Most people know that coral reefs are important.
But surprisingly few people know what a coral actually is.
Ask a group of snorkelers or new divers and you’ll hear all kinds of answers.
“A plant.”
“A rock.”
“A shell.”
“A type of seaweed.”
The truth is far more interesting.
Corals are animals.
Tiny animals.
And together they create some of the largest living structures on Earth.
The reefs surrounding Gili Trawangan, Gili Meno, and Gili Air were built by billions of tiny organisms working together over thousands of years.
Understanding how corals function completely changes the way you look at a reef.
Coral Polyps: The Builders of the Reef
Every coral colony consists of thousands of individual animals called polyps.
A coral polyp looks a little bit like a miniature sea anemone.
Each polyp has:
- A mouth
- Tentacles
- A simple digestive system
The tentacles capture plankton and tiny food particles drifting through the water.
Most coral polyps are only a few millimetres across.
Individually they seem insignificant.
Together they create entire ecosystems.
The World’s Most Successful Partnership
Corals have a secret.
They don’t survive alone.
Inside their tissues live microscopic algae called zooxanthellae.
These algae use sunlight to perform photosynthesis.
Just like plants on land.
The algae produce nutrients.
The coral uses those nutrients to grow.
In return, the coral provides shelter and access to sunlight.
This partnership is called a symbiotic relationship.
It is one of the most important biological relationships on Earth.
Without it, coral reefs would not exist.
Why Coral Reefs Need Sunlight
Because corals rely on photosynthetic algae, most reef-building corals need sunlight.
This explains why coral reefs are found primarily in shallow tropical waters.
The reefs around the Gili Islands are concentrated in depths where sunlight can easily penetrate.
The clearer the water, the deeper corals can grow.
This is one reason why the excellent visibility often experienced during scuba diving in the Gili Islands is so beneficial for reef growth.
How Corals Build Reefs
Corals are constantly building.
Each polyp extracts calcium and carbonate from seawater.
These minerals are used to create a limestone skeleton.
When a coral polyp dies, its skeleton remains behind.
New polyps grow on top.
Generation after generation.
Century after century.
Eventually entire reef systems form.
Some coral colonies visible today may have started growing long before modern diving equipment existed.
Are Corals Plants or Animals?
Again,corals are animals.
This often surprises people because corals do not move.
But movement is not required to be classified as an animal.
Corals belong to the phylum Cnidaria.
This group also includes:
- Jellyfish
- Sea anemones
- Hydroids
In fact, corals are more closely related to jellyfish than to plants.
Learn more about different hard coral shapes and the difference in between soft and hard corals.
How Corals Reproduce
Corals have developed remarkable ways of reproducing.
Some reproduce by cloning themselves.
Others release eggs and sperm into the water during mass spawning events.
During these events, entire reefs may release reproductive cells simultaneously.
The sea becomes filled with millions of coral larvae.
Only a tiny fraction survive.
Those that do settle onto suitable surfaces and begin building new colonies.
Every healthy reef depends on successful coral recruitment.
What Is Coral Bleaching?
Coral bleaching occurs when corals become stressed.
Usually due to elevated water temperatures.
When stressed, corals expel their zooxanthellae.
Without the algae, the coral becomes white.
This is bleaching.
A bleached coral is not immediately dead.
But it is weakened.
If stressful conditions continue, the coral may die.
This is why climate change represents the greatest threat to coral reefs worldwide.
Why Corals Matter
Corals create habitat.
Without coral reefs there would be:
- Fewer fish
- Fewer turtles
- Fewer sharks
- Less biodiversity
The reefs of the Gili Islands support an incredible variety of marine life precisely because they provide structure, shelter, and food.
Every turtle encountered at Turtle City.
Every reef shark at Shark Point.
Every frogfish at Bounty Wreck.
Every pygmy seahorse at Soraya.
Ultimately depends on healthy coral ecosystems.