Coral reef facts for kids
A coral reef is a type of biotic reef developing in tropical waters. Although corals are major contributors to the overall framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault by ocean waves are algae, especially, although not entirely, species of red algae.
Water temperature of 20–28 °C (68–82 °F) is an optimal range for proper growth and health of coral reefs. Coral reefs are found in all oceans of the world, except the Arctic Ocean, generally between the Tropic of Cancer and the Tropic of Capricorn, because reef-building corals live in these waters.
Such reefs take a variety of forms, defined as follows:
- Apron reef — short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
- Fringing reef — reef extending directly out from a shoreline, and more or less following the trend of the shore.
- Barrier reef — reef separated from a mainland or island shore by a lagoon; see Great Barrier Reef.
- Patch reef — an isolated, often circular reef, usually within a lagoon or embayment.
- Ribbon reef — long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
- Table reef — isolated reef, approaching an atoll type, but without a lagoon.
- Atoll reef — a more or less circular or continuous barrier reef surrounding a lagoon without a central island; see atoll.
Contents
Formation
Most present-day coral reefs were formed after the last ice age when melting ice caused the sea level to rise and flood the continental shelves. This means they are less than 10,000 years old. As coral reef communities were established on the shelves, they built reefs that grew upwards, keeping pace with the rise in sea level. Reefs that did not keep pace became drowned reefs, covered by so much water that there was insufficient light for further survival.
Coral reefs are also found in the deep sea away from the continental shelves, around oceanic islands and as atolls. The vast majority of these ocean coral islands are volcanic in origin. The few exceptions have tectonic origins where plate movements have lifted the deep ocean floor on the surface.
Material
The building block of coral reefs are the skeletons of generations of hard corals, composed of calcium carbonate. As each polyp dies, it leaves behind its skeletal structure, upon which the next generation of polyps grow, enlarging the reef. Grazing fish (such as parrotfish), sea urchins, sponges and other organisms break down the coral skeletons into fine fragments, which settle into spaces in the reef structure.
Biodiversity
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Coral reefs form some of the world's most productive ecosystems, providing complex and varied marine habitats that support a wide range of organisms. Fringing reefs just below low tide level have a mutually beneficial relationship with mangrove forests at high tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and sea grass protect the coral from large influxes of silt, fresh water and pollutants. This level of variety in the environment benefits many coral reef animals, which, for example, may feed in the sea grass and use the reefs for protection or breeding.
Reefs are home to a variety of animals, including fish, seabirds, sponges, cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, cleaner shrimp, spiny lobsters and crabs), mollusks (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins the main exception. A few species feed directly on corals, while others graze on algae on the reef. Reef biomass is positively related to species diversity.
The same hideouts in a reef may be regularly inhabited by different species at different times of day. Nighttime predators such as cardinalfish and squirrelfish hide during the day, while damselfish, surgeonfish, triggerfish, wrasses and parrotfish hide from eels and sharks.
The great number and diversity of hiding places in coral reefs, i.e. refuges, are the most important factor causing the great diversity and high biomass of the organisms in coral reefs.
Coral reefs also have a very high degree of microorganism diversity compared to other environments.
Threats
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From 2009 to 2018, coral reefs worldwide declined 14%. Human activities that threaten coral include coral mining, bottom trawling, and the digging of canals and accesses into islands and bays, all of which can damage marine ecosystems if not done sustainably. Other localized threats include blast fishing, overfishing, coral overmining, and marine pollution. Chemicals in sunscreens may awaken latent viral infections in zooxanthellae and impact reproduction.
Greenhouse gas emissions present a broader threat through sea temperature rise and sea level rise, resulting in widespread coral bleaching and loss of coral cover. Climate change causes more frequent and more severe storms, also changes ocean circulation patterns, which can destroy coral reefs.Ocean acidification also affects corals.
Conservation
Coral reefs are fragile ecosystems. Because corals need warm, sunlit water to live, they often grow close to the top of the water. Being so close to land makes them often be damaged by poisons and dirt that can come from boats and the land nearby. Dirt makes the water more cloudy, which makes the sunlight less. Poisons can bleach and kill corals. Also, they are hard for ships to see, but easy to hit, which makes ships often run into the coral, damaging both the boats and coral. Because of this, many countries are trying to lessen the kinds of building that usually happen near beaches that have coral reefs nearby, and be more careful about the boats that go around reefs.
Designating a reef as a biosphere reserve, marine park, national monument or world heritage site can offer protections.
In Australia, the Great Barrier Reef is protected by the Great Barrier Reef Marine Park Authority, and is the subject of much legislation.
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Brain coral
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Staghorn coral
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Spiral wire coral
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Pillar coral
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Mushroom coral
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Maze coral
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Black coral
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Fluorescent coral.
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Significant coral reefs
- The Great Barrier Reef—largest, comprising over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometers (1,600 mi) off Queensland, Australia
- The Mesoamerican Barrier Reef System—second largest, stretching 1,000 kilometers (620 mi) from Isla Contoy at the tip of the Yucatán Peninsula down to the Bay Islands of Honduras
- The New Caledonia Barrier Reef—second longest double barrier reef, covering 1,500 kilometers (930 mi)
- The Andros, Bahamas Barrier Reef—third largest, following the east coast of Andros Island, Bahamas, between Andros and Nassau
- The Red Sea—includes 6,000-year-old fringing reefs located along a 2,000 km (1,240 mi) coastline
- The Florida Reef Tract—largest continental US reef and the third-largest coral barrier reef, extends from Soldier Key, located in Biscayne Bay, to the Dry Tortugas in the Gulf of Mexico
- Blake Plateau has the world's largest known deep-water coral reef, comprising a 6.4 million acre reef that stretches from Miami to Charleston, S. C. Its discovery was announced in January 2024.
- Pulley Ridge—deepest photosynthetic coral reef, Florida
- Numerous reefs around the Maldives
- The Philippines coral reef area, the second-largest in Southeast Asia, is estimated at 26,000 square kilometres. 915 reef fish species and more than 400 scleractinian coral species, 12 of which are endemic are found there.
- The Raja Ampat Islands in Indonesia's Southwest Papua province offer the highest known marine diversity.
- Bermuda is known for its northernmost coral reef system, located at 32°24′N 64°48′W / 32.4°N 64.8°W. The presence of coral reefs at this high latitude is due to the proximity of the Gulf Stream. Bermuda coral species represent a subset of those found in the greater Caribbean.
- The world's northernmost individual coral reef is located in the Finlayson Channel, in the inside passage of British Columbia, Canada.
- The world's southernmost coral reef is at Lord Howe Island, in the Pacific Ocean off the east coast of Australia.
Images for kids
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Fringing reef
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Barrier reef
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Platform reef
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The three major zones of a coral reef: the fore reef, reef crest, and the back reef
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Water in the reef surface zone is often agitated. This diagram represents a reef on a continental shelf. The water waves at the left travel over the off-reef floor until they encounter the reef slope or fore reef. Then the waves pass over the shallow reef crest. When a wave enters shallow water it shoals, that is, it slows down and the wave height increases.
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Locations of coral reefs
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Boundary for 20 °C isotherms. Most corals live within this boundary. Note the cooler waters caused by upwelling on the southwest coast of Africa and off the coast of Peru.
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This map shows areas of upwelling in red. Coral reefs are not found in coastal areas where colder and nutrient-rich upwellings occur.
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Zooxanthellae, the microscopic algae that lives inside coral, gives it colour and provides it with food through photosynthesis
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Close up of polyps arrayed on a coral, waving their tentacles. There can be thousands of polyps on a single coral branch.
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Corals are animals and not plants. They can appear like plants because they are sessile and take root on the ocean floor. But unlike plants, corals do not make their own food.
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Table coral -
Corraline algae Lithothamnion sp.
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Deep-water cloud sponge
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Eastern oysters (Crassostrea virginica)
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Fluorescent coral -
The colour of corals depends on the combination of brown shades provided by their zooxanthellae and pigmented proteins (reds, blues, greens, etc.) produced by the corals themselves.
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Coral polyps -
Most coral polyps are nocturnal feeders. Here, in the dark, polyps have extended their tentacles to feed on zooplankton.
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A diversity of corals -
Coral fragments growing on nontoxic concrete
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Deep sea corals at the Wagner Seamount. These corals are well adapted to deep water conditions where substrates are plentiful.
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Coral in preparation of being relocated
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Students from Nā Pua No‘eau remove invasive algae from Kāne‘ohe Bay. Programs could be created to remove algae from Caribbean reefs
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Ancient coral reefs
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Darwin's theory starts with a volcanic island which becomes extinct
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As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef.
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As the subsidence continues, the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside.
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Ultimately, the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon.
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Coralline algae
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Elkhorn coral
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Caribbean reef squid
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Soft coral, cup coral, sponges and ascidians
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The shell of Latiaxis wormaldi, a coral snail
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See also
In Spanish: Arrecife de coral para niños
