Cities beneath the sea

Imagine animals -- about the size of pencil erasers -- so efficient in their environment so harmonious with the plants living within their tissue, that they can fashion some of the largest hard structures on Earth out of seawater and sunlight. Some of their creations, such as Australia's Great Barrier Reef, stretch up to 1,250 miles in length. By placing tiny calcareous house upon house, these little builders maintain their position in the well-lighted, shallow layer of ocean, pushing their cities upward toward the sun as the sea floor sinks beneath then. Eniwetok Atoll, supreme example of animal architecture in the South Pacific, is a circle of calcium carbonate rock twenty-one miles wide, twenty-six miles long, and five thousand feet thick.

These master builders are corals. Soft-bodied invertebrates related to jellyfish, they filter tiny organisms from the water and secrete hard, calcite shells around themselves. Once a year, some corals reproduce by broadcasting millions of sperm and eggs into the open sea, while others brood their larvae to an advanced stage and their release their. Coral colonies cover about 232,000 square miles of the sea floor and produce between 100,000 and 500,000 tons of limestone per square mile each year. Their constructions remain unparalleled in size by the works of any living creatures on Earth, including humans. Geologic processes have folded some ancient reefs into mountains and buried others, compressing the organic material into some of our major oil deposits.

Reef-building corals have changed little since the modern species evolved some 230 million years ago. Distributed within the tissues of all reef-building corals, and vital to their existence, are a group of single-celled plants, or zooxanthellae, that convert sunlight into energy, for the corals. These algae are found in concentrations of up to a half million cells per cubic inch; about 8,500 of them would fit on the period at the end of this sentence. The tiny plants impart the rich hues of oranges, reds, purples, and yellows to corals and apparently energize them to build their limestone apartments at a speedy pace.

Keystone species of one of the planet's richest ecosystems, corals foster a biodiversity that surpasses even that of the rain forests. Reefs provide shelter, food, and breeding grounds for an estimated 35,000 to 60,000 species world-wide, including almost a third of the world's estimated 12,000 kinds of marine fishes. They not only are home to many species but also contain an uncommonly high number of phyla -- creatures as diverse as anemones, starfish, crabs, eels, sea slugs, and sponges.

Life on the coral reef follows a predictable cycle of day and night activities. As dawn breaks, oceangoing predators such as hammerheads, gray reef sharks, and barracuda patrol the reef lagoons and channels, looking for a morning meal of fish. Manta rays, mouths gaping to strain out plankton carried by the tides, cruise along the reef. Hawksbill turtles graze on sponges, bobbing along the walls and crevices to avoid predators.

As the sun rises higher, the day grazers emerge from their resting places. Brightly colored Caribbean long-snout butterfly fish use their snouts to pluck out soft coral animals from their homes, while parrotfish employ their beaklike mouths to bite off algae growing on the reef. Spotted eagle rays glide along the walls and nearby sand flats, stopping occasionally to forage for a mollusk meal.

Within entangled coral branches, territorial damselfish prune the algal gardens they so carefully maintain, even removing any trespassing sea urchins. The urchins themselves meander along and among the corals', grazing on the blue-green algae that rapidly accumulate. Pufferfish brave the urchins' spines to feast on their vulnerable undersides. Should a passing predator nab a balloonfish for a meal, the prey will quickly inflate to two or three times its normal size, causing the hunter to spit it out. Resembling elongated plants, trumpetfish fool predators by hovering vertically just above the reef while searching for a suitable meal. Sea horses wrap their tails around sea fans or grasses, where they remain anchored, waiting for small invertebrates carried by the currents.

As the sun sets, the night shift begins. Unfolding their tentacles in a hydraulic yawn, corals begin to capture passing plankton. An octopus ventures from its burrow in search of a feast of crab or claim, overturning rubble and groping in crevices with its sensitive arms. In turn, goldentail moray eels emerge from their rocky dens to hunt octopuses. Snappers, porgies, and jacks exit their nooks on the reef to graze nearby grass beds. Spiny lobsters crawl forth, foraging for invertebrate prey. Undulating along the reef wall, a Spanish dancer -- a type of sea slug -- grazes on corals, sponges, and anemones.

A reef is in some ways like a city, with various parts functioning to help maintain the whole system. Sponges act as water-treatment plants, circulating water and removing suspended particles that can reduce light. Parrotfish keep the coral skeletons free of suffocating algae, as did the long-spined Caribbean sea urchin until a still unexplained die-off of these creatures started in Panama. In the late 1970s, rampant algae began to dominate and destroy many of the Caribbean reefs, but in the past ten years, the sea urchin has been making a comeback.