Many types of algae, seaweed, sponge, sediment and even mollusks like giant clams and oysters, add to the architecture of a coral reef. When these organisms die, they also serve as foundations for new corals. Coral reefs are one of the most ancient ecosystems on the planet. The first stages of coral reef evolution began 350 million years ago. Modern coral reefs have been in existence for over 50 million years.
Corals grow at different rates.
The massive corals are the slowest species, growing anywhere between 5 and 25 millimeters per year. Branching and Staghorn corals can grow much faster than this, adding as much as 20 centimeters to their branches each year. The variety of shapes and sizes of coral colonies largely depends on their species, with some shaped into hard, pointed forms, while others are shaped into soft, rounded forms. The shape of coral colonies also depends on the location of the coral. For example, where there are strong waves corals tend to grow into robust mounds or flattened shapes.