Knowing this past, we can see in the present a repetition of the pattern, a recurrence of earth processes of an earlier day. Now, as then, living reefs are building up offshore; sediments are accumulating in shallow waters; and the level of the sea, almost imperceptibly but certainly, is changing.
Off this coral coast the sea lies green in the shallows, blue in the far distances. After a storm, or even after a prolonged southeasterly blow, comes “white water.” Then a thick, milk-white, richly calcareous sediment is washed out of the reefs and stirred from its deep beds over the floor of the reef flat. On such days the diving mask and the aqualung may as well be left behind, for the underwater visibility is little better than in a London fog.
“White water” is the indirect result of the very high rate of sedimentation that prevails in the shallows around the Keys. Anyone who wades out even a few steps from the shore notices the white, siltlike substance adrift in the water and accumulating on the bottom. It has visibly rained down on every surface. Its fine dust lies over sponge and gorgonian and anemone; it chokes and buries the low-growing algae and lies whitely over the dark bulks of the big loggerhead sponges. The wader stirs up clouds of it; winds and strong currents set it in motion. Its accumulation is going on at an astonishing rate; sometimes, after a storm, two or three inches of new sediment are deposited from one high tide to the next. It comes from various sources. Some is mechanically derived from the disintegration of dead plants and animals—mollusk shells, lime-depositing algae, coral skeletons, tubes of worms or snails, spicules of gorgonians and sponges, skeletal plates of holothurians. It is also derived in part from chemical precipitation of the calcium carbonate present in the water. This, in turn, has been leached out of the vast expanses of limestone rock that compose the surface of southern Florida, and has been carried to the sea by rivers and by the slow drainage of the Everglades.
A few miles outside the chain of the present Keys is the reef 5 of living coral, forming the seaward rim of the shallows, and overlooking a steep descent into the trough of the Florida straits. The reefs extend from Fowey Rocks, south of Miami, to the Marquesas and Tortugas and in general they mark the 10-fathom depth contour. But often they rise to lesser depths and here and there they break the surface as tiny offshore islands, many of them marked by lighthouses.
Drifting over the reef in a small boat and peering down through a glass-bottomed bucket, one finds it hard to visualize the whole terrain, for so little of it can be seen at a time. Even a diver exploring more intimately finds it difficult to realize he is on the crest of a high hill, swept by currents instead of winds, where gorgonians are the shrubbery and stands of elkhorn coral are trees of stone. Toward the land, the sea floor slopes gently down from this hilltop into the wide water-filled valley of Hawk Channel; then it rises again and breaks water as a chain of low-lying islands—the Keys. But on the seaward side of the reef the bottom descends quickly into blue depths. Live corals grow down to a depth of about 10 fathoms. Below that it is too dark, perhaps, or there is too much sediment, and instead of living coral there is a foundation of dead reef, formed at some time when the sea level was lower than it is today. Out where the water is about 100 fathoms deep there is a clean rock bottom, the Pourtal[[[grave.gif]]]s Plateau; its fauna is rich, but the corals that live here are not reef builders. Between 300 and 500 fathoms sediments have again accumulated on a slope that descends to the trough of the Florida straits—the channel of the Gulf Stream.
As for the reef itself, many thousand thousand beings—plant and animal, living and dead—have entered into its composition. Corals of many species, building their little cups of lime and with them fashioning many strange and beautiful forms, are the foundation of the reef. But besides the corals there are other builders and all the interstices of the reef are filled with their shells or their limy tubes, or with coral rock cemented together with building stones of the most diverse origin. There are colonies of tube-building worms and there are mollusks of the snail tribe whose contorted, tubular shells may be intertwined into massive structures. Calcareous algae, which have the property of depositing lime in their living tissues, form part of the reef itself or, growing abundantly over the shallows on the landward side, add their substance at death to the coral sand of which limestone rock is later formed. The horny corals or gorgonians, known as sea fans and sea whips, all contain limestone spicules in their soft tissues. These, along with lime from starfish and sea urchins and sponges and an immense number of smaller creatures, will eventually, with the passage of time and through the chemistry of the sea, come to form part of the reef.
Along with those that build are others that destroy. The sulphur sponge dissolves away the calcareous rock. Boring mollusks riddle it with their tunnels, and worms with sharp, biting jaws eat into it, weakening its structure and so hastening the day when a mass of coral will yield to the force of the waves, will break away, and perhaps roll down the seaward face of the reef into deeper water.
The basis of this whole complex association is a minute creature of deceptively simple appearance, the coral polyp. The coral animal is formed on the same general lines as the sea anemone. It is a double-walled tube of cylindrical shape, closed at the base and open at the free end, where a crown of tentacles surrounds the mouth. The important difference—the fact on which the existence of coral reefs depends—is this: the coral polyp has the ability to secrete lime, forming a hard cup about itself. This is done by cells of the outer layer, much as the shell of a mollusk is secreted by an outer layer of soft tissue—the mantle. So the anemone-like coral polyp comes to sit in a compartment formed of a substance as hard as rock. Because the “skin” of the polyp is turned inward at intervals in a series of vertical folds, and because all of this skin is actively secreting lime, the cup does not have a smooth circumference, but is marked by partitions projecting inward, forming the starlike or flowerlike pattern familiar to anyone who has examined a coral skeleton.
Most corals build colonies of many individuals. All the individuals of any one colony, however, are derived from a single fertilized ovum that matured and then began to form new polyps by budding. The colony has a shape characteristic of the species—branched, boulderlike, flatly encrusting, or cup-shaped. Its core is solid, for only the surface is occupied by living polyps, which may be widely separated in some species or closely crowded in others. It is often true that the larger and more massive the colony, the smaller the individuals that compose it; the polyps of a branching coral taller than a man may themselves be only an eighth of an inch high.
The hard substance of the coral colony is usually white, but may take on the colors of minute plant cells that live within the soft tissues in a relation of mutual benefit. There is the exchange usual in such relations, the plants getting carbon dioxide and the animals making use of the oxygen given off by the plants. This particular association may have a deeper significance, however. The yellow, green, or brown pigments of the algae belong to the group of chemical substances known as carotinoids. Recent studies suggest that these pigments in the imprisoned algae may act on the corals, serving as “internal correlators” to influence the processes of reproduction. Under normal conditions, the presence of the algae seems to benefit the coral, but in dim light the coral animals rid themselves of the algae by excreting them. Perhaps this means that in weak light or in darkness the whole physiology of the plant is changed and the products of its metabolism are altered to something harmful, so that the animal must expel the plant guest.