Coral reefs are underwater ecosystems made from the skeletons of hard coral colonies. Each colony is composed of multiple polyps called corals—animals with tentacles around a mouth at one end and sac-like bodies at the other that attach to a surface and secrete calcium carbonate for protection. Over thousands of years, these secretions accumulate to form habitats that support about 25% of marine species, even though they cover less than 1% of the ocean floor.
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Formed from calcium carbonate skeletons deposited by hard corals, some estimates suggest these reefs provide habitats for over a million marine species. Spanning massive areas, coral reefs also serve as buffers against extreme weather, such as tropical storms.
The mouth is surrounded by tentacles that help clear away debris and defend the organism, using stinging cells called nematocysts that resemble those of jellyfish. Connective tissue called coenosarc connects individual polyps within colonies and allows them to share nutrients.
These organisms—also known as stony corals—form a type of calcium carbonate called aragonite from ions brought to the ocean by rivers that have broken down terrestrial rocks. The accumulation of aragonite protects the soft parts of coral polyps' bodies and creates reef structures when colonies of polyps cement themselves together.
These microorganisms enter the cells lining corals' digestive tracts and provide corals with sugars produced via photosynthesis in exchange for a home within this protective tissue. Like sea anemones and jellyfish, each coral species forms a symbiotic relationship with distinct species of zooxanthellae, which provide the coral with unique coloration.
Developed by The Ocean Agency, the Ocean Image Bank launched in 2021 with the help of more than 200 ocean photographers, videographers, and science organizations to help engage the public and inspire support for oceans as part of the UN Decade of Ocean Science.
Microorganisms called zooxanthellae are in a symbiotic relationship with coral polyps and provide them with food created through photosynthesis. When ocean temperatures or sunlight exceed an optimal range, zooxanthellae release harmful chemicals that irritate coral cells. To protect themselves, corals sacrifice their food source and expel the colorful microorganisms, becoming white.
The Allen Coral Atlas provides imagery of shallow water reefs, along with maps of potential bleaching events, bleaching alert areas, and bleaching hotspots derived from satellite imagery of whitening. Coral reefs are most prevalent in the Indo-Pacific, the Caribbean, the Red Sea, and the tropical coasts of South America and Africa.
Parrotfish and surgeonfish feed on algae that grow on coral reefs to prevent overgrowth while serving as prey for larger fish across the food chain. Reefs also serve as structures for organisms like sea anemones, which provide shelter for clownfish, and homes for invertebrates such as crabs, lobsters, and shrimp, which help recycle nutrients.
The National Oceanographic and Atmospheric Administration estimates that US commercial and recreational fisheries valued at $200M depend on coral reefs, which provide up to $1.8B in flood protection by absorbing 97% of wave energy. Combined with the benefits from tourism, US coral reefs provide $3.4B in value annually.
When diving and snorkeling in environments containing reefs, avoid touching reefs, anchoring on them, or using sunscreen with active ingredients harmful to marine life. Properly recycling and disposing of trash in bins helps reduce microplastics and chemicals that can run off into wastewater and into the oceans.
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