ANIMAL, VEGETABLE, OR MINERAL?

Related to jellyfish and sea anemones, most corals are colonial animals made up of multitudes of coral polyps, each of which has a typically sea anemone-like structure.

Most corals get most of their energy from tiny algal cells – zooxanthellae. Hosted in the coral tissues and one hundredth of a millimetre across, these algae photosynthesise, passing on up to 95 per cent of sugars they produce to their coral host.

Corals can also catch food from the plankton using stinging cells in their tentacles, but most are not able to survive for long without the energy the algae provide, which is why loss of these algae during coral bleaching is a problem.

Some deep-water corals can survive without photosynthesis in near or complete darkness, but these species are generally very slow growing and fragile.

In ‘hard corals’, the polyps sit in a carbonate coral skeleton, which over time builds the fabric of the reef. Another order, the ‘soft corals’, are often brightly coloured, lack hard skeletons and don’t generally contribute to the structure of the reef.