The beauty of the coral reefs is matched only by their delicacy, making them particularly vulnerable in our increasingly polluted world. Global warming has been named as the chief culprit in the diminishing health of the reefs. But there are a number of other factors at work too, as University of Illinois researchers revealed last week. They conducted a study off the coast of the island of Curacao, near the Venezualan coast, and found that human sewage and shipyard discharge are giving rise to a lethal disease in coral. What is coral, and what are the many threats it faces?
oral is not the plant or rock that many people think it is. Rather, it is composed of fragile animals called coral polyps, each no larger than a pinhead. These animals form a thin layer on large coral reefs, which are the mounds of dead coral polyp skeletons, built up slowly layer upon layer. Different reef species grow between 5-200 millimeters (up to 8 inches) per year. Some of the oldest existing reefs are thought to be 5,000 to 10,000 years old.
There are over 2,500 species of coral. These invertebrate animals have soft, sacklike bodies. They have a mouth encircled by stinging tentacles called cnidae, which they use for feeding. Hard coral polyps use the calcium carbonate from the seawater to build a hard, cup-shaped skeleton. These limestone skeletons attach themselves to the reef, while the top part waves freely for the coral to feed.
When corals die, their skeletons remain behind on the coral reef, contributing to its slow formation. There are also soft, non-reef building corals such as sea fingers and sea whips. (References to coral in this article mean the more common hard coral.)
Coral polyps eat tiny single-celled algae called zooxanthellae, which live within the coral's tissue. These tiny algae are plants that use sunlight in the photosynthetic process, thereby requiring that corals grow in clear, shallow water, where the sun can reach them.
The zooxanthellae give coral its color. They share a symbiotic relationship: the algae provide the coral polyps with nutrients and the oxygen and carbohydrates required for producing the skeletons; the polyps provide the algae with a home and with carbon dioxide for photosynthesis.