Education: coral.
Learn about the fascinating world of these tiny creatures that form colourful reefs in the sea
LIFE under the surface of the sea is quite fascinating. You might have read about sharks and large mammals such as whales that inhabit the oceans, but if you’ve seen pictures of people snorkelling or scuba-diving, or if you’ve tried it yourself, you’ll know there’s a lot more to see. Apart from schools of tiny fish dashing around, it’s often colourful coral reefs that attract attention. Let’s find out more about coral and how these reefs are formed.
WHAT’S CORAL?
Coral is made up of invertebrate organisms called polyps. Polyps are tiny soft-bodied organisms related to jellyfish and sea anemones. A polyp is cylindrical or sac-like in shape. On one end is its mouth surrounded by a set of tentacles, and on the other end is its base. The coral polyp’s base consists of a skeleton of limestone (calcium carbonate) called a corallite. This is what eventually forms a coral reef. The polyp attaches its skeleton base to a rock on the ocean floor then reproduces to form thousands more polyps. The corallites are all connected and in this way the polyps form a colony that functions like a single organism.
When a polyp dies it hardens and other polyps grow on top of it. A colony develops over millions of years and joins up with other colonies to form a coral reef. In this way a coral reef is continuously expanding.
Coral has various shapes – some look like small trees, fingers or mushrooms, while othersresemble tables, flowers or honeycombs. The massive ball- or boulder-shaped corals are formed quite slowly.
HOW DOES CORAL EAT?
Coral polyps need a source of food to thrive and grow. They’re sustained by special algae that live inside them. Like plants on land, algae use sunlight for photosynthesis, converting carbon dioxide and water into oxygen and nutrients.
The algae supply glucose, glycerol and amino acids to the polyps who then use this to manufacture protein, fat and carbohydrates as their energy source. Thanks to this process they also
secrete calcium carbonate (limestone) to form corallites.
Coral polyps also eat tiny organisms known as plankton, and sometimes even small fish. This is why polyps are considered carnivorous.
They usually catch their food at night with their tentacles. The tentacles have hooks and sensors that detect movement. As soon as food is snared in the tentacles it’s brought to the mouth and eaten.
Symbiosis The system of cooperation between polyps and algae is called symbiosis – when two organisms in nature live in close proximity and help each other to survive.
The polyps receive food and the algae have a place to live. The polyps release carbon dioxide which the algae absorb and convert into oxygen. The polyps in turn use the oxygen.
Scientists believe the algae also help in the calcification of the coral reef – the process through which the polyps harden and eventually form a reef.
WHERE DO YOU FIND IT?
For coral to grow and reefs to develop a few things are required. The first is sunlight. Algae work the same way as plants do on land, so they need sunlight to photosynthesise. This is why coral reefs are normally found in shallower water, no deeper than about 46 m, and not in pitch-dark depths where sunlight can’t penetrate to the ocean floor.
Because sunlight is vital there’s a better chance of coral growing in clear water – sunlight can penetrate to the ocean floor more easily.
Reef-forming coral also prefers warmer water of 20-28 °C. This is why reefs often occur near the equator, where it’s warmer, or around islands. The majority of coral reefs are found in Southeast Asia or around Australia.
Coral reefs also occur in areas where there’s strong or regular wave action. Waves help create a good habitat for the coral: they provide food and oxygen, help spread coral larvae and flush away waste products.
THE THREATS FACING CORAL REEFS
Coral reefs are an exceptional natural habitat – not only for polyps, but also for many other marine animals and organisms that live among them. This is why they’re so popular among divers: you’re assured of seeing a wide range of sea life when you dive there. Coral reefs occupy just one percent of the ocean floor, but they sustain about 25 percent (a quarter!) of marine life.
Unfortunately all isn’t well with the coral reefs of the world. Scientists estimate that we’ve lost about half the globe’s coral reefs in the past 30 years. They estimate that at the current rate of destruction, almost 90 percent of our coral reefs will be lost by 2050.
One of the main causes of damage is pollution. Waste products such as sewage, for example, that get pumped into the sea compromise the purity and clarity of the water. As a result the algae and polyps don’t grow well.
Other pollutants such as plastic bags get snagged on the coral and suffocate the tiny animals. Harmful fishing practices such as dragnets, poison and dynamite damage many coral reefs and tourists who dive at them sometimes break off pieces of the reef or damage it.
Remember, it takes thousands of years to establish a reef. It grows just a few centimetres a year, which means it doesn’t recover quickly.
Global warming is also detrimental to reefs – when the water gets warmer the reefs bleach and die. A clear sign that a reef is struggling or dying is when its colours start to fade. This indicates the algae have died or are no longer healthy.