What are coral reefs?
Why are coral reefs important?
Coral reefs are often called the 'rainforests of the sea' because of the astonishing variety of life they support. They provide food, shelter, and nursery grounds for countless fish, invertebrates, and other marine species. Reef-building corals also create natural breakwaters that protect coastal communities from storms and wave damage.
Beyond their ecological importance, coral reefs hold cultural, spiritual, and economic significance for coastal peoples. They sustain fisheries, draw millions of visitors each year, and support research that advances our understanding of biodiversity, changing ocean conditions, and ecosystem resilience. Mesophotic and deep coral ecosystems, once thought barren, are now recognized as potential refuges that may help replenish degraded shallower reefs.
What pressures do coral reefs face?
Coral reefs are among the most pressured ecosystems on Earth. Rising ocean temperatures cause coral bleaching, a stress response that can lead to widespread coral death after prolonged heat periods. Ocean acidification reduces corals' ability to build their skeletons, while coastal pollution, sedimentation, and overfishing disrupt delicate ecological balances.
At greater depths, external pressures include bottom-contact fishing, deep-sea mining, and energy exploration, which can damage fragile corals that take centuries to grow. National marine sanctuaries help protect coral ecosystems through marine zoning, installing and maintaining buoys, restoration projects, research and monitoring, and public education that encourages responsible recreation and stewardship.
Coral Ecosystems: From Shallow to Deep
From shallow lagoons to deep seamounts, coral ecosystems across the National Marine Sanctuary System form a network that sustains ocean life. Coral reefs provide food and shelter for a range of species, from seahorses and tropical fish to sea turtles and other wildlife that migrate across vast ocean distances. While these zones may seem like distinct parts of the ocean, scientists are finding that important ecological connections can link shallow, mesophotic (twilight), and deep-sea coral ecosystems. Safeguarding coral reefs at all depths ensures that the ocean's most biodiverse habitats continue to thrive for future generations.
Shallow Coral Reefs
Where they grow:
In clear, sunlit waters less than 100 feet deep, reef-building corals create vibrant, three-dimensional habitats. In Florida Keys and Flower Garden Banks national marine sanctuaries, species such as great star coral (Montastrea cavernosa), massive brain coral (Diploria spp.), and purple sea fan (Gorgonia ventalina) are found throughout shallow reefscapes. While thickets of staghorn (Acropora cervicornis) and elkhorn (Acropora palmata) coral used to dominate reefs in the Keys, today they are more scarce. There are efforts underway to restore these coral populations.
In the Pacific, shallow reef flats and fringing reefs of National Marine Sanctuary of American Samoa and Papahānaumokuākea National Marine Sanctuary host thriving communities of finger corals (Porites spp.), rice coral (Montipora capitata), and lobe coral (Porites lobata)—species specially adapted to warm, wave-exposed tropical waters.
Why they matter:
Shallow reefs support hundreds of fish and invertebrate species, protect coastlines from waves and storms, and provide vital feeding and nursery habitat. Their structure underpins tourism, fisheries, and cultural heritage in the U.S. throughout the Atlantic, Gulf, and Pacific.
Mesophotic Coral Ecosystems (The "Twilight Zone")
Where they grow:
In the area between sunlight shallow reefs and deep-sea reefs, mesophotic coral ecosystems flourish along steep slopes, submerged banks, and volcanic ridges where dim blue light still penetrates the water column.
In Flower Garden Banks National Marine Sanctuary, mesophotic ecosystems include star corals, black corals, and sponges scattered across deep escarpments. In National Marine Sanctuary of American Samoa and the sanctuary waters of Papahānaumokuākea, extensive mesophotic zones support species such as Leptoseris, Pavona, Antipathes (black corals), and crustose coralline algae that thrive in low-light environments.
Why they matter:
Some studies suggest that mesophotic ecosystems may serve as refuges for fish during bleaching events in shallower waters and provide genetic and ecological links across depths. They also host unique communities that include rare, specialized, and even newly discovered or endemic species—particularly in places like Papahānaumokuākea, where extensive mesophotic zones have revealed coral, sponge, and algae species found nowhere else on Earth. These ecosystems contribute significantly to biodiversity and deepen our understanding of how life adapts to low light and high-pressure environments.
Deep Coral Reefs (Cold-Water Corals)
Where they grow:
Hundreds to thousands of feet below the surface, deep-sea corals such as Lophelia pertusa, Paragorgia arborea, and Chrysogorgia spp. grow on canyon walls, volcanic seamounts, and rocky slopes. These cold-water coral ecosystems occur in Monterey Bay, Greater Farallones, Cordell Bank, Channel Islands, Chumash Heritage, and Olympic Coast national marine sanctuaries.
Across the Pacific, deep seamounts within Papahānaumokuākea and National Marine Sanctuary of American Samoa also harbor diverse deep-sea coral forests, including delicate glass sponges, gorgonians, and stalked crinoids, shaped by strong currents, geology, and nutrient-rich deep waters.
Why they matter:
Hidden from sunlight, deep-sea coral communities are essential to our ocean. They form ancient, cathedral-like structures that provide habitat for deep-water fish, crabs, shrimp, and other invertebrates. Their slow growth means they record long-term ocean history in their skeletons, and their presence signals stable, healthy ecosystems that play an important role in global carbon storage and biodiversity. Deep coral ecosystems are also a frontier for medical research, with several deep species found to produce unique chemical compounds that scientists are studying for potential medical applications.
