Corals are tiny fixed animals called polyps that secrete a hard calcium carbonate skeleton serving to provide protection and a base for colony formation. These skeletal structures form reefs over time.

Coral gardens in the early morning light at Kanawa Island near Flores, Indonesia. The island is located in the Komodo National Park.

© Paul Hilton / Greenpeace

Estimated Area of Coral Reefs

109,768.84 square miles of shallow coral reefs worldwide; 2,573.76 square miles and 5,317 square miles inside the 100-fathom (600 feet) depth curve around the Main Hawaiian Islands and the Northwestern Hawaiian Islands, respectively.


Corals feed by extending their tentacles to catch small fish and planktonic species. Almost all corals that inhabit reefs have a symbiotic relationship with algae. The algae photosynthesizes inside the polyp to produce food for the coral in exchange for protection.

The Northwestern and Main Hawaiian Island reefs support between 57 and 59 different coral species, many of which are endemic species. The waters of the Pacific/east Asia region of the world have the most diverse reefs with 500 – 600 species of coral.

These highly important and productive ecosystems support about 25 percent of all marine life and around 2 million types of species.


Coral reefs occupy less than 1 percent of the planet’s surface, almost all of which are found in tropical and subtropical oceans (between 30ºS and 30ºN latitude) at depths less than 100 feet to allow significant light penetration and in seawater temperatures ranging from 64° – 84°F.

Reefs can be classified into three categories: the most common, fringing reefs, which surround the shores of ocean islands; barrier reefs, which are platforms separated from the mainland by a bay or lagoon; and atolls which rest on the tops of submerged volcanoes.

Reefs in the western Pacific are believed to be 50 – 59 million years old.


Human-related stressors on coral reefs include disease; urban and tourism-related coastal development; sedimentation; toxic chemical pollution; overfishing; physical damage from ships, bottom trawling and anchors; invasive species; sedimentation; trade in coral and live reef species; and marine debris.

Due to their low tolerance for very warm water, global warming and coral bleaching currently pose a major threat to reef ecosystems worldwide. In the northern three atolls of the Northwestern Hawaiian Islands, 77 percent of corals were bleached in 2002 with another bleaching event in 2004.

Although the reefs of the NWHI are considered to be some of the last relatively pristine ecosystems in the world, guano mining, fishing camps, U.S. navy airfields and bases, and landfilling threaten and plague many atolls with contamination.

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