Describing the sensation of diving among a coral reef, David Attenborough wrote: "There is nothing on land that can prepare you for the profusion of shapes and colors of the corals themselves. There are domes, branches and fans, antlers delicately tipped with blue, organ pipes that are blood red. Some seem flower-like yet when you touch them they have the incongruous scratch of stone."

"Often different coral species grow beside one another, mingled with seas pens arching above and beds of anemones that wave long tentacles in the current. Sometimes you swim over great meadows that consist entirely of one kind of coral; sometimes in deeper water, you discover a coral tower hung with fans and sponges that extends your sight into depths of darkest blue."

Coral reefs are sometimes called the rain forests of the sea because of their beauty, biodiversity and the stresses they are under. They contain more species per unit than any other marine environment. They go a step further than rainforests in arguably being the most colorful place on earth. In the rainforest you generally don’t find the brilliant yellows, shocking blues and brilliant reds that you find in the reef.

What makes reefs so colorful and why? In recent years scientists have begun to develop a better understanding into these questions by studying how light and color appear at different depths to the eyes of the creatures that inhabit the reef. See Reef Fish

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Books: “Coral Reefs: Cities Under the Sea” by Richard C. Murphy (The Darwin Press, 2002); “Palaces Under the Sea: A Guide to Understanding the Coral Reef Environment” by Joe Strykowski and Rena M. Bonern (Star Throw Foundation, 1993); “ Field Guide to Coral Reefs: Caribbean and Florida” (Peterson Field Guide Series) by Eugene Kaplan and Susan Kaplan (Houghton Mifflin, 1988); “ Coral Reef: A City that Never Sleeps” by Mary Ceruilo and Jeffrey Rotman (Cobblehill, 1996); “ Greenpeace Book of Coral Reefs” by Sue Wells and Nick Hanna (Sterling Publications, 1992).

Websites and Resources: National Oceanic and Atmospheric Administration ; Smithsonian Oceans Portal ; Ocean World ; Woods Hole Oceanographic Institute ; Cousteau Society ; Montery Bay Aquarium

Websites and Resources on Fish and Marine Life: MarineBio ; Census of Marine Life ; Marine Life Images ; Marine Species Gallery

Coral polyp
Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ; Coral Reef Pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.


Sea anemones, jellyfish and corals belong to the family of colonizing organisms called coelenterates (Greek for cavity) and the 9,000-species phylum Cnidarians (meaning "stringing thread"), a group of tentacled creatures which also includes anemones, jellyfish and corals and hydriods. Most reproduce asexually without mating by producing buds from their own bodies.

All coelenterates are simply a hollow sac or shallow cup of cells with a mouth at one end surrounded by tentacles. Armed with stinging cells, the tentacles help them to paralyze small swimming animals which are then pushed into its mouth. Coelenterates have a primitive gut for digestion and their mouth also serves as an anus.

Coelenterates take in food through their mouthes and ingest it in the stomach, with the indigestible parts being expelled back out the mouth. They are almost exclusively carnivores but have no teeth. Instead they have tentacles lined with whiplike structures called nematocysts that release poison barbs that are strong enough to paralyze prey and allow it to be pulled into the coelenterate’s mouth and gut. Jellyfish have their tentacles pointed downward while anemones and corals have theirs pointed upwards.

Cnidarians are essentially the same: cup-like animals but looked at in a slightly different way. The tube can either be a medusa, flattened into a bell shape, or a polyp, with the closed end attached to a hard surface. Corals and sea anemones are polyps. Medusas are mostly jellyfish. Some hydriods and jellyfish exist in both medusa and polyp forms in their lifetimes.

Coral and Coral Polyps

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coral polyps
Coral is the hard skeleton secreted by a tiny organism called a polyp. The scientific name for coral, “ Anthozoa”, means "flower animals." These animals are partly carnivorous with stinging cells that catch microscopic prey with tentacles and mucous coating. They get 90 percent of their nourishment from algae. Corals have changed little since they first evolved around 230 million years ago. There are deep-sea corals that are more than 8,000 years old.

Coral looks like plants but in fact are animals. This fact is evident at night when the polyps emerge and cover the rocky exoskeletons like fur. Attenborough wrote: "Each polyp is connected to its neighbor by strands that extend laterally. As the colony develops, new polyps form, often on these connecting sections and their skeletons grow over and stifle earlier polyps."

Polyps are about the size of pencil erasers. They extract calcium carbonate (limestone) from seawater and deposits this beneath themselves as a exoskeletons (protective calcium shells). The polyps live inside the limestone exoskeletons during the day and emerge at night to feed.

Coral Polyps and Algae

Coral polyps in symbiosis
with unicellular dinoflagellates
The outside layer of each polyp is inhabited by tiny one-celled plants, an algae called “ zooxanthellae”, which give the coral its rich orange, purple, red and yellow colors. In this way coral is part animal, part plant and part mineral. The name of most kinds of corals — elkhorn, staghorn, brain, blue, black, sea fans, star — describes the appearance and color of the coral colonies. Algae lies in tissues of these colonies at a density of millions per square inch. [Source: Walter A. Stark II, National Geographic, November 1966 [╆]

The polyps in coral secrete a digestive solution that weakens the skins of the algae and caused 80 percent of the food produced by the algae to leak out and into the polyps cells, nourishing the polyps. Algae provided coral with 90 percent of its energy needs. The remaining 10 percent comes from plankton gathered by the polyps tentacles when they emerge at night. The relationship between coral and algae has allowed coral to thrive in an environment that has relatively few nutrients. Without algae the coral eventually starves, dies and turns white.

Coral polyps and the algae living within the polyps's tissue have a symbiotic relationship. The algae provides fat and sugars for the polyp and gives off oxygen though photosynthesis that the polyps need to survive when they are is inside their exoskeletons during the day. The algae also absorbs the waste of the polyps, converting phosphates and nitrates into proteins and, with the help of the sun, uses carbon dioxide to make carbohydrates through photosynthesis. The algae also supplies the polyp with chemicals that act as sun cream against harmful ultraviolet radiation from the sun.

The algae also accelerate the conversion of calcium, providing the coral with minerals that help it build it exoskeletons and produce larger and stronger reefs. The polyps in turn produce carbon dioxide through respiration which the algae needs to sustain itself. "Coral eating" fish such as parrotfish, triggerfish and surgeon fish feed on the algae rather than the coral polyps themselves.

Scientists used to think there was just one species of zooxanthellae but research in the 2000s revealed that there are more than a dozen different kinds and some of them have significant attributes such as surviving better in warm water than others.

Coral Characteristics

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elkhorn coral
During the day polyps expel water from their bodies and sink as deeply into their exoskeletons as possible, where they are reasonably safe from the jaws of grazing fish. After sunset, when the coral-eating fish hide in their crevasses to escape nighttime predators, the colorful polyps inflate their bodies with water and emerge from their exoskeletons, their stinging tentacles snatching plankton in the water.

Corals fight off competitors with chemical weapons. Some species extends threads from the gut, called mesenterial filaments, that deposit mucus with tissue-dissolving enzymes on competitors, which sometimes fight back with extra-long stinging tentacles. Corals deal with stresses like silt and pollution by secreting mucous proteins on their outer tissues.

Some corals turn strange colors — like black light posters from the 1960s — when ultraviolet light is shined on them. The orange, blue and green colors are caused by fluorescence, a phenomena in which a surface absorbs ultraviolet light and emits lights of different colors. Corals exude chemicals that protect them from sunburn during low tide. Scientists are examining these chemicals to develop a new waterproof protection cream.

Ove Hoegh-Guldberg, a biologist at Queensland University, told National Geographic, "At shallow depths sunlight here can actually be toxic. Corals have special pigments to absorb ultraviolet rays, and their symbiotic algae hide in the shade beneath bundles of these pigments for protection. Conversely, at depths where little sunlight penetrates, the algae nestle right inside the coral's pigment bundles. Then, as the pigments re-radiate the light energy they have gathered, the algae can use it for photosynthesis.” This re-radiation causes corals to fluoresce.

Coral Growth and Movement

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pretty algae that looks like coral
Corals are very delicate and very slow growing. This is why it is so important not to touch or disturb them when snorkeling or diving. Most corals only grow about one centimeter a year. Staghorn corals are among the fastest growing. The grow as fast as 10 centimeters a year. Those that grow fast also are among the most sensitive. Nancy Knowlton, a biodiversity expert at Scripps Institute, told the New York Times, “They can slip from growing really fast to dying really fast.”

Coral generally don't move because their exoskeletons are fixed to a reef or coral head. There is one species of "walking" coral (“ Heteropsammia michelenii” ) that is about an inch-long and moves around with the help of a specific worm which has a symbiotic relations with the coral polyp and live in its exoskeleton.

Since coral doesn’t swim or fly, scientists have long wonder how different species are transported around the globe and how remote islands can have so many species of coral. Based on the fact that some places down current from relatively recent volcanic eruptions and pieces of coral have been observed floating on pumice, it is believed that fertilized coral eggs and coral itself are transported through the water and carried like a hitchhiker by various means, reaching destinations a considerable distance from where they originated.

Coral. It is believed, can be transported by flotsam such as pieces of driftwood and manmade objects like plastic and litter. Christmas Island, which is in the South Pacific, down current, from Krakatau, which erupted violently in 1883, possesses an especially rich variety of coral.

Coral Reproduction

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coral bubble
Some corals reproduce asexually (without mating) by budding in which mature corals sends out shoots called buds from their own body that develop into new polyps. In some cases the new polyp separates completely; in many cases the budding is incomplete and individual remain joined together in a colony.

Most of the reef-building species are hermaphroditic. Some species have polyps that contain reproductive bundles with around 100 eggs and 2 million sperm. The bundles are released once a year and drift in the open sea to a new homes. Others raise their larvae to an advanced stages and then release them.

Coral eggs are filled with nutrients and fat. In addition to providing food the fat makes the eggs buoyant and able to float to the ocean surface. After being released by the coral, the egg-and-sperm bundles can collect en mass and form a spongy carpet on the ocean surface. After a half hour or so the bundles break open allowing the eggs and sperm of different polyps to mix.

Coral Mass Spawns

Several species of reef-building star corals reproduce once a year by releasing billions of eggs and sperm into the water at one time, a phenomena that wasn't described in scientific journals until 1984. Before then most scientists thought that coral reproduced like plants. [Source: John Ross. Smithsonian magazine]

Describing a mass-spawning event in the Caribbean, John Ross wrote in Smithsonian magazine, "I can see the slightly pink, tiny egg-sperm bundles in the stars of the coral, ready to fire like cannonballs...I half expect to hear a noise then the corals launch their precious packages...I watch a Montastraea coral head as the eggs pop out in a quick wave that moves across its face. Suddenly streams of these tiny balls are wafting up and around me, and I'm reminded of a snow-globe toy...The egg bundles rise to the surface like tiny bubbles." [Ibid]

In the Caribbean, “ Acropora” (a common elkhorn species) and “ Montastraea” mass spawn according to the lunar calendar. Acropora begins to spawn two to four nights after the full moon in August. Montastraea spawn six nights after the same moon.

In addition to reproducing on a certain day, mass-producing coral species also release their eggs and sperm at a specific time. How they coral know when to do all this is still a mystery. Coral species in Japan release eggs en mass during high tides with a full moon in late July or early August. The one-millimeter-in-diameter eggs emerge from the coral for about 30 minutes.

The mass spawning provides a feast for a number of sea creatures. Every year whale sharks gather in large numbers at Ningaloo reef in Western Australia to a feast on spawning of coral. This is the largest known gathering of whale sharks in the world. It is not unusual for divers to see 20 or 30 of them in a single day, and manta rays, minke whales and mobula rays also show up at the 160 mile long reef for the mass spawning which only occurs once every year. Groups at Ningaloo tend to be immature males.

Coral Eggs and Larvae

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About 24 hours after being fertilized coral polyp eggs evolve into larvae with cilia hairs that allow them move about. After five to eight days the larvae have developed enough to "settle" and attach themselves to a hard surface and grow into a mature coral animal. Before then many are gobbled up by a host of creatures. The survival rate for coral larvae is generally only around 1 or 2 percent.

For many years scientists believed that the larvae found their homes by chance like windblown-seeds. But that doesn't always seem to be the case. Recent research has shown that the larvae have a sophisticated development system that triggers metamorphosis after the larvae comes in contact with a hard surface with algae. If it hits a hard surface with no algae, the metamorphosis mechanism is not activated and the larvae keeps drifting and eventually dies or is eaten.

During metamorphosis the larvae is transformed from a soft bodied organism into a young polyp with an exoskeleton. The upper part of the body becomes dome-shaped. The creature also develops a stomach and a mouth. Around the mouth grows a number tentacles. After the metamorphosis is mostly complete the polyp builds a platform from lime between its body and the surface to which it is attached. Then it builds up an exoskeleton of lime. When the skeleton is complete only the tentacles reach above it.

Black Corals and Other Kinds of Coral

Hard coral with extended polyps at night
There are several kinds of corals, of which hard (stony) species are the most numerous. Solitary corals are found in most of the world's oceans but they only form large reefs in tropical waters above 68̊F. The distribution of corals is relatively uneven. There are 67 coral species in the Caribbean Sea, compared around 450 in the Pacific and Indian Oceans around Malaysia, Indonesia, the Philippines, Papua New Guinea and Australia.

There are about 4,000 coral species. Each coral has its own system of budding and this produces the shape of the colonies unique to each kind of coral. Beautiful organ-pipe coral, which expands like a flower, is a close relative of soft corals (See Below). The organ pipes are a mass of vertical tubes, joined at intervals throughout their length by thin horizontal plates. The purplish skeleton contains a pale lilac-colored polyp.

Living black coral can be white, gray, green, orange and even bright yellow. Their name comes from the color of their exoskeletons. By some reckonings black corals are not "true" corals. They are anatomically different and do not have a skeleton of calcium carbonate but rather one made of a semiprecious, ebonylike protein that is similar to insect cuticle. The protein is yellow-brown in color but looks black when layers of it are bonded together. Black corals form shrublike colonies with well-defined branches. They release both eggs and larvae at dusk and dawn around the time of ceratin full moons.

Rose, black and blue coral are prized by jewelry makers. Fire coral cause a burning sensation on skin. Among the more colorfully-named corals are sea whip (which can can reach lengths of five meters); “bubble” coral (comprised of skeletal ridges filled with water that resemble bubbles), "daisy" coral (crowned by anemone-like tentacles), "castle" coral (stony corals that grow in pinnacles and are covered with linear patterns), "leather" coral (a soft coral with a spongelike consistency), and "wire" coral (which often spirals out of caves to pull in food from passing currents).

Hard, Stony, Horny and Branching Corals

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pink coral
True corals are divided into four groups: 1) stony, or hard, coral (Order “ Madreporaria” and “ Sceleractinia” ); 2) horny corals ( “ Gorgonacea” ); 3) branching corals (“ Antipatharia” ); and 4) one species of blue coral (“ Coenothecalia” ).

Stony coral, the builders of reefs, form colonies in mound like shapes that sometimes don’t get as much access to sunlight as other kinds of coral but are more likely to weather storms. There are about a thousand species of hard , stony coral, making them the most diverse and widely distributed coral. Living either solitary or alone, hard corals are found in all tropical oceans and grow best at depths of less than 30 meters though there are some species that grow to depth of 7,000 meters. Stony coral can be shaped like mushrooms, moose antlers, cabbages, tabletops, fluted pillars and wrinkled brains.

The 1,200 species of horny coral thrive in shallow waters. They include varieties include elkhorn, and staghorn corals. Many species of branching corals have ribbon-like branches that occasionally grow to lengths of three meters. This adaption help the coral thrive after getting a small toe hold on the reef and deprives competitors of sunlight. But its shape also makes it vulnerable in storms. There are about a hundred species of branching coral.

Soft Coral, Sea Fans and Sea Pens

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sea fan
Soft corals are not true corals. What sets soft coral polyps apart from other normal corals are their tentacles, which are more complex and fringed than tentacles on true corals. In addition, soft coral polyps have eight tentacles as opposed to six or multiples of six as is the case with true corals.

Soft corals inflate their entire bodies. At night, soft coral expand their bodies to four or five times their daytime size, extending their polyps to snag more plankton and waving in the current as they do so. Some of the toxins found in soft corals are anti-inflammatory agents that have the potential of treating cancer, AIDS and a host of other ailments.

Sea pens are a kind of soft coral. They are ancient, primitive water creatures that were given their names in an age when people wrote with quill pens. They have a flexible, horny stem and feathery appendages. They vary in height from a few centimeters to about a meter.

Sea pens are particularly interesting at night. Many kinds glow with a bright purple luminescence. If you touch them, waves of light pulsate along their arms sort of like some of the plants in the film “Avatar”.

Sea pens can reach a height of about 70 centimeters. Their “feathers” house polyps that snag zooplankton, fish and jellyfish that float its way. If threatened they expel pulses of water that fill up their bodies or hide in mud or a crevice of a rock.

Sea fans and sea whips are tree-like soft coral known as gorgonians that don't build reefs. They jut out from reef walls and catch plankton as it rises during the night and descends during the day. Gorgorians, like a jellyfish, are colonies of small individual organisms that behave like a larger organism. Their flexible skeletons drift in the currents and polyps grab food and feed it to a common digestive track.


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finger sponge
Mostly anchored to reefs or other hard surfaces, sponges are plant-like animals that live in water and survive by drawing water through small pours of their tubelike walls and expelling it through openings at the top, in the process filtering out the plankton it feeds on. Sponges can grow to the size of barrels. For a long times they were thought to be plants. [Source: Henry Genthe, Smithsonian]

Sponges are colonies of single cells with a porous structure. There are several thousand species of marine and freshwater sponge, many of which form spectacular, bright colored masses on reefs around the world. Most sponges live in salt water but a few species live in fresh water. Sponges belong to the phylum porifera, meaning "pore-bearing animals.” These are animals with porous bodies and specific cells for extracting plankton from seawater.

Sponges are among the world's oldest creatures. Along with jellyfish they first emerged between 800 million and 1 billion years ago. They are more primitive than coral, sea urchins and jellyfish in that they don't have stomachs or tentacles and are regarded as the simplest of all living animals. Sponges are immobile, living attached to solid surface. Instead of organs or tissues that have colonies of cells that perform specific tasks.

There are around 5,000 species of marine sponge. They include glass sponges, with fragile but delicate matrixes of spicules; calcareous sponges, the only sponges with spicules made of calcium carbonate; demosponges, that compete with coral to dominate reefs and make up 90 percent of all sponges; Venus-flower baskets, one of the most beautiful glass sponges; bath sponges, used to make shingles; and horny sponges that you should keep away from your girlfriend. Deep sea sponges gave been found at deep sea vents and in the Southern Ocean abyss.

Some sponges have symbiotic relationships with crabs and shrimp that extract food as they clean algae and parasites and tend and prune the sponges themselves. Most sponges contains toxins to protect them from grazing fish and mobile invertebrates. Without the toxins the sponges are vulnerable and perfect food for many fish to munch on. Sponges also defend themselves with tough layers of skin and sharp spicules.

For more Information see a separate article on Sponges

Image Source: National Oceanic and Atmospheric Administration (NOAA)

Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011

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