Coral & Coral Reefs (Types, Threats & Surprising Facts)

Coral and coral reefs

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Dive under the ocean and you’ll find an array of marine life, much of which we’re all incredibly familiar with. But what about coral? These seemingly inanimate lifeforms are an important, albeit primitive part of the underwater ecosystem.

What is Coral?

Coral reef

It is thought that there are around 6000 species of corals in the world’s oceans; but what are they? They might look like plants, but corals are actually animals. These animals are made up of small invertebrates known as polyps which largely feed on plankton.

They are classified as animals because they feed off other lifeforms, have a digestive system, have an embryonic reproductive process, and can independently move around. What’s fascinating is that each polyp that makes up a coral is its own individual creature.

These polyps take ions from the ocean water in order to create limestone exoskeletons for themselves.

While they may not look the same, corals are members of the same family as creatures like sea anemones and jellyfish; the cnidaria phylum family. Much like jellyfish, coral eats and excretes through the same opening which is located at one end of the polyp’s cylindrical body.

Coral Anatomy

Corals typically thrive in tropical waters. In fact, they are unable to survive in water temperatures lower than 64ºF (18 °C) and prefer to live in very salty waters. That said, some corals are able to survive in temperatures as high as 104ºF (40 ºC), although they can only cope with this for a limited period. Corals are found in clear waters since they require a lot of light in order for photosynthesis to take place since part of their make up, the zooxanthellae, is a type of algae.

While most coral prefer to live in shallow tropical waters, there are examples, among the 6000 species that live at depths of up to 20,000 feet (6000 meters) and even in polar seas. Surprisingly, however, corals only make up around 0.1% of the ocean but are incredibly important for marine biodiversity. Among their species, a further 98,000 subspecies of plants and animals can be found.

What is the Diet of Coral & How do they Feed?

There is a complex relationship between many types of hard corals and a form of algae known as zooxanthellae. Both the algae and the corals benefit from this symbiotic relationship, with millions of algae living in just a single square inch of any given coral.

The zooxanthellae help to produce much needed calcium in order that the coral can produce its exoskeleton, as well as other important nutrients. While the coral provides the algae with shelter as well as providing its waste to help the algae with photosynthesis.

What is the diet of coral & how do they feed?
NOAA / Wikimedia Commons / Public Domain

There are other types of coral that use filter feeding, a process through which they literally filter plankton out of the surrounding water. This usually happens at night when corals can be observed stretching out limbs and tentacles to reach for the plankton.

How Does Coral Reproduce?

How does coral reproduce?

It is only possible for corals to reproduce if they are healthy and have the right conditions. Poor water quality, overfishing, or high temperatures can all decrease the chances of reproduction as the corals will focus their energy on survival.

However, healthy corals reproduce in one of two ways; sexually and asexually.

Sexual Reproduction

Sexual reproduction requires male and female cells, and in the case of coral, this process happens through spawning. Amazingly, corals mass reproduce with around 130 different species releasing sperm and eggs cells on the same night every year!

The cells combine near the surface of the water where they turn into larvae. While many will be preyed upon by fish and other marine life, those that survive sink down to the bottom within a few days where they develop into polyps.

Asexual Reproduction

In a process known as fragmentation, corals can break off a part of themselves which will then turn into its own colony. This is a very effective and reliable form of reproduction but does come with the downside of less diversity.

Thankfully, humans are able to use this method of coral reproduction to help reefs or coral colonies that are under threat. By simply pruning the corals and ‘replanting’ parts in surrounding substrate, there is a chance to aid a colony to thrive once again.

How Does a Coral Reef Form?

How does a coral reef form?

Look at some of the largest coral reefs in the world, and you can see just how gigantic these natural phenomena can become. But this doesn’t happen overnight. In fact, coral reefs may only grow a maximum of around four inches (10 cm) every year.

But how do they form? Not all corals will form into a reef. This is typically something that happens with those found in shallow tropical waters.

Over time, the spatfall, which are the fertilized larvae, begin to sink and settle in colonies. It takes a serious amount of time but eventually, these will form thickets on top of the old remains of dead coral which then creates a reef.

Why are Coral Reefs So Important

Why are coral reefs so important

Without thinking too deeply about it, you could be forgiven for thinking that coral reefs were just ocean structures that don’t provide any benefit to anything or anyone else. But this is a common misconception. These living reefs provide shelter to other animals, boost the tourism industry, and even protect against flooding and erosion. Let’s find out more!

Provide Habitats & Shelter to Marine Organisms

Per unit area in the marine environment, coral reefs support more life than anything else in the world. This includes around 4000 species of fish as well as a whole host of other animals. That’s around 25% of all the fish in the ocean! So while they may only cover a minute area of the ocean floor (around 0.1%), they are essential to marine life. 

Without the support from coral reefs, these species would dwindle in number and potentially fail to survive at all. Coral reefs are often referred to as the rainforests of the ocean because of the amount of life they support.

Carbon & Nitrogen Fixing

Coral reefs are responsible for a lot of things, and that includes carbon and nitrogen fixing. These reefs absorb carbon dioxide from the atmosphere and this is one of the gasses that contribute to global warming. As a result of this, they’re often named carbon sinks.

Absorbing nitrogen means that they are able to pass on this resource to the many lifeforms that rely on the reef for survival.


With more than 700 million people going hungry every day, it’s interesting to learn that a lot of thought is now going into sustainable production of food from coral reefs. When properly managed, each square kilometer of a coral reef can produce up to 15 tons of fish every year; that’s more than enough to go around!

Protection against Storms & Coastal Erosion

For coastal towns and cities, coral reefs provide a barrier from waves and bad weather. Without them, these areas would be at a higher risk of flooding. More than 200 million people living in coastal regions rely on coral reefs for protection, and many of them aren’t even aware of it!

In the event of a tsunami, coral reefs can act as something of a buffer. Not only does this benefit the settlements on land but also any ecosystems found between the land and the coral reef.

Bad weather coming in from the water, such as hurricanes or storms, can also be somewhat quelled by the presence of a coral reef. On top of this, the potential erosion that would be caused by the weather and the water is lessened significantly with a reef acting as a barrier.

People’s Livelihoods

Coral reefs are a huge support for fisheries creating jobs all over the world.  It is estimated that around half of the gross national product from countries that border reefs comes from the reef alone. In fact, there are around 6 millions fisheries in over 100 countries that are supported by a coral reef.

What’s more, with their attractive appearance and being home to thousands of species, people will travel to see coral reefs, therefore boosting the tourism industry in these areas. Around 350 million people travel to areas with coral reefs every year and a staggering 275 million people live within just 19 miles (30 km) of a coral reef!

In Florida, coral reefs bring in more than $4 billion each year in the southern part of the state alone. Globally, it is believed that coral reefs are responsible for around $172 billion going into the economy; that’s more than the global profit from the music industry in a single year.

Medical Research & New Medicines

On top of this, it is believed that there are still thousands, if not millions of undiscovered microorganisms living among coral reefs. These discoveries could be of significant benefit to humans in that they could aid in medical treatments, including possible cancer medications and medicines for viruses and bacteria.

There are many species that live within coral reefs that contain biomedical compounds; these are used to create much needed medicines. It’s thought that coral reefs and their supported life may be invaluable in the creation of medicines for conditions previously thought of as ‘untreatable.’

Furthermore, the skeletal structure of coral reefs has proved incredibly useful in human bone grafting.

Types of Coral Reefs

To the untrained eye, a coral reef may just look like a coral reef. But did you know that there are different types of reefs?

Fringing Reefs

Types of Coral Reefs - fringing reef

Fringing reefs are usually found on stable or rising coasts as they are more easily able to develop here. You’ll usually find it attached to the shore, and there are two parts; the flat rear part of the reef and the sloping part closer to the shore.

They are very common but are one of the youngest types of coral reefs found on earth. While there are fringing reefs across the planet, the most well-known one is located on the western coast of Australia. The Ningaloo reef is known to support around 1400 species, including types of fish, mollusks, and corals.

Bank or Platform Reefs

Types of Coral Reefs - platform reef

Bank reefs, also known as platform reefs have a semi-circular shape and are usually found in deeper waters. They begin at the ocean floor with corals that prefer darkness and rise up from the water from here.

As a result of this, there are usually three levels to these reefs. The base is formed of skeletal remains while further up, you’ll find white limestone and at the very top, star corals benefit from the greater light levels coming from the surface. The top of the platform reef tends to be flat with several shallow lagoons.

One of the most famous bank reefs is located off the coast of Florida called the Carysfort reef. It’s currently incredibly healthy and is a popular diving site.

Barrier Reefs

Types of coral reefs - barrier reef

Just like fringing reefs, barrier reefs form closer to the coast, but they are not attached. Instead, they are separated from the land by deep lagoons, and their tops can often reach the surface. It is for this reason that they earned their name as they can often act as a barrier for boats and ships.

These are among some of the largest reefs, with some stretching for hundreds of kilometers along the coast. One of the most famous in the world is the Great Barrier Reef located off the coast of North East Australia. Not only is this one of the largest, covering more than 133,000 square miles, but it’s also one of the biggest natural tourist attractions in the world.


Types of Coral Reefs - atoll reef

An atoll is a coral reef that is shaped like a ring and forms a lagoon in the center. There are channels that lead out into the open ocean but the inner part is largely protected by the reef.

Atolls typically form when islands that have fringing reefs have been submerged by the sea but their fringing reefs carry on growing, forming atolls.

Pretty much all of the atoll reefs are located in the Pacific and Indian oceans. However, one of the most notable is the Great Chagos Bank which is in the Indian Ocean which covers more than 12,000 square miles!

Types of Corals

As we mentioned earlier on, there are more than 6000 species of coral that we are currently aware of. Likely there are plenty more yet to be discovered. In any case, corals are classified into two main groups; hard corals and soft corals.

Hard Coral

Hard corals are far more widely distributed than their soft counterparts, and there are currently more than 3000 discovered species. Hard corals are also the only types of corals that are able to form reefs and have that limestone exoskeleton we talked about earlier on.

While soft corals tend to have eight tentacles, their hard cousins only have a total of six per polyp and are only ever found in warm, tropical seas. Without zooxanthellae, hard corals would not be able to survive, and once they die, their skeletons become part of the reef for further colonies to develop.

Types of Hard Corals

With more than 3000 species of hard coral known to humans, it would be impossible for us to discuss them all. However, here comes a selection of some of the most interesting hard corals in our oceans.

1. Staghorn Coral (Acropora cervicornis)
Staghorn Coral (Acropora cervicornis) have long branches, some of which may grow up to two meters in length.

Staghorn coral grows in waters as deep as 99 feet (30 meters) but can be found very close to the surface as well. They’re usually in a back or fore reef position. The staghorn corals have long branches, some of which may grow up to 7 feet (two meters) in length.

Typically found in Caribbean waters as well as along the coast of Florida and in the Gulf of Mexico, the staghorn coral is one of the fastest growing in these regions. Sadly, since the early 1970s, there has been a notable decline of the species, particularly around the Florida coast.

2. Tan Lettuce-leaf Coral (Agaricia agaricites)
Tan Lettuce-leaf Coral (Agaricia agaricites) is found in the Caribbean Sea and the western parts of the Atlantic.

Found in the Caribbean Sea and the western parts of the Atlantic, tan lettuce leaf coral is one of the most common types of hard coral and isn’t considered to be at risk.

As the name suggests, they have a leaf-like appearance and irregular shapes. These corals can grow in either a horizontal or vertical direction which largely depends on the flow of the water. The closer to the surface the coral grows, the more tubercles the coral will form. They tend not to live any deeper than around 33 feet (10 meters).

3. Pillar Coral (Dendrogyra cylindricus)
Pillar Coral (Dendrogyra cylindricus) looks like a cluster of cigars or fingers.

Often said to look like a cluster of cigars or fingers, the pillar coral is found in the West Atlantic and the Caribbean.

These corals are considered an endangered species, and will grow as deep as 65 feet (20 meters). Larger corals, the pillar coral, will grow on both flat and sloped surfaces. Interestingly, the polyps are among some of the only ones that feed during the day.

4. Elkhorn Coral (Acropora palmata)
Elkhorn Coral (Acropora palmata) is highly endangered, and since the mid-70s, numbers have reduced by as much as 95%.

Elkhorn coral is highly endangered, and since the mid-70s, numbers have reduced by as much as 95%. As a result, there are lots of conservation efforts in progress.

These corals have branches that look very similar to the antlers of an elk, which is where they get their name. They’re found exclusively in the Caribbean and can grow as wide as 43 feet (13 meters). They prefer shallow waters with lots of light as this is required to help the zooxanthellae photosynthesise since the elkhorn requires a lot of oxygen to thrive.

Soft Coral

Unlike hard corals, soft corals cannot form a reef; this is one of the major differences. Another key difference is that these corals do not have a hard exoskeleton but rather a flexible one which is more reminiscent of a tree.

The polyps have a total of eight tentacles and these corals are able to thrive in a variety of water types including much cooler climates.

Types of Soft Corals

There are not as many soft corals as there are hard. However, there are still thought to be around 800 species. Let’s get better acquainted with some of them.

1. Bubble Coral (Plerogyra sinuosa)
Bubble Coral (Plerogyra sinuosa) l is made up of several bubbles, which are normally grape-sized.

As the name suggests, these corals have a bubbly appearance. Each coral is made up of several bubbles, which are normally grape-sized. However, depending on how much light the coral is exposed to, this size could vary.

Bubble coral can be found in the Red Sea, the Pacific, and the Indian Oceans and gets smaller at night time when it uses its tentacles to feed. They prefer shadier areas in gently flowing water and like a shallow location.

2. Venus Sea Fan (Gorgonia flabellum)
Venus Sea Fan (Gorgonia flabellum) is most commonly found around the Bahamas but is also spread along the Florida coastlines.

The venus sea fan is most commonly found around the Bahamas but is also spread along the Florida coastlines. These filter-feeding corals like a shallow spot in more tumultuous waters and won’t usually grow any deeper than 33 feet (10 meters).

Looking like a fan with ‘built-in’ branches, the venus sea fan is a delicate-looking coral with an irregular shape and can grow to around 5 feet (1.5 meters) in height.

3. Dead Man’s Fingers (Alcyonium digitatum)
Dead Man’s Fingers (Alcyonium digitatum) has finger-like lobes that come in a range of colors, including cream and yellow.

With a rather morbid name, it won’t come as a surprise that the dead man’s fingers coral has finger-like lobes that come in a range of colors, including cream and yellow.

These corals are usually found in the North Atlantic as well as some parts of the South Pacific. They grow on boulders and stones but are one of the rarer species of coral that can grow as deep as 329 feet (100 meters).

4. Sea Whip Coral (Leptogorgia virgulata)
Sea Whip Coral (Leptogorgia virgulata) has hin wavy stems that are brightly colored.

With thin wavy stems that are brightly colored, the sea whip coral is certainly one of the most interesting in terms of appearance. The stems can grow to a meter but usually don’t get much bigger than 24 inches (60 cm), in most cases.

Sea whip coral can be found in the western parts of the Atlantic Ocean and won’t usually grow much deeper than around 65 feet (20 meters). It prefers shallow waters and is quite common in bays and estuaries.

Threats to Coral Reefs

Shockingly, in the last 30 years, as many as 50% of the world’s coral reefs have died out. Even more frightening is that it is estimated that, without help, a further 90% could die within the coming half a century.

But it isn’t just a danger from one thing. Coral reefs are being bombarded from every angle with a whole host of threats that could wipe them out entirely.


Threats to coral reefs - pollution

There’s been a huge focus on how the oceans are being polluted in recent years, so it’s time we all started taking better care of our waters.

While you may not think that you can do a lot, even something as simple as the sunscreen you choose can pose a risk to coral reefs as this can bleach them. In fact, manufacturers have even started making reef-friendly sunscreens as this has become such a significant problem when humans bathe in the sea. 

We hear a lot about plastic waste in our oceans and this is an issue for coral since many of them are becoming wrapped in plastic, preventing them from getting light and releasing toxins.

Humans have a huge impact on the quality of the water which directly affects coral reefs. Even industries that are taking place far from the coast play a role in this since run-off quickly gets into the oceans. Industries such as logging, mining, and farming are to blame with things as toxic as raw sewage being pumped directly into the sea.

What’s more, with so many oil spills, these chemicals are taking a serious toll on water quality and preventing corals from getting everything they need.

Climate Change

Climate change threats to coral reefs
NOAA: National Ocean Service / Wikimedia Commons / Public Domain

With global temperatures soaring, corals that are used to certain conditions are now struggling to thrive. In the Mediterranean, recent temperatures were 6 degrees higher than usual, turning the sea into something more comparable to the Red Sea. Native corals struggle to cope with this change, but it’s happening all over the world. 

When the temperature rises, coral becomes a lot more susceptible to certain diseases such as skeletal eroding band and white band disease. With the rapid rising temperatures, it’s almost impossible for species to adapt quickly enough to keep up.

But it isn’t just the temperature that’s a problem. As a direct result of globally higher temperatures, sea levels are rising and this means that corals which need to be closer to the surface are being starved of light. Furthermore, this is one of the most common causes of coral bleaching.


Overfishing a threat to coral reefs

Shockingly, more than 15 countries around the world use a method known as cyanide fishing which is as brutal as it sounds. Cyanide is pumped into the reef to stun fish and make them easier to catch but of course, this toxin also has a negative effect on the reef itself.

Blast fishing is another common method used to stun fish, but at the same time, these underwater explosions are blasting chunks off the reefs.

Catching fish for aquariums is usually done far more than is healthy for the reef and this overfishing can destroy the ecosystem. Things like a reduction in the number of grazing fish that remove algae from the corals are a serious issue here.

Even the very presence of things like nets and cages can cause destruction to the reef. Once this has happened, it can take an incredibly long time to recover since coral grows so slowly.

Coral Bleaching

Coral bleaching

Coral bleaching refers to the coral losing its color and becoming paler. When this happens, it’s not long before the corals die off, and it is incredibly difficult for the reef to recover. Where the once bright and bountiful Great Barrier Reef was a paradise for marine life, much of it is now devoid of color and life owing to as many as six mass bleachings. 

This bleaching typically happens when the water temperature rises as this causes the corals to lose their zooxanthellae which they rely on to survive. Moreover, other stressors, such as too little or too much light, exposure to the atmosphere, or a dilution of the seawater can all cause problems.

Unsustainable Tourism & Overexploitation

Unsustainable tourism is a serious threat to coral reefs

We all want to get up close and personal with coral reefs to experience their beauty, but business is booming so much that humans seem to have lost their moral direction. Tourism is massively unsustainable around coral reefs, and these natural wonders are being exploited to the point that it’s threatening their survival.

Boat trips out onto the ocean may seem like a good idea, but when you think about how the anchor or propellers impact the reef and damage it, it suddenly doesn’t seem as attractive an afternoon out.

There are tourist resorts being built right next to coral reefs, and while that’s OK in itself, the waste that’s being pumped into the ocean is directly impacting the coral. With so many tourists wanting to take home a souvenir, corals are being harvested for sale.

While you might not think that merely entering the water would be a problem, you are bringing in sediment from the land, which can cause damage to the coral. In fact, it has been demonstrated that corals in a tourist spot are 12% more likely to suffer necrosis than those in other areas. So things like swimming and snorkeling may not be such a good idea.

Furthermore, when snorkeling or diving, flippers and other equipment often make contact with the reef. This can cause pieces of coral to be torn off, affecting the health of the reef which again, takes a long time to recover.


Pathogen threats to coral reefs including black-band disease (BBD)

Pathogens in the water are responsible for an astonishing number of coral reef deaths, and it’s happening all over the world. In Florida, a 330 year old reef suddenly died away when it was whitened (not bleached) by an unknown pathogen. Over in the Indian Ocean, there are four reefs that are currently suffering as a result of pathogens and other threats.

Black-band disease (BBD) is something that affects corals and was one of the first coral diseases to be discussed in a scientific capacity. Studies have shown that the prevalence of the disease could be worsened by the presence of cyanobacterial mats found on the ocean floor. These mats contain several types of bacteria known to cause the disease. However, other factors like high temperature can also cause the condition.

Ocean Acidification

Ocean acidification makes it more difficult for hard corals to create their limestone exoskeletons

Ocean acidification takes place when there is too much carbon dioxide. Since the ocean absorbs a lot of this gas from the atmosphere, up to one-third to be precise, rising levels are causing problems. When carbon dioxide enters the water, it turns into carbonic acid, which alters the pH level of the water affecting how well coral can survive.

Ocean acidification makes it more difficult for hard corals to create their limestone exoskeletons. Without this, they have no protection or support. Research has shown that a change in the pH levels of the water can cause the corals’ skeletons to be up to 20% less dense!


Crown of thorns starfish (Acanthaster planci) has become such a serious threat to coral reefs

Coral reefs are part of the ecosystem where everything (aside from those at the very top and bottom of the food chain) is a predator and prey. It’s only natural that coral reefs would be attacked by predators, but this is happening in excess and causing them to deteriorate.

One of the biggest threats is the lionfish which are invading non-native waters and have serious effects on the coral reefs. And they are growing in numbers since females are known to lay as many 2 million eggs each year. With such a large number of lionfish feeding off the reef, it’s little surprise the impact they’re having.

But it isn’t just the lionfish that’s to blame. The crown of thorns starfish has become such a serious threat down at the Great Barrier Reef that the Australian government has pledged more than $162 million to tackle the problem. These starfish are able to eat through as much as 10 square meters per year; each! What’s more, their venom is toxic to marine life, which further upsets the balance of the ecosystem. 

Other predators include different types of fish, snails, crabs, and marine worms.

What Actions are Being Taken to Protect Coral Reefs?

Actions are being taken to protect coral reefs

The decline in the number of healthy coral reefs is certainly worrying, but there are conservation efforts in place to try to protect reefs and even restore those that have already been damaged.

Educating people on the threats to coral reefs is one of the most important places to start as many people aren’t even aware of their own impact on the situation. Hawaii was the first place to introduce a ban on sunscreens that contain the ingredients octinoxate and oxybenzone which are responsible for causing damage to the reefs.

Authorities in Indonesia have implemented a sustainable tourism program on Komodo Island to better manage the situation and improve water quality which will provide a better environment for the corals. In other countries, such as Belize, there have been efforts to quell the number of tourists and fishing activities by banning fishing in certain spots as well as charging a fee for the privilege of diving or snorkeling.

Bali, another Indonesian island, banned the harvest of coral back in 2018. While this has had a negative impact on those who farmed it for a living, it has had a significant impact on illegal harvesting which is nothing but a positive thing.

Scientists are also trying to assist coral reefs by bringing them back to life through aided reproduction. A team in Florida has been cultivating corals and helping them to become more resistant to things like changes in the water temperature as well as other stressors. It’s possible to grow coral in nurseries before replacing it back in the ocean to boost the recovery of reefs. 

Plenty of other countries are jumping on the bandwagon in an attempt to save coral reefs by reducing overfishing and tourism to better education. While the future of the world’s coral reefs did look grim, with all of these initiatives, one has to ask the question of whether we can be successful in restoring them; let’s hope it’s not too late.

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