How Are Coral Reefs Formed?

How are coral reefs formed? Coral reef structure often seem infinitely variable, particularly from the relatively small scale perspective of a person observing reef environments at close range, as would a scuba diver or snorkeler. Yet many or most coral reefs within the larger regions of coral reef development share a surprising degree of similarity in terms of basic reef types and ecological interactions.

The formation of coral reefs is an ancient and complex process involving numerous kinds of marine life along with a variety of chemical and physical processes. Thus, to fully answer the question, “how are coral reefs formed?” we must look in turn at each of the main components of coral reefs, as well as the physical and oceanic processes that contribute to reef formation.

Hard Corals

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 the slow, cumulative efforts of just one particular group of simple, tiny animals called the hard (Scleractinian) corals.


Coral Reef Info

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.


A most unusual and advantageous aspect of hard coral biology is that their body tissues serve as a home for numerous, tiny algal plant cells called zooxanthellae. These biological partners (symbionts) provide the coral polyp with vital nutrients produced through photosynthesis. In return, the plant cells gain the benefit of a ready source of otherwise scarce nutrients which are produced by the polyps’ metabolism.

zooxanthellae coral anatomy

Tiny algal cells called zooxanthellae live within hard coral polyp tissues.

In truth then, hard corals are composite organisms, part animal and part plant; and this is in no small part a reason for their tremendous success in waters where dissolved nutrients are far from plentiful.

Coral polyps also feed on tiny planktonic creatures suspended in the water column. These are captured by the tentacles and passed to the gut for digestion, with the resultant nutrients shared by the plant cells. In most species the tentacles are used only at night, when they are relatively safe from hungry fish; during the daylight hours they are retracted into the safety of the protective skeleton.

As adults, hard coral animals exist as parts of large cohesive assemblages called colonies, in which only the outermost layer is composed of living polyps. Each coral colony is composed of many genetically identical individual polyps, with succeeding generations 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.

Each species of hard coral forms colonies of characteristic shape, size, and color, but these traits are subject to modification by local environmental conditions such as wave action, currents, prevailing winds, etc., and are therefore often quite variable. Oceanic and geomorphic processes are therefore partially responsible for explaining how are coral reefs formed.

Hard coral colonies generally exhibit one of three basic growth forms: massivebranching, or plate-like.

However, some species form colonies that do not readily fit into any of these general “shapes”. Although many species may usually be readily recognized by the characteristic appearance of the colony, identification in some cases requires close and careful scrutiny of the form and pattern of the individual polyps that make up the colony’s surface.

Coralline Algae

A key part of answering the question “how are coral reefs formed?” lies in the fact that growing upon and amidst the coral colonies are an unusual group of plants, called coralline or calcareous algae. These coral reef plants, like the hard corals themselves, also incorporate calcium carbonate into their bodies, thereby a pivotal role in helping to build and cement the reef into a cohesive structure.

Other Contributors

Another quite different type of coral (Millepora spp., commonly known as fire coral, also contributes to the framework of many shallow reefs of both the Indo-Pacific and Greater Caribbean regions.

Upon the solid structural foundation built mainly by hard corals and calcareous algae grows a superficial layer of spongesoctocorals, and numerous smaller less conspicuous kinds of animals and plants that together provide additional dimensions of structural and biological complexity and diversity to coral reefs.

So, to answer the basic question “how are coral reefs formed?”, we must consider not only the growth of hard coral colonies, but the contributions of a host of different types of marine life as well as dynamic forces of the ocean and the earth itself.

Our section on “Additional Resources” can provide more information related to the question, “how are coral reefs formed?”


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