Coral reefs are one of the world’s most important ecosystems, sheltering and sustaining about a quarter of all ocean species.

The reefs are formed over thousands of years, mainly by colonies of animals called polyps. These creatures make stony corals, the foundation for most coral reefs, by secreting protective skeletons of limestone. As these skeletons accumulate over time — with new colonies of polyps growing on top of the skeletons of older ones — they build up the base of coral reefs.

The Coral Reefs stamps feature highly stylized digital portraits that depict four types of stony corals and associated reef fish:
• Elkhorn coral and two French angelfish
• Brain coral and a spotted moray eel
• Staghorn coral and bluestriped grunts
• Pillar coral and a coney grouper and neon gobies

Elkhorn coral is one of the most important corals in the Caribbean.  It, along with staghorn coral and star corals (boulder, lobed, and mountainous), built Caribbean coral reefs over the last 5,000 years. Elkhorn coral can form dense groups called “thickets” in very shallow water.  These provide important habitat for other reef animals, especially fish.

In the early 1980s, a severe disease event caused major mortality throughout its range and now the population is less than 3 percent of its former abundance. The greatest threat to elkhorn coral is ocean warming, which cause the corals to release the algae that lives in their tissue and provides them food, usually causing death. Other threats to elkhorn coral are ocean acidification (decrease in water pH caused by increased carbon dioxide in the atmosphere) that makes it harder for them to build their skeleton, unsustainable fishing practices that deplete the herbivores (animals that feed on plants) that keep the reef clean, and land-based sources of pollution that impacts the clear, low nutrient waters in which they thrive.
Elkhorn coral used to be a dominant coral on Caribbean reefs and was so abundant that an entire reef zone is named for it. Beginning in the 1980s, the elkhorn coral population declined 97 percent from white band disease. This disease kills the coral’s tissues.

Currently, there are locations such as the U.S. Virgin Islands where populations of elkhorn coral appear stable at low abundance, and some such as the Florida Keys where population numbers appear to be decreasing. Successful reproduction is very rare, so it is hard for elkhorn coral populations to increase.