Coral reef facts for kids

Some of the biodiversity of the Great Barrier Reef, off Queensland, Australia

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.

The Importance of Coral Reefs
Conservation
Biology
- Deep water reefs
Formation
Images for kids
See also

The Importance of Coral Reefs

Coral reefs provide a natural habitat and massive protection for different species of fish. They are not plants; they are living organisms that provide an environment for fishes to breed. Without them, fishes in the ocean are homeless.

According to professor Christie’s lecture, corals are colonial organisms that need to be exposed to sun, in order for them to grow. The corals also influence the amount of carbon dioxide in the ocean. This is a major process that acts as a 'carbon sink' for excess carbon dioxide in the atmosphere, as well as maintaining an environment of exceptionally high biodiversity despite relatively poor nutrient availability.

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.

Brain coral
Staghorn coral
Spiral wire coral
Pillar coral
Mushroom coral
Maze coral
Black coral
Fluorescent coral.

Biology

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.

Coral reefs are among the most diverse and productive biological communities on Earth. The reef structure provides habitat and shelter to a wide variety of marine plants and animals - it is estimated that a quarter of marine life lives in or around coral reefs. Additionally, corals act as water filters, increasing water quality in the vicinity of the reef.

Offshore, the reef structure acts as a breakwater, protecting coastlines from ocean waves. The wide variety of flora and fauna has turned some coral reefs and the islands and coastal areas near them into popular tourist attractions.

Deep water reefs

In December 2004, United States Geological Survey (USGS) researchers announced the confirmation of the discovery of the deepest coral reef ever found in the United States. The reef is in the Pulley Ridge area, a north-south-trending drowned barrier island, more than 60 miles (100 km) long, approximately 40 miles (70 km) west of Dry Tortugas National Park. It is up to three miles wide and about 20 miles long (5 km wide and 30 km long), and located at a depth that ranges from 200 to 250 feet (60 to 80 m). Unlike most coral reefs, which tend to grow vertically, Pulley Ridge coral grows flat, an adaptation to the limited penetration of light at that depth to increase surface area exposed to sunlight.

Similar deep reefs occur in other parts of the world, e.g. the Mingulay reef complex and the Darwin Mounds located off the west of Scotland in about 150 metres of water.

It is estimated that about 60% of the world’s reefs are at risk due to destructive, human-related activities. The threat to the health of reefs is particularly strong in Southeast Asia, where an enormous 80% of reefs are considered endangered. Various efforts are being made to grow coral reefs and coral, a difficult task; they are fragile, react to small changes in their biological environment.

Coral reefs are being grown using a process called "mineral accretion". Applying a low voltage to a metallic structure causes limestone to build on the metal, upon which baby coral can attach, settle and feed. This greatly speeds the coral reef growth process. A coalition of scientists known as the Global Coral Reef Alliance, GCRA, has been growing coral reefs, as well as teaching the techniques. Numerous photos of the results are available at their website.

Formation

A fringing reef can take ten thousand years to form, and an atoll can take up to 30 million years

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.

Images for kids

Fringing reef
Fringing reef at Eilat at the southern tip of Israel
Barrier reef
Platform reef
A small atoll in the Maldives
Inhabited cay in the Maldives
The three major zones of a coral reef: the fore reef, reef crest, and the back reef
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.
Locations of coral reefs
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.
This map shows areas of upwelling in red. Coral reefs are not found in coastal areas where colder and nutrient-rich upwellings occur.
Diagram of a coral polyp anatomy
Zooxanthellae, the microscopic algae that lives inside coral, gives it colour and provides it with food through photosynthesis
Close up of polyps arrayed on a coral, waving their tentacles. There can be thousands of polyps on a single coral branch.
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.
Table coral
Corraline algae Lithothamnion sp.
Deep-water cloud sponge
Eastern oysters (Crassostrea virginica)
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.
Coral polyps
Most coral polyps are nocturnal feeders. Here, in the dark, polyps have extended their tentacles to feed on zooplankton.
Island with fringing reef off Yap, Micronesia
A major coral bleaching event took place on this part of the Great Barrier Reef in Australia
A diversity of corals
Coral fragments growing on nontoxic concrete
Deep sea corals at the Wagner Seamount. These corals are well adapted to deep water conditions where substrates are plentiful.
Coral in preparation of being relocated
Students from Nā Pua No‘eau remove invasive algae from Kāne‘ohe Bay. Programs could be created to remove algae from Caribbean reefs
Ancient coral reefs
Darwin's theory starts with a volcanic island which becomes extinct
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.
As the subsidence continues, the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside.
Ultimately, the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon.
Coralline algae
Elkhorn coral
Schooling reef fish
Caribbean reef squid
Banded coral shrimp
Whitetip reef shark
Green turtle
Giant clam
Soft coral, cup coral, sponges and ascidians
Banded sea krait
The shell of Latiaxis wormaldi, a coral snail