• Coral Reefs

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The Rainforests of the Sea

When you walk into a forest, you notice many different species living close together, depending on one another. On land, the rainforest is the place where more different species live together than anywhere else on Earth. In the sea, corals are the same. More species of marine organisms live on coral reefs than any other biome on the planet. Thousands of species depend on their existence to survive.

The Coral Polyp

Corals have many shapes and sizes, but they all have the same basic form. It is called a polyp, and is a small organism made of many cells. They all look alike, and in fact are genetically identical. Each polyp is only a few millimeters in diameter, and consists of tentacles surrounding a central mouth. The mouth is the only opening to the stomach, and serves as both a way for food to be ingested and waste expelled.

A cup coral polyp, whic is one of the larger coral polyps.

A Colonial Lifestyle

A polyp on its own looks like a baby jellyfish. However, aside from starting out coral polyps are never found on their own. They live in colonies, and arrange themselves in many different shapes. Some look like brains, others like fans, and others like the antlers of a deer. Even though each polyp is an animal in its own right, working together they grow to amazing sizes. They share nutrients through channels that work like veins throughout the coral.

Sex in Every Way Possible

Where humans have a single way to reproduce, the options corals have is amazing. They can reproduce on their own (asexually) or combine their DNA to form a new organism (sexually). Corals can break off and regrow, sometimes colonies divide into two separate colonies, and a single polyp can even do something called a “bailout”, where they leave their colony to start their own. When it comes to sexual reproduction, coral have many more options. One amazing feat is that many coral have timed their spawning, so that at a given time all the corals release their eggs and sperm at once. This could be because the day is longer, or the moon is full, or maybe coral even use chemicals to talk to one another!

Corals off the coast of Belize

Soft and Hard Corals

Most people think of a table or brain coral when asked to picture a coral. There are so many more types though! One way to divide the corals are by looking at their bodies. Are they hard, like brain coral, staghorn coral, and table coral? Or are they soft, like sea fans, mushroom coral, and sea whips? The soft coral are an essential part of the reef community, but the hard corals leave a legacy that lasts much longer than the life of one colony.

A gorgonian, a type of soft coral, with cyanobateria growing alongside it.

The Coral Triangle

Located in between the Pacific and Indian ocean are tens of thousands of islands, comprising the countries Indonesia, the Philippines, Malaysia, and Papua New Guinea to name a few. While these islands are beautiful, their real treasure lies just beneath the surface. Some of the largest reef structures are found in this area, and the most diverse coral reefs are located in the coral triangle as well. Unfortunately, the whole area is under threat. Humans depend on the reefs, but also exploit them for their fish.

Island Builders

This era in the world is named the anthropocene... meaning that we humans have created such a mark on the planet that it will be noticeable long into the future. Corals have done this like no other animal. In fact, many of the islands in world owe their existence to coral reefs. As the volcano starts to sink back into the sea after it has died, the corals continue to build the island upwards towards the surface. These islands are called atolls, and have a characteristic ring shape often associated with idyllic islands.

A Pillar of the Community

The first thing that people notice when they dive into a reef are the thousands and thousands of fish flying around everywhere, but they would all not exist if not for the coral. The coral polyp, although a simple structure, provides the building block to all the inhabitants of the reef. They provide homes for fish, either through their complex maze of passageways or through a symbiotic relationship, like that of the coral and damselfish.

Penrhyn Island is a model atoll... it once represented a large volcano surrounded by a reef.

Concerns about Coral Reefs

Coral Bleaching

Many people think that white is a sign of purity and beauty, but underwater, white coral means dead coral. This phenomenon has been happening more and more around the world. What could be causing it? Well, the coral polyps are in fact incredibly sensitive to temperature. A couple degree rise in temperature causes the polyps to leave the skeleton, taking all the colour out of the coral. They leave behind the white calcium carbonate structure. The polyps can recolonize this structure, but it needs to be algae free. This means that fish in fact play a critical role in helping the coral increase resistance to climate change.

Ocean Acidification

Corals have proven to be pretty tough... they have lasted over millions of years and through many ice ages and warming events. However, humans have found their weak spot and gone for the jugular. The basic building block of a coral reef is the polyp, which uses calcium carbonate to create a skeleton, thus building these amazing reefs. As atmospheric carbon dioxide increases as does the absorption of the greenhouse gas into the sea. It turns into carbonic acid, which reacts with the calcium carbonate in the water and reduces the amount available to corals and other animals. This small piece could ultimately stop reefs from growing. Nobody knows for sure, but the signs are already on the wall that this reaction is occurring at an ever increasing rate.


Aside from a few picky eaters, most people find fish a delicious and healthy meal. Indeed, many of the people on the planet depend on them as their main source of protein. Our dependence on this food has become all too great, and now we have taken many more fish than are being replaced by repopulation. Many fish populations are crashing, and if we don't stop overfishing soon it could cause chain reactions throughout the seas. Just as fish depend on the coral to live, the coral also needs the fish to keep the algae off them so that they can regrow.

Descructive Fishing Practices

Imagine you are on holiday looking at a beautiful reef. You see fish darting in and out, and the whole scene looks more colorful and vibrant than you could ever imagine. Suddenly a CRACK sound shoots through the water. You have just heard a dynamite fisherman making a beautiful piece of reef into a cemetery. Your snorkeling tour leads you to a piece that was once bombed... the fish are fewer and the bottom of the ocean is a dull gray. The coral have been blown to bits. This and cyanide fishing are two major problems affecting reefs throughout the tropics.

Bleached Acropora coral stands out compared to its healthy neighbour behind it.

Ways we can Save Coral Reefs

Marine Protected Areas

Many countries are now looking for ways to stop the damage being done to reefs. The main way this is occurring is through the establishment of marine protected areas. New Zealand is leading the way, with over 10% of its coastline deemed as marine sanctuaries. Since the fish can swim in and out of these, they actually serve to boost the local fisherman’s catches. One country, Kiribas, has created the largest conservation area on the planet, called the Phoenix Islands. At 408 000 square kilometers, it is one of the last intact oceanic atoll reefs left in the world.

Sustainable Fishing Practices

Sure, you love your fish. But, you can be conscientious about what you eat. Some fish are easily farmed, or reproduce quickly. Others are delicacies for a reason: they take a long time to mature and are rare. One way to help promote reef conservation is through buying fish that are listed as safe to eat on the Seafood Watch list.

Reducing Atmospheric Carbon Dioxide

Although this will take a global concerted effort, the world is made up of independent thinking people all able to make good decisions in conserving the environment. Part of that is lowering your carbon footprint, which would help directly in slowing the rate of ocean acidification. While the coral reef may be threatened by human actions, it can also be conserved by concerted effort of humans to do the right thing: protect our ocean ecosystems.

A coral reef community is always bright and cheerful.

Interesting Links

  • Reef Relief
  • Ecokids - Reefs
  • Kids do Ecology - Coral Reefs
  • Ducksters - Biomes - Coral Reefs
  • Kids Discover - Spotlight: Coral Reefs

  • Image Credits

  • 01 - Colorful underwater landscape of a coral reef
  • 02 -By jayhem from Antibes, France (Ocean Brain) [CC BY-SA 2.0, via Wikimedia Commons
  • 03 - By Nhobgood (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons
  • 04 - By Derek Keats from Johannesburg, South Africa [CC BY 2.0], via Wikimedia Commons
  • 05 - By Mfield, Matthew Field, www.photography.mattfield.com (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons
  • 06 - Uploaded to Wikimedia by Acropora