CORAL POLYPS

A coral reef is an offshore ridge which is composed mainly of calcium carbonate. 

It is formed from the secretions of small marine animals called coral polyps. 

Coral polyps thrive in shallow, warm, nutrient poor, tropical waters. 

These tiny animals attach themselves to objects such as rocks or existing reefs and build tiny shells of calcium carbonate around themselves. 

They sit in these shells and use their tiny tentacles to catch food. 

When coral polyps die, new coral polyps build their shells on top of the old shells of the previous generation. 

In this way the reef expands over time. Take a look at the cross section of a coral polyp below.

Diagram showing a cross section of a coral polyp. The polyp is shown sitting in its calcium carbonate shell with its  tentacles outstretched.

Coral reefs are made up of millions of individual animals called coral polyps. These give reefs their structure. One of the features of coral is the way it is able to get energy from the Sun through algae called zooxanthellae. This is one of the special relationships that exist in all reef environments.

BUILDING BLOCKS OF THE REEF

Coral polyps

The basic building blocks of the coral reef are coral polyps. Coral polyps have clear bodies and white skeletons. The calcium carbonate on coral reefs is actually the skeleton of coral polyps, or more correctly the materials left behind by coral polyps as they grow. This calcium carbonate gives the coral their shape. The polyps feed off zooplankton, tiny animals that are in reef waters. 

Up to 90 per cent of the nutrients needed by the coral polyp to grow actually come from the zooxanthellae algae that live within it.

Zooxanthellae

Zooxanthellae are algae that actually grow in the coral polyp. Algae require sunlight to grow. It is algae that give coral polyps their huge range of colours. Millions of zooxanthellae algae live within just one square centimetre of coral. *

Symbiotic relationship builds reef

Nutrients in the waters around coral reefs must be low or else the coral will die. This is unusual because coral reefs are one of the most productive environments on the planet. The way nutrients are cycled around makes coral reefs unique.

Coral polyps and the zooxanthellae have a symbiotic relationship. Each organism benefits the other—the wastes of the polyp are used by the zooxanthellae to grow, and the wastes of the zooxanthellae are used by coral polyps to grow. As zooxanthellae are plants and coral polyps are animals, it is actually the zooxanthellae that are at the base of the coral reef food web.