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What are Corals?


If you ask most people this question, they will probably suggest that corals are plants. After all, they take root on the floor of the ocean (or your marine tank) and grow from their fixed spot. They don’t have recognisable faces or features other than sprawling, leaf-like branches. Strangely enough, however, they are animals!

The secret to the survival of coral is a process called symbiosis, a term for the partnership between corals and the surrounding life forms which allows them all to thrive. Unlike plants, corals have the ability to take in food from their environment.

The colourful structures we think of in the oceans and use as decorations in our marine tanks are actually huge clusters of small corals.These tiny polyps combine to create large forms, although the process depends on the type of coral.

For example, hard or stony corals secrete a substance that gradually becomes limestone, so the polyps build up hard skeletons for themselves. As the polyps multiply, the older ones die leaving the bed of limestone behind and the new ones continue to build on the structure. This allows hard corals to build up huge reefs and survive on the seabed for a very long time. Stony corals require a fair amount of light, which they convert into energy using a similar process to photosynthesis, so they are normally found in shallow waters.

In contrast, soft corals do not produce a limestone skeleton. They instead rely on sclerites to support their larger structures, which are tiny organic skeletal elements. When the polyps die, the sclerites can consolidate at the base of the structure so they are able to gradually build bases on the seabed from organic material rather than rock. Soft corals are able to thrive in deeper waters away from sunlight as long as there are enough nutrients in the water to sustain them.

If you want to add corals to your marine tank then soft corals are normally considered the best choice, since you can collect small cuttings (known as frags) with a lower risk of them picking up infections or being damaged compared with hard corals. They also multiply quickly under controlled conditions so it’s easy to grow more corals and move pieces into other tanks where they will take hold and keep multiplying. They are certainly an impressive life form, and thanks to their unique ways of surviving in the wild they have been thriving for approximately 25 million years.