Spectacular snorkeling and scuba diving bring many tourists to St. John each year. They eagerly anticipate the experience of exploring the underwater wonderland that they have read about in travel books or heard about from friends. Where do these visitors go when they don their masks, fins and snorkels? First and foremost they seek out that diverse and colorful underwater community called the coral reef.
If our underwater explorers were to venture just a short distance away from the reef, they would encounter an almost barren sandy bottom with a lot less going on and a lot less to see. In general, life is sparse in tropical seas; except for around the coral reef, which is the underwater equivalent of an oasis in the dessert.
There is a richness of life on the reef that defies the imagination. The coral reef provides an environment for all the major phyla, or classifications, of plants and animals on the planet including thousands of species of fish, corals, sponges and marine plants. In fact, twenty five percent of all marine species live around coral reefs even though they cover only a tiny fraction (0.2%) of the ocean floor. The only other biological community on Earth containing such a large diversity of life forms and having an equivalent ecological importance is the tropical rainforest.
The coral reef is especially important to us on St. John. The reef protects our coastline from the full force of the sea, preventing erosion of the coast and allowing for the establishment of other important marine environments such as mangroves and undersea grasslands, which serve as nurseries for most of our marine life. Moreover, the soft white sand of our world-renowned beaches is a product of the coral reef. Without the coral reef there would be no beaches, no fish, no fishing and more than likely, no tourism, no jobs and no money.
The basis of the coral reef is a limestone mass formed by layer upon layer of the skeletal remains of generations of tiny animals called coral polyps.
Coral polyps are members of the Phylum Cnidaria (Nigh-DARE-ee-uh). Animals of this group live in the sea. They are simple bag or cup-shaped animals with only one opening into their digestive tracts. All Cnidarians have tentacles surrounding this opening, which contain coiled threads of stinging cells called nematocysts.
There are two basic types of Cnidarians, medusa and polyps. Medusa float around freely and include jellyfish of all types. Polyps attach themselves to the bottom and live rooted to one spot. Examples of polyps are sea anemones and corals.
The main difference between sea anemones and corals is that sea anemones live individually, while coral polyps form colonies, which over time may become quite large. Generations of coral colonies make up the coral reef, which can be immense. Australia's Great Barrier Reef, for example, would cover an area extending from St. John to Miami with a width of more than 40 miles in some places.
In her book, The Nature of the Islands, Virginia Barlow writes, "Some Caribbean reefs contain several times the building materials that exists in New York City".
The basic element of this magnificent environment is the diminutive coral polyp, an animal so small that as many as 250 could occupy an area of one square inch.
To find out why are reefs found where other life is scarce, see the Why
Coral Reefs are Only Found in the Tropics page.