Coral reefs: How they work, why they're in danger, and why it matters
Coral reefs are some of the world's most diverse ecosystems— even more diverse than some tropical rainforests. They offer habitat and protection for many underwater organisms and, from a purely visual standpoint, are beautiful to behold. These reefs are made up of the skeletons of coral polyps and are found all over the world. Most coral live in warm, tropical seas, but some can survive way down at the bottom of the ocean where it is dark and cold. About 25% of marine life rely on coral reefs for food, shelter, and breeding. People around the world also rely on coral reefs for food, medicine, and environmental protection like preventing coastal erosion.
The biggest coral reef is the Great Barrier Reef, which is located off the coast of Australia and is over 1,500 miles long. Unfortunately, the Great Barrier Reef is struggling with the impacts of climate change. In fact, the UNESCO World Heritage status of the Great Barrier Reef is being reconsidered because a large portion of the reef has been degraded. In a report to UNESCO, the Queensland government referred to the reef as "an icon under pressure with a deteriorating long-term outlook."
This description of the Great Barrier Reef has become apt for coral reefs all around the world, which are being threatened by overfishing, pollution, storms, diseases, predators, and rising sea temperatures due to climate change. The Earth has already lost 27% of the world's live coral, and conditions are not improving. Using data from a range of sources, including the National Oceanic and Atmospheric Administration (NOAA), the International Union for Conservation of Nature (IUCN), Conservation International, the Coral Reef Alliance, and others, Stacker compiled a list of things readers should know about coral reefs. This article reveals how coral reefs work, why they're in danger, and why it matters.
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Coral reefs are made of skeletons
Corals comprise of the skeletons of generations of hard corals. When an individual coral polyp dies, it leaves its skeleton behind, and the next generation grows on that skeleton. Then fish, such as parrotfish and sea urchins, along with other organisms break these skeletons down into smaller pieces, which end up in the gaps of the reef.
[Pictured: Macro image of coral polyps.]
Coral reefs grow by latching onto rocks
Reefs are created and grow when coral larvae latch onto submerged rocks and other hard surfaces in the ocean. The coral polyps then secrete skeletons, which are made from calcium carbonate. These skeletons protect coral from predators and also provide a surface to which the coral polyps attach themselves, thus growing the reef.
[Pictured: A bommie reef in the Egyptian Red Sea.]
Corals (generally) like it warm
Warm water is best for coral reef growth, and most thrive between 70 and 85 degrees Fahrenheit. They prefer to be close to the surface where sunlight can come through. There are corals in the ocean that are as deep as 300 feet, but in general, reef-building corals do not grow well below 60-90 feet.
[Pictured: Reef-building coral colonies grow on the edge of a low-lying island just off of Raiatea and Tahaa in French Polynesia.]
Today’s coral reefs are pretty young
While the geological record shows that the predecessors of today's coral reefs were formed at least 240 million years ago, most of the coral reefs we see today are between 5,000 and 10,000 years old. And size doesn't necessarily indicate how old a reef is.
[Pictured: A Ph.D. student conducts research on the coral reefs of the Gulf of Eilat at the Interuniversity Institute for Marine Sciences.]
Coral reefs come in different shapes and sizes
There are many kinds of coral reef formations, which scientists generally split into four groups: atolls, barrier reefs, fringing reefs, and patch reefs. Atolls are rings of coral that create lagoons, and they are usually located out in the sea. Barrier reefs grow near the coastline and are separated from the shore by wide lagoons. Fringing reefs also grow near the coasts but are separated from the shore by narrow lagoons, and patch reefs grow in isolated "patches" and are small, usually growing up from the bottom of an island platform.
[Pictured: Aerial views of The Great Barrier Reef in Cairns, Australia captured on Aug. 7, 2009.]
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Coral reefs are very biodiverse
Coral reefs are thought by many scientists to have the highest level of biodiversity of any ecosystem on Earth, including tropical rainforests. Although they only occupy less than 1% of the ocean, coral reefs are home to over a quarter of all marine life.
[Pictured: A sea turtle swims in the Ras Mohammed protection area near Sharm el-Sheikh in Egypt.]
Coral reefs cultivate symbiotic relationships
Many reef-building corals contain photosynthetic algae called zooxanthellae in their tissue. The corals and the algae have a symbiotic relationship, meaning that the coral provides the algae with the compounds needed for photosynthesis along with a protected environment. In exchange, the algae produce oxygen and help the coral dispose of its waste. This mutualistic relationship recycles nutrients in nutrient-deficient tropical waters.
[Pictured: Bright sunlight shines on a fragile colony of staghorn coral.]
Coral reefs provide food
The fish that depend on coral reefs provide food for over a billion people around the world. They are vital to the world's fisheries and form the nurseries for approximately a quarter of the fish in the ocean. If properly managed, reefs can provide up to 15 tons of fish and seafood per 247 acres each year. The NOAA National Marine Fisheries Service estimates that in the United States, the annual commercial value of coral reefs to national fisheries is over $100 million.
[Pictured: A Rusty Parrotfish (Scarus ferrugineus) feeds on algae.]
Coral reefs help make medicine
Some species that live in coral ecosystems produce chemical compounds useful in pharmaceuticals. Some of these medicines are being developed to treat cancer, arthritis, asthma, bacterial infections, heart, and other diseases. This potential has led coral ecosystems to be dubbed by some as the "medicine cabinets of the 21st century."
[Pictured: A coral sample in a test tube is pictured at the National Sequencing Center (Genoscope) in Evry, France.]
Coral reefs create and protect land
Long after volcanic islands sink back into the sea, the atoll islands created by coral growth and other reef-associated organisms can remain above the ocean's surface. In addition, coral reefs can protect adjacent land from storms and tsunamis and prevent coastline erosion.
[Pictured: An aerial view of Lady Elliot Island, Australia photographed in 2012.]
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