Coral Reef Formation

Coral reefs are the biologically richest and most diverse of all marine biomes on our planet.

Nonetheless, coral reef formation is only made possible by the cumulative efforts of millions of individual organisms working over very long periods of time.

Although a wide variety of marine creatures ultimately contribute to the complexities of coral reef structure, most of the reef's underlying solid framework is constructed by a combination of just a few specialized kinds of animals (reef-building corals) and plants (coralline algae).

Reef-building Corals

The vast bulk of the reef base is formed by just one of the two types of reef-building (hermatypic) corals. These primary reef-builders are known as the scleractinian (or "hard") corals.

In these animals, the body of each individual (called a polyp) is encased in a hard external skeleton composed of calcium carbonate formed by the animal itself from substances extracted from seawater.

Close up view of hard coral polyps with feeding tentacles exposed. Photo: Courtesy NOAA

As adults, individual hard coral polyps are very small, perhaps averaging about the size of a standard pencil eraser. However, these animals exist as parts of much larger, cohesive colonies.

Hard coral colonies are composed of countless thousands of genetically identical individuals, and may attain sizes of several meters in diameter. Only the outermost layers of hard coral colonies are composed of living polyps. Succeeding generations are built upon the skeletal remains of their forbearers.

Occasionally, isolated colonies of hard coral may be found, but far more typically they grow in larger assemblages that we call coral reefs, composed of many colonies of the same or different species.

Hard coral colonies generally exhibit one of three basic growth forms, called massive, branching, or plate-like.

Massive (left), branching (center) and plate-like (right) are the most common growth forms of hard coral colonies.

The shape, size, and color of these colonies are subject to modification by local environmental conditions such as wave action, currents, prevailing winds, etc., often leading to substantial variability in the appearance of colonies of the same species.

A smaller contribution to coral reef formation is made by a second group of hermatypic corals called the Milleporina ("fire corals". etc.) that grow atop the underlying reef base.

Coralline Algae: Reef-Building Plants

Growing upon and amidst the coral colonies are an unusual group of plants, called coralline algae. As with the reef-building coral animals, these plants also incorporate calcium carbonate into their bodies.

Coralline algae. Courtesy NOAA

Coralline algae are a form of red algae, and appear reddish to pink in color (see photo; left). These plants are major contributors to coral reef formation; in some cases they comprise almost half of the biomass of the reef framework.

Coralline algae also play a pivotal role in helping to build and cement the reef into a strong cohesive structure, and are the chief builders of algal ridges on Indo-Pacific reefs.

Other Contributors To Coral Reef Formation

Ultimately, a great variety of organisms assist the primary reef builders (discussed above) in coral reef formation.

Octocorals adorn a shallow Caribbean reef, adding physical complexity and biodiversity to the reef structure. © Fotolia.com

Carbonate particles contributed by a variety of marine organisms other than reef building corals and coralline algae also make minor contributions to building the reef framework, although to a far less degree than the main contributors.

On many shallow reefs, a superficial layer of sponges, octocorals, and other invertebrate animals and macroalgae grows from the reef base. Together, these assemblages add substantially to the structural and biological complexity and diversity of coral reefs.

The processes involved in the formation of different types of coral reefs are discussed on another page of our site.