What is Coral?

Corals belong to a group of animals called cnidarians. This group encompasses hard and soft corals., sea fans, gorgonians, hydroids, jellyfish, and sea anemones Although the group is remarkably diverse, there are a few features shared by all. They have a free-swimming larval stage and a simple body plan: a central mouth, through which material passes in and out of the body, and a ring of tentacles. The other distinguishing feature of the cnidarians is the presence of nematocysts (stinging cells), which are used to catch prey.

Individual corals are known as coral polyps: each is a single animal. In isolation a coral polyp looks very similar to a sea anemone, but unlike sea anemones, which live separated from each other, corals form colonies. A few species remain solitary, but in most cases, new polyps bud off the initial founding polyp, and gradually colonies of thousands or even millions of polyps will grow, each connected to its neighbors by living tissue.Freed from the limitations of living alone, colonies can grow to immense sizes and live a very long time.

Corals come in all shapes and sizes, but the basic plan is the same, polyps live as a surface layer on some sort of structure, be it hard and inflexible or rubbery. The hard, or true corals, for which reefs are most famous, build limestone skeletons beneath the living tissue. Soft corals, as their name implies, do not form solid skeletons; instead they secrete limestone crystal structures called sclerites, which are embedded within a jellylike matrix beneath the polyps.When they die, little is left of most soft corals as the limestone element of their makeup is so small and easily breaks up.

By contrast, some sea whips and fans and, notably, the black corals, have such dense skeletons of limestone, protein, and minerals that they are highly durable and are collected and polished for jewelry. The skeleton of black corals is more dense the slower it grows, and it grows more slowly at depth. So it is the very deep, older colonies that are most highly prized.

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