Corals are small animals (polyps) that are closely related to sea anemones and jellyfish.
Most corals create colonies, by growing a large rocky skeleton that we see as a solid coral rock. The soft bodied polyps live in the outer edge of this skeleton, on top of the empty skeletons of previous generations. Only the outer layer of a coral boulder is alive.
The skeleton made by the polyps is white (calcium carbonate). Corals get their wide range of colours from single-cells plants (symbiotic algae called Zooxanthellae) living in the skin of the corals.
For more Butterflyfish information:
A Guide to Angelfishes and Butterflyfishes
Author: Gerald R Allen, Roger Steene, Mark Allen.
Author: Terrence M. Gosliner, David W. Behrens
Josh Jensen and Liz Harlin