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 bottom 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.