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Corals are probably the only animal group that has evolved a wide range of different reproductive strategies to persist over time. Besides many asexual modes, corals are known for their variability in sexual reproduction. The majority of all species is hermaphrodites (polyps are both male and female), only about one third of all species has seperate sex (gonochoric). There are two modes of sexual reproduction: broadcast spawning and brooding.
After fertilization, for weeks coral larvae may remain as plankton in the water until they find a potential substrate for settlement. A complex searching behaviour helps to determine whether the substrate is suitable for the particular species. Finally the larva settles and metamorphizes into a coral polyp, not bigger than one millimeter - a new coral is born!
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The following period, from weeks to months, is an extremely crucial phase. The miniature coral might become overgrown by algae or covered by sediment, causing it to die. Therefore it has to grow quickly, usually developing daughter polyps that lead to a colony. Once the coral has reached the size of a few centimetres, it has probably passed probably the most dangerous time in its life.
In order to compensate for the loss of offspring during their early stages of development, corals may develop huge quantities of gametes. In some regions, one night of coral spawning causes beaches to turn red by the gametes that wash ashore. The evolutionary strategy of producing high amounts of gametes has helped corals to ensure that enough propagules will survive, thereby ensuring the survival of the species.
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