Although the best known reefs are those in warm, tropical waters, coral reefs exist in other locations. Deepwater coral reefs can be found near landmasses around the globe in waters from 656.17 feet (200 m) to 4,921.26 feet (1,500 m) in depth. Deepwater reefs are similar in many ways to those in shallow waters. The most obvious differences in the two environments are temperature and available light.
A rocky or firm surface provides deepwater coral animals a point of attachment. For this reason, most reefs in deep water are located on underwater mounds, ridges, slopes, and mountains. Strong, fast-moving currents are almost always associated with these communities because they continuously supply water that is laden with oxygen and particles of food. Strong currents also help disperse the reproductive cells of corals and keep their surfaces free of sediments.
Scientists have been aware of deepwater reefs for more than 200 years, but gathering information on them has been a challenge. These habitats are widely scattered throughout the ocean and located at depths that make them hard to study. A reef associated with the Dry Tortugas, a cluster of islands near Key West, was first sighted in 1999 by a team of researchers from the University of South Florida. Located on an underwater barrier island called Pulley Ridge off the southwest coast of Florida, this reef is the deepest in U.S. waters. Little was known about Pulley Ridge until 2004 when further studies showed it to be a thriving deepwater community. Unlike other deepwater reefs, Pulley Ridge is the only one known to be dependent on light filtering from the surface. Because the light at this depth is extremely low, corals, sponges, and algae assume flattened shapes to maximize their surface area. In shallow-water systems, corals structures are taller and thinner.