Gorgonian Coral
Among the more familiar corals are the gorgonian, or horny, corals of the order
Gorgonacea. This group includes corals commonly known as sea feathers, sea fans, and the
red coral (genus Corallium) often used in jewelry. Members of the gorgonian coral group
are often present in reefs. The body consists of an erect central rod of organic material
called gorgonin, surrounded by a cylinder of calcareous spicules and by the polyp
organisms, in branching, plantlike forms. The spicules contain a pigment that gives the
gorgonian an orange or purple color. Yellow or brown colors may be caused by
zooxanthellae. In Corallium, the red jewelry coral, the central rod of gorgonin is
replaced by a solid rod of fused red calcareous spicules that can be polished.
Soft Corals
Soft corals differ from gorgonian corals in that they lack an axial skeleton. They derive
their support from a gelatinous, spicule-filled mesoglea. Soft corals form rubbery,
irregularly shaped colonies.
Stony Corals
Stony corals, or the true or hard corals, remove calcium from sea water and deposit it
beneath the living tissue as a white, external skeleton of calcium carbonate (limestone).
Many kinds of organisms bore into coral skeletons, leading to the coral's eventual death.
Several groups of stony coral are known to have been abundant in ancient seas. Some types
are commonly found in limestone formations. Best known are the reef-forming corals that
contain zooxanthellae which aid in very rapid skeletal growth, as compared to
non-reef-forming species which exhibit slower growth.
Fire Corals
The fire, or stinging, corals, belong to the class Hydrozoa, whereas all other corals
belong to the class Anthozoa. Fire corals produce hard, smooth skeletons containing many
tiny polyps. The skeleton is light brown because of zooxanthellae. The common name is
derived from nematocysts that can inject poison through the skin. Fire corals bud off
tiny, sexually reproducing jellyfish, and the fertilized egg becomes a planula larva which
develops into a new colony.