As one does in dreams, floating effortlessly above the bright and unknown treasures of a new world, the diver hangs above a coral reef.
From the sunlit reef platform only a few feet below, down into darker and deeper zones at the foot of the reef, the strange shapes of a stone forest provide cover for small, brilliantly colored darting fish, and for larger ones that loom through the landscape like finny zeppelins. Squeaks, chirps, crackles, grunts and crunches make a continuous background of noise that reinforces the impression of a busy world hurrying about its business unconcerned - a world too interesting to leave unexplored.
Coral reefs are the mightiest structures ever built by any life form on this planet. The largest - Australia's 1,200-miIe-long Great Barrier Reef - contains more than 5,000 cubic miles of solid rock: limestone deposited a molecule at a time by small flower-like animals called polyps. They are the architects and landlords of the reef, for they created the habitat that shelters hundreds of other animal species - from shellfish that live imprisoned inside the coral's stony branches, to harpoon-throwing snails, to the pastel-colored coral-crunching parrot fish.
This is very much an inhabited and dynamic world, and the relationships among its creatures are complex. Indeed, the coral reef is the sea world's most complicated biotope, a word meaning a region with uniform environmental conditions for the plants and animals that live there. A coral reef is very high in the number of different species it includes. And its inhabitants exhibit a tremendous variety of diverging adaptations and fine specializations that suits each of them neatly for its particular role in the coral reef ecology.
That ecology is now a little better understood than it was, thanks to Aramco scientists' studies of the untypical coral reefs of the Arabian Gulf which were included in a 285-page illustrated book called Biotopes of the Western Arabian Gulf.: Marine Life and Environment of Saudi Arabia. Coral reefs occur in all the world's warm oceans where conditions are right - in particular where water salinity is not too high and water temperature does not vary too widely during the year - and the reefs of the Red Sea are famous for their variety and color. Yet large and healthy coral reefs and coral islands are also common in the salty, shallow Gulf, where conditions are far from ideal for their growth.
In adversity, character is revealed; that generalization is often useful in studying ecological systems. Thanks to the Gulf's adversely high salinities and adversely large swings in water temperature (see Aramco World, July-August, 1978), coral reefs there are less overwhelming than elsewhere in the world in the numbers of species of coral and associated creatures they support - and they are thus easier to study. Where a Pacific Ocean reef may support as many as 3,000 different species, a typical Gulf coral reef is home to between a third and half as many.
Even so, that is about twice as many different species as the Aramco study found in any other Gulf biotope. What accounts for the richness of life forms and the variety of species that make coral reefs a favorite goal of both amateur divers and scientists?
Coral polyps, according to their species, lay down their limestone deposits in different patterns; each type of coral has its own unique growth pattern that produces a characteristically shaped structure. Many of the creatures that live in, on or among the shapes exploit the structural peculiarities for shelter in their own, similarly individual, ways.
Branching corals, for example, are home to fish, shrimp and crabs that shelter in their branches. Massive brain corals are inhabited by shellfish that bore into the limestone and make their homes in the little caverns they excavate; some barnacles, and the females of a crab species, allow themselves to be overgrown, enclosed, protected and - ultimately - entombed by the living coral. Coral crabs, spiny lobsters and many fish species live in the innumerable nooks and crannies formed by the intermingling of coral species of different shapes, as do brilliant sponges, sea squirts, and some sea fans.
Other animals rely on the coral for food. Parrot fish feed directly on the living coral, biting off chunks of limestone, crunching it up with their horny beaks like children eating peanuts, digesting the living "skin" of polyps, and voiding fine white coral sand. Other species of fish poach the tiny plankton animals captured by the coral polyps' tentacles. Still other creatures inhabit the reef because the species they prey on live there: the beautiful textile cone snail prowls the coral for fish and other active animals, which it harpoons with a poisoned dart and swallows whole. Surgeon-fish, carrying scalpel-sharp bony knives on their tails, graze on the algae that grow on top of the sunlit coral.
The sunlight is important, for all reef-forming coral species are, literally inhabited. Within the polyp's tissues are microscopic photo-synthetic algae; these algae consume the carbon dioxide that the plankton-eating polyps give out, and themselves liberate oxygen that the polyps can use. It is the rapid removal or carbon dioxide by the algae, scientists have found, that makes it possible for the polyps to deposit limestone fast enough to outgrow their competitors for seafloor elbow room. Thus if there is no light for the algae, there is no reef-building by the polyps; coral reefs only exist in water shallow enough and clear enough for sunlight to penetrate it - 30 to 50 feet of depth in the Arabian Gulf.
Because different coral species - with their symbiotic algae - have different light requirements, a coral reef with many sorts of corals growing on it exhibits "depth zonation," with bright-light corals growing near the surface and corals that need less light found further down toward the base or the reef. All the various creatures associated with the coral are also "depth zoned" - they are found only where their coral is. Species of fish, crabs, snails, worms and so on are thus geographically spread out around the reef, which permits more different species to be in the same general area at the same time, and contributes to the enormous diversity of reef life.
Especially in the northern part of Saudi Arabia's Gulf coast, coral reefs are numbered in the hundreds. Most of them are platform reefs, flat-topped structures that rise from the sea floor to just below the low-tide water surface, but there are also fringing reefs, bordering parts of the shore, and a few coral islands. Few have been explored by divers, and fewer still thoroughly studied by Aramco's ecologists and marine biologists. They lie there, sunlit, teeming and productive, waiting for explorers from the small dry-land fraction of our planet.
Robert Arndt is Assistant Editor of Aramco World.
