Corals can reproduce both sexually and asexually. An individual polyp may use both reproductive modes within its lifetime.
Corals reproduce sexually by either internal or external fertilization. The reproductive cells are borne on mesenteries (membranes) that radiate inward from the layer of tissue that lines the stomach cavity.
- Internally fertilized eggs are brooded by the polyp for days to weeks. Free-swimming larvae are released into the water and settle within hours.
- Externally fertilized eggs develop while adrift. After a few days, fertilized eggs develop into free-swimming larvae. Larvae settle within hours to days.
Some corals are hermaphroditic (having both male and female reproductive cells). Others are either male or female. Both sexes can occur in a colony, or a colony may consist of individuals of the same sex.
Synchronous spawning occurs in many corals. Polyps release eggs and sperm into the water at the same time. This spawning method disperses eggs over a larger area. Synchronous spawning depends on four factors: time of the year, water temperature, and tidal and lunar cycles.
- Spawning is most successful when there is little variation between high and low tides. The less water movement over the reef, the better the chance that an egg will be fertilized.
- At least one-third of the reef-building corals of the Great Barrier Reef are synchronous spawners. These corals spawn (release eggs) annually in the spring. Spawning occurs on the third through sixth nights after a full moon. Larvae usually settle in four to ten days.
Once the larva settles on a substrate, it develops into a polyp. Some scientists believe that most larvae settle within 2,000 ft. (600 m) of the parent reef. Others contend that some larvae travel longer distances. Research is ongoing.
Environmental disturbances may dislodge some polyps or portions of colonies from the parent colony and deposit them on another part of the reef.
Sometimes, newly developing coral colonies split and form separate colonies.
Often a polyp produced by sexual reproduction initiates growth of a colony asexually by budding. Budding occurs when a portion of the parent polyp pinches off to form a new individual. Budding enables the polyp to replicate itself several times and at the same time maintain tissue connections within the colony. Later, the same polyp may reproduce sexually.
Coral colonies growing in shallow water are often heavily branched. In contrast, deeper water corals often grow in sheets or plates. These flattened forms allow for more efficient use of lower light intensities in deeper waters.
The growth rate of corals and coral reefs depends on factors such as light intensity, water temperature, salinity, turbidity, food availability, competition for space, and predation. Upward growth of coral colonies is generally between 0.5 to 4 in. (1-10 cm) a year.