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

The cerebral-looking organisms known as brain corals do not have brains, but they can grow six feet tall and live for up to 900 years! Found in the Caribbean, Atlantic, and Pacific Oceans, brain corals display what is known as Meandroid tissue integration. This means that the polyps, which are the basic living unit of corals, are highly associated to one another. Their tissues are more closely connected than those of other corals and are not separated by skeletal structures. Many researchers think that the more integrated a coral's polyp tissue is, the more advanced the coral species.

Tissue integration is advantageous because the coral polyps are able to transfer molecules such as nutrients, hormones, and oxygen—making it easier for the brain coral colony to communicate. In some cases, however, this can lead to vulnerability, because if even one polyp gets sick, the pathogen can quickly spread to the rest of the colony. Some species of brain corals suffer mass mortalities due to diseases such as black band disease, white plague, and thermal bleaching.