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.

Coral spawning

Coral spawning is a remarkable natural phenomenon that occurs in certain species of corals, typically during specific times of the year. It refers to the synchronized release of reproductive cells, including eggs and sperm, by the coral colonies, resulting in the potential for fertilization and the creation of new coral offspring.

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.


Hard corals:

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.


Titan triggerfish

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.


Cone snail

Cone snail

The cone snail is a fascinating marine gastropod mollusk that belongs to the family Conidae. These snails are known for their beautifully patterned shells, which often resemble a cone shape, hence their name.

Cone snails are found in various warm, tropical waters around the world, including the Indian Ocean, the Pacific Ocean, and the Caribbean Sea. They inhabit coral reefs, sandy bottoms, and rocky areas, where they hunt their prey.

What makes cone snails particularly intriguing is their venomous nature. They possess a specialized structure called a radula, which is essentially a harpoon-like tooth that they use to inject venom into their prey. This venom is extremely potent and can quickly paralyze or kill small marine organisms, such as fish and other snails.

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.

Sea Turtles

Marine turtles, also known as sea turtles, are captivating creatures that have captured the fascination of people around the world. These majestic reptiles are adapted for life in the ocean and play a crucial role in marine ecosystems. There are seven recognized species of marine turtles, each with its own unique characteristics and distribution.  Six of the World’s seven species can be found on the Great Barrier Reef

Marine turtles are remarkable ambassadors for the conservation of our oceans. Their grace, resilience, and ecological significance make them a symbol of the fragile beauty and interconnectedness of marine ecosystems. By working together to mitigate threats and protect their habitats, we can ensure the survival and well-being of these magnificent creatures for generations to come.



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.  Over half of all fish caught this way die in the poisoning process or shipping.


The Great Barrier Reef faces various natural threats that impact its health and biodiversity. While it is a resilient ecosystem, these challenges can have significant consequences for its long-term survival. Some of the key natural threats to the Great Barrier Reef include:

Climate Change

The warming of the Earth’s climate is one of the most significant threats to the reef. Rising sea temperatures can lead to coral bleaching, a phenomenon where corals expel the symbiotic algae that give them their vibrant colours and provide them with essential nutrients. Severe or prolonged bleaching events can result in coral death and hinder the reef’s ability to recover.

Ocean Acidification

Increased carbon dioxide (CO2) emissions are causing global warming and leading to ocean acidification. As the oceans absorb more CO2, they become more acidic, affecting the growth and calcification of corals and other marine organisms. Acidic conditions make it harder for corals to build and maintain their calcium carbonate structures, weakening their resilience.

Severe Weather Events

The Great Barrier Reef is vulnerable to cyclones, storms, and extreme weather events. Severe weather can cause physical damage to the reef structure, break corals, and dislodge them from their substrate. Large waves and storm surges can also contribute to erosion and sedimentation, impacting the health of corals and other reef organisms.

Crown-of-Thorns Starfish:

The crown-of-thorns starfish is a native species that can become a threat when populations grow excessively. These starfish feed on coral polyps and can cause extensive damage to reef ecosystems, leading to coral loss and reduced biodiversity. Outbreaks of crown-of-thorns starfish can occur due to various factors, including nutrient runoff and overfishing of their natural predators.

Disease Outbreaks

Like any living organism, corals are susceptible to diseases. Pathogens and bacteria can infect corals, leading to tissue loss, discolouration, and mortality. Disease outbreaks can spread quickly across the reef, impacting multiple coral species and reducing overall resilience.

Predation and Competition

Predatory organisms, such as certain fish species and invertebrates, can prey upon corals and other reef organisms. Increased predation pressure, combined with competition for resources, can affect the delicate balance of the reef ecosystem.

Looking after the Great Barrier Reef

Recognizing the importance of protecting this unique ecosystem, various conservation and management initiatives are in place. The Great Barrier Reef Marine Park, a World Heritage Site, encompasses a network of protected areas and regulations to safeguard the reef. Research and monitoring programs help understand the reef’s dynamics, while education and awareness campaigns promote responsible practices among locals and visitors.

Preserving the Great Barrier Reef requires a concerted effort from governments, communities, and individuals worldwide. By addressing climate change, reducing pollution, implementing sustainable fishing practices, and promoting conservation, we can contribute to the long-term survival and health of this extraordinary natural treasure.