Hard Corals

in July 2011 - Marine Scene

Some say coral reefs are the only living structures visible from space. Whether this is true or not, they can certainly be massive. The Barrier Reef off Australia runs for over 2 600km and all of it has been made by the action of tiny individual corals building together to form the hard colonial corals of the reef. Many tropical islands owe their existence to the combined action, over millennia, of tiny coral polyps, which have constructed their minute calcium carbonate cottages, died, and had their skeletons used for the foundations of the next generation of polyps.

Each small individual of a hard coral colony consists of a small polyp, of less than 3mm, with symbiotic algae living in its tissues. These are the zooxanthellae ('zoox' for short) and they provide the vast majority (some estimates suggest up to 95 per cent) of the polyp's food from photosynthesis. It is their photosynthetic pigments that give corals their lovely colours. This ready source of sugars allows hard corals to grow fast, but restricts them to clear warm shallow waters. It's a delicate balance. Too little sunlight and the zoox can't function. Too much, or too high a water temperature, and the stressed corals expel their zoox in a phenomenon known as bleaching.

Though the polyps extend their stinging tentacles nightly to catch zooplankton from the water, without the zoox they don't get enough food and so starve to death. Average temperature increases of as little as 1.5-2ºC are enough to tip the balance.