Corals can reproduce asexually and sexually
Spawning in Boulder star coral
Corals reproduce asexually by budding or fragmentation. Through budding, new polyps “bud” off from parent polyps to form new colonies. In fragmentation, an entire colony (rather than just a polyp) branches off to form a new colony. This may happen, for example, if a larger colony is broken off from the main colony during a storm or boat grounding.
In terms of sexual reproduction, some coral species, such as Brain and Star coral, produce both sperm and eggs at the same time. For other corals, such as Elkhorn and Boulder corals, all of the polyps in a single colony produce only sperm and all of the polyps in another colony produce only eggs.
Coral larvae are either fertilized within the body of a polyp or in the water, through a process called spawning. In some areas, mass coral spawning events occur one specific night per year and scientists can predict when this will happen.
Once in the water, larvae ‘swim’ to the ocean surface. If they are not eaten, they eventually settle to the ocean floor and attach to a hard surface. Once attached, they metamorphose into a coral polyp and begin to grow, dividing in half. As more and more polyps are added, a coral colony develops and eventually begins to reproduce.