Coral reefs are vast underwater structures composed of calcium carbonate skeletons secreted by tiny marine invertebrates called coral polyps. Often referred to as the “rainforests of the sea,” they form complex, three-dimensional ecosystems with vibrant colors and diverse inhabitants.
The Organisms That Build Reefs
The builders of coral reefs are small, soft-bodied animals called coral polyps, related to sea anemones. Each polyp secretes a hard, cup-shaped calcium carbonate exoskeleton, providing protection and structural support. As generations of polyps grow, die, and secrete new layers, their accumulated skeletons form the stony framework of the reef.
Stony corals grow into massive reef structures through a symbiotic partnership with microscopic algae called zooxanthellae. The coral polyp provides the zooxanthellae with a protected environment and access to compounds like carbon dioxide, nitrogen, and phosphorus, byproducts of the coral’s metabolism.
In return, the zooxanthellae perform photosynthesis to produce energy-rich compounds such as sugars, fats, and amino acids. These organic materials are transferred to the coral host, supplying up to 90% of its metabolic requirements. The photosynthetic pigments within these algae also give corals their vibrant colors. This allows corals to thrive and build massive structures in nutrient-poor tropical waters.
Global Distribution and Reef Types
Reef-building corals flourish in shallow, clear, warm, and sunlit tropical and subtropical waters. Most large reef systems are found between 30° North and 30° South latitude. These corals prefer water temperatures ranging from 20° to 29° Celsius (68° to 82° Fahrenheit). Sunlight is necessary for the coral’s symbiotic zooxanthellae, so most reef-building corals thrive at depths less than 46 to 50 meters (150 to 160 feet) where light penetrates.
Coral reefs are classified into three main types based on their proximity to land and structural formation. Fringing reefs grow directly along coastlines or are separated from the shore by a narrow, shallow channel or lagoon. They are the most common type and can extend for many kilometers, less than 100 meters wide.
Barrier reefs are distinguished by their separation from the mainland by a deeper, wider lagoon, with Australia’s Great Barrier Reef being a notable example. Atolls are ring-shaped reefs that enclose a central lagoon, often forming around volcanic islands that have subsided below the sea surface.
Importance to Marine Life and Humans
Coral reefs are diverse ecosystems, supporting an estimated 25% of all marine species despite covering less than 0.1% of the ocean floor. They serve as nurseries, feeding grounds, and shelters for an array of organisms, including over 4,000 species of fish. Regions like the Coral Triangle in Southeast Asia are recognized as the most biologically diverse marine ecosystems on Earth due to their extensive reef systems.
Beyond their biological richness, coral reefs provide substantial benefits to human communities. Their structures act as natural breakwaters, absorbing ocean wave energy and protecting coastlines from storms, erosion, and tsunamis. Healthy reefs can reduce wave energy by up to 97%, preventing billions of dollars in storm damage annually.
Reefs also hold economic and social value worldwide. They support commercial and subsistence fisheries, providing food and livelihoods for hundreds of millions of people. The commercial value of U.S. fisheries dependent on coral reefs exceeds $100 million, while global fisheries generate approximately $6.8 billion annually. Reefs generate billions in tourism revenue, supporting local economies through activities like diving, snorkeling, and recreational fishing. Marine organisms found on reefs are also a source for potential new medicines, with compounds explored for treatments for various ailments, including cancer, arthritis, and viral infections.
The Phenomenon of Coral Bleaching
Coral bleaching is a process where corals lose their vibrant colors, appearing stark white. This occurs when corals experience stress, most commonly due to elevated ocean temperatures. When water temperatures rise by as little as one degree Celsius above the seasonal average for about four weeks, corals expel symbiotic zooxanthellae from their tissues.
The expulsion of zooxanthellae removes the primary source of color from the coral, revealing the white calcium carbonate skeleton underneath. While not immediately dead, a bleached coral is in a weakened and starving state. Zooxanthellae provide up to 90% of the coral’s energy needs, so their absence leaves the coral susceptible to disease and mortality if stressful conditions persist. Rising ocean temperatures associated with climate change are the main cause of mass coral bleaching events globally. Other stressors like increased ultraviolet radiation, poor water quality, or pollution can also contribute to bleaching severity.