Coral reefs are massive structures made of limestone deposited by living things. Although thousands of species inhabit coral reefs, only a fraction produce the limestone that builds the reef. The most important reef-building organisms are the corals.
Coral reefs support approximately 25 percent of all known marine species. As one of the most complex ecosystems on the planet, coral reefs are home to more than 4,000 species of fish, 700 species of coral, and thousands of other plants and animals.
Imagine a coral reef like a bustling city: the buildings are made of coral and thousands of inhabitants live in, on, or near the buildings. In this sense, a coral reef is like a metropolis under the sea.
Although coral is often mistaken for a rock or a plant, it is actually composed of tiny, fragile animals called coral polyps. When people say coral, they are referring to these little animals and the skeletons they leave behind after they die.
Although there are hundreds of different species of corals, they are generally classified as either hard coral or soft coral.
Hard corals grow in colonies and are the architects of coral reefs. Including such species as brain coral and elkhorn coral, hard coral create skeletons out of calcium carbonate (also known as limestone), a hard substance that eventually becomes rock. Hard corals are hermatypes, or reef-building corals, and need tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee) to survive. Generally, when we talk about coral, we are referring to hard corals.
Soft corals, such as sea fingers and sea whips, are soft and bendable and often resemble plants or trees. These corals do not have stony skeletons, but instead grow wood-like cores for support and fleshy rinds for protection. They are referred to as ahermatypes, or non–reef building corals, and they do not always have zooxanthellae. Soft corals are found in both tropical seas and in cool, dark regions.
A coral polyp is an invertebrate (spineless animal; they are cousins to anemones and jellyfish) that ranges in size from tiny (no bigger than a pinhead) up to a foot in diameter. One coral branch or mound is covered by thousands of coral polyps, which when grouped together, is called a coral colony. Thus, each coral branch or mound is one colony of coral polyps. A polyp has a saclike body and an opening, or mouth, encircled by stinging tentacles called cnidae. The polyp uses calcium carbonate (limestone) from seawater to build itself a hard, cup-shaped skeleton. This limestone skeleton protects the soft, delicate body of the polyp. Coral polyps are usually nocturnal, meaning that they stay inside their skeletons during the day. At night, polyps extend their tentacles to feed.
Coral reefs are found in more than 100 countries around the world. Most reefs are located between the Tropics of Cancer and Capricorn, in the Pacific Ocean, the Indian Ocean, the Caribbean Sea, the Red Sea, and the Persian Gulf. Corals are also found farther from the equator in places where warm currents flow out of the tropics, such as in Florida and southern Japan. Worldwide, coral reefs cover an estimated 110,000 square miles (284,300 square kilometers).
Coral reefs grow best in warm water (70–85° F or 21–29° C). It is possible for soft corals to grow in places with warmer or colder water, but growth rates in these types of conditions are very slow. Corals prefer clear and shallow water, where lots of sunlight filters through to their symbiotic algae. It is possible to find corals at depths of up to 300 feet (91 meters), but reef-building corals grow poorly below 60–90 feet (18–27 meters). Corals need salt water to survive, so they also grow poorly near river openings or coastal areas with excessive run-off.
The geological record indicates that ancestors of modern coral reef ecosystems were formed at least 240 million years ago. Most established coral reefs are between 5,000 and 10,000 years old. Although size sometimes indicates the age of a coral reef, this is not always true. Different species of coral grow at different rates depending on water temperature, oxygen level, amount of turbulence, and availability of food.
Coral reefs are complex, multistory structures with holes and crevices shared by various creatures. If a coral reef is likened to a bustling city, then a coral colony is like a single apartment building with many rooms and hallways that house different marine species. Not all coral species build reefs. The actual architects of coral reefs are hard or stony corals, which are referred to as hermatypic, or reef-building corals. As the polyps of stony corals grow, they produce limestone skeletons. When they die, their skeletons are left behind and used as foundations for new polyps, which build new skeletons over the old ones. An actual coral branch, or mound, is composed of layer upon layer of skeletons covered by a thin layer of living polyps.
Other types of animals and plants also contribute to the structure of coral reefs. Many types of algae, seaweed, sponge, sediment, and even mollusks like giant clams and oysters add to the architecture of coral reefs. When these organisms die, they also serve as foundations for new corals.
Corals grow at different rates depending on water temperature, salinity, turbulence, and the availability of food. The massive corals are the slowest growing species, adding between 5 and 25 millimeters (0.2–1 inch) per year to their length. Branching and Staghorn corals can grow much faster, adding as much as 20 centimeters (8 inches) to their branches each year.
The variety of coral shapes and sizes largely depends on the species. Some corals form hard and pointed shapes, while others form soft and rounded shapes. The shape of coral colonies also depends on the location of the coral. For example, in areas with strong waves corals tend to grow into robust mounds or flattened shapes. In more sheltered areas, the same species may grow into more intricate shapes with delicate branching patterns.
Coral polyps eat in two different ways according to the species. Many coral polyps are nourished by a tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee). The algae live within the coral polyps, using sunlight to make sugar for energy just like plants. Zooxanthellae process the waste of the polyp to retain important nutrients and in turn provide the polyp with oxygen. Meanwhile, the coral polyps provide the algae with carbon dioxide and a safe, protected home. Zooxanthellae living within the tissue of hard corals can supply them with up to 98 percent of their nutritional needs.
Another way that corals eat is by catching tiny floating animals known as zooplankton. At night , coral polyps come out of their skeletons to feed, making the reef look like a wall of hungry mouths. The polyps stretch out their long, stinging tentacles to capture the zooplankton that are floating by. The captured plankton are then put into the polyps' mouths and digested in their stomachs.
Most coral polyps have clear bodies and their skeletons are white, like human bones. Generally, their brilliant color comes from the zooxanthellae living inside their tissues. Several million zooxanthellae live and produce pigments in just one square inch of coral. These pigments are visible through the clear body of the polyp and are what gives coral its beautiful color.
Coral reproductive methods vary according to the species. Some species, such as Brain and Star corals, are hermaphrodites, meaning they produce both sperm and eggs at the same time. Other corals, such as Elkhorn and Boulder corals, are gonochoric, meaning that they produce single-sex colonies. In these species, all of the polyps in one colony produce only sperm, and all of the polyps in another colony produce only eggs.
Coral larvae are formed in two different ways. The larvae are either (1) fertilized within the body of a polyp or (2) fertilized outside of the polyp's body in the water. Fertilization of an egg within the body of a coral polyp is achieved from sperm that is released through the mouth of another polyp. The sperm and egg merge and form a planula larva, which matures inside the body of its mother. When the larva is ready, it gets spit out into the water through the mouth of its mother. O
Other species of coral reproduce by ejecting large quantities of eggs and sperm into the surrounding water. When this happens, the eggs and sperm fertilize in the water. This process is called coral spawning. In some areas, mass coral spawning events occur on one particular night per year and scientists can predict exactly when this will happen. Trillions of eggs and sperm are simultaneously released into the water in one of the most astounding acts of synchronicity in the natural world!
Once in the sea, larvae are naturally attracted to the light. They swim to the surface of the ocean, where they remain for days or even weeks. If predators do not eat the larvae during this time, they fall back to the ocean floor and attach themselves to a hard surface. An attached planula metamorphasizes into a coral polyp and begins to grow—dividing itself in half and making exact genetic copies of itself. As more and more polyps are added, a coral colony develops. Eventually the coral colony becomes mature, begins reproducing, and the cycle of life continues.
Sunlight: Corals need to grow in shallow water where sunlight can reach them. Corals depend on the zooxanthellae (algae) that grow inside of them for oxygen and other things, and since this algae needs sunlight to survive, corals also need sunlight to survive. Corals rarely develop in water deeper than 165 feet (50 meters).
Clear water: Corals need clear water that lets sunlight through to survive; they don't thrive well when the water is opaque. Sediment and plankton can cloud water, which decreases the amount of sunlight that reaches the zooxanthellae.
Warm water temperature: Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand various temperature fluctuations. However, corals generally live in water temperatures of 68–90° F or 20–32° C.
Clean water: Corals are sensitive to pollution and sediments. Sediments can settle on coral, blocking out sunlight and smothering coral polyps. Pollution from sewage and fertilizers increase nutrient levels in the water, harming corals. When there are too many nutrients in the water, the ecological balance of the coral community is altered.
Saltwater: Corals need saltwater to survive and require a certain balance in the ratio of salt to water. This is why corals don't live in areas where rivers drain fresh water into the ocean.
Scientists generally divide coral reefs into four classes: fringing reefs, barrier reefs, atolls, and patch reefs.
Fringing reefs grow near the coastline around islands and continents. They are separated from the shore by narrow, shallow lagoons. Fringing reefs are the most common type of reef that we see.
Barrier reefs also parallel the coastline but are separated by deeper, wider lagoons. At their shallowest point they can reach the water's surface forming a "barrier" to navigation. The Great Barrier Reef in Australia is the largest and most famous barrier reef in the world.
Atolls are rings of coral that create protected lagoons and are usually located in the middle of the sea. Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (these islands are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside.
Patch reefs are small, isolated reefs that grow up from the open bottom of the island platform or continental shelf. They usually occur between fringing reefs and barrier reefs. They vary greatly in size, and they rarely reach the surface of the water.