Reef-building corals (hermatypic corals) are found globally, but are confined to shallow waters in the photic zone. In 2008, all 845 species were assessed against the IUCN Categories and Criteria in a project led by the Global Marine Species Assessment program (GMSA) and completed in 2008 (Carpenter et al. 2008 Science 321: 560-563) .
Reef-building corals occur through a symbiotic relationship between coral polyps, and the algae zooxanthellae. Fourty percent of adult reef-building corals are restricted to shallow reefs of less than 20m in depth, with 60% of species surviving on deeper reefs, (>20m depth). Temperature, salinity and water quality all have to fall within narrow environmental parameters to ensure the survival of both the coral and the zooxanthellae. Where this is achieved, coral reefs provide the basis for a highly diverse and superabundant ecosystem, supporting intricate food webs on which millions of people depend for protein worldwide.
33% of reef-building corals are estimated as threatened with extinction. Half of the species are highly susceptible to bleaching caused by the stress-induced expulsion or death of the symbiotic algae. The build up of carbon dioxide in the atmosphere is leading to rising sea surface temperatures which increases the likelihood of mass coral bleaching. Ocean acidification also reduces the carbonate ion concentrations and thus the ability for the corals to build skeletons. Additionally, local-scale anthropogenic activities such as coastal development, pollution and exploitation reduce the ability of corals to withstand global threats such as climate change.