The island of Moorea (French Polynesia) is in a geographic area that facilitates reef growth. (Photo credit: Dr. Anthony Picciolo)
Reef-building corals are restricted in their geographic distribution. This is because the algal-cnidarian symbiotic machinery needs a narrow and consistent band of environmental conditions to produce the copious quantities of limestone necessary for reef formation. The formation of highly consolidated reefs only occur where the temperature does not fall below 18°C for extended periods of time. This specific temperature restriction -18°C- does not, however, apply to the corals themselves. In Japan, where this has been studied in detail, approximately half of all coral species occur where the sea temperature regularly falls to 14°C an approximately 25% occur where it falls to 11°C (Veron 2000). Many grow optimally in water temperatures between 23° and 29°C, but some can tolerate temperatures as high as 40°C for limited periods of time. Most require very salty (saline) water ranging from 32 to 42 parts per thousand. The water must also be clear to permit high light penetration. The corals’ requirement for high light also explains why most reef-building species are restricted to the euphotic (light penetration) zone, approximately 70 m (Lalli and Parsons, 1995).