Boulder and massive corals, like this boulder star coral (Montastrea annularis), are the "builders" of the reef. A coral head is a colony of small animals called polyps. Polyps the size of a pencil eraser build an external skeleton of calcium carbonate around their bodies. The walls of these skeletons form a rock-like structure. Over time, as new polyps build their skeletons on top of each other, a large coral head is formed. The boulder and massive coral skeletons develop the main reef structure. Coralline algae and encrusting corals glue everything together. (photo: William Harrington)