribbean coast of Belize) is a distant second at only 290 kilometres, and Ningaloo Reef, off the coast of Western Australia, is just 280 kilometres.
The reef contains over 2,900 reefs, including 760 fringing reefs and 300 coral cays. There are also 618 continental islands once part of the mainland.
As the world’s largest coral reef ecosystem, the Great Barrier Reef is home to approximately 1,500 species of fish, 400 species of corals, 4,000 species of molluscs, 500 species of seaweed, 215 species of birds, 16 species of sea snake, six of the world’s seven species of threatened marine turtles and some of the largest populations of dugong in the world.
There are more species of animals and plants in a cubic metre of the Great Barrier Reef than in any other environment, including tropical rainforests.
Over one-third of the reef’s 350 species of coral, each year reproduce sexually during a mass spawning event. Most inner reefs spawn around November, with the outer reefs spawning later in December. Spawning always takes place at night and follows any time up to six days after the full moon. Eggs and sperm are released into the water, where they combine to form a free-swimming planktonic larval stage.
Biggest but also the youngest
The Great Barrier Reef may be the most extensive coral Reef system in the world, but it’s also the youngest. The Reef’s underlying basement began growing about 18 million years ago in the north and two million years ago in the south. The current Great Barrier Reef structure grew over the older platform about 18000 years ago.
Many of the places that support reefs today were part of the land during the Ice Age. As global temperatures increased and the ice melted and retreated to the poles and mountain tops, sea levels rose to their present levels, creating ideal conditions for corals to develop along the tops of formerly low coastal hills.
Turtles and sharks are the marine ‘dinosaurs’ of the Great Barrier Reef. Turtles have been swimming around in its water for 150 million years, while sharks have been around for 400 million years – that’s 100 times longer than humans.
How are coral reefs formed?
Reefs are masses of limestone made from skeletons of millions upon millions of tiny marine animals and plants. Colonies of tiny, living coral polyps grow on a reef’s surface. These animals are the primary reef builders. They extract dissolved limestone from the water and, with the help of single-celled plants living inside them, lay it down as hard limestone around the lower half of their bodies.
Polyps can pull their whole bodies inside these limestone cups for protection, if necessary. The combined skeletons of many colonies of polyps form the large corals seen on reefs. When coral polyps die, their limestone skeletons and the remains of other animals and plants add to the framework of the Great Barrier Reef.
When is a reef not a reef?
Only about five per cent of the Great Barrier Reef World Heritage Area area is coral reefs. Islands also represent a small proportion, and most of the remaining 95 per cent is seabed between reefs. This seabed is ecologically complex and comprises many different habitat types. Generally, be divided into the inter-reef area and the lagoon.
Each no bigger than a fingernail, hard corals build reefs by growing atop the stony skeletons of previous coral colonies. Coral colonies have created the most significant structures ever made by living things, assuming such shapes as plates, domes, and branches. But like any masterpiece, a coral reef takes time, growing about a half inch (1.3 centimetres) a year. Armed with tentacles that help them “fish” for meals of minute plankton, the individual corals, or polyps, are tubelike animals related to jellyfish and sea anemones. Corals harbour their built-in food factories, unlike most of their cousins. 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 even gain a bit of added colour.
Psychedelic skin tones tell predators this shell-less snail packs a poisonous punch. But most nudibranchs weren’t born toxic—Nudibranchs snack on sea squirts, sponges, and hydroids poisonous to other animals. But instead of breaking down their prey’s toxins, nudibranchs incorporate them into their armouries. And that’s not all they can recycle. So-called solar-powered nudibranchs eat soft corals, which generally have algae in their tissues. When solar-powered nudibranchs eat soft corals, they don’t digest the algae. They keep them in their outer tissues. The algae continue photosynthesising, converting sunlight into food for themselves and their new host.
Up to 2.5 feet (75 centimetres) long, titans are the largest triggerfish, which wield an intimidating arsenal of hunting behaviours. Squirting water from their mouths and flapping their fins, triggerfish “dig” for crabs, worms, and other prey. When attacking sea urchins, triggers often flip them over, exposing their less spiny undersides. The triggerfish is equally resourceful and tenacious when it comes to egg rearing. After female triggerfish lay their eggs in nests on the seafloor—a rarity among reef fish—they continually blow water on them to ensure a good oxygen supply. They’re also known to put the bite on approaching fish or photographers swimming in for a close-up.
After sniffing out worms and other prey on the seafloor with its tubelike siphon, this toxic mollusc extends a sharp, spear-like tooth at the end of an organ called a proboscis. When the fang finds its mark, the cone snail injects a neurotoxin that induces immediate muscle spasms and quick death. The same fate may await humans who encounter especially venomous species of cone snails, though fatalities are rare.
Sea anemone and anemone fish
The stinging tentacles of a sea anemone offer the immune anemone fish shelter and a safe place to lay eggs. In return, the anemone gains a “guard dog.” Anemone fish prune harmful parasites from their hosts and drive off fish that prey on anemones, such as some butterfly fish. After a day of feeding on plankton, an anemone gathers itself into a bunch for the night. Anemone fish, protected by mucus on their skin, snuggle into the stinging mass, protected till daybreak.
This type of grouper may be overfished in some parts of the Great Barrier at least partly due to its popularity in Asia, where diners often order it live from restaurant aquariums. Concerns for the coral trout aside, fishing is strictly regulated on the Great Barrier Reef, but countless other reefs aren’t so lucky.
Elsewhere many are obliterated by blast fishing—in which reefs are often blown to bits in hopes of an easy catch—and cyanide poisoning, the dark secret of the aquarium trade. In cyanide fishing, divers generally squirt the toxin onto coral reefs, then search the newly dead reef for live-but-stunned fish—or so they hope. More than half of all fish caught this way die in the poisoning process or shipping.
Croplands (as on this farm near the town of Cairns) and development along north-eastern Australia’s coastal plain have replaced many seaside wetlands, the natural filters for freshwater coming from the continent. Along with deforestation, overgrazing by livestock, and runoff from upstream towns, farms, and industries, this sends more sediments and nutrients to the Great Barrier. The total has quadrupled since colonial times. Corals can persist in surprisingly murky water as long as tides and currents periodically sweep the sediments off. It’s the nutrients that wreck a reef. Anything beyond moderate nitrogen levels hurts growth and reproduction in corals while fertilising free-floating algae that can smother their neighbours.
—Text adapted from “The Kingdom of Coral” (NATIONAL GEOGRAPHIC Magazine, January 2001)
Crown-of-thorns sea star
The Crown of thorn sea star has poison spikes and an appetite for hard coral. Adult crown-of-thorns sea stars can kill a reef.
By projecting their stomachs out of their mouths and wrapping them around coral, they slowly devour it—or not so slowly if they’re out in force, which is increasingly the case.
No one’s quite sure why crown-of-thorns epidemics are on the rise. Some believe the Crown of thorn sea star is part of a natural cycle, like wildfires. Others think humans inspire the outbreaks, possibly by overfishing species that cull juvenile crown-of-thorns sea stars. Dive crew on many tourist boats are trained to kill Crown of thorn starfish. A large-scale, responsible weapon against crown-of-thorns epidemics doesn’t seem to be. Conservationists now watch and wait—and hope afflicted areas bounce back.
Corals expel the algae in their transparent outer tissues when stressed, exposing the corals’ limestone skeletons. The most common cause of this bleaching is abnormally high ocean temperatures—other culprits: inflows of fresh water, increased ultraviolet radiation, and changes in salinity.
Bleached coral isn’t necessarily dead; it often regains its healthy colour when conditions return to normal. But prolonged bleaching can lead to coral death—one more way global warming and periodic climate shifts such as El Niño can change the face of the Earth.
You can’t beat snorkelling and scuba diving. It’s one of the world’s most amazing and diverse scuba diving locations. From beginners to experienced divers, snorkellers and non-swimmers, dive trips and Great Barrier Reef tours suit all budgets and levels of experience.