Coral

V

Types of Corals

The classification of corals is complex. Most belong to a group or class known as Anthozoa, which has two broad groupings, the octocorals and the hexacorals. The octocorals have polyps with eight tentacles and a body cavity separated by eight mesenteries. The octocorals include soft corals and their relatives, the blue coral and the sea pens. The hexacorals typically begin life with six tentacles and mesenteries, but these quickly subdivide. The hexacorals include the stony and hard corals, but also noncoral groups such as the familiar sea anemones. A few other corals, notably the fire corals, belong to the class Hydrozoa.

Although the individual coral animal is described as a polyp, many colonial species form distinctive communal skeletons. In some cases scientists have found it more helpful to describe the shape and structure of these skeletons rather than the individual polyps.

A

Soft Corals, Sea Fans, and Whip Corals

Soft corals lack a distinct skeleton. Although they live in colonies, the individual polyps are fused into a complex body, usually strengthened by small lumps or spikes known as sclerites, which are made of protein and calcite. Soft corals come in a variety of shapes, including undulating sheets, upright mushroomlike shapes, and beautiful shapes that form branches.

A number of other octocorals have skeletons made from a hard or horny protein, sometimes strengthened with more brittle calcareous deposits. Some octocorals have skeletons that form branches. Branching forms include the red coral from the Mediterranean Sea. The skeleton of the red coral has been used to make jewelry for thousands of years. Sea fans are typically finely branching colonies that form broad sheets, measuring up to 2 m (6.5 ft) or more in diameter. Sea fans thrive where there are strong currents, which the polyps filter for food. Whip corals are a group of species that form long, whiplike strands with few or no branches.

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The organ-pipe coral is a relative of the soft corals and sea fans. It lays down a stony skeleton of limestone with distinctive red tubes around each polyp.

B

Blue Corals

The blue coral is another octocoral but one that has a limestone skeleton. It is classed in its own group or order known as Helioporacea. Blue coral colonies form pillars or wide ridges. When alive the corals are brownish in color. The limestone skeleton is a highly distinctive blue.

C

Stony Corals

Stony corals are the best-known and one of the largest groups of corals. They produce a limestone (calcium carbonate) skeleton, and most live in close association with zooxanthellae. Nearly 800 species of these corals are builders of coral reefs. Stony corals come in a vast array of shapes and sizes. Massive corals are often boulder-shaped, with smooth surfaces, or with deeply complex twisting lines earning them the name of brain corals. These corals may only grow a few millimeters a year, but may eventually reach several meters in diameter, as they can live for several hundred years.

Branching stony corals are much faster growing and some may extend their branches by 15 cm (6 in) a year. Other stony corals are also commonly named after their appearance, such as pillar corals, plate corals (resembling broad circular dinner plates), and lettuce corals (with twisting sheets). Encrusting corals tend to grow over the bottom and simply follow the contours of whatever lies beneath.

D

Black Corals

Black corals consist of about 200 species, all of which live in colonies. They have a stiff but flexible skeleton made of protein that is usually either a branching skeleton, as in the bushy black corals, or a long twisting thread, as in the wire corals. Black corals lack zooxanthellae and typically grow at depths of about 30 m (98 ft) since they do not require sunlight. The skeleton in some species is very dense and black in color, and is often collected to make jewelry. These corals are uncommon, however, and are slow growing, so they are quickly decimated unless the harvest collection is well managed.

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