by Sandy Fritz
As the sun rises, the sea turns aqua, revealing dark shapes grouped offshore: an entire city - the reef. Fluorescent sea fans sway in the currents, blood-colored crabs amble across sandy patches, yellow- spotted fish, silvery barracuda, giant manta rays, and a host of other, improbable creatures dart from point to point, overwhelming the eye.
While the land surfaces of our planet have been extensively mapped and explored, much of the sea and its marine metropoli are still worlds waiting. A mere 10 percent of the known reefs in the Pacific Ocean have been visited by scientists; the extensive reefs rimming the Bahama Banks archipelago are poorly known; and large segments of the most luxuriant array of reefs in the Western Hemisphere and the second largest barrier reef in the world, the Belize Barrier Reef complex, remain unmapped.
This is a surprising state of affairs for an ecosystem thought to house one in every four marine species on our planet. For species diversity, coral reefs rival terrestrial rain forests, and as with rain forests, the untapped potential for science, especially medical science, is enormous. (See "Fishing for Cures.")
Coral reefs also resemble rain forests in the way the lush, complex ecosystem sustains itself despite the paucity of nutrients. The crystalline waters of the tropics achieve their breathtaking clarity because there are virtually no nutrients in the water. Yet reef communities thrive in these ecological deserts, largely because of the ceaseless labors of an animal called the coral polyp.
Builders of the stony masses we call reefs, polyps are the cornerstone of the community. Colonies of coral polyps enter into remarkable partnerships with algae, allowing them not only to use solar energy to manufacture food by photosynthesis, but to consume small planktonic animals as well. (See "The Reef Builders.") One of the most beneficial results of the partnership is its mysterious ability to accrete calcium carbonate from seawater to fashion stony strongholds. The shelter coral polyps provide for themselves attracts the myriad creatures that compose the reef ecosystem.
Many creatures that make coral reefs their homes live in symbiotic relationships. Symbiosis - literally, "together living" - takes many forms on the living reef. Mutualistic symbiosis, where both partners benefit from the association, exists not only in corals but in some jellyfish and even giant clams, which incorporate algae in their tissues to reap the benefits of photosynthesis.
Another remarkable form of mutualistic symbiosis can be observed between small fish, such as gobies, and bottom-dwelling shrimp. Both creatures live together, sharing the same burrow, and as the nearsighted shrimp spends most of its time digging and maintaining the dwelling, the sharp-eyed fish keeps a wary watch for predators. At the first sign of danger, the fish flicks its tail, and both creatures dive for cover.
Commensal symbiosis, in which one partner benefits while the other is neither harmed nor benefited, is especially common. Examples include shrimp that ward off predators by nestling in the venomous spines of sea urchins, or crabs that protect themselves by pirating formidable, stinging sea anemones and securing them to their shells. A healthy reef supports these and many other symbiotic relationships. But when the reef is stressed by nature or by human abuse, the relationships can break down and the reef becomes a poor relic of its former wondrous self.
Human-generated stress can be especially damaging for coral reefs.
Pollution from pesticides and soil runoff is smothering sections of the small reef that fringes Costa Rica in the Caribbean. Reefs off Jakarta, the capital of Indonesia and a city of 9.5 million, were slowly killed by untreated sewage, reef mining, and soil runoff. Dynamite fishing and large-scale, illegal harvesting of coral for the aquarium trade have devastated reef communities ringing the Philippines. Even ecotourism - hoped by many to be the saving grace for the world's wild places - scars reefs, as inexperienced divers inadvertently mar the very places they treasure. A single touch can be fatal to ultrasensitive reef builders.
"Reefs are among the oldest ecosystems on earth," says Ian Macintire, a marine biologist with the Smithsonian Institution in Washington, D.C. "Yet we are rapidly destroying an ecosystem we are only beginning to understand."
Nature itself can be cruel to the coral reefs. Tropical Storm Gordon, with its three days of strong winds and pounding high seas in 1992, did even more damage to the reefs in Florida's Biscayne National Park than did Hurricane Andrew's three-hour tantrum in 1993. Gordon shattered forests of elkhorn coral and fatally enveloped the heads of other hard corals in sand. Recovery is proceeding very slowly.
Recent discoveries have linked over-warm water, sometimes spawned by the annual event known as El Niño: (see "El Niño: The Weathermaker"), to a blight known as coral bleaching, in which the algal symbiont in coral polyps dies, turning the colony bone white. Should the warmth linger too long, entire reefs can be killed, as has happened in the Caribbean and elsewhere. And diseases, such as the recently discovered coralline lethal orange, can threaten whole communities. Even diseases that attack just one member of a coral community can open the way to general destruction. In the waters off Jamaica, herbivorous sea urchins fell victim to a complex sickness that thinned their ranks, causing the unchecked growth of algae that overran heads of coral, all but slaying the island's reefs.
These and other natural disasters have long challenged the hardiness of reef communities but have never managed to snuff out the spark of life. The immediate ancestors of today's reef-building corals date back 200 million years and have weathered no fewer than three mass extinctions, including the catastrophe 65 million years ago that wiped out the dinosaurs and many other creatures. And as continents drifted and jostled, as global temperatures waxed and waned, as ocean currents shifted and sea levels rose and fell, the reef survivors had always managed to regroup and form new communities - at least until now.
Ocean Planet Exhibition Floorplan
gene carl feldman (gene@seawifs.gsfc.nasa.gov) (301) 286-9428