Updated by: Arabinda Naik
Skeleton of a solitary coral is known as corallite. It is a calcareous exoskeleton secreted by epidermis. In a colonial coral, corallites of individual polyps fuse together to form a skeletal mass, called corallum. Each corallite is like a stony cup with a basal part or basal plate, and a cup wall or theca, enclosing the aboral portion of polyp. Cavity of cup contains several vertical radiating ridges called sclerosepta, proceeding from theca towards the centre of cup. Inner ends of sclerosepta are fused to form an irregular central skeletal mass or columella. The corallite provides protection for the polyp and its soft tissues.
The coral skeleton is a complex structure made up of several different parts. These parts work together to provide support for the coral polyps and to protect them from harm.
Corallite: The corallite is the skeleton of an individual coral polyp. It is a cup-shaped structure with a central cavity called the calyx, in which the polyp sits. The walls of the corallite are made up of several different structures, including:
Septa: Septa are vertical ridges that project into the calyx from the walls of the corallite. They provide support for the polyp and help to protect it from predators.
Costae: Costae are extensions of the septa that extend beyond the walls of the corallite. They help to give the coral its characteristic shape and to increase its surface area for capturing sunlight and plankton.
Theca: The theca is the outer wall of the corallite. It is made up of a layer of calcium carbonate that is secreted by the polyp.
Coenosteum: The coenosteum is the porous skeleton that links polyp corallites within a colony. It is made up of a network of interconnected spaces that provide support for the colony and help to protect it from damage.
Basal plate: The basal plate is the foundation of the corallite. It is a flat plate of calcium carbonate that is secreted by the polyp and that attaches the coral to the substrate.
Epitheca: The epitheca is a thin layer of calcium carbonate that covers the outside of the corallite. It helps to protect the coral from erosion and from damage caused by algae and other organisms.