The Story of a Coral's Life Cycle
Corals can reproduce either asexually by budding or sexually by releasing gametes (sperm and eggs). Budding is the replication of new individuals and is the method by which coral colonies grow. But where did the first coral polyp, from which the others were budded, originally come from? This first individual is the product of sexual reproduction.
Like all animals, corals will take time to reach sexual maturity. Massive hard corals such as Brain corals, grow slowly and it will take about 8 years before they are sexually mature. Since branching corals grow faster, they reach sexual maturity a few years earlier. The individual coral polyp can be male, female, both or may not be reproductively active at all. If a polyp is just of one sex then it is termed gonochoric. A polyp that is both male and female is known as a hermaphrodite. The coral colony is made up of many of these individual coral polyps (or modules). Therefore, the sex of a coral is described at both the polyp and the colony level. A coral colony may be comprised of all female or of all male polyps, thereby being of one sex, or gonochoric. Some colonies, however, are made up of both individual male and female polyps, or of hermaphroditic polyps, and so the colony as a whole is a hermaphrodite.
A coral polyp's reproductive organs are contained inside the body cavity and lie on the mesenteries (or septa). Fertilization of the mature eggs by male sperm may take place within the female coral polyp (internal fertilization) or may be external, occurring in the water column. These are two major contrasting modes of reproduction and have many implications in reproductive ecology. A coral that releases all of its gametes into the water so that fertilization occurs externally is known as a broadcaster. Internal fertilization is achieved by only the male gametes of the species being liberated from the polyps. These mature sperm swim through the water and find a polyp of the same species that has ripe eggs within it. The sperm then enter the polyp via the mouth to fertilize the eggs internally. A coral adopting this strategy is known as a brooder. (An example of the approximately 15% of corals that use this reproductive mode are Porites spp.)
The differences between broadcasting and brooding are shown by the following illustration. The zygote that is formed after fertilization will develop into a larva which is known as a planula. If the planula is the result of external fertilization of a broadcasting coral then this development will take place entirely within the water column. If fertilization was internal the planula will develop within the maternal polyp (known as brooding) and will later be released into the water column. Many brooding corals will only release planulae over discrete seasons whilst others will planulate all year long. Broadcasting corals will release gametes at very specific times so as to ensure fertilization. If the broadcasting species is a hermaphrodite (the individual polyps being both male and female) thegametes are often released together in packages which will also enhance the success rate of fertilization. The corals co-ordinate this timing by the lunar (moon) cycle and by the light-dark regime.
Larval development occurs within the maternal polyp of brooding species, so the planulae that are released spend very short periods of time in the water (a few days). In comparison, the planulae of broadcast corals spend longer in the water column (several days to months) as they mature. The length of time that the planulae spend in the water column will determine the distance in which they are dispersed away from the parent colony. The planulae have limited powers of locomotion and drift with the plankton. They are preyed upon by many reef invertebrates and by fish. The survivors will settle on the bottom to become polyps and start a new colony. The polyps have a measure of control over selecting a surface suitable for settlement. This is important, as once the planula has landed and metamorphosis has taken place, the coral can never move again. Growth of the new colony then takes place through asexual reproduction and the life cycle begins again.