Hard Corals

Some species of coral have hard protective skeletons around them, and others do not. Although both kinds are found on coral reefs, it is the hard corals that actually build the reef structure and are responsible for creating the reef environment.

Looking more like rocks or plants than animals, hard corals are polyps surrounded by protective skeletons. They vary in diameter from the size of a pinhead to more than 3 feet (0.9 m).

Coral skeletons are made of calcium carbonate, also known as limestone. Each individual polyp continually secretes a cupshaped skeleton, or corallite, in which it sits. Throughout the polyp’s life, the cup expands in height and diameter. With growth of the cup, the polyp is pushed up so that it always remains on the surface of the skeletal mass. Polyps often grow on top of large colonies, each new generation building atop the previous ones. The skeletons of the entire colony are called a corallum.

To construct their skeletons, corals absorb calcium ions from the seawater. Within their cells, they assemble these ions into crystals of calcium carbonate. Each newly formed crystal is held in the cell within a membrane-lined sac. Eventually, a crystal is pushed out through the cell membrane to the external surface. Once extruded to the coral’s exterior, the crystal acts as a nucleus for continued crystal growth. The process of gathering calcium ions and using them to make skeletons is very efficient, and the amount of calcium carbonate that coral forms is staggering. Each day, the corals within 1 square mile (2.59 sq. km) of a reef are capable of creating almost 16 tons of calcium carbonate.

The presence of zooxanthellae in corals speeds the rate at which calcification occurs. In experiments, scientists removed the zooxanthellae by keeping corals in darkness for several months. Results show that corals without zooxanthellae grow very slowly. When their zooxanthellae are intact, corals can grow 14 times faster in the light than in dark. Furthermore, corals grow faster at noon, when the sun is bright, than at other times of the day.

There are hundreds of species of hard corals, and they show tremendous variation in size, shape, and color. Most hard corals are assigned common names based on something they resemble; for example, one of the branched species, shown in the lower color insert on page C-1, is called elkhorn coral (Acropora palmata) because it has thick and solid antlerlike branches. Colonies of elkhorn coral prefer water in the shallow, wave-pounded regions of the reef. Elkhorn corals are broadcast spawners that reproduce in August or September.

Acropora palmata
Acropora palmata

New colonies are fast growing, elongating their branches at a rate of 2 to 4 inches (5 to 10 cm) per year, and most colonies reach their maximum size in 12 years. Because elkhorn coral lives in high-energy areas, storms often break apart the colonies. This process, known as fragmentation, is an important part of the coral life cycle. Broken branches reattach to the reef and grow into new coral colonies.

Another branching variety, staghorn coral (A. cervicornis), lives on the upper reef slope and produces cylindrical branched colonies in blue, green, or cream colors. Finger coral (Porites compressa) forms branched, fingerlike skeletal structures. The tips of each branch are rounded or flattened, and the color of the coral can vary from light brown to yellow. Many coral species are shaped like rocks. Boulder coral (Montastrea annularis) is a brown to green species that can develop into mounds up to 10 feet (about 3 m) high. The yellow porous coral (P. astreoides) forms small, bright yellow boulders with bumpy surfaces. Common brain coral (Diploria strigosa), featured in the upper color insert on page C-2, is a medium-sized hemisphere outlined with regular hills and valleys that resemble brain matter. Other species of brain corals include large-grooved brain coral (Colpophyllia natans), which is pale brown with green-tinted valleys, and depressed brain coral (D. labyrinthiformis), which has hills with distinctive shallow depressions.