The reef is made up of small colonial animals, called coral polyps. Coral polyps live on plankton and also on sunlight, utilizing symbiotic algae in their skin tissue to augment their diet. This makes reef growth possible in the nutrient-poor tropical ocean water. The reef then provides nesting areas and hiding spots for numerous species of small fish and invertebrates. These in turn attract larger animals to the reef in search of food. Therefore, a coral reef can turn an otherwise barren patch of sand into a bustling marine metropolis. All of this is made possible by the tiny coral polyps, most no larger than a single pearl.