Reefs of the Yucatan Peninsula

Undersea

The material that forms all reefs is essentially the same as the substrata on the peninsula: limestone. On the dry land the limestone is formed by untold generations of tiny sea crustaceans, mollusks and polyps that died eons ago. But on the underwater shelf that runs down Quintana Roo's eastern Caribbean coast, those sea creatures are alive and growing in one of nature's strangest life form combinations: coral. Coral is the common name for several species of coelenterates, a large group of invertebrate animals. The class has over 6,000 living species and scientists have identified over 6,000 different extinct species. The phylum include jellyfish, hydras, sea cucumbers and sea anemones. What they all have in common is a cylindrical shape, a gastrovasular cavity and a 'mouth' surrounded by tentacles. Coral polyps (the living organism) extract calcium from sea water and create calcium carbonate 'skeletons' that offers physical support and protection when they're threatened.

Most coral live in colonies, attaching themselves first to limestone on the sea bed, then to each other. As polyps die, leaving hard stony limestone skeletons as a base, new living coral builds upon them. In time, this forms the enormous underwater structures we know as reefs. Coral reproduce either asexually from buds or with a larval polyp, called a planula, which floats away and settles elsewhere to begin a new reef colony.

To form and grow, coral reefs need four critical environmental factors; the first is sea water temperature between 23ø and 27øC (73 - 80øF). The warmest temperature that coral will endure is 30øC (86øF). Coral also needs water shallow enough to allow sunlight to penetrate. Symbiotic algae, (zooxanthellane) lives in the tissues of the living coral and photosynthesizes nutrients and oxygen that the coral uses. These algae provide 'stony,' or hard coral, with a brown or yellow color. Other colors found in coral - red, pink, orange, black and purple - come as a result of pigment-colored calcareous spicules. (Got all that? There will be a test at the end of the book.)

Another condition effecting coral growth is salinity, which should be between 30 and 40 parts per thousand. If the sea water is either too saline or not saline enough - perhaps due to freshwater runoff from the mainland or to unusually heavy amounts of rainfall - coral will not grow.

Lastly, coral thrives in areas of strong wave action, which aerates the water. Along the Yucatan coast the Caribbean Sea, unlike the Gulf, has some very active surf. Waves help prevent silt from accumulating in the reef and suffocating the living coral.

In response to environmental stresses - such as higher water temperatures - coral will expel their algae, thus 'bleaching' the reef of its color, leaving it ghostly white. Unfortunately, without its beneficial algae the reef begins to die. A reef can come back from occasional mild periods of bleaching, but a sustained warming of the water will kill it completely. Scientists, marine biologists and zoologists are still unsure how global warming is affecting the oceans or whether it is a major contributor to bleaching. However, oil spills, water pollution, damage from ships and attacks of the coral-eating starfish known as the 'crown of thorns,' has placed many of the world's reefs under additional threat.

The reefs off Quintana Roo, which are known collectively as the Belize Reef, have battled some bleaching in recent years. Another threat comes from divers and snorkelers, usually amateurs, who break off pieces and touch or stand on the reef, killing the fragile living organisms. If you're underwater, look but do not touch.

The Yucatan claims all three of reef types: Fringe reefs, located close to the shore in very shallow water, like El Garraf¢n Park on Isla Mujeres; barrier reefs, which lie farther offshore and are separated from the land by lagoons or water generally no deeper than 10 meters (about 30 feet), such as at Akumal: and atolls, ring islands that form a natural lagoon in the middle. Atolls are often associated with extinct underwater volcanoes and are found mostly in the South Pacific, but Quintana Roo's Chinchorro coral atoll sits only 22 km (14 miles) off Xcalak. Its inner lagoon varies in depth from two to eight meters. Beyond the reef, the ocean floor drops away to depths of 150 to 200 meters (450-600 feet).