- 1 Coral Reproduction
- 1.1 Sexual Reproduction
- 1.2 Asexual Reproduction
- 1.3 Abiotic Factors
- 1.4 Biotic Factors
- 1.5 Notes
Sexual reproduction diversifies the gene pool. Approximately 75% of stony corals are hermaphrodite, while others have separately sexed colonies or individuals.   This distinguishment tends to be by species, but sometimes is by geographical region.  Many corals partake in “broadcast spawning,” where gametes that can be released into the water column and fertilize externally and is known as such because this method reaches a wider geographical range based on currents, climate, and conditions.  Because the cells are less dense than the water, they float towards the surface, and upon fertilization, become a “planula.”  The high numbers of planulae then search for a place to settle, but many die in the process, which explains the mass reproductive strategy. Surprisingly, planulae can survive for months before settling because of their symbiotic relationship with zooxanthelae, even this early in the life cycle. 
Mass Spawning Events
Most importantly, concerning sexual reproduction, are the mass spawning events of corals. There is synchronization not only between colonies of the same species, but also of different species. These events are controlled by several factors: the maturation of the coral gonads by means of temperature and/or day length/rate of change, the lunar cycle and the time of day (where germ cells are usually released at sunset), and other biological, chemical, and physical signals likely come into play. However, for this last category, we don't know enough yet to say how, why, which ones for sure.
Cross fertilization is actually possible between coral species. This is because long-distance dispersal of gametes can contribute to links between reefs. The extent of hybridization is not fully known; in other words, we know it can occur within the same genus, but not much can successfully hybridize beyond that. There are also other mechanisms for species distinctions, but scientists only have hypotheses about what those might be such as, but not limited to: reduced fertility and lower survival rate of hybrids, thereby allowing natural selection to come into play.
Asexual reproduction propagates successful genotypic polyps within a coral head through budding and fragmentation. In asexual reproduction, new clonal polyps bud or fragment off from their parent polyp in order to expand current colonies or begin new ones. The extent of asexual reproduction is related to habitat conditions, day length, and the rate of temperature change. Asexual reproduction methods are often used when conditions are relatively stable in order to rapidly expand, and switch to sexual reproduction in order to produce genetically diverse offspring. 
Budding involves the formation of a daughter polyp from a parent polyp, through intra- or extra-tentacular budding. Different species may bud through either process, or both. 
Intratentacular budding occurs when the parent polyp divide itself into two or more daughter polyps. 
Extratentacular budding occurs when daughter corallites form adjacent to the parent colony, external to the wall.  The resulting daughter corallite is generally smaller than its surrounding neighbors, and will grow over time. Only coral species that possess separate walls will reproduce through extratentacular budding. 
Fragmentation can be both intentional or unintentional. Intentional fragmentation occurs when localized skeletal dissolution occurs along the corallum for easy breakage, and pieces of the coral fall away.  Unintentional fragmentation occurs when corals are subject to physical disturbances, such as storms. The success of fragmentation depends largely on the substrate upon which the coral fragments settles. Success is much greater for fragments that settle on top of living colonies, as opposed to sand. If conditions are favorable, fragmentation allows a portion of one colony to establish a new coral colony that is genetically identical to its parent colony. 
Coral reproduction is affected by small and large-scale disturbances in the ecosystem. Upon changes in ocean conditions, corals are forced to react in order to survive, using energy that could instead be used for reproduction. 
Coral have a very specific temperature that they thrive in, between 20ºC and 32ºC. As the ocean temperatures are rising, corals are less able to survive. As temperature increases, the symbiotic zooxanthellae leave the coral, causing coral bleaching. Without this algae, the corals do not have a source of photosynthetic material.  The temperature has a direct affect on the density of zooxanthellae.  Studies have found, however, that corals can rebound from the effects of dramatic temperature change when the ocean temperature returns to normal. 
Human pollution, especially runoff and untreated sewage, causes stressors on the reefs.  Researchers are currently studying the effects of salinity, nutrients, and sediments, among other factors, that interdependently affect coral reproduction. 
Rising CO2 levels, as a result of human pollution, prevent coral from reproducing as effectively as possible. In particular, excess CO2 damages tissue regeneration of the corals. 
Eutrophication, the excessive enrichment of nutrients, can primarily increase coral reproductive success due to abundance of nutrients. However, these nutrients eventually limit coral growth, as they harm coral calcification rates, reduce exposure to sunlight, and overall decrease the quality of the water. As a result, eutrophication stunts coral growth. .
While coral have functions that allow them to respond to high levels of UV radiations, it is shown that abnormal rays lead to coral vulnerability, which in turn affects reproduction rates. In particular, UV rays degenerate the skeletal of corals.  However, another experiment concludes that light has little effect on coral reproduction. 
Storms are crucial in coral reproduction, since they act as a distributing agent for the coral polyps. Tropical storms also provide for genetic diversity, as seen in the Pocillopora verrucosa population in the Gulf of California. 
Zooxanthellae are photosynthetic algae that are symbiotic with most reef-building corals. This mutualistic relationship involves corals providing zooxanthellae with protection, a habitat, and carbon dioxide needed for photosynthesis. In exchange, the zooxanthellae act as the coral's primary energy source by providing oxygen, nutrients, and waste recycling mechanisms.  High levels off stress cause zooxanthellae to vacate their habitats, leaving corals with very little resources for obtaining food and energy. Corals will lose their color in the absence of zooxanthellae, and undergo a process known as "coral bleaching".
While the causes of coral disease are unknown, disease is a huge stressor on coral populations. Scientists have shown a significant correlation between susceptibility to disease and ocean pollution, including pesticides, sewage, and fertilizer runoff. 
Porites Case Study
The reproduction of the Porites evermanni, a coral species found in the eastern Pacific was found to have a survival rate much higher than that of a closely related species, Porites lobata, which is more susceptible to the effects of coral bleaching. Researchers concluded that the housing of mussels by Porites evermanni boosts reproduction rates. Triggerfish bite on the coral in order to feed on these mussels. They then spit out the coral, spreading them onto the ocean floor and allowing the polyps to grow into colonies.  The two Porites species are shown; the Porites Lobata (left) is bleached while the Porites evermanni (right) remains unbleached.
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