<P> Internally fertilized eggs develop in the polyp for a period ranging from days to weeks . Subsequent development produces a tiny larva, known as a planula . Externally fertilized eggs develop during synchronized spawning . Polyps release eggs and sperm into the water en masse, simultaneously . Eggs disperse over a large area . The timing of spawning depends on time of year, water temperature, and tidal and lunar cycles . Spawning is most successful when there is little variation between high and low tide . The less water movement, the better the chance for fertilization . Ideal timing occurs in the spring . Release of eggs or planula usually occurs at night, and is sometimes in phase with the lunar cycle (three to six days after a full moon). The period from release to settlement lasts only a few days, but some planulae can survive afloat for several weeks . They are vulnerable to predation and environmental conditions . The lucky few planulae which successfully attach to substrate next confront competition for food and space . </P> <P> There are eight clades of Symbiodinium phylotypes . Most research has been completed on the Symbiodinium clades A--D. Each one of the eight contributes their own benefits as well as less compatible attributes to the survival of their coral hosts . Each photosynthetic organism has a specific level of sensitivity to photodamage of compounds needed for survival, such as proteins . Rates of regeneration and replication determine the organism's ability to survive . Phylotype A is found more in the shallow regions of marine waters . It is able to produce mycosporine - like amino acids that are UV resistant, using a derivative of glycerin to absorb the UV radiation and allowing them to become more receptive to warmer water temperatures . In the event of UV or thermal damage, if and when repair occurs, it will increase the likelihood of survival of the host and symbiont . This leads to the idea that, evolutionarily, clade A is more UV resistant and thermally resistant than the other clades . </P> <P> Clades B and C are found more frequently in the deeper water regions, which may explain the higher susceptibility to increased temperatures . Terrestrial plants that receive less sunlight because they are found in the undergrowth can be analogized to clades B, C, and D. Since clades B through D are found at deeper depths, they require an elevated light absorption rate to be able to synthesize as much energy . With elevated absorption rates at UV wavelengths, the deeper occurring phylotypes are more prone to coral bleaching versus the more shallow clades . Clade D has been observed to be high temperature - tolerant, and as a result it has a higher rate of survival than clades B and C . </P> <Ul> <Li> <P> Brain coral </P> </Li> <Li> <P> Staghorn coral </P> </Li> <Li> <P> Spiral wire coral </P> </Li> <Li> <P> Pillar coral </P> </Li> <Li> <P> Mushroom coral </P> </Li> <Li> <P> Maze coral </P> </Li> <Li> <P> Black coral </P> </Li> <Li> <P> Fluorescent coral </P> </Li> </Ul>

When did earth's first coral reefs develop