Corals

Where Are Reef Building Corals Found?

The majority of reef-building corals are found in tropical and subtropical waters. Click the image for a larger view.

Reef-building corals are restricted in their geographic distribution by their physiology. For instance, reef-building corals cannot tolerate water temperatures below 18⁰ Celsius (C). Many grow optimally in water temperatures between 23⁰ and 29⁰C, but some can tolerate temperatures as high as 40⁰C for short periods. Most also require very saline (salty) water ranging from 32 to 42 parts per thousand, which must also be clear so that a maximum amount of light penetrates it. The corals’ requirement for high light also explains why most reef-building species are restricted to the euphotic zone, the region in the ocean where light penetrates to a depth of approximately 70 meters (Lalli and Parsons, 1995).

Corals have recently been investigated at previously unimagined depths. These Lophelia corals were discovered in 1,250 feet of water off the coast of North Carolina. Click the image for an image of a single branch of this coral.

The number of species of corals on a reef declines rapidly in deeper water. Corals are also generally absent in turbid, or murky waters, because high levels of suspended sediments smother them, clogging their mouths, impairing feeding and decreasing the depth to which light can penetrate. In colder regions, murkier waters, or at depths below 70 m, certain species of corals still exist on hard substrates, but their capacity to secrete calcium carbonate is greatly reduced (Barnes, R.D., 1987).

With such stringent environmental requirements, reefs generally are confined to tropical and semitropical waters. The number of species of stony corals decreases in higher latitudes up to about 30⁰ north and south. Beyond these latitudinal boundaries, reef corals are usually not found. Bermuda, at 32⁰ north latitude, is an exception to this rule because it lies directly in the path of the Gulf Stream’s warming waters (Barnes, R.D., 1987).

Not only are reef-building corals confined by a specific range of environmental conditions, but as adults, almost all of them are sessile. This means that for their entire lives, they remain on the same spot on the sea floor. Reef-building corals have developed reproductive, feeding and social behaviors that allow them to deal favorably with this situation.

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