The shipworm’s place in history is secure. It was the scourge of the Romans with their galleys, of the seagoing Greeks arid Phoenicians, of the explorers of the New World. In the 1700’s it riddled the dikes that the Dutch had built to keep out the sea; by so doing it threatened the very life of Holland. (As an academic by-product, the first extensive studies of the shipworm were made by Dutch scientists, to whom knowledge of its biology had become a matter of life and death. Snellius, in 1733, pointed out for the first time that this animal is a clamlike mollusk, not a worm.) About 1917 the shipworm invaded the harbor of San Francisco. Before its inroads were even suspected, ferry slips had begun to collapse, and wharves and loaded freight cars fell into the harbor. During the Second World War, especially in all tropical waters, the shipworm was an unseen but powerful enemy.
The female of the common shipworm retains the young in her burrow until they have attained the larval stage. Then they are launched into the sea—each a tiny being enclosed in two protective shells, looking like any other young bivalve. If it encounters wood when it has reached the threshold of adulthood, all goes well. It puts out a slender byssus thread as an anchor, a foot develops, and the shells become modified into efficient cutting tools, for rows of sharp ridges appear on their outer surfaces. The burrowing begins. With a powerful muscle, the animal scrapes the ridged shell against the wood, revolving meanwhile so that a smooth, cylindrical burrow is cut. As the burrow is extended, usually with the grain of the wood, the body of the shipworm grows. One end remains attached to the wall near the tiny point of entrance. This bears the siphons through which contact with the sea is maintained. The penetrating end carries the small shells. Between stretches a body that is thin as a lead pencil, but may reach a length of eighteen inches. Although a timber may be infested with hundreds of larvae, the burrows of the shipworms never interfere with each other. If an animal finds itself coming close to another burrow, it invariably turns aside. As it bores, it passes the loosened fragments of wood through its digestive tract. Some of the wood is digested and converted into glucose. This ability to digest cellulose is rare in the animal world—only certain snails, certain insects, and a very few others possess it. But the shipworm makes little use of this difficult art, and feeds chiefly on the rich plankton streaming through its body.
Other timbers on the beach bear the marks of the wood piddock. These are shallow holes that penetrate only the outer portions just beneath the bark, but they are broad and cleanly cylindrical. The boring piddock is seeking only shelter and protection. Unlike the shipworm, it does not digest the wood, but lives only on the plankton that it draws into its body through a protruding siphon.
Empty piddock holes sometimes attract other lodgers, as abandoned birds’ nests may become homes for insects. On the muddy banks of salt creeks at Bears Bluff in South Carolina, I have picked up timbers riddled with holes. Once stout little white-shelled piddocks dwelt in them. The piddocks were long since dead and even the shells were gone, but in each hole was a dark glistening body like a raisin embedded in a cake. They were the contracted tissues of small anemones, finding there, in this world of silt-laden water and yielding mud, that bit of firm foundation which anemones must have. Seeing anemones in such an improbable place, one wonders how the larvae happened to be there, ready to seize the chance opportunity presented by that timber with its neatly excavated apartments; and one is struck anew by the enormous waste of life, remembering that for each of these anemones that succeeded in finding a home, many thousands must have failed.
Always, then, in this flotsam and jetsam of the tide lines, we are reminded that a strange and different world lies offshore. Though what we see here may be but the husks and fragments of life, through it we are made aware of life and death, of movement and change, of the transport of living things by ocean currents, by tides, by wind-driven waves. Some of these involuntary migrants are adults. They may perish in mid-journey; a few, being transported into a new home and finding there conditions that are favorable, may survive, may even produce surviving young to extend the range of the species. But many others are larvae, and whether or not they will make a successful landing depends on many things—on the length of their larval life (can they wait for a distant landfall before they reach the stage when they must take up an adult existence?)—on the temperature of the water they encounter—on the set of the currents that may carry them to favoring shoals, or off into deep water where they will be lost.
And so, walking the beach, we become aware of a most fascinating problem—the colonization of the shore, and especially of those “islands” of rock (or the semblance of rock) that occur in the midst of a sea of sand. For whenever a seawall is built, or a jetty, or pilings are sunk for a pier or a bridge, or rock, long hidden from sun and buried even beneath the sea, emerges again on the ocean floor, these hard surfaces immediately become peopled with typical animals of the rocks. But how did the colonizing rock fauna happen to be at hand—here in the midst of a sandy coast that stretches for hundreds of miles to north and south?
Pondering the answer, we become aware of that ceaseless migration, for the most part doomed to futility, yet ensuring that always, when opportunity arises, Life shall be waiting, ready to take advantage. For the ocean currents are not merely a movement of water; they are a stream of life, carrying always the eggs and young of countless sea creatures. They have carried the hardier ones across oceans, or step by step on long coastwise journeys. They have carried some along deep, hidden passageways where cold currents flow along the floor of the ocean. They have brought inhabitants to populate new islands pushing above the surface of the sea. These things they have done, we must suppose, since first there was life in the sea.
And as long as the currents move on their courses there is the possibility, the probability, even the certainty, that some particular form of life will extend its range, will come to occupy new territory.
As almost nothing else does, this to me expresses the pressure of the life force—the intense, blind, unconscious will to survive, to push on, to expand. It is one of life’s mysteries that most of the participants in this cosmic migration are doomed to failure; it is no less mysterious that their failure turns into success when, for all the billions lost, a few succeed.
V. The Coral Coast
I DOUBT that anyone can travel the length of the Florida Keys without having communicated to his mind a sense of the uniqueness of this land of sky and water and scattered mangrove-covered islands. The atmosphere of the Keys is strongly and peculiarly their own. It may be that here, more than in most places, remembrance of the past and intimations of the future are linked with present reality. In bare and jaggedly corroded rock, sculptured with the patterns of the corals, there is the desolation of a dead past. In the multicolored sea gardens seen from a boat as one drifts above them, there is a tropical lushness and mystery, a throbbing sense of the pressure of life; in coral reef and mangrove swamp there are the dimly seen foreshadowings of the future.
This world of the Keys has no counterpart elsewhere in the United States, and indeed few coasts of the earth are like it. Offshore, living coral reefs fringe the island chain, while some of the Keys themselves are the dead remnants of an old reef whose builders lived and flourished in a warm sea perhaps a thousand years ago. This is a coast not formed of lifeless rock or sand, but created by the activities of living things which, though having bodies formed of protoplasm even as our own, are able to turn the substance of the sea into rock.