What is a coral reef?

Appearing as solitary forms in the fossil record more than 400 million years ago, corals are extremely ancient animals that evolved into modern reef-building forms over the last 25 million years. Coral reefs are unique (e.g., the largest structures on earth of biological origin) and complex systems. Rivaling old growth forests in longevity of their ecological communities, well-developed reefs reflect thousands of years of history (Turgeon and Asch, in press).

grande barrière de corail vue depuis l’espace

The Great Barrier Reef from space.

Corals and their Kind

Corals are anthozoans, the largest class of organisms within the phylum Cnidaria. Comprising over 6,000 known species, anthozoans also include sea fans, sea pansies and anemones. Most stony corals are under the group “scleractinians” and are primarily responsible for laying the foundations of, and building up, reef structures (notable exceptions of stony corals outside this scleractinian order include Fire corals, which fall under the Hydrozoan, and Blue and Organ Pipe corals, which are Octocoral). For the most part, scleractinians are colonial organisms composed of hundreds to hundreds of thousands of individuals, called polyps (Barnes, R.D., 1987; Lalli and Parsons, 1995).

Close-up of acropora sp polyps. 

As members of the phylum Cnidaria, corals have only a limited degree of organ development. Each polyp consists of three basic tissue layers: an outer epidermis, an inner layer of cells lining the gastrovascular cavity which acts as an internal space for digestion, and a layer called the mesoglea in between (Barnes, R.D., 1987).

All coral polyps share two basic structural features with other members of their phylum. The first is a gastrovascular cavity that opens at only one end. At the opening to this cavity, commonly called the mouth, food is consumed and some waste products are expelled. A second feature all corals possess is a circle of tentacles, extensions of the body wall that surround the mouth. Tentacles help the coral to capture and ingest plankton for food, clear away debris from the mouth, and act as the animal’s primary means of defense (Barnes, R.D., 1987; Levinton, 1995).


While coral polyps have structurally simple body plans, they possess several distinctive cellular structures. One of these is called a cnidocyte—a type of cell unique to, and characteristic of, all cnidarians. Found throughout the tentacles and epidermis, cnidocytes contain organelles called cnidae, which include nematocysts, a type of stinging cell. Because nematocytes are capable of delivering powerful, often lethal toxins, they are essential to capturing prey, and facilitate coralline agonistic interactions (Barnes, R.D., 1987).


Structure of a typical coral polyp. 

Structure ofZooxanthellae

A Symbiotic Relationship

Most corals, like other cnidarians, contain a symbiotic algae called zooxanthellae, within their gastrodermal cells. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis. These include carbon dioxide, produced by coral respiration, and inorganic nutrients such as nitrates, and phosphates, which are metabolic waste products of the coral. In return, the algae produce oxygen and help the coral to remove wastes. Most importantly, they supply the coral with organic products of photosynthesis. These compounds, including glucose, glycerol, and amino acids, are utilized by the coral as building blocks in the manufacture of proteins, fats, and carbohydrates, as well as the synthesis of calcium carbonate (CaCO3). The mutual exchange of algal photosynthates and cnidarian metabolites is the key to the prodigious biological productivity and limestone-secreting capacity of reef building corals (Barnes, R.D., 1987; Barnes, R.S.K. and Hughes, 1999; Lalli and Parsons, 1995; Levinton, 1995; Sumich, 1996).

La relation symbiotique

Zooxanthellae often are critical elements in the continuing health of reef-building corals. As much as 90% of the organic material they manufacture photosynthetically is transferred to the host coral tissue (Sumich, 1996). If these algal cells are expelled by the polyps, which can occur if the colony undergoes prolonged physiological stress, the host may die shortly afterwards. The symbiotic zooxanthellae also confers its color to the polyp. If the zooxanthellae are expelled, the colony takes on a stark white appearance, which is commonly described as “coral bleaching” (Barnes, R.S.K. and Hughes, 1999; Lalli and Parsons, 1995). However, if the stressors are removed within a short period of time, corals may gain zooxanthellae back and, under favorable conditions, they can eventually recover.

Close-up ofZooxanthellae

Massive reef structure of the Great barrier reef, Australia.

From Polyp to Reef

Massive reef structures are formed when each stony coral polyp secretes a skeleton of CaCO3. Most stony corals have very small polyps, averaging 1 to 3 mm in diameter, but entire colonies can grow very large and weigh several tons. Although all corals secrete CaCO3, not all are reef builders. Some corals, such as Fungia sp., are solitary and have single polyps that can grow as large as 25 cm in diameter. Other coral species are incapable of producing sufficient quantities of CaCO3 to form reefs. Many of these corals do not rely on the algal metabolites produced by zooxanthellae, and live in deeper and/or colder waters beyond the geographic range of most reef systems (Barnes, R.D., 1987; Sumich, 1996).

The skeletons of stony corals are secreted by the lower portion of the polyp. This process produces a cup, called the calice, in which the polyp sits. The walls surrounding the cup are called the theca, and the floor is called the basal plate. Thin, calcareous septa (sclerosepta), which provide structural integrity, protection, and an increased surface area for the polyp’s soft tissues, extend upward from the basal plate and radiate inward from its outer wall. Periodically, a polyp will lift off its base and secrete a new floor to its cup, forming a new basal plate above the old one. This creates a minute chamber in the skeleton. While the colony is alive, CaCO3 is deposited, adding partitions and elevating the coral. When polyps are physically stressed, they contract into the calice so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements (Barnes, R.D., 1987; Sumich, 1996).

Solitary coral Fungia sp. showing its outer covering of living tissue, and clearly visible central mouth.

Most of the reefs, with a few exceptions are found in tropical and semitropical waters, between 30° north and 30° south latitudes.

At other times, the polyp extends out of the calice. The timing and extent to which a polyp extends from its protective skeleton often depends on the time of the day, as well as the species of coral. Most polyps extend themselves furthest when they feed on plankton at night.


In addition to a substantial horizontal component, the polyps of colonial corals are connected laterally to their neighbors by a thin horizontal sheet of tissue called the coenosarc, which covers the limestone between the calices. Together, polyps and coenosarc constitute a thin layer of living tissue over the block of limestone they have secreted. Thus, the living colony lies entirely above the skeleton (Barnes, R.S.K. and Hughes, 1999).

Close-up of acropora sp coenosarc and polyps.

Colonies of reef-building (hermatypic) corals exhibit a wide range of shapes, but most can be classified within ten general forms.

  • Branching corals have branches that also have (secondary) branches.
  • Digitate corals look like fingers or clumps of cigars and have no secondary branches.
  • Table corals are table-like structures of fused branches.
  • Elkhorn coral has large, flattened branches.
  • Foliose corals have broad plate-like portions rising above the substrate.
  • Encrusting corals grow as a thin layer against the substrate.
  • Submassive corals have knobs, columns or wedges protruding from an encrusting base.
  • Massive corals are ball-shaped or boulder-like corals which may be small as an egg or large as a house.
  • Mushroom corals resemble the attached or unattached tops of mushrooms.
  • Cup corals look like egg cups or cups that have been squashed, elongated or twisted (McManus et al. 1997).

While the growth patterns of stony coral colonies are primarily species-specific, a colony’s geographic location, environmental factors (e.g., wave action, temperature, light exposure), and the density of surrounding corals may affect and/or alter the shape of the colony as it grows (Barnes, R.D. 1987; Barnes, R.S.K. and Hughes 1999, Lalli and Parsons, 1995).

In addition to affecting the shape of a colony’s growth, environmental factors influence the rates at which various species of corals grow. One of the most significant factors is sunlight. On sunny days, the calcification rates of corals can be twice as fast as on cloudy days (Barnes, R.S.K. and Hughes, 1999). This is likely a function of the symbiotic zooxanthellae algae, which play a unique role in enhancing the corals’ ability to synthesize calcium carbonate. Experiments have shown that rates of calcification slow significantly when zooxanthellae are removed from corals, or when corals are kept in shade or darkness (Lalli and Parsons 1995).


In general, massive corals tend to grow slowly, increasing in size from 0.5 cm to 2 cm per year. However, under favorable conditions (high light exposure, consistent temperature, moderate wave action), some species can grow as much as 4.5 cm per year. In contrast to the massive species, branching colonies tend to grow much faster. Under favorable conditions, these colonies can grow vertically by as much as 10 cm per year. This fast growth rate is not as advantageous as it may seem, however. Mechanical constraints limit the maximum size that branching corals can achieve. As they become larger, a heavier load is placed on the relatively small area attached to the substratum, rendering the colony increasingly unstable. Under these circumstances, the branches are prone to snapping off during strong wave action. The opposite is true of the massive-shaped corals, which become more stable as they grow larger (Barnes, R.S.K. and Hughes, 1999).

Coral core samples reveal horizontal growth lines.


Darwin’s three stages of atoll formation.

The Structure of Coral Reefs

Coral reefs begin to form when free-swimming coral larvae (planulae) attach to the submerged edges of islands or continents. As the corals grow and expand, reefs take on one of three major characteristic structures—fringing, barrier or atoll. Fringing reefs, which are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. Barrier reefs also border shorelines, but at a greater distance. They are separated from their adjacent land mass by a lagoon of open, often relatively deep water. If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms. Atolls are usually circular or oval, with a central lagoon. Parts of the reef platform may emerge as one or more islands, and breaks in the reef provide access to the central lagoon (Lalli and Parsons, 1995; Levinton, 1995; Sumich, 1996).


In the 1830s, Charles Darwin distinguished between the three main geomorphological categories of reefs, and suggested that fringing reefs, barrier reefs, and atolls were all related stages in the sequence of atoll reef formation.


All three reef types—fringing, barrier and atoll—share similarities in their biogeographic profiles.Bottom topography, depth, wave and current strength, light, temperature, and suspended sediments all act to create characteristic horizontal and vertical zones of corals, algae and other species. While these zones vary according to the location and type of reef, the major divisions common to most reefs, as they move seaward from the shore, are the reef flat, reef crest or algal ridge, buttress zone, and seaward slope.


Three different types of Reef

Fringing Reef

It runs directly along the coast and is often separated from it by a small shallow channel.

For example, there is this type of coral mainly in Reunion and the Caribbean.


Barrier Reef

Unlike the fringing reef, the barrier reef is separated from the coast by an inland sea: the lagoon.

They are found in New Caledonia, around the high islands of French Polynesia, Wallis, Mayotte.

A barrier is also present on the east coast of Martinique and Guadeloupe (Grand Cul-de-sac Marin).

And also the dual barrier reefs, which consist of two parallel barrier reefs. The New Caledonia and Mayotte each have double coral reefs, extremely rare in the world (there are less than 10).



The Atoll is a coral reef offshoreringshaped, it encircles a lagoon size and variable depth.

The reef can be continuous and form a closed or it may be discontinuous ring and form a set of separated passes connecting the lagoon to the ocean islands. There are atolls in French Polynesia and New Caledonia. Clipperton Island and the Europa and Bassas da India (scattered islands) are also atolls.


Atoll formation


Graphic of typical coral reef zones.

The reef flat zone, or back reef, is located on the sheltered side of the reef. It extends outward from the shore; and may be highly variable in character. Varying in width from 20 or 30 meters to more than a few thousand, the reef flat may range from only a few centimeters to a few meters deep, and large parts may be exposed at low tide. The substrate is formed of coral rock and loose sand. Beds of sea grasses often develop in the sandy regions, and both encrusting and filamentous algae are common.