Figure 1. The life cycle of the coral, Acropora millepora.

The Acropora life cycle begins with a mass spawning event, when all of the corals in a given area spawn simultaneously. Buoyant egg-sperm bundles are released which break up on their way to the surface, releasing the sperm to fertilize eggs from other colonies. Once fertilized, the egg soon begins to divide, reaching the prawn chip stage in about 12 hours. Then, in a process which is not fully understood, the prawn chip rounds up to form a gastrula. As the blastopore (white arrowhead) closes, cilia develop and the spherical embryo begins to rotate. Over the next several days the sphere gradually elongates to form a spindle-shaped planula larva and the nervous system and nematocysts develop. After a period of a few days to a few months in the plankton, where it swims aboral end first, the planula begins a searching behavior of corkscrew swimming into the bottom. When it finds appropriate chemical settlement cues it rounds up and metamorphoses into a primary polyp. During this process the aboral end of the planula is resorbed and the oral end expands, resulting in a post-settlement, crown-shaped form from which the primary polyp arises. This polyp grows and produces others, eventually resulting in a colony with thousands of polyps.