Marine Science Chapters


Australia's Great Barrier Reef

Map showing Australia
Australia is in the southern hemisphere.

The Great Barrier Reef in Australia is an immense and very unique coral reef. The continent of Australia, in the southern hemisphere, is about as big as the entire United States. One of Earth's wonders - the Great Barrier Reef - is located off the northeastern side of Australia. It is the largest biological structure on Earth and one of the only naturally occurring structures that can be seen by satellite.

Australia The Great Barrier Reef
Australia's northeast coastline is where the Great Barrier Reef is located (shadowed in blue on the left). The Great Barrier Reef is a chain of reefs along the coastline ranging from a few miles to over 150 miles from the shore (right). Both maps (above) from the Australian Tourist Commission.

The Great Barrier Reef is not one reef but a chain of over 2,000 reefs located anywhere from 10 to 150 miles off the northeastern coast of the territory of Queensland in Australia - extending some 1,250 miles from north to south. It is actually not a true barrier reef as defined by the 'classic' coral reef formation and originally described by Charles Darwin in the 1800's.

Wistari and Heron Reef Heron Reef with Heron Island
Wistari and Heron reefs (left) are two of the reefs that make up the Great Barrier Reef. Heron reef has a very stable cay called Heron Island (right).

Most of the Great Barrier Reef is almost entirely underwater. Wistari Reef (left) is in the foreground and Heron Reef is in the background separated by a deep channel. Part of Heron Reef filled in with debris (mostly broken coral) and a cay, knows as Heron Island, formed. These reefs are typical of the Great Barrier Reef. Each of the 2,000 reefs making up the Great Barrier Reef has its own particular shape and name.

Australia is not a true barrier reef because it is a continental island (not volcanic) and is not sinking. It is unique among coral reefs. Its history began some 15,000 years ago when Earth was experiencing an ice age and much of North America was covered with glaciers. These glaciers held a lot of Earth's water so that the oceans were some 300-400 feet lower than they are today. The area that is now the Great Barrier Reef was a broad, flat, coastal plain then - land, not sea. Coral larvae, in the ocean, had settled along the edge of this plain and grown into coral heads as a reef (similar to the formation of a 'classical' fringing reef).

Gradually, the glaciers melted and sea level rose to its present height where it stabilized about 6,500 years ago. As the sea covered the coastal plain on Australia's northeast coast, many of the coral heads grew upward. About 2,000 of these grew fast enough to keep the upper (living) part of the reef in the lighted surface waters so the symbiotic zooxanthellae, in the coral animal's tissues, could live.

As sea level stabilized these isolated individual reefs began to grow out. Their tops were "mowed down" by waves, tides and storms so they remained just under the surface of the water but expanded in width (just as you saw Wistari Reef in the foreground of the last image). Each reef is unique - many are table reefs (like Wistari) but some are long, thin, ribbon reefs.

Heron Island
Heron Island.

Unaware of the spectacular reef below, visitors approaching the cay, called Heron Island, are in for a treat. Heron Island has been quite stable for a long time and has a lot of plant growth on it contributing to its stabilization. There is a marine lab and a resort on this tiny island.

High Tide Low Tide
High tide (left) and low tide (right) on Heron Island.

People used to row across the reef before the harbor was built on Heron Island. This view from Heron Island at high tide shows the structure used to hoist baggage and supplies out of the boats at high tide while people waded ashore. At low tide the baggage and supplies could be retrieved without any chance of it getting soaked.

High Tide Low Tide
High tide (left) and low tide (right) on Heron Island (picture taken from the same location in both images).

The reef is exposed at low tide, often leaving over 100 yards of low tide reef area to walk across before getting to the edge of the reef.

Edge of the reef Man feeding fish on the edge of the reef
The edge of the reef as viewed from below the water (left). A man feeding fish from the edge of the reef (right).

The edge of the reef has the most life as the reef continues to grow and expand. Jumping off the edge of the reef one is immediately amazed at the diversity of coral. Each reef has its own diversity and patchiness. Although the coral makes the reef, there are many species, like these fish, who make their home in and amongst the coral.

Staghorn coral reef Staghorn coral close up
A part of the reef made up almost exclusively of staghorn coral (left). Staghorn coral close-up (right).

Some areas of the reef are dominated by a few species. Here, at high tide, a snorkler dives down to the flat top of this part of the reef made up almost entirely of staghorn coral. A close-up of staghorn coral shows its colonial nature - each flower-like cluster is a single coral animal called a polyp, living condo-style, one next to the other protected by their fused corallites.

Diverse reef
Diverse reef with many types of coral.

Some areas of the reef are diverse. Coral comes in many shapes, sizes and colors.

Brain coral
Brain coral, a colonial reef building coral.

Brain coral closer Brain coral closest
Brain coral, close-up (left), and closer (right).

Brain coral polyps are hard to distinguish. The polyps of brain coral form rows (from asexual reproduction called cloning) where they do not separate their corallites next to each other (an unusual growth pattern). These rows look like the ridges in a vertebrate brain. If you look closely in the above image you can see where the individual polyps would be found. A close-up of brain coral reminds you of the colonial nature of most reef building corals and shows one polyp dividing in the center.

Fire coral
Fire coral looks like a miniature castle.

Elephant ear coral
Elephant ear coral is another interesting species.

Tube coral
Tube coral, Lobophyllia, is one of the more delicate species. Each polyp makes a long tube attaching to its neighbor at the bottom only - making it easily broken in storms, as in this picture.

Table coral with fish Table coral without fish
Table coral is closely related to staghorn coral but grows like a table from a stump. These table corals are often the home for small fish hiding in cracks and crevices. Here (right) a diver just swam over this table coral and the fish "dove for cover" in the cracks. New research done at Heron Island shows that most of these little fish never leave their table coral - they come in as babies and spend their entire life on one table coral.

Staghorn coral with fish
Fish may depend on the coral for protection. This mound of staghorn coral has a small group of fish that use it for cover.

Staghorn coral with fish Staghorn coral without fish
Staghorn coral is a favorite place for fish (left) and when they are disturbed they all duck in side the coral branches for protection (right).

Staghorn coral with fish
This mounding coral is called a "bommie" in Australia - after the aboriginal word for mountain. This bommie is 40 feet across and 20 feet tall - all one species of coral - thousands of polyps originally coming from the one polyp that started this colony with each cementing its hard corallite skeleton to its neighbor's.

Bommie close up
Close up view of the bommie shows it is made of polyps, each with their corallite under the skin of the polyp. The radiating septa of each corallite can be seen as ridges. Note the center polyp that is dividing into three - you can see the three different mouths and the radiating septa are starting to grow to separate the different polyps.

Arches in the reef Cave in the reef
Archs and underwater caves form in areas where these corals may grow out, up and together - creating many nooks and crannies for other species to use as a habitat. Through time parts of the archs become space for other species to grow on such as these sponges. Thus, the reef continues to change and grow.

All this change and growth is due to as many as 70 or more species of reef building coral (hermatypic) building the reef up and out as they grow. Each polyp secretes its skeleton (the corallite) with thin walls (called radiating septa) pointing toward the mouth. Each species competes for food and space, often becoming (or creating) a home or hiding place for other species. Physical breakdown offsets this biological buildup as tides, waves and storms "mow down" the upward growth, depositing rubble to fill in channels or form cays on the reef. Biological breakdown occurs on reefs as organisms eat each other. Over time each reef changes both in shape and species as all these forces interact. There are many other species of animals besides the reef building corals that are important on the reef. Several soft corals (some called sea fans or gorgonians) contribute greatly. The soft corals do not form the stony corallite skeletons but they do secrete some calcium - that may end up cemented as part of the reef.

Purple sea fan Orange sea fan
Purple sea fan (left) and orange sea fan (right) add color to the reef.

Purple sea fan close up
Purple sea fan close up, you can even see the tiny polyps along the branches.

Gorgonians (sea fans) are similar to true coral. The many identical, very tiny, polyps of sea fans all came from one parent - together they secrete a firm, but flexible skeleton (with calcium) to hide in. Pieces of these sea fan skeletons can help to form the reef. Soft coral polyps always have eight (pennately branched like an antennae) tentacles on each polyp and are almost always colonial (cloning).

Cauliflower coral
Cauliflower coral in the middle of a hard coral reef.

Cauliflower coral close up Cauliflower coral and its trail
When cauliflower coral lives on a reef building coral (left) it competes easily for its space and causes the polyps under it to secrete a solid wall of calcium and die. When cauliflower coral moves along the surface of a reef building coral it leaves a trail (right) of solid calcium made by the coral.

Other soft corals have no skeletal hiding place, such as this cauliflower coral. Their tissues have chemicals called terpenes that are toxic to most fish and other would-be predators. When in contact with soft corals the hard corals lay down calcium carbonate and put up a wall to imprison it. These soft corals can move and may leave a trail on the hard coral.

Besides corals, the Great Barrier Reef is alive with an incredible diversity of creatures including invertebrates (like sponges, worms, slugs, clams, seastars and anemones), and vertebrates (like fish, sharks and birds).

Reef with a green sponge
Green sponge growing on the reef.

Orange sponge Brown sponge
An orange sponge (left) and a brown sponge (right). The close-up of this brown sponge shows the large excurrent pore and the small incurrent pores that sponges use to filter water through their bodies to get the plankton they eat.

This brightly colored flatworm is crawling across a lavender sponge on the edge of the reef. It is a carnivore who eats only tiny worms and other small animals. These flatworms have their mouth in the middle of the underside of their body so must crawl over their prey before consuming it.

These two Tridacna clams are perpendicular to each other. Can you see them in the reef? Their 'lips' are wavy. These clams can grow to a yard or more across and typically have their shells in the reef.

<i>Tridacna</i> lips <i>Tridacna</i> lips <i>Tridacna</i> lips
Sometimes feared as "killer" clams by divers, it is only the ignorant person that might get into trouble by sticking their arm inside the clam, then the clam may close on their arm, and keep the diver under the water until they run out of air. This is the only danger and easily avoided. Usually they cannot close their shells all the way as they get older. Their mantle looks like lips along their wavy shell and is extremely variable in color (above). Many Tridacna clams have a symbiotic relationship with zooxanthellae in their 'lips.'

Sea slug Sea slug
Sea slugs, or nudibranchs, are some of the more delicate creatures on the reef. This is Chromodoris daphne (left). Chromodoris coi is pictured on the right. It is a sea slug and a sponge eater -- it often raises its mantle as you see here to expose its foot and head.

Orange seastar
This orange sea star provides some color on the reef.

Red feather star
A red feather star, or crinoid, attached to staghorn coral. The feathery legs are collecting plankton to eat.

Red feather star
This anemone can also sting but doesn't usually hurt humans (unless you have a cut). It can sting and kill a small fish though. After killing a small fish or other prey, the prey is transferred to the mouth that is in the middle of all the tentacles.

Anemonefish in its host anemone Anemonefish in its host anemone
Two different species of anemonefishes (above) are hiding in their host anemone without being stung.

Tomato clown Tomato clown
The tomato clown (above left and right) hides with its host anemone.

Clownfishes (anemonefishes) and sea anemones have a unique and interesting symbiotic relationship in the tropical Pacific Ocean. This results in the clownfish's ability to hide in the anemone's tentacles for protection. In return for the protection given by the anemone, the clownfish often catches food and brings it to the anemone. Sometimes a clownfish will grab another fish by the fin and pull it into the stinging anemone tentacles where it is killed. But the resident clownfish is not hurt.

Angel fish
Angel fish.

Puffer fish inflated
Puffer fish hide in cracks in the reef with their spines pressed down along their body. They puff up and stick out their spines when pulled from their hiding place.

Spotted grouper
Spotted grouper.

Whitetip shark Gray reef shark
Whitetip sharks (left) and gray reef sharks (right) patrol the reef for food.

Sharks are one of the major predators on the Great Barrier Reef. These whitetip and gray reef sharks are usually on the prowl looking for food. They may congregate, at low tide, in small indentations in the reef edge that collect the warm water running off the reef flat. There is often a lot of food in this reef runoff. These sharks remind us that all is not safe on the Great Barrier Reef.

Sea wasp
This sea wasp is the most deadly jellyfish in the world. Although its bell is only a few inches across, it can kill a man in ten minutes with its sting. Many of Australia's beaches are unsafe to swim in during sea wasp season. Warning signs are usually posted, during the dangerous periods, at the beaches where sea wasps can be found.


The lionfish (or turkeyfish), above, has poison in the spines that support its fins.

Another deadly reef creature is the stonefish. Resting on the bottom, this fish looks just like the bottom of the ocean. It is highly camouflaged by both the color and texture of its body. If stepped on, the poison injected by the 13 spines supporting the dorsal fin is extremely painful and can be deadly to humans.

Blue-ringed octopus
The blue-ringed octopus can also be deadly. It was featured in the James Bond movie thriller called Octopussy. Usually not aggressive, this octopus must grab you with all eight tentacles and press its mouth in the middle down to bite you.

Sea snake

Sea snakes are also found on the Great Barrier Reef but, like the blue-ringed octopus, they are not aggressive. However, the poison from their bite is more poisonous than a cobra.

Crown of thorns sea star Crown of thorns sea star
Crown of thorns sea star.

Crown of thorns seastar Crown of thorns seastar
Crown of thorns sea stars eat coral, leaving just the white corallites when they are finished (above).

Although this crown of thorns sea star is not deadly - its spines can inflict a painful wound on humans that usually kills the skin area. The crown of thorns is a coral eater. In recent years there has been an increase in its population on many Pacific coral reefs, including the Great Barrier Reef. Some of these reefs have been heavily damaged and may be endangered from this predator.

Marine Turtle with SBCC student
SBCC student enjoying a marine turtle experience while diving on Australia's Great Barrier Reef.

Marine turtles are common in Australian waters and provide a wonderful experience for divers. The tend to 'hang out' near particular beaches where they nest onshore but it is only the females who come ashore. The males stay in the water and mating is in the water. Thanks to the summer 2006 Marine Science student who donated her photo for inclusion in this lesson.

Marine Turtle
Marine turtle returning to the ocean after laying eggs on Heron Island (a cay on the Great Barrier Reef).

Marine turtles may use Australian beaches, during the Australian summer from November to February, for reproduction sites. Six species of marine turtles are found in Australia but only 3 use the Great Barrier Reef for nesting - the green, loggerhead and hawksbill.

Black noddy nesting colony Black noddy
Black noddy nesting colony (left). Black noddy incubating its egg (right).

Many seabirds may nest on the islands of the Great Barrier Reef. On Heron Island 120,000 black noddys arrive during Christmas time. 50,000 mate, build nests in the trees and raise their young each year. The noddys stay on their nest until the baby hatches. They feed the baby regurgitated fish caught in the surrounding waters. In March or April they all leave to fly north.

Mutton bird
Mutton bird.

Many other birds are found along the Great Barrier Reef. Most are fish eaters, like this underground nesting mutton bird, and most use the prolific reef for their dinner table.

As we say good-bye to the Great Barrier Reef we are reminded of its origin as sea level rose over a continent, in the warm water tropics; its true coral that forms the reef itself; the many species of marine life living in and on the reef including poisonous forms; and creatures like the turtles and birds that rely on the reef for food and use the island for reproduction.

Great Barrier Reef sunset
We are also glad to know that this area is protected as the Great Barrier Reef Marine Park - some areas are completely off limits to everyone, some are limited to tourists or scientists, and some are strictly for fishing. But, thanks to the Australian government, the underlying philosophy is to use the Great Barrier Reef today in a way that it will be the same for generations to come.

 Copyright and Credits
(Revised 6 August 2007)
 Page Back  Top  Page Forward