Brain Star Coral

There are three main types of reefs that form as hermatypic coral colonies grow: fringing reefs, barrier reefs, and atolls.
Charles Darwin determined that these three reef types are also related stages in the process of atoll formation. Fringing reefs, which are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. Barrier reefs are separated from their adjacent land mass by a lagoon of open, often deep water. Atolls are usually circular or oval, with a central lagoon.

Image:
Individual polyp of Goniastrea sp. (commonly known as brain star or honeycomb) colonial coral showing deep-set mouth surrounded by corallite ridges. Live coral specimen in artificial seawater imaged with Olympus MVX Fluorescent Macroscope