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Coral polyps are the animal that make up the reef. Individual polyps range in size depending on the species. Most hard corals have very small polyps, measuring just 1mm to 3mm across, about the size of a pinhead. However, mushroom corals, composed of a single polyp, can grow to over 30cm in diameter.

In this photograph, you can also see the zooxanthellae within the coral tissue.

This photo is a close-up of a section of a branching coral. It shows the hard structure of the coral and the individual calcium carbonate cups that 'house' each coral polyp. During the day, the polyps hide in their carbonate homes, receiving energy via photosynthesis, and then at night, they extend their tentacles to catch any plankton drifting past.

Most corals are colonial animals. This means that the individual coral polyps that form coral are still connected and share nutrients. A coral colony starts with a single coral polyp settling on the seafloor. As it grows and forms its calcium carbonate structure, it starts to divide in a process known as 'budding'. This is a type of asexual reproduction.

Once the coral polyp has divided in two, both new polyps stay connected. In this photo, you can see an example of staghorn coral, created over many years of individual polyps growing and dividing and continuing to grow their hard calcium carbonate structure at a rate of 10-20cm a year. It may look like a tree, but is in fact made up from many tiny individual animals.

The coral reef does not always grow in a constant line, but is more often broken up into separate areas. Each of these areas is known as a patch. Habitat patches on the reef crest will appear very different to those on the fore reef or reef slope. These patches can merge into one another, but can and do function as individual units.

In terms of habitat scales, the reef is a self-contained habitat, although it may be part of a larger reef mosaic, such as a barrier reef system. This is an example of a fringing reef developed around an atoll.

Historically, the term reef referred to a navigational hazard and these would be marked on a nautical chart, so that the captains of sailing ships would be able to avoid them.

The term reef mosaic refers to the pattern of connected individual reefs that make up a reef system. The largest of these is the Great Barrier Reef that stretches over 2,300km along the east coast of Australia. It is called a mosaic as it looks like a collection of tiny coral reef tiles when viewed from space or high up.