What are coral reefs?

Corals are extremely ancient animals that appeared as solitary polyps more than 400 million years ago, and evolved into modern reef-building forms over the last 25 million years.

Coral reefs are unique: they are the largest structures on earth of biological origin and are complex systems.

They rival with old forests in terms of the longevity of their ecological communities; well-developed reefs reflect thousands of years of history (Turgeon and Asch, in press).

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 and anemones. Most stony corals are known under the group “scleractinians” and are primarily responsible for laying the foundations of, and building up, reef structures. Scleractinians are colonial organisms composed of hundreds, sometimes thousands of individuals, called polyps (Barnes, R.D., 1987; Lalli and Parsons, 1995).

As members of the phylum Cnidaria, corals only have 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).

A symbiotic relationship

Most corals, like many cnidarians, host a symbiotic algae called zooxanthellae inside the cells of the endoderm. Coral provides protection for the algae and the nutritional elements necessary for photosynthesis, such as nitrogen and phosphates.

In return, the algae provide organic products from photosynthesis. These products include glucose, glycerol and amino acids, which are used by corals to make proteins, fats and carbohydrates, as well as to synthesize calcium carbonate in the original form of aragonite (CaCO3).

The mutual exchange of algal photosynthesis and cnidarian metabolism is a source of prodigious organic production. It has the ability to deposit limestone from reef-building corals (Barnes, R.D., 1987; Barnes, R.S.K. and Hughes, 1999; Lalli and Parsons, 1995; Levinton, 1995; Sumich, 1996). Zooxanthellae are key components of healthy reef-building corals. More than 80% of the carbon resulting from photosynthetic activity is used for coral cellular respiration. In addition, it is used to produce mucus which has various functions: protection against aggression, food capture, excretion of waste and photosynthesis that cannot be used by the coral. Its growth comes from the capture of external, both dissolved and solid, elements.

A coral hosts various clades of zooxanthellae and regulates its algal population by expelling it if necessary. Zooxanthellae are honey-colored. It is the photoprotective pigments, including pocilloporins, which give the color of a polyp. During an episode of stress, the coral no longer recognizes the algae as its host, and thus expels its entire algal population. However, if all the zooxanthellae are expelled, the colony loses its pigments, and its calcareous skeleton appears. This phenomenon is commonly referred to as coral bleaching (Barnes, R.S.K. and Hughes, 1999; Lalli and Parsons, 1995). However, if stress factors are quickly eliminated, corals can quickly regain a population of zooxanthellae, and under favourable conditions, can even regain its health.

From polyp to reef

Most coral reefs, with the exception of a few, are found in tropical and semi-tropical waters between the 30th degrees north and south latitudes.

Massive reef structures are formed by the secretion of each polyp from a calcium carbonate skeleton in the form of aragonite (CaCO3).

Most hard coral polyps are very small: between 1 and 3 mm in diameter, but whole colonies can exceed total weights of several tons. Although all hard corals synthesize limestone, not all hard corals are involved in reef construction. Some, such as Fungia sp., are solitary polyps that are not attached to the substrate and can exceed 25 cm in size.

The skeleton of hard corals is synthesized by the lower part of the polyp. Periodically, the polyp raises its base and synthesizes a new floor above the old one.

As long as the polyp is alive, it creates new divisions and rises. Thus, colonies live only on the surface of the skeleton (Barnes, R.S.K. and Hughes, 1999).

Colony growth varies from a few centimetres only (for massive corals) to about 10 centimetres per year (for branching corals).

Colonies of reef-building, or hermatypic, corals exhibit a wide range of shapes, but most can be classified within ten general forms (Mc Manus & al. 1997).

Branching corals
Digitate corals
Table corals
Elkhorn coral
Foliose corals
Encrusting corals
Submassive corals
Massive corals
Mushroom corals
Cup corals

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).

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 characteristic structures – fringing, barrier or atoll.

Fringing reef

Fringing reefs, the most common, extend directly along the coast and are often separated from it by a small shallow channel. For example, this type of reef is found mainly in La Réunion and the West Indies.

Barrier reef

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

Dual barrier reefs 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).

Atoll

If a fringing reef forms around an island that subsequently sinks completely below the water level, an atoll is formed. Atolls are usually circular or oval and have a central lagoon. Parts of the reef plain may emerge to form islands, and passes through the reef may provide access to the central lagoon (Lalli and Parsons, 1995; Levinton, 1995; Sumich, 1996).

Fringing reef Barrier reef Atoll

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

Dual barrier reefs 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).