However, coral polyps do have skeletons, which they make with limestone. Thousands of these tiny skeletons combine to become the structure of a reef. Ever so slowly, over hundreds or thousands of years, the coral polyps add limestone to their skeletons in layers, and grow outward and upward, expanding the coral colonies and the reef. The limestone is made of calcium carbonate. Calcium is extracted from the seawater by the polyps and combined with carbon, which is a by-product of their respiration, to produce this limestone.
Corals can be either rock hard or soft.
The combined skeletons of hard corals produce a rigid, boulder-like reef. The spicules are embedded in the colony to give it strength. But the space in between these spicules enables the colony to bend and sway with the currents. Soft corals look like trees or bushes. There are many varieties of soft corals, including branching corals and gorgonians, like these sea whips.