All You Ever Wanted To Know About Corals

Asides from promoting conservation through research we also believe that education is a critical tool to inspiring individuals to care for the underwater world. This is why we’ve come up with some fun and not-so-fun facts about coral reefs to help you to gain a better understanding of what they are and why they are so important.

How a small group of people is trying to restore the reefs in Indonesia


Image by: Kasper Prijs

Are Corals Plants Or Animals?

It’s time for a 1-minute biology lesson about corals. Understanding the physiology of a coral is not easy, but let’s try anyway. Corals are animals. Invertebrates in fact, which is in the same family as jellyfish.

There are soft corals and hard corals. Hard corals are the ones that build the reef… Wait! Build the reef? Are reefs and corals not the same thing? No, actually the coral is the animal and the reef is the structure that the corals live on, which is built by the coral. The coral reef is the whole ecosystem.

The body of a coral is called a polyp. The body is pretty much only made up of a stomach and a mouth. Doesn’t that sound ideal? We bet even if they had a brain, all they could think about would be food! The mouth has tentacles, which the coral uses for hunting. It eats anything from microscopic plankton to tiny fish.

Each polyp is one marine animal. Some polyps, like the mushroom coral, live alone. But most polyps live in big colonies and are dependent on each other like one organism.

learn more about what we do to restore the reefs of the Gili Islands in Indonesia

What’s The Difference Between A Hard Coral And A Soft Coral?

It may seem simple enough, one is soft and one is hard, but can you really spot the difference between these two distinct groups of coral?

Hard Corals

Hard corals are so-called because they build a hard skeleton as they grow, which is left behind when they die. Hard coral skeletons are what people used to use as ornaments in the 1970s and they are characteristically white and spiky.

This type of coral is known as reef-building coral, as it’s those skeletons that remain on the reef, building it up in height, before more hard corals colonize and grow on top of it. 

Hard corals have hard, calcium-based skeletons. Most hard corals — also called stony corals — consist of numerous single polyps living together in colonies. A single polyp consists of a sea-anemonelike organism that secretes the calcium-based structure of the colony’s skeleton.

If you look very closely at the polyp, all hard corals’ polyps have rings of six smooth tentacles. Hard corals provide the majority of structure on coral reefs, with their dead skeletons becoming the anchors for other corals, including both hard and soft coral.

Close up Diploastrea heliopora - Hard Coral

Soft Corals

Soft corals’ chief difference from hard corals is structural. While hard corals secrete calcium-based skeletons, soft corals do not. Instead, soft corals contain structures within their tissues called spiracles that support their bodies.

Did you know that soft corals have eight fuzzy tentacles for feeding? Soft corals produce a broad range of chemicals to avoid predators. There are some nudibranch and snails that feed on the soft coral tissue and these chemicals can deter pests.

Also, soft corals need to protect their territory so chemicals help deter other corals from getting too close. Some of these chemical compounds are even studied for medical purposes.

So the next time you go diving, try to spot the difference between hard and soft corals by looking for the number of tentacles on each polyp. If you find a polyp with eight tentacles you know you’ve spotted a soft coral. Any polyp with more tentacles than you can find is a hard coral (even if the body appears soft).

How Are Coral Reefs Created?

A coral reef begins when a coral polyp attaches itself to a hard substrate on the ocean floor. Once the polyp has comfortably settled it will then divide asexually into thousands of clones, similar to how cells multiply. These clones then attach together and become a tight bond community that acts as a single organism.

These colonies then join with other colonies creating a larger more diverse community and critical habitat for hundreds of thousands of marine animals.

FAQ about corals and coral restoration


Image by: Kasper Prijs

How Fast Do Corals Grow?

Some coral species have faster growth rates than others. There is a difference between hard corals and soft corals.

The rate at which a stony coral colony lays down calcium carbonate depends on the species, but some of the branching species can increase in height or length by as much as 10 cm a year (about the same rate at which your hair grows). Other corals, like the dome and plate species, are bulkier and may only grow 0.3 to 2 cm per year.

A soft coral colony has a growth potential of two to four centimeters per year.

Did you know that it can take up to 10,000 years for a coral reef to form from a group of larvae? Depending on their size, barrier reefs and atolls can take from 100,000 to 30,000,000 years to fully form.

Would you like to know more about corals? Check out our FAQ about corals.


Close-up of a Coral Polyp
How Old Are Coral Reefs?

It is said that modern coral reefs can be traced back to 10,000 years ago, during which time sea levels rose significantly at the end of the ice age, allowing coral reefs to grow rapidly in subtropical and tropical geographical regions.

Close up of soft corals on a tropical reef

Why Are Coral Reefs Important?

Coral reefs are an intricate environment and home to countless species of marine life. They can be found in less than 0.1% of the ocean and yet up to one-third of all marine fish can be found in these extraordinary environments.

The existence of coral reefs is critical to the health of the marine ecosystem; almost all marine fish rely on the coral reef to survive which in turn directly impacts our own wellbeing.

A close up of luminescent coral

What Are The Threats To Coral Reefs?

Coral reefs need fish in order to remain healthy and survive just as much as fish need coral reefs to exist for their own wellbeing and we in turn heavily rely on this ecosystem for our own survival. The heartbreaking truth is that over the last century, fish populations have decreased rapidly as has the existence of coral reefs.

Humans play a huge part in the destruction of the marine environment and declining populations of marine life inhabiting coral reefs.

As human populations grow, sadly so does the demand for operations of industries that create environmental damage. Such industries exist on both a local and global scale such as overfishing and unsustainable fishing methods, drilling for oil, polluted runoff from factories entering the ocean, unsustainable tourism in tropical locations, coastal development, poor waste management strategies, Global warming, and the list goes on.

Human impact on coral reefs is significant.

What Is Coral Bleaching?

Coral bleaching is the term given to the event of coral colonies experiencing thermal extremes due to global warming, resulting in coral communities expelling their symbiotic zooxanthellae as a defense.

As zooxanthellae supply their coral host with organic carbon, their absence leads to starvation of the coral and in many cases death. 

most coral reefs worldwide will be bleached every year by 2070

What Do We Do To Protect The Coral Reefs Around The Gili Islands?

As marine conservationists, it is difficult to accept the damage that our own kind is causing to the world we love so dearly. We refuse to sit back and watch this devastation unfold and will do anything in our power to implement positive changes to the environment through research and education. You can read here what we exactly do to protect and restore the coral reefs around the Gili Islands.

What Can You Do To Help?

There are many actions that you can take to help to protect marine environments:

  • Wear reef-safe sunscreen
  • Practice good buoyancy while diving
  • Join dives against debris to remove trash from the ocean
  • Eat sustainable seafood
  • Join and organize beach clean-ups
  • Reduce your use of one-use plastic items
  • Spread the word; educate yourself and others about important environmental issues putting pressure on the coral reef
  • Don’t step on or touch corals while diving or snorkeling
  • Encourage the use of mooring lines during diving or snorkeling excursions
  • Dispose of waste properly, re-use products, up-cycle, and re-cycle