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Coral reefs in danger!

What is coral?

Corals are tiny animals (polyps) that live together in colonies. They are related to sea anemones. There are soft corals and hard corals; only the latter build reefs.

Coral reefs are usually found in shallow tropical waters, so are found around the equator, generally between the Tropics of Cancer and Capricorn. Recent years have seen the discovery of deep sea corals, and these occur in waters throughout the world. When corals die they leave behind a hard limestone (calcium carbonate) structure.

How does coral reproduce?

The most common way is sexual reproduction, where eggs and sperm are released into the water. Sperm fertilises the egg and produces a planula, which floats around for several weeks before attaching to rocks or shipwrecks and producing polyps.

Asexual reproduction is where a polyp can clone itself, giving coral the ability to repair damaged parts. This is how a reef grows. It can grow up to 20cm (8 inches) in a year.

What do corals eat?

Corals feed like sea anemones, using tentacles to catch food such as plankton. In addition they have tiny plants called Zooxanthellae living inside them. These provide them with nutrients. The tiny plants use sunlight and coral waste materials to feed themselves. This is known as a symbiotic relationship.

What eats corals?

Parrotfish and Butterflyfish are well adapted coral eaters. They have evolved strong front teeth for this purpose.

The seastar called Crown of Thorns is a well known coral pest. It eats the coral heads, leaving the brittle skeleton, which can get worn away.

Some marine snails e.g. Drupella can also cause large amounts of damage to reefs. They have specialised mouthparts (or radula) and feed on live coral heads.

Why are reefs in danger?

Physical damage by coralliphores (coral eaters) is a natural process, but powerboating, snorkelling and coral collecting all have a much more devastating effect on the reef.

Coral 'bleaching' means that the tiny animal has died and left behind its calcium carbonate skeleton. This bleaching is caused by an increase in temperatures and UV rays from the sun - both caused by the greenhouse effect.

Extreme weather causes waves to smash the reef and excessive rain dilutes lagoons, causing further bleaching. Pollution from industry and homes is also affecting coral reefs. Increased levels of chemicals and sediments kills the coral.

Why should we care?

Reefs house unique communities of plants and animals that are found nowhere else on the planet. They provide physical protection for small island communities, and provide food for the people there.

Coral reef communities have produced many substances new to science, e.g. a pain killer developed from Cone shell venom, and a sunscreen made from coral.

Article Date: 23 July 2007

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