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The term hermatypic was originally proposed by Wells (as cited in Schuhmacher & Zibrowius, 1985) to describe corals that contributed to reef growth and possessed symbiotic zooxanthellae in their tissues. In contrast, ahermatypic corals are those that may live in either deep or shallow water conditions yet are not reef building. Ever since, there has been much confusion as to what the true definition of hermatypic is.^[1] Hermatypic corals are stony corals, belonging to the order scleractinia, however not all stony corals may be hermatypic. Reef-building corals are generally colonial organisms. They are able to build up their skeletons by adding calcium carbonate to their corallite.^[2] The body forms of hermatypic corals are generally species specific, however geographic locations and environmental factors, such as wave action and temperature, may influence the growth of a colony. Hermatypic corals occur in 10 major body forms ^[3]

• Branching
• Digitate
• Table
• Elkhorn
• Foliose
• Encrusting
• Submassive
• Massive
• Cup
• Mushroom

High rates of sedimentation impair the photosynthetic capabilities of hermatypic coral and reduce the potential for colonial growth. This environmental factor is the primary limiting control on the global distribution of coral reefs. Hermatypic corals, like most corals are confined to areas of shallow, clear, and warm water. Sediment influx is generally promoted by increased levels of turbidity. Therefore, turbidity, from natural or artificial sources, is an important environmental factor in sediment influx and subsequent coral growth. High levels of sedimentation and turbidity lead to decreased growth rate and ultimately coral death, due to reduced illumination necessary for zooxanthellae and/or increased energy expenditure by the coral to remove impacted sediments.^[4] Hermatypic corals thrive in warm waters. At these ideal temperatures calcium carbonate is readily precipitated to form the carbonate structures that define hermatypic corals. A study by Miller (as cited by Dodge, & Vaisnys,1975), shows a positive effect on growth of coral with increasing temperatures over a time period of three years.^[4]

Eutrophication processes, including nutrient enrichment, sedimentation, turbidity, and toxicity, are detrimental to hermatypic corals. These processes interfere with the sediment rejection abilities and feeding and reproductive strategies of hermatypic corals. Eutrophication is associated with high phytoplankton biomass, which can negatively affect survival rates of hermatypic corals. Eutrophication is often attributed to anthropogenic activities, including usage of fertilizers on crops and dumping of raw sewage. As a result, the coral community is affected directly through competition for space or light with the algae that benefit from nutrient enrichment, or indirectly, through sediment landing on the corals. When corals are subject to turbid waters, their symbiotic zooxanthellae suffer due to a decreased intensity of light. If these zooxanthellae are not able to complete photosynthesis and die, the corals will experience mass bleaching events. They too will die as well if they are unable to regain live zooxanthellae. Furthermore, sediment landing on corals can clog their feeding apparatuses, effectively choking them. ^[5]

Hermatypic corals are able to reproduce both asexually and sexually. Sexual reproduction is accomplished through annual mass spawning events for many corals. Spawning events occur solely during the night. During these times corals release large bundles of male and female gametes to increase the chances of fertilization. The successful recombination of two gametes is less likely to occur during times of high winds as the gametes are dispersed too rapidly by surface currents and waves. As fertilization and development of coral embryos are both external processes, warming oceans pose a threat to the reproduction of hermatypic corals. Sexual reproduction is advantageous to corals as the recombination of different gametes produces more robust offspring that are able to better adapt to changing environments. More thermally tolerant genotypes able to adapt to warming ocean conditions are an example of this.^[6] Asexual reproduction occurs when a fragment of a coral is broken off of an existing colony under high wave energy conditions, such as a storm event, and the fragment is able to recolonize elsewhere. This method of reproduction is unfavourable as the coral is essentially cloning itself, and the resulting new colonies all have the same susceptibility to disease.^[7]

Due to their significant contribution to reef growth, there is a reason to suspect that calcification rates are higher in hermatypic corals. However, this theory is hard to test since calcification rates can vary by up to a factor of ten between various hermatypic species. For example, corals with porous skeletons will have higher apparent calcification rates than non-porous corals. This results in some hermatypic species being more significant in reef construction than others, and some barely contributing at all. ^[8] It was determined by Goreau (1961) (as cited by Carlon et al., 1996) that hermatypic corals calcify 3.58 times faster in daylight versus being in the dark. This suggests the importance of zooxanthellae in some hermatypic corals. In addition, it was determined that hermatypic corals calcify 5.27 times faster than ahermatypes. .^[8]

Hermatypic coral fossils can be utilized as paleoclimatic indicators. Systematic assessment of d13C , d18O , and MLSE enhances our understanding of the natural variation in isotopic signatures and growth patterns in corals used for coral-based, tropical paleoclimate reconstruction ^[9]. The specific geographical distribution of these coral fossils is indicative of the depositional environment. Hermatypic corals are most commonly formed in warm, clear and shallow waters found in equatorial regions. This specific geographical distribution allows paleoclimatologists to understand the climate of the fossil-bearing regions at the time of deposition. One study of hermatypic coral fossils from the Paleocene to the Oligocene in the Eastern Pacific found that fossil-bearing units are spatially restricted to Washington-Seattle, south and central California, Gulf of California and Chiapas, and there is a lack of outcrops in western México and Central America.^[10] In this study, the species richness increases from Paleocene to Oligocene followed by a loss of species towards the Pleistocene. The presence of coral fossils found in higher latitudes shows a significant shift in climate of these regions. These fossils also provide a geologic record of relative abundances of coral formation in different epochs.