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?

Coral: Fragile Animals
Coral 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.

• You can learn about the role of sunlight and carbon dioxide in photosynthesis in the Biology Gateways activities, The Role of Light in Photosynthesis and Fueling Photosynthesis with Carbon Dioxide and Water.

Coral reefs provide a barrier for coastal areas by breaking the waves of severe storms. They help prevent coastal erosion, flooding, and the loss of property along the shore. The species in the reef — like seawood and mollusks — contribute a valuable source of protein in the diets of people living along the coast. Many coastal communities also benefit from the tourism and fishing generated by coral reefs.

Like tropical rainforests, the rich biodiversity of the coral reef ecosystem may serve as a source for unique chemicals with medicinal properties, just waiting to be explored and discovered. Coral itself can be used in human bone grafts. Sponges growing on reefs can provide virus-fighting agents.

• Cyanide fishing: Fishermen stun tropical fish by squirting cyanide into the reefs. This technique enables them to capture live fish for aquariums. In the process, the cyanide poisons both the reef and many of the invertebrate species living in its ecosystem.



• Pollution: Sewage, fertilizers, and pesticides all originate on land and often run off into the ocean. Since coral reefs are close to the shoreline, they are exposed to these pollutants in high concentrations. The University of Illinois study found that human sewage and shipyard discharge are making coral vulnerable to a deadly disease known as "black band disease." Another danger to the reefs are oil spills: these generally occur close to shore, often when the tanker runs into rocks in the shallow waters.



• Global warming: As global warming causes ocean temperatures to rise, coral loses the zooxanthellae that gives it its nutrients and colors. The coral begins to starve and turn white, a phenomenon called "coral bleaching." Higher sea temperatures and the spread of dust blowing from drought-ridden Africa are also causing a rise in coral disease. As the atmosphere's temperature rises, scientists expect that rapid melting of glaciers and sea ice will result in a rise in sea level. Since sea level may riser faster than reefs grow, the coral reefs may find themselves in water too deep for the zooxanthellae to receive the sunlight necessary for photosynthesis.



• Overdevelopment: Development of coastal communities has also taken a toll on coral reefs. As humans build extensively near the shoreline, there is more runoff polluting the water. Channels are dredged for commercial and recreational ship travel, damaging reefs in the process.



• Reckless recreation: People themselves pose a threat to coral reefs. Tourists collect pieces of coral as souvenirs — whether by breaking a piece directly from the reef or by buying it in a tourist shop. Boats sometimes anchor on reefs, damaging them in the process. Divers sometimes entangle their gear in reefs. Snorkelers also present an unexpected problem: reefs close to shore near major tourist areas have been damaged by poisons from snorkelers' sunscreen. The irony, of course, is that people go to enjoy the reefs and then inadvertently destroy them by not showering before they snorkel.

Such conservation efforts will be key to securing the future of the world's coral reefs. Protecting this delicate web of marine life is part of our essential duty on Earth. If you're interested in finding out more or becoming involved in conservation efforts, you'll find plenty of useful resources above, on the right-hand side of this article.