Corals reproduce sexually, but are at a disadvantage as they are fixed to the sea bed. So the use the rising water temperatures of spring and the phases of the moon as a cue to synchronise their reproductive activity to maximise the chance of success. Species that have separate sexes must release eggs and sperm at similar times for them to be fertilised. Some species of coral are both male and female, and they release eggs already prepackaged in sperm. Bundles of eggs and sperm float to the surface to mix with others along the reef. Each kind of coral times its release to a certain hour on a certain night, maximising the chance of cross-fertilisation. The fertilised eggs drift away from the reef.