Virgin Islands National Park - Castles of Stone

Castles of Stone
Throughout the park the sea is never far from sight or mind. Sometimes during the winter months, surf caused by swells from distant Atlantic storms pounds the northern and eastern shorelines with waves up to eight feet high, but such fury occurs infrequently and lasts only a day or two. Normally, these waters are calm and quiet, with tidal ranges averaging only a foot or less. Warm, placed, and crystal clear, the sea is ideal for snorkeling or scuba diving. And anyone who tries either sport soon discovers that these Caribbean waters contain a fascinating and stunningly beautiful world, the realm of the coral reef.

People often have the mistaken impression that reefs are merely rocks, but in fact they contain and are constructed by living creatures. The individual coral animal, called a polyp, is small (generally a fraction of an inch in size) and usually tubular in shape, with a ring of tentacles resembling those of its larger relatives, sea anemones. Equipped with stinging capsules, the tentacles sway about, gathering the tiny drifting animals that are the coral's food. But that is only part of the story, for certain species of algae grow inside the coral itself. Numbering in the thousands in a single polyp, these minute plants are responsible for the great variety of vivid colors among the corals. More important, the algae absorb carbon dioxide and through the process of photosynthesis produce oxygen and certain nutrients vital to both the coral and themselves.

The coral animal protects its soft, vulnerable body from predators in two ways: it secretes calcium carbonate, the substance that forms limestone, to create a hard shell around itself, and it finds safety in numbers by living in colonies of thousands -- sometimes millions -- of its kind that grow into large, formidable rocky masses. What we see, then, beneath these waters are complex and colorful fortresses created by a simple animal. They are, however, castles that take a long time to build; a brain coral just a few feet in diameter may be centuries in age.

Not all corals form such hard, rocky masses. Polyps of the type known as gorgonians -- sea fans, sea whips, and the like -- secrete only small amounts of carbonate and are instead connected together by a flexible horny material that allows them to sway rhythmically with the undersea currents like grasses in the wind.