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. This brilliantly colored sea fan is another type of soft gorgonian coral. It is easy to see how soft corals could be mistaken for plants. But just like hard corals, they are a collection of coral polyps. While soft corals also grow in colonies, they do not form reefs like hard coral.