Corals are renowned for being vibrant habitats for fish and crustaceans to live in, and while they look like colourful ocean plants, they are actually animals.

Related to sea anemones and jellyfish, each animal, called a polyp, lives with other coral polyps in a skeleton which they secrete to give their hard shape.

Some of their colour comes from a protein pigment they release in intense sunlight. Like tanning in humans, they make this as protection from sunlight. They produce pink and purple which protects another creature living within the coral, zooxanthellae (Zoo-zan-thelly).

"Zooxanthellae are single-celled plant micro-organism that live inside the tissue of the coral host, and they give the coral most of their colour" says Jessica Conlan, coral researcher for the Australian Institute of Marine Sciences.

"Different coral are different colours because they have different clades, or types, of zooxanthellae living within them.

"The zooxanthellae live in mutual symbiosis with the coral; the coral provides shelter and nutrients that the plant can't make itself, and in return the zooxanthellae provides nutrients the animal can't make in the form of carbohydrates."

The zooxanthellae provide 60-90per cent of the corals food source through energy from the sun via photosynthesis, and the coral gets the rest of its nutrients by trapping plankton in its tentacles. In a bleaching event, coral loses its zooxanthellae, revealing the stark white of its calcium carbonate skeleton.

"When the coral is stressed, it wants all the nutrients for itself and gets rid of the zooxanthellae" Conlan says. This can happen when the water temperature increases for extended periods.

Not all coral lives within the reach of sunlight, and for those living below 1000 metres, it's a different story. There are thousands of different deep sea corals, from Hawaii to Antarctica. Because they live without sunlight they also live without zooxanthellae, but are still colourful, mainly in reds.

Why deep sea corals have colour isn't clear although there a number of possible reasons. One is the lack of light means colour isn't as important, and colours which take less energy to produce are the most beneficial. Red is the new black; because red light doesn't travel as far into the water as blue, red appears black in the low light. This helps hide red animals from predators.

Another idea is it might be as simple as a genetic carry-over from their evolutionary past, when colours were more useful.

Response by: Jesse Jorgensen-Price, Fuzzy Logic

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