But that which seems quiescent—a dream world inhabited by-creatures that move sluggishly or not at all—comes swiftly to life when the day ends. When I have lingered on the reef flat until dusk fell, a strange new world, full of tensions and alarms, has replaced the peaceful languor of the day. For then hunter and hunted are abroad. The spiny lobster steals out from under the sheltering bulk of a big sponge and flashes away across the open water. The gray snapper and the barracuda patrol the channels between the Keys and dart into the shallows in swift pursuit. Crabs emerge from hidden caverns; sea snails of varied shape and size creep out from under rocks. In sudden movements, swirling waters, and half-seen shadows that dart across my path as I wade shoreward, I sense the ancient drama of the strong against the weak.
Or if I have listened from the deck of a boat anchored at night among the Keys, I have heard splashings of large bodies moving in the shallows nearby, or the slap of a broad form striking water as a sting ray leaps into the air and falls, leaps and falls again. One of those whom the night stirs to activity is the needle-fish, long, slender, and powerful of body, armed with a sharp beak that would seem more appropriate in a bird. By day the small needle-fish may be seen from wharves and sea walls as they come close inshore, floating at the surface like straws adrift in the water. At night the large fish, that have ranged far to sea, come in to feed in the shallows, sometimes singly, sometimes in large schools. They leap out of the water or go skipping along the surface, making a disturbance that can be heard for a long distance on a calm night. Fishermen say that the needle-fish jump toward a light—that if one is out in a small boat at night where the needle-fish are hunting, it is dangerous, if not suicidal, to show a light, for the fish will leap across the boat. Probably there is an element of truth in the belief, for in some places in the Keys the beam of a searchlight thrown out across the water on a calm night—even if no fish have been heard about—will often be greeted by a series of splashes as a dozen or more large fish leap out of the water. The leaps, however, are usually at right angles to the beam, and the fish seem to be trying to escape the light.
This coral coast is the drowned world of the offshore reef and the world of the shallow reef flats with their fringing, rocky rim; it is also the green world of the mangrove, silent, mysterious, always changing—eloquent of a life force strong enough to alter the visible face of its world. As the corals dominate the seaward margin of the keys, the mangroves possess the sheltered or bay shores, completely covering many of the smaller keys, pushing out into the water to lessen the spaces between the islands, building an island where once there was only a shoal, creating land where once there was sea.
Mangroves are among the far migrants of the plant kingdom, forever sending their young stages off to establish pioneer colonies a score, a hundred, or a thousand miles from the parent stock. The same species live on the tropical coasts of America and the west coast of Africa. Probably the American mangroves crossed from Africa eons ago, via the Equatorial Current—and probably such migrants continue to arrive unnoticed from time to time. How the mangroves got to the Pacific coast of tropical America is an interesting problem. There is no continuous system of currents that would have carried them around the Horn, and besides the cold water to the south would be a barrier. It is not certain how early the mangroves arose, but definite fossil records seem to go back only to the Cenozoic, whereas the Panama Ridge, separating Atlantic from Pacific waters, probably arose much earlier, toward the end of the Mesozoic. By some means, however, the mangroves made the journey to Pacific shores, where they became established. Their further migrations also are mysterious. They must have dispatched their migrant seedlings into the great currents of the Pacific, for at least one American species grows on the islands of Fiji and Tonga and seems to have drifted as well to Cocos-Keeling and Christmas Islands. And some appeared as new colonists on the devastated island of Krakatoa, after it was virtually destroyed by volcanic eruption in 1883.
The mangroves belong to the highest group of plants, the spermatophytes or seed-bearers, whose earliest forms developed on land, and as such they are a botanical example of that return toward the sea that is always fascinating to observe. Among mammals, the seals and whales made such a return to the habitat of their ancestors. The marine grasses have gone even farther than the mangrove, for they live permanently submerged. But why this return to salt water? Perhaps the mangroves or their ancestral stock were forced out of more crowded habitats by the competition of other species. Whatever the reason, they have invaded and established themselves in the difficult world of the shore with such success that no plant now threatens their dominance there.
The saga of an individual mangrove begins when the long pendent green seedling, produced on the parent tree, drops to the floor of the swamp. Perhaps this happens at low tide when all the water has drained away; then the seedling lies amid the tangled roots, waiting till the salt flood comes in to lift it and later float it seaward on the turn of the tide. Of all the hundreds of thousands of red mangrove seedlings produced annually on the southern Florida coast, probably less than half remain to develop near the parent trees. The rest put out to sea, their buoyant structure keeping them in the surface waters, moving with the flow of the currents. They may drift for many months, being able to survive the normal vicissitudes of such a journey—sun, rain, the battering of a rough sea. At first they float horizontally, but with increasing age and the development of their tissues for a new phase of life they gradually come to lie almost vertically with the future root end downward, ready for that contact with earth upon which their future existence depends.
Perhaps in the path of such a pelagic seedling there may lie a small shoal, a little ridge off an island shore, deposited, grain by grain, by the waves. As the tide floats the young mangrove into the shallows, the downward-pressing tip touches the shoal; the sharp point, pressing earthward, becomes embedded. The water movements of later tides rising and falling press the young plant firmly into the receptive soil. Later, perhaps, they bring other seedlings to lodge beside it.
No sooner have the young mangroves anchored themselves than they begin to grow, sending out tiers of roots that arch out and downward to form a circle of supporting props. Among this rapidly increasing tangle of roots, debris of all sorts comes to rest—decaying vegetation, driftwood, shells, coral fragments, uprooted sponges and other sea growths. From such simple beginnings, an island is born.
In twenty to thirty years the young mangroves have acquired the stature of trees. These mature mangroves can resist the battering of a considerable surf, and probably are destroyed only by violent hurricanes. Once in many years such a hurricane comes. Because of the efficiency of their buttressing roots, few mangroves are uprooted even by a violent blow. But the high storm tides press far inland through the swamp, carrying the salt of the open sea into the forested interior. Leaves and small branches are stripped off and carried away, and if the wind is truly violent the trunks and limbs of the great trees are shaken and battered until the bark separates and blows away in sheets, exposing the naked trunk to the burning salt breath of the storm. This may be the history of some of the mangrove ghost forests bordering the Florida coast. But such catastrophes are rare, and in southwestern Florida whole islands of mangroves come to maturity without any serious interruption of their growth.
A mangrove forest, its fringing trees literally standing in salt water, extending back into darkening swamps of its own creation, is full of the mysterious beauty of massive and contorted trunks, of tangled roots, and of dark green foliage spreading an almost unbroken canopy. The forest with its associated swamp forms a curious world. On their flood the tides rise over the roots of the outermost trees and penetrate into the swamplands, carrying many small migrants—the pelagic larvae of sea creatures. Over the ages many of these have found a suitable climate for their survival and have become established, some on the roots or trunks of the mangroves, some in the soft mud of the intertidal zone, some on the bott
om of the bay offshore. The mangrove may be the only kind of tree, or the only seed plant growing there; all the associated plants and animals are bound to it by biological ties.
Within the range of the tides the prop roots of the mangroves are thickly overgrown with an oyster whose shell has fingerlike projections to grasp these firm supports and so to remain above the mud. On the night ebb tides, raccoons follow the water down, leaving meandering tracks across the mud as they move from root to root, finding food within the shells of the oysters. The crown conch also preys heavily on these oysters of the mangroves. Fiddler crabs dig tunnels in the mud, sheltering deep within them when the salt tide rises. These crabs are remarkable for the possession, by the males, of one immense claw—the "fiddle"—that is incessantly waved about, apparently serving for communication as well as for defense. Fiddlers eat plant debris picked from the surface of sand or mud. For this the female has two spoon claws; the male, because of his fiddle, only one. By their activities the crabs help to aerate the heavy mud, which is saturated with organic debris and so deficient in oxygen that the mangroves must breathe through their aerial roots to supplement what their buried roots can obtain. Brittle stars and strange burrowing crustaceans live among the roots, while overhead in the upper branches great colonies of pelicans and herons find roosting and nesting places.
Here on these mangrove-fringed shores some of the pioneering mollusks and crustaceans are learning to live out of the sea from which they recently came. Among the mangroves and in marshy areas where the tides rise over the roots of sea grasses there is a small snail whose race is moving landward. This is the coffee-bean shell, a small creature within a short, widely ovate shell tinted with the greens and browns of its environment. When the tide rises the snails clamber up on the mangrove roots or climb the stems of the grasses, deferring as long as possible the moment of contact with the sea. Among the crabs, too, land forms are evolving. The purple-clawed hermit inhabits the strip above the highest tidal flotsam, where land vegetation fringes the shore, but in the breeding season it moves down toward the sea. Then hundreds of them lurk under logs and bits of driftwood, waiting for the moment when the eggs, carried by the female under her body, shall be ready to hatch. At that time the crabs dash into the sea, liberating the young into the ancestral waters. Nearing the end of its evolutionary journey is the large white crab of the Bahamas and southern Florida. It is a land dweller and an air-breather, and it seems to have cut its ties with the sea—all its ties, that is, but one. For in the spring the white crabs engage in a lemming-like march to the sea, entering it to release their young. In time the crabs of a new generation, having completed their embryonic life in the sea, emerge from the water and seek the land home of their parents.
For hundreds of miles this world of swamp and forest created by the mangroves extends northward, sweeping from the Keys around the southern tip of the Florida mainland, reaching from Cape Sable north along the coast of the Gulf of Mexico through all the Ten Thousand Islands. This is one of the great mangrove swamps of the world, a wilderness untamed and almost unvisited by man. Flying above it, one can see the mangroves at work. From the air the Ten Thousand Islands show a significant shape and structure. Geologists describe them as looking like a school of fish swimming in a southeasterly direction—each fish-shaped island having an "eye" of water in its enlarged end, the heads of all the little "fish" pointing to the southeast. Before these islands came to be, one may suppose, the wavelets of a shallow sea heaped the sand of its floor into little ridges. Then came the colonizing mangroves, converting the ripple marks to islands, perpetuating in living green forest the shape and trend of the sand ripples.
Today we can see, from one generation of man to another, where several small islands have coalesced to form one, or where the land has grown out and an island has merged with it—sea becoming land almost before our eyes.
What is the future of this mangrove coast? If it is written in its recent past we can foretell it: the building of a vast land area where today there is water with scattered islands. But we who live today can only wonder; a rising sea could write a different history.
Meanwhile the mangroves press on, spreading their silent forests mile upon mile under tropical skies, sending down their grasping roots, dropping their migrant seedlings one by one, launching them into the drifting tides on far voyages.
And offshore, under the surface waters where the moonlight falls in broken, argent beams, under the tidal currents streaming shoreward in the still night, the pulse of life surges on the reef. As all the billions of the coral animals draw from the sea the necessities of their existence, by swift metabolism converting the tissues of copepods and snail larvae and minuscule worms into the substance of their own bodies, so the corals grow and reproduce and bud, each of the tiny creatures adding its own limy chamber to the structure of the reef.
And as the years pass, and the centuries merge into the unbroken stream of time, these architects of coral reef and mangrove swamp build toward a shadowy future. But neither the corals nor the mangroves, but the sea itself will determine when that which they build will belong to the land, or when it will be reclaimed for the sea.
The Enduring Sea
NOW I HEAR the sea sounds about me; the night high tide is rising, swirling with a confused rush of waters against the rocks below my study window. Fog has come into the bay from the open sea, and it lies over water and over the land's edge, seeping back into the spruces and stealing softly among the juniper and the bayberry. The restive waters, the cold wet breath of the fog, are of a world in which man is an uneasy trespasser; he punctuates the night with the complaining groan and grunt of a foghorn, sensing the power and menace of the sea.
Hearing the rising tide, I think how it is pressing also against other shores I know—rising on a southern beach where there is no fog, but a moon edging all the waves with silver and touching the wet sands with lambent sheen, and on a still more distant shore sending its streaming currents against the moonlit pinnacles and the dark caves of the coral rock.
Then in my thoughts these shores, so different in their nature and in the inhabitants they support, are made one by the unifying touch of the sea. For the differences I sense in this particular instant of time that is mine are but the differences of a moment, determined by our place in the stream of time and in the long rhythms of the sea. Once this rocky coast beneath me was a plain of sand; then the sea rose and found a new shore line. And again in some shadowy future the surf will have ground these rocks to sand and will have returned the coast to its earlier state. And so in my mind's eye these coastal forms merge and blend in a shifting, kaleidoscopic pattern in which there is no finality, no ultimate and fixed reality—earth becoming fluid as the sea itself.
On all these shores there are echoes of past and future: of the flow of time, obliterating yet containing all that has gone before; of the sea's eternal rhythms—the tides, the beat of surf, the pressing rivers of the currents—shaping, changing, dominating; of the stream of life, flowing as inexorably as any ocean current, from past to unknown future. For as the shore configuration changes in the flow of time, the pattern of life changes, never static, never quite the same from year to year. Whenever the sea builds a new coast, waves of living creatures surge against it, seeking a foothold, establishing their colonies. And so we come to perceive life as a force as tangible as any of the physical realities of the sea, a force strong and purposeful, as incapable of being crushed or diverted from its ends as the rising tide.
Contemplating the teeming life of the shore, we have an uneasy sense of the communication of some universal truth that lies just beyond our grasp. What is the message signaled by the hordes of diatoms, flashing their microscopic lights in the night sea? What truth is expressed by the legions of the barnacles, whitening the rocks with their habitations, each small creature within finding the necessities of its existence in the sweep of the surf? And what is the meaning of so tiny a being as the transparent wisp of protopl
asm that is a sea lace, existing for some reason inscrutable to us—a reason that demands its presence by the trillion amid the rocks and weeds of the shore? The meaning haunts and ever eludes us, and in its very pursuit we approach the ultimate mystery of Life itself.
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Appendix: Classification
Protophyta, Protozoa: One-celled Plants and Animals
THE SIMPLEST FORMS of cellular life are the one-celled plants (Protophyta) and one-celled animals (Protozoa). In both groups, however, there are many forms that defy attempts to place them definitely in one category or another because they display characteristics usually considered animal-like along with others usually thought definitive of plants. The Dinoflagellata form such an indeterminate group, and are claimed both by zoologists and by botanists. Although a few are large enough to be seen without magnification, most are smaller. Some wear shells with spines and elaborate markings. Some have a remarkable, eye-like sense organ. All dinoflagellates are immensely important in the economy of the sea as food for certain fishes and other animals. Noctiluca is a relatively large dinoflagellate of coastal waters, where it produces brilliant displays of phosphorescence, or by day reddens the water Sphaerella the abundance of its pigmented cells. Other species are the cause of the phenomenon known as "red tide," in which the sea is discolored and fishes and other animals die from poisons given off by the minute cells. The red or green scum of high tide pools, "red rain," and "red snow" are growths of these forms, or of green algae (e.g., Sphaerella). Much phosphorescence or "burning" of the sea is caused by dinoflagellates, which create a uniformly diffused light, lacking large spots of illumination. Examined closely, in a vessel of water, the light is seen to consist of tiny sparks.