An offshore ridge, mainly of calcium carbonate, formed by the secretions of small marine animals. Corals flourish in shallow waters over 21 °C and need abundant sunlight, so the water must be mud free, and shallow. Fringing reefs lie close to the shore, while barrier reefs are found further from the shore, in deeper water. Reef health is mapped by: live coral cover (Mumby et al. (2004) Marine Pollut. Bull. 48), diversity (Hodgson et al. 2004), and remote sensing (Kutser et al. (2006) Limnol. & Oceanog. 48).

Coral reefs are the most biodiverse marine ecosystems on the planet; M. L. Reaka-Kudla (1997) estimates that they harbour nearly one million species globally. However, the health of coral reefs is declining (see C. Wilkinson 2004), due to overfishing, nutrient enrichment, and coral diseases, at the local scale, and ocean warming, acidification, and sea-level rise at the global scale; see also Hoegh-Guldberg (1999) Marine & Freshw. Res. 50, Hempel and Morozova (2001) Bull. Marine Sci. 69, 2 on coral reef management from the benthos up, and the US NOAA Coral Reef Conservation Programs.

A coral atoll is a horseshoe-shaped ring of coral which almost encircles a calm lagoon. Many coral reefs are hundreds of metres deep; Charles Darwin’s theory (1842) is that deep reefs formed during a long period of subsidence. Thus, coral forms in shallow waters and then sinks. Woodfine et al. (2008) Sedimentol. 52 seem to support the subsidence theory. See Braithwaite et al. (2000) Int. J. Earth Scis 89, 2 on revisiting Darwin’s model. Woodruffe et al. (1999) Marine Geol. 160, 1 dispute Darwin’s theory. There is also the possibility of a positive feedback loop: sea level rises, providing more habitat for coral reefs, the reefs flourish and produce more CO₂, the additional CO₂ causes additional temperature increase, and sea levels rise even more; see G. Camoin and P. J. Davies (1998).