Rhizangiidae, Caryophylliidae, and Oculinidae - Hard corals
Hard corals, also called stony corals, are in the Order Scleractinia. Most are colonial polyps that are housed in a solid calcareous skeleton (similar to an apartment building with many tenants). The polyps secrete a calcareous cup-like structure, the corallite, around itself for protection. The corallites are usually joined to each other by a calcareous material called the coenosteum. Many hard corals build massive reefs, but some build smaller structures or are encrusting (growing on other hard objects). A few form large individual polyps. Hard corals feed by extending their polyps to capture food from the water column, though many corals get most of their energy from tiny microscopic algae (called zooxanthellae) that live within the polyp's tissue. Zooxanthellae are also responsible for the beautiful colors exhibited by corals, colors ranging from orange, yellow, green, blue, red and purple. These corals can expel their zooxanthellae when under stress producing a phenomena called "coral bleaching". Generally, non-reef-building corals lack zooxanthellae while reef builders possess numerous in their tissue. Because of the light requirement of the zooxanthellae, most reef-building corals are limited to shallow depths (<50 m) and clear water.
Within the colonies, reproduction is asexual. Polyps reproduce by budding off a part of its body which develops into a new polyp. The new polyps stay with the colony and extend the colony's size. Sexual reproduction may also occur where sperm and eggs are "broadcast" into the water, and the sperm fertilizes the egg in the water column. These new polyps, called planulae, can swim or crawl and generally initiate a new colony.
There are 29 families of scleractinians. Corals of the family Rhizangiidae are non-reef builders. They form small, encrusting colonies of corallites joined at their bases. Their septa (skeletal radiating plates in the corallites) are not fused together and are dentate. They are a shallow water species.
The family Caryophylliidae are encrusting corals that are knob-like in appearance. They may be colonial or solitary. Some of the solitary species are free-living, unattached to hard substrates. Most caryophyllids lack zooxanthellae, thus lacking the brilliant colors of some other coral families. All have a fine external membranous layer, called the epitheca, surrounding the corallites. The corallites of some of the solitary species are among the most beautiful designs in nature, the septa usually displaying a symmetry in multiples of six.
The family Oculinidae are colonial corals that can be encrusting or branching and form a multitude of shapes such as bushes, columns, flat structures or domes. The corallites are widely space and connected by a smooth skeletal wall (coenosteum). The walls of the corallites are smooth but may have small costae (ridges) at the top rims. Their septa extend upward and outward, giving the corallites a spiky appearance, which is characteristic of the family.