Easy Answer:

Coral Reefs are one of the world's oldest ecosystems and in and around the reefs are more kinds of life than at any other place in the oceans. Coral reefs are normally found in the warm, shallow seas of the tropics. The reefs themselves are actually made of the hard bony skeletons of coral polyps. As coral polyps die, they leave behind their skeletons for new corals to grow on top of. Over a long period of time, this pile of skeletons forms underwater walls, platforms, and ridges that other animals and plants attach to, forming a kind of underwater city. Oftentimes, coral reefs are surrounded by nothing but enormous areas of sand or sea grass and so all underwater life becomes centered around the reef as a source of food, shelter, and protection.

Most coral reefs are made up of different "colonies" that are recognized by the specific type of coral polyp living there. You can picture this by imagining a lot of different neighborhoods thrown together side-by-side and even on top of one another. All the houses in one neighborhood look exactly alike, but are very different looking from the houses in the other neighborhoods. One coral colony might look like an elk's horn, while other colonies might resemble plants or even brains.

There are many different types of coral. Corals come in every color of the rainbow and almost every size and shape you can imagine. Although corals can look very different from one another they do have one thing in common. All corals are made by tiny sea animals called coral polyps. Don't be confused by this, because even though a lot of corals resemble plants, they are actually tiny animals. The coral polyps take calcium (the same substance your bones are made of) from the sea water and mix it with other minerals to build limestone "houses" around their bodies which are very soft and need protection. The polyp is rooted, or attached, at the base of its house and can only move in and out by contracting its body. Outside its protective house, each tiny polyp looks like a strange and mushy flower with soft, waving arms and a mouth in the center. Coral polyps are related to jellyfish and their arms are called tentacles. Like their relatives, they have stinging cells all along their tentacles for catching food. Some of these stinging cells are so powerful that they can hurt, and even kill, large fish and humans.

Of course, coral polyps are so tiny that you almost need a microscope to see each individual animal. Thousands and even millions of polyps live together to form colonies. It can take 50 years or more for a coral colony to grow about three feet. When a coral polyp dies, its limestone house is left behind and these skeletons are what will eventually form a reef.

Our sources of information for this answer are:

Collard, Sneed B. and
James M. Needham (Illustrator) Our Wet World: Exploring Earth's Aquatic Ecosystems . Charlesbridge Publishing: Watertown, MA (1998).

Davis, Gary W. Habitats: Coral Reef: Children's Press: New York (1997).





















Advanced Answer:

Coral reefs have been around for more than 200 million years and are among the oldest ecosystems of our planet. They host an incredible amount and variety of plant and animal life and, indeed, reefs and tropical rain forests are the two most biologically diverse ecosystems on the planet. According to scientists, the number of species in the tropical rain forest just slightly outnumbers those of the coral reef. Unfortunately, the two ecosystems share another commonality; they are both among Earth's most threatened and rapidly disappearing ecosystems.

Coral reefs contain two types of corals: hard corals and soft corals. The individual polyps of both types are tiny, cylindrical animals with a very simple body plan. They have a central mouth that is used for both the consumption of food and the elimination of waste. The mouth is surrounded radially by tentacles armed with harpoon-like nematocysts (stinging cells) that sting and impale plankton and other small animals.

There are also many different types of algae on each and every coral reef. In fact, there are actually more algae than coral on a reef so a more proper name for this structure would be a "coral-algal reef." Lacking the many algae-grazing fish and invertebrates that are so common to a coral reef, the soft algae would undoubtedly overgrow the reef. Hard algae are also important in the reef building process. They aid in reef construction by producing calcium carbonate plates that fill in holes or gaps in the reef and help consolidate it. Some species of red algae cement the reef pieces together.

Hard corals -- brain, boulder, elkhorn, pillar, staghorn, and star, to name but a few -- are the main reef builders. Most of these tiny polyps are smaller than a fingernail, but with the help of microscopic algae growing in their tissue, they build rock-hard reefs. The symbiotic (mutually dependent) coral-algae relationship is complex and bizarre. The coral polyps supply the algae in their tissue, called zooxanthellae, with carbon dioxide. The zooxanthellae uses sunlight to convert the carbon dioxide and water into oxygen and carbohydrates. Sound familiar? It should. This is the exact same process by which green land plants turn carbon dioxide and water into oxygen and food -- photosynthesis. The coral polyps then absorb the carbohydrates which allow them to secrete limestone skeletons through a process called calcification.

What happens next is debatable. Some scientists suggest that corals get all the food they need from the zooxanthellae algae that live in their tissue. Other scientists argue that corals are mainly plankton feeders. Dr. Richard Chester, a specialist in systematics and tropical ecology, asserts in his book, Living Corals, that both arguments are right. He states that hard corals do feed on plankton, but not very often and not very efficiently. He goes on to say that soft corals, who are true filter feeders, are better adapted to eating plankton. Soft corals, in true filter feeder style, extend their food-gathering tentacles throughout both the day and the night. Hard corals, who have a food source in the algae-produced carbohydrates, extend their plankton gathering tentacles only at night when photosynthesis is not occurring in their tissue due to lack of sunlight.

Scientific studies have shown that reef-forming corals do not grow as quickly without zooxanthellae, although it should be stated that most corals are very slow growers to begin with. On the reef, some corals grow as rapidly as five to ten inches per year, but most grow only about an inch per year. A coral reef "grows" as hard corals move upward and outward. Most types of hard corals regularly "hoist themselves" up or out and secrete new limestone floors beneath themselves in the process. Reefs also expand when hard coral polyps die and other hard coral polyps grow over them.

If hard corals are the reef builders, then soft corals are definitely the reef "decorators." The hard corals have thin, colorless tissue in order to allow the sunlight to shine through and reach the zooxanthellae. In fact, it is the golden-brown algae living within the animals' tissue that give hard corals their bland and drab color. Some soft corals do not have zooxanthellae and instead of clear, colorless tissue, have evolved a dazzling array of diverse and vibrant colors that adorn the reef. Every color in nature can be found on the reef and, as a result, many would argue that the reef is the most beautiful and amazing ecosystem on the planet.

Of course, there are exceptions to every generality listed above. Organ-pipe coral, although a soft coral, has a hard red skeleton that helps build and form the reef. Hard corals in the Caribbean and elsewhere have dazzling colors because they lack zooxanthellae. However, for the most part soft corals are not reef builders and they tend to display brighter colors than reef-building hard corals.

Sources of info for this answer are:

Sammon, Rick Rhythm of the Reef: A Day in the Life of the Coral Reef. Voyageur Press: Stillwater, MN (1995).









Coral reefs vary immensely in structure, density, and area from one location to another. Some reefs are massive and cover many miles in diameter, while others may be no larger than a few yards across. Reefs may be elongated, following along a general shoreline, or they may start at the shoreline itself and work their way seaward to deeper waters. As a rule, the large, massive, spectacular reefs will nearly always be on the Eastern shore of an island, country, or continent. This is probably due to the fact that prevailing winds and currents nurture the reef building polyps by constantly renewing the food supply -- a condition that is less prevalent on the Western coasts. The tremendous difference in reefs just a few miles apart is amazing. One reef may consist of enormous brain or boulder coral with heads as large as ten to twenty feet in diameter, while another reef in the same area would be nothing but sea fans and a few other soft corals.

The largest reef ecosystem in the world is actually a continuous series of smaller reefs located off Australia's northeast coast and called the Great Barrier Reef. The world's second largest reef runs from the tip of Mexico's Yucatan Peninsula south to the Gulf of Honduras and is known as the Belize Barrier Reef.

Sources of info for this answer are:

Straughan, Robert Exploring the Reef. A.S. Barnes and Company: New York (1968).

Sammon, Rick Rhythm of the Reef: A Day in the Life of the Coral Reef. Voyageur Press: Stillwater, MN (1995).



























There are four major types of natural coral reefs in the world's oceans: fringing reefs, barrier reefs, atolls, and platform reefs. Each reef type has its own distinct structure. There are also many types of artificial reefs created by sunken ships and planes, piers and docks, oil rigs and, of course, the Reef Ball.

Fringing reefs border islands and continents a relatively short distance from the shoreline. They do not include a substantial lagoon area as the water between this type of reef and the shoreline tends to be rather shallow and narrowly elongated following the shoreline. They are found around many Caribbean islands and are probably the most common type of reef that we see. These coral reef communities are easily accessible to divers and snorkelers who don't want to travel far from shore. Put on your gear at the beach, take a five-minute swim, and you're on the reef. The fringing reef starts at a depth of about twenty feet and offers a wide variety of marine life up to about eighty feet.

As some coral reefs grow older and expand, the near-shore coral polyps can not compete for food with the outermost corals. After all, the outermost corals can grow farther and farther seaward, supported by the skeletons of their ancestors, whereas the near-shore corals are stuck between a reef and a dry place. As the corals grow seaward, a lagoon develops between the outer reef and the island. This type of reef formation is called a barrier reef. Because they are really a sort of extension of a fringe reef, barrier reefs also parallel the coastline but at a much greater distance and depth. However, at their shallowest points, the reefs can reach the water's surface creating a "barrier" to navigation. The largest barrier reef in the world is the Great Barrier Reef off the northeastern coast of Australia. Actually, the Great Barrier Reef is not a single continuous reef, but a series of smaller, closely set reefs that make up one enormous reef ecosystem. The Belize Barrier Reef, in the Caribbean, is the second-largest barrier reef in the world. This reef runs along the coast of southern Mexico from the tip of the Yucatan peninsula to the southern end of Belize. The Great Barrier Reef and the Belize Barrier Reef both offer a large number of endemic species of marine life (an endemic species is a species that is found nowhere else in the world).

Atolls are usually formed when a tropical island (or islands) surrounded by a fringing reef sinks into the ocean under its own weight. The sinking motion is due either to erosion of the island or shifts in the tectonic plates that make up the earth's crust beneath the island. When an island sinks, the resultant geography is a somewhat circular or oval-shaped chain of islands surrounding a lagoon. You see, the tallest points on the now-sunken island are still high enough to be above the water's surface. These tall points, which were often volcanic mountain peaks on the original island, now form the new chain of islands. As the fringing reefs continue to grow, they eventually join together forming a circular enclosure that holds the lagoon. Atolls, which are typically located in the middle of the sea, tend to support less marine life than barrier reefs do because the food supply is limited in the sheltered lagoon. However, land need not sink to form an atoll. A rise in sea level can produce the same effect. It is interesting to note here that only 21,000 years ago, a mere blink in geologic time, sea level was four hundred feet lower than it is now.

Platform reefs (also called patch reefs) and coral cays, although common in the Caribbean, Pacific, and Indian Oceans, are small and isolated and, therefore, tend to offer a lot less in terms of marine life diversity. Platform reefs begin to form on underwater mountains or other rock-hard outcrops between the shore and a barrier reef. When the living corals reach the water's surface, they grow outward and form a sort of shallow and platform-like reef flat. Coral cays begin to form when broken coral and sand wash onto these flats; cays can also form on shallow reefs around atolls. Over time, the coral skeletons and sand are cemented together and the beachrock breaks the surface to form the cay. The cays, which are only a few feet above sea level at best, attract sea birds, crabs, and other marine life .

In more recent times, artificial reefs have been developed. These reefs provide hiding places, shelter, and food and attract many corals, fish, and other marine plants and animals. Depending on what is being used as the artificial structure for the corals to grow on, some artificial reefs can become polluted as they deteriorate or react with elements found in the ocean water. The artificial Reef Ball is an environment-friendly cement aggregate structure that is totally safe from the threats of self-pollution and looks to be one of the most promising artificial reef structures of the future.

Source of information for this answer are:

Sammon, Rick Rhythm of the Reef: A Day in the Life of the Coral Reef. Voyageur Press: Stillwater, MN (1995).
































Most coral reefs are found in the relatively shallow and warm seas of a part of the world that we call the tropics. This is the area between the latitude lines known as the Tropic of Cancer (north of the equator) and the Tropic of Capricorn (south of the equator). Corals thrive in these latitudes because the water is warm and there is plenty of sunlight for the zooxanthellae algae (see "What is a coral reef?"). Corals like a water temperature between 78 and 80 degrees Farenheit (26-27 degrees Celsius), and the algae that live in the corals' tissues need sunlight to conduct photosynthesis.

Of course, as with all rules, there are exceptions to coral reef locations. The Atlantic current known as the Gulf Stream brings warm water from the tropics to Florida and Bermuda, and Indo-Pacific currents allow reefs to grow as far north as the southern tip of Japan and as far south as the southern tip of the Great Barrier Reef. Some corals even grow in areas such as the Arabian Gulf, where the water temperature is 55 degrees Farenheit (13 degrees Celsius) in winter and 100 degrees (38 degrees Celsius) Farenheit in summer. These corals are not as colorful as those found in the tropical regions, but they are true corals that have somehow managed to adapt to temperatures that are well outside the typical coral reef range.

Coral reefs are not found in all tropical waters. For example, cold currents prevent the formation of reefs along the western coasts of South America and Africa. Even where the water is warm, coral can not tolerate large amounts of fresh water. That is why we do not find coral reefs for miles around the estuaries (mouth regions) of major rivers such as the Amazon. In addition, the soft surfaces of the ocean floor and the large amounts of sediment near the mouths of rivers discourage the formation of reefs.

Source of information for this answer are:

Sammon, Rick Rhythm of the Reef: A Day in the Life of the Coral Reef. Voyageur Press: Stillwater, MN (1995).



























Easy Answer:

The coral reef often provides the only food and shelter for marine plants and animals for miles around. This means that the reef becomes a very crowded place. It becomes the center of life for hundreds of different kinds of plants and animals, much like a modern city serves this same purpose for humans. Day or night, there is always something exciting going on around the reef.

All kinds of living things can be found on or around the reef. Some animals that live attached to the reef include sea anemones (a relative of the coral polyp), sea fans, and sponges. Animals that live on or around the reef include the sea horse, giant blue clam, clown fish, hermit crab, sea slug, shrimp, sea cucumber, emperor angelfish, mandarin fish, octopus, lobster, butterfly fish, flounder, scorpion fish, moray eels, goby fish, blenny fish, sea urchins, parrotfish, triggerfish, damselfish, barracuda, sharks, flounder, grouper, tube worms, crabs, squirrelfish, grunts, and many, many more. Remember that the coral reef is second only to the tropical rain forest in terms of the number of animal and plant species that are found living there. Like the coral polyps themselves, these creatures come in every color of the rainbow and account for a lot of the natural beauty of the coral reef.

Many of the reef dwellers live in symbiotic (this is the scientific term for cooperative) relationships with other reef animals, others are hunters, and still others feed on the coral itself. Regardless of a reef dweller's immediate food source, any change in the food chain can trigger enormous consequences for the entire ecosystem.

Sources of information for this answer are:

Taylor, Barbara Coral Reef: A Close-up Look at the Natural World of a Coral Reef. Dorling Kindersley, Inc.: New York (1992).

Davis, Gary W. Habitats: Coral Reef. Children's Press: New York (1997).






















Advanced Answer:

Life on the coral reef is so varied and diverse that it is doubtful whether an entire series of books could describe the different types of marine life found there. The corals themselves, the main reef-builders, are comprised of thousands on tiny anemone-like polyps. There are also sea fans, gorgonians, and other soft corals to add variety and beauty to the reefs. Thousands of tiny shrimps and crabs live within the reef and countless worms, mollusks, algae and sponges add to the heterogeneous variety of creatures who make up the coral reef. Of course, the reef would appear relatively bare without its fish population.

Among the most common reef dwellers are the small six-to-eight-inch blue or yellow-striped grunts that inhabit the reefs in schools of hundreds, sometimes thousands, all swimming about in close proximity in an attempt to attain maximum protection from the reef's predators. In amongst the grunts, you will usually see members of the snapper family, including the mangrove snapper, the schoolmaster, the lane snapper and the larger mutton snapper. Occasionally, yellowtails will congregate with them at the edge of the reef. The grunt, named after a sound they emit, is familiar to every fisherman who fishes the reef.

The next most common group of fish on the reef is the wrasse family. At four-to-six-inches, these are even smaller than the grunts, and they resemble slender, hyperactive, and colorful cigars in their appearance. Common wrasses include the yellow wrasses, blueheads, slippery dicks, green wrasses, neon wrasses and varigated wrasses. Some are very colorful and all are extremely fast. One of the most colorful fish in the sea is a large wrasse (fifteen-to-eighteen-inches) known as a pudding wife. It contains nearly every color in the rainbow and prefers the outer reefs of the Gulf Stream. Wrasses are widely known for their peculiar habit of diving under the sand when pursued, and at night they sleep under the sand.

Next in popularity are the parrotfish. They range in length from a few inches to two or three feet and are the true rainbow fish of the reef. Some are brilliant orange or blue, others are bright red, green, spotted, gray, dark blue or a stunning, luminescent and shimmering blue. Like their namesake feathered counterparts, even the gaudy beaks of the parrot fish are brightly colored. The beak is used as a powerful cutting edge to help the parrotfish actually eat the hard coral which forms its main diet. Divers have reported seeing a large school of parrotfish eat away an entire elkhorn or staghorn coral formation in one setting, and the crunching sounds produced from this seemingly strange appetite can be heard a long way off underwater.

The most graceful and, as a result of their popularity as aquarium species, perhaps the most recognizable of the reefs' highly colored fish are the angelfish. They are not as common as the other fish, but their beautiful colors make them stand out from the crowd. Members of this family include the queen angelfish, rock beauty, townsend angelfish, french angelfish, and black or gray angelfish. Angelfish are not a schooling fish and usually swim by themselves or in mated pairs, although they may be seen in groups of a dozen to a hundred when they congregate to select a mate.

Other common groups of reef fish include the porkfish, demoiselles, gobies, surgeon fish, grouper, and barracuda. Generally, the younger and smaller fish are found close to shore in very shallow water and tend to seek safety in numbers. As they grow larger, the fish gradually work towards deeper water. Another common reef visitor is the shark. Nurse sharks are very common and are often spotted in groups of a dozen or more. Other sharks known to make appearances on the reef include the lemon shark, bull shark, sandbar shark, hammerhead, tiger shark, and white tipped shark. These animals may be found from the very shoreline, in water barely deep enough for them to swim, all the way out to deep water.

Many of the pelagic fish (open sea dwellers) are also known to visit the coral reefs, especially the deeper reefs near the Gulf Stream. These include the sailfish, wahoo, kingfish, dolphin, tuna, giant tarpon, jewfish, and amberjack. Many other marine animals are common visitors to the coral reef. These include stingrays of all types, sea turtles, the sea horse, lobsters, crabs, shrimps, the octopus, and many, many more. As mentioned earlier, we have only scratched the surface of the enormous variety of life to be found on the coral reef.

Scientists divide the reef fishes into six major trophic (method for obtaining food) categories: the planktivores, herbivores, omnivores, piscivores, invertebrate predators and cleaner fish. Planktivores, such as some groupers, butterfly fish and damselfish, tend to feed in large schools and consume plankton found near the coral reef. The herbivores, which include some surgeon fish, feed primarily on the algae associated with the coral reef. Omnivores, such as some triggerfish and puffers, feed on a variety of crustaceans in addition to reef algae. Piscivores, which include many groupers, survive by eating other fish and the invertebrate predators, like some parrotfish, eat coral polyps and other sessile and mobile invertebrates. The cleaner fish feed on the mucous and parasites that are found on the skin of other fishes.

Sources of information for this answer is:

Straughan, Robert Exploring the Reef. A.S. Barnes and Company: New York (1968).


















Be careful of this question because it is probably a trick. Coral is often mistaken for a plant. If you were to look at pictures of some of the different types of corals, such as sea fans and gorgonians, you might think they look a lot like underwater ferns or other types of water plants. Coral is also mistaken for rock by a lot of people. After all, it can be as hard as a rock, and when the polyps are inside their protective limestone "houses" the coral can appear can dull and lifeless. However, it is important to remember that coral is actually composed of tiny animals called polyps and their skeletons.
















Try these books (listed alphabetically):


 Coral Reef: A Close-up Look at the Natural World of a Coral Reef
by Barbara Taylor
Dorling Kindersley, Inc.: New York (1992).

Exploring the Reef
by Robert Straughan
A.S. Barnes and Company: New York (1968).

Habitats: Coral Reef
by Gary W. Davis
Children's Press: New York (1997).


 Our Wet World : Exploring Earth's Aquatic Ecocsystems
by James M. Needham (Illustrator)
Sneed B. Collar, III
Charlesbridge Publishing
: Watertown, MA (1998).

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