a reef covered in different corals, mainly branching and table corals


Corals are essential reef-builders, providing homes and food for a quarter of the world's marine life.

What is a coral?

Corals may look like colourful rocks or plants, but they are animals that form colonies of identical polyps.

Fully grown, individual polyps can range from a few millimetres to a few centimetres in diameter. Corals grow by asexually reproducing polyps, which means a polyp will ‘bud’ and form another polyp, which is an exact replica of itself. This process grows the coral into what is known as a colony. Some colonies, especially smooth rounded coral of the Porites genus, can grow several meters in height.

different shaped corals on a reef
Hard corals come in a variety of different shapes and sizes. The colonies are built by coral polyps.

These animals, while seemingly simple, are the builders of coral reefs which provide a home to over a quarter of the marine species on the planet.

Animal, vegetable, mineral – the wonderful combination of coral

Hard corals produce a skeleton made of calcium carbonate, or limestone. Coral skeletons grow in many different shapes and form the hard surface of the reef which provides food and shelter to many reef animals and plants.

Soft corals lack a hard limestone skeleton but are incredibly colourful and often live in places protected from waves and currents.

Most shallow water hard coral species have a symbiotic relationship with microscopic single-celled algae, commonly known as zooxanthellae. Zooxanthellae (also referred to as Symbiodiniaceae) live within the corals’ tissues and give them their colour. They also provide most of the coral’s nutrition through photosynthesis. Breakdown of this symbiotic relationship is called coral bleaching which can lead to starvation of coral, and over extended periods, death.

Learn more about coral bleaching.


A young hard coral with polyps. The small, brown flecks inside the coral are single-celled algae called zooxanthellae. Image: Katarina Damjanovic

Solar powered and plankton hungry

As well as receiving energy from the sun via the zooxanthellae, shallow water corals also feed on small animals and plants floating past. This often occurs at night, when the stinging tentacles of the coral extend out of their skeleton, and sting passing morsels, feeding it into their centralised mouth.

Spectacular spawning

Many corals breed sexually via ‘broadcast’ spawning where eggs and sperm are released into the water to fertilise. The fertilised egg changes into a free-swimming larva before settling onto the reef’s surface and growing into a single coral polyp.

Mass coral spawning is a spectacular annual phenomenon where colonies from multiple species synchronise release of sperm and eggs over several nights following a full moon. The resulting coral larvae (known as a planula) travel with the currents and eventually settle on the surface of a coral reef. Here, they metamorphose into a coral polyp, then grow through budding, creating new coral colonies. Spawning is a key event in creating future coral generations and replenishing coral ecosystems.


Coral larvae, or planula, settling on a prepared surface in an aquarium in the National Sea Simulator.

On the Great Barrier Reef, mass coral spawning can happen after the full moons in the late spring or early summer. In Western Australia, mass spawning occurs in the autumn months.

Not all corals spawn during mass spawning. Some species spawn outside the mass coral spawning window and are called asynchronous spawners. Other species will fertilise eggs internally, and release larger, more developed larvae (planulae). These corals are called brooders.

Why are coral reefs important?

Coral reefs might only occupy 1% of the world’s marine environment but they’re home to about a quarter of the world’s marine species and an important nursery for many fish species.

They’re also very important to the wellbeing of humans. An estimated 500 million people who live in coastal communities depend on coral reef fisheries for their livelihoods. They also help protect our coastlines from erosion and flooding.

What threats do they face?

Climate change is widely recognised as the greatest threat to coral reefs worldwide. Warming oceans around the world are causing more frequent and serious and severe bleaching events which threaten to outpace corals’ ability to regenerate and adapt to warmer temperatures. Climate change is also driving ocean acidification.

Other threats to the world’s coral reefs include destructive fishing practices, pollution, poor water quality and outbreaks of predators, such as the native crown-of-thorns starfish.


Coral reefs are threatened by many disturbances such as coral bleaching caused by marine heatwaves driven by climate change (left), cyclone damage (center), and coral-eating crown-of-thorns starfish outbreaks (right).

What is AIMS doing to help coral reefs?


AIMS monitors many aspects of the Great Barrier Reef and other reef ecosystems and the open ocean in Australia.

This contributes to the sustainable use and development of the tropical marine environment by setting baselines and by advising managers of changes in ecosystems and the environment.

AIMS' long-term data on the Reef is the most comprehensive dataset covering the condition and trends of the Great Barrier Reef, spanning more than three decades.

AIMS holds the largest collection of coral cores from the southern hemisphere, which provides important insights into historical coral growth rates and climate impacts.

Reef restoration and adaptation

We are developing knowledge and tools to support coral reef resilience in the face of climate change and other threats. This work builds on years of AIMS research investigating how coral respond to changing environments.

Reef spawning research

Assisted evolution

Reef Restoration and Adaptation Program

Australian Coral Reef Resilience Initiative

Coral Aquaculture