Anatomy of Corals

Coral reefs consist of many diverse species of corals. These corals in turn are made up of tiny organisms called polyps. The structure of the polyps and the skeleton of the coral is a rather simple combination. A polyp is made up or two cell layers: the epidermis and the gastrodermis. The non-tissue layer between the gastrodermis and the epidermis is called the mesoglea.

The polyp contains mesentery filaments, which contain nematocysts used in food capture, a pharynx, endothecal dissepiments (horizontal layers of skeletal material) and the columella (the central axis of the corallite found below the mouth). The corallite is the part of the skeleton deposited by one polyp. The skeletal wall around each polyp is called the theca. Other structures include the calice (the upper opening of the corallite), the coenosarc (the coral tissue that stretches over the surface of the coral between the polyps), the coenosteum (the skeletal material around the corallites), and the corallum, which is the skeleton of the coral. The coral anatomy also includes calcareous plate-like structure known as septa. The septa radiate from the wall to the center of the corallite. There are two types of septa: insert septa which lie below the corallite wall and exsert septa which protrude above the corallite wall.

Corals are of two types: perforate and imperforate. Perforate corals have porous skeletons with connections between the polyps through the skeleton. Imperforate corals have solid skeletons.

Many corals have different growth forms. They can be plocoid as in Tubastrea coccinea (orange cup coral) and Favia fragum (golf ball coral). They can also be meandroid in which corallites form a series within the same walls, as in the species Dendrogyra cylindrus (pillar coral). Other growth forms include cocoid, spherical shaped and phalecoid, as in Eusmilia fastigiata.