As noted in the introduction to this web site, most people's impressions of corals and coral reefs are governed by their perception of the solid reef structure primarily composed of calcium carbonate. Over thousands of years, scleractinian corals and coralline algae have built the framework that we recognize as a coral reef.

The white calcium carbonate skeleton of a hard coral is deposited by a thin layer of cells known as the calicoblastic epithelium. Skeletal morphology is primarily controlled by genetics. The shape of the skeleton which protects the colony of polyps varies with the species, resulting in a number of characteristic shapes that allow even the uninitiated observer to distinguish between many coral species.

The process of skeleton deposition appears to be similar for different coral species, and it can be altered by environmental factors. For example, corals that live in deeper waters tend to form plate-like colonies, probably to maximize exposure of the symbiotic algae to available light, whereas in shallow waters the typical colony shape for the same species is more hemispherical. Wave action, sedimentation, and tissue injuries also contribute to shaping the colony.

Skeletal deposition can change as a result of the actions of mussels, barnacles, Christmas tree worms, and commensal crabs, all of which may bore holes in the coral skeleton and cause the coral to change the pattern of skeletal deposition. To the untrained eye, any of these changes in the shape or form of the skeleton may appear to be a "disease." However, the presence of a distinct hole in otherwise healthy coral skeleton often is a sign that the boring organism is a commensal one rather than a pathogenic one.

Other skeletal anomalies are caused by changes in the coral cells that deposit the carbonate skeleton. Two such changes are hyperplasia and neoplasia.

Hyperplasia is:

A biological process that leads to an increase in the number of cells in a tissue or organ, thereby increasing the bulk of the tissue or the organ.

A hyperplasm is a mass formed through the increase in the number of cells.

Neoplasia is:

A pathologic process that results in the formation and growth of an undifferentiated mass of cells.

A neoplasm is an abnormal mass of tissue that grows through the proliferation of undifferentiated cells. These cells grow and multiply more rapidly than normal and lack the structural organization and function of the normal tissue.

Skeletal anomalies have been described in hard corals that are the results of both processes. A hyperplasm often is apparent because portions of the coral tissue and skeletal elements look larger than those around them. The tissue and skeleton of a coral affected by a neoplasm is unlike the surrounding portions of the coral.

To see examples, click on a topic below: