NPS Photos/S. Allred 
NPS Graphic/S. Allred What is Coral?Despite their rock or plant-like appearance, corals are in fact animals.Most corals are made up of hundreds of thousands of tiny individuals called polyps that grow together to form a colony. Corals belong to a group called Cnidaria (the “C” is silent), which makes them closely related to other marine animals like jellyfish, and sea anemones. If you look very very closely, you might be able to see the tiny parts of each polyp. Like their relatives, each polyp has a body made of soft tissue, retractable tentacles and a mouth. Unlike animals like jellyfish and anemones, corals have a hard internal skeleton made of calcium carbonate. Over time, this skeleton accumulates and creates the sturdy, rock-like structure we recognize as coral. It takes decades, and sometimes even centuries, for them to grow big! Like all animals, corals need food. They gather part of that by waving their tiny tentacles in the water to catch microscopic plankton to eat. They get the rest of their nutrients using another interesting method. Corals have a symbiotic relationship with certain kinds of microscopic algae called zooxanthellae which they let live inside their tissues. Zooxanthellae can photosynthesis, meaning they make food or energy out of sunlight like plants do. They share this energy with the corals in exchange for a safe place to live. Corals get their vibrant colors from the zooxanthellae living inside them. 
NPS Photo/S. Wolfe What is a Reef?As corals grow they build large underwater structures called reefs. Reefs are home to many marine species including fish, turtles, crabs, and octopus. There are three main types of reefs– fringing reefs, barrier reefs, and atolls– each defined by how they form and their distance from land.Reefs are important for several reasons. They provide homes for the fish we eat and other sea creatures and they create jobs for scientists, fishermen and others who rely on the ocean. Reefs also protect coastlines by slowing destructive waves from large swells and storms, reducing erosion and flooding. When a wave rushes to shore it hits the edge of the reef, called the reef crest, which absorbs about 97% of that wave energy and spreads it out so it is no longer a problem by the time it reaches the shore.
A fringing reef forms near the shore and grows outward. This is the most common type of reef and is the one most beachgoers are familiar with.
A barrier reef also grows around a shoreline, but they sit farther from land and are usually separated from the coast by deep waters called a lagoon.
An atoll, forms through a process called subsidence. This happens when a volcanic island stops growing and slowly starts to sink over millions of years.
While the island sinks, the fringe reef around it does not, instead it continues to grow upward and outward. Eventually, when the island is completely gone, all that remains is a ring of reef around where it used to be. This ring of reef is called an atoll. 
NPS Photo/S. Wolfe Our Unique ParkThe National Park of American Samoa is home to over 250 different species of coral. Three of these are currently listed as endangered, and another thirty-eight are classified as vulnerable or endangered under the Endangered Species Act (ESA). This means that these species are protected due to how few there are worldwide.The Park boundaries contain three separate reef systems across the islands of Tutuila, Ofu, and Ta’u. In total we manage and protect 27 km of coastline. From the shore, our boundaries extend another 0.4 km, giving us a total of 10 km² (2550 acres) of marine area. With shallow, mid-marine and deep marine habitats, the Park is home to an impressive diversity of corals, fish, algae, turtles, crustaceans, mollusks and other invertebrates. We also have some of the largest known corals in the world off the coast of Ta’u. The biggest one documented so far is a species of Porites measuring 8 meters tall and 22.4 meters in diameter. This coral colony has been estimated to be between 420-652 years old, meaning it has been growing at least a century to several centuries longer than Samoa has had a written history. The Park also contains an area along the south coast of Ofu known as the Ofu pools. This area is unique because of the extreme fluctuation in water temperature, regularly rising above 31°C (88°F) at low tide when the pools are cut off from the ocean, then dropping back down as the tides come back in. The corals living in these back water pools have demonstrated remarkable resilience to the temperature shifts and an unusually high heat tolerance compared to corals found elsewhere in the world. Scientists study the corals here to better understand coral resilience and the effects of rising water temperatures. 
NPS Photo/M. Morikawa Challenges Our Coral Reefs FaceCorals are extremely sensitive creatures.They prefer warm tropical waters between 73°and 84°F (23°-29°C), plenty of sunlight for their zooxanthellae, and clean, clear water. Any changes in these conditions can stress them leading to a phenomenon called “bleaching”. This is when the stressed corals expel their zooxanthellae which turns them white. Without their zooxanthellae, corals lose their main food source and unless they take them back in, will starve to death. What can you do to help protect our coral reefs? See more at: https://www.nps.gov/subjects/oceans/living-blue.htm
Some of the major stressors that impact our corals are man-made problems like marine debris. Our trash can get tangled in the corals, smashing and breaking them.
Fertilizers and pollutants that run off the land when it rains can trigger algal blooms that block the sunlight needed by the zooxanthellae.
These increases of nutrients can also encourage the growth of Crown of Thorn Starfish, or Alamea. These starfish eat coral tissues and are a natural part of the marine ecosystem, but when there are unusually high levels of nutrients in the water, their population can increase by thousands. Outbreaks like these can devastate our reefs with recovery taking up to 50 years. There have been several outbreaks over the years; Tutuila lost up to 90% of its coral in the late 1970s and in the early 1980s due to one. The most recent outbreak occurred between 2011 and 2015. In an effort to protect our reefs the National Park killed over 26,000 starfish at that time.
Corals also face environmental stressors. Global warming makes the water hotter than the corals can tolerate, leading to bleaching. Extreme low tides leave corals exposed above the water line, killing them. Storms can break corals and stir up sediment, which can smother polyps and block sunlight. There has also been a rise in coral diseases which make the corals sick.
One particularly serious threat is Stony Coral Tissue Loss Disease (SCTLD). This fast spreading disease can kill 70-80% of all the domed corals (species like Porites) in an area in a matter of weeks.
It is spread by cruise ships that have passed through infected waters or taken in ballast water from infected areas. Fortunately, SCTLD has not reached American Samoa yet, but because it is aggressive and spreads rapidly, it is likely only a matter of time before it arrives. If caught early, SCTLD can be treated with antibiotics. Scientists and Research in the ParkCoral Reef Studies and Products - National Park of American Samoa (U.S. National Park Service) |
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Last updated: May 6, 2026