Coral polyps are able to reproduce two ways, asexually and sexually.

 

Asexual Reproduction

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During asexual reproduction, matured coral polyps will make clonal polyps that will leave the parent polyp through budding or fragmentation to then grow and possibly start a new colony. Coral budding (also called fission) is the process where the parent polyp splits apart carrying a genetic copy of the parent polyp. Coral fragmentation differs by instead of singular polyps, it is a whole colony that separates to form a new colony. 

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Sexual Reproduction 

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Most stony corals are able to make male or female gametes, even both! Large amounts of eggs and sperm are dispersed into the water in order to expand over a large geographical area to increase successful fertilization and to colonize more habitats. When the egg and sperm fuse, they form larvae also called planulae. One disadvantage to spawning is that many predators will consume planulae, making it difficult to successfully develop and settle. Timing is very crucial to mating since corals are sedentary. Different stages of spawning have different influential factors that coral polyps need to be attentive to in order to increase the chance of fertilization. This includes temperature, day length, lunar cues, and time of sunset. Before planulae settle at the bottom of the seafloor, they will swim towards the surface and sunlight, float, then swim back down and settle. They will then start to develop into young polyps, establish a symbiotic relationship with zooxanthellae and calcify to grow. With continuous calcification, colonies of corals will start to develop and expand. 

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Life Cycle

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There are four different classes of coral reefs: fringing, barrier, atolls, and patch reefs. Although they vary in shape, size, and habitat location, their life cycles are the same. Through sexual reproduction, an egg will develop through the fertilization of a male and female gamete through the process of spawning. A mature planula will settle to the seafloor and start to transform into a polyp via metamorphosis. Coral polyps will begin to expand and replicate through asexual reproduction to eventually form colonies. The longest stage of a coral's life is growing into a mature polyp with a mouth and tentacles. Coral colonies can live extremely long lives from decades to centuries. The oldest coral reef is the Chazy Reef in Isle La Motte, where corals first were discovered, this reef is 480 million years old. (Press, 2015)

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Effects of Coral Runoff on Coral Reproduction

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Different forms of human-driven pollution have affected coral populations. Reefs are dying from excessive sediment burial and heterotrophic feeding. With the continuous construction of resorts, buildings, etc. on tropical islands, there has been an increase in erosion and runoff into the ocean. The dumping of excessive sediment, chemicals, and trash into the sea, has buried corals as well as blocked sunlight that is essential to photosynthesis. An experiment done by R. H. Richmond proved that these forms of pollutions have led to the decline in reproductive success. Fertilization rate fell by 86% and larval development by as high as 50%. Failure in the management of ocean health has led to mass-spawning events to have a higher mortality rate since the water quality has been extremely affected by runoff and sediment dumping.