III. The Rocky Shores 1 страница

W HEN THE TIDE is high on a rocky shore, when its brimming fullness creeps up almost to the bayberry and the junipers where they come down from the land, one might easily suppose that nothing at all lived in or on or under these waters of the sea’s edge. For nothing is visible. Nothing except here and there a little group of herring gulls, for at high tide the gulls rest on ledges of rock, dry above the surf and the spray, and they tuck their yellow bills under their feathers and doze away the hours of the rising water. Then all the creatures of the tidal rocks are hidden from view, but the gulls know what is there, and they know that in time the water will fall away again and give them entrance to the strip between the tide lines.

When the tide is rising the shore is a place of unrest, with the surge leaping high over jutting rocks and running in lacy cascades of foam over the landward side of massive boulders. But on the ebb it is more peaceful, for then the waves do not have behind them the push of the inward pressing tides. There is no particular drama about the turn of the tide, but presently a zone of wetness shows on the gray rock slopes, and offshore the incoming swells begin to swirl and break over hidden ledges. Soon the rocks that the high tide had concealed rise into view and glisten with the wetness left on them by the receding water.

Small, dingy snails move about over rocks that are slippery with the growth of infinitesimal green plants; the snails scraping, scraping, scraping to find food before the surf returns.

Like drifts of old snow no longer white, the barnacles come into view; they blanket rocks and old spars wedged into rock crevices, and their sharp cones are sprinkled over empty mussel shells and lobster‑pot buoys and the hard stipes of deep‑water seaweeds, all mingled in the flotsam of the tide.

Meadows of brown rockweeds appear on the gently sloping rocks of the shore as the tide imperceptibly ebbs. Smaller patches of green weed, stringy as mermaids’ hair, begin to turn white and crinkly where the sun has dried them.

Now the gulls, that lately rested on the higher ledges, pace with grave intentness along the walls of rock, and they probe under the hanging curtains of weed to find crabs and sea urchins.

In the low places little pools and gutters are left where the water trickles and gurgles and cascades in miniature waterfalls, and many of the dark caverns between and under the rocks are floored with still mirrors holding the reflections of delicate creatures that shun the light and avoid the shock of waves–the cream‑colored flowers of the small anemones and the pink fingers of soft coral, pendent from the rocky ceiling.

In the calm world of the deeper rock pools, now undisturbed by the tumult of incoming waves, crabs sidle along the walls, their claws busily touching, feeling, exploring for bits of food. The pools are gardens of color composed of the delicate green and ocher‑yellow of encrusting sponge, the pale pink of hydroids that stand like clusters of fragile spring flowers, the bronze and electric‑blue gleams of the Irish moss, the old‑rose beauty of the coralline algae.

And over it all there is the smell of low tide, compounded of the faint, pervasive smell of worms and snails and jellyfish and crabs–the sulphur smell of sponge, the iodine smell of rockweed, and the salt smell of the rime that glitters on the sun‑dried rocks.

One of my own favorite approaches to a rocky seacoast is by a rough path through an evergreen forest that has its own peculiar enchantment. It is usually an early morning tide that takes me along that forest path, so that the light is still pale and fog drifts in from the sea beyond. It is almost a ghost forest, for among the living spruce and balsam are many dead trees‑some still erect, some sagging earthward, some lying on the floor of the forest. All the trees, the living and the dead, are clothed with green and silver crusts of lichens. Tufts of the bearded lichen or old man’s beard hang from the branches like bits of sea mist tangled there. Green woodland mosses and a yielding carpet of reindeer moss cover the ground. In the quiet of that place even the voice of the surf is reduced to a whispered echo and the sounds of the forest are but the ghosts of sound–the faint sighing of evergreen needles in the moving air; the creaks and heavier groans of half‑fallen trees resting against their neighbors and rubbing bark against bark; the light rattling fall of a dead branch broken under the feet of a squirrel and sent bouncing and ricocheting earthward.

But finally the path emerges from the dimness of the deeper forest and comes to a place where the sound of surf rises above the forest sounds–the hollow boom of the sea, rhythmic and insistent, striking against the rocks, falling away, rising again.

Up and down the coast the line of the forest is drawn sharp and clean on the edge of a seascape of surf and sky and rocks. The softness of sea fog blurs the contours of the rocks; gray water and gray mists merge offshore in a dim and vaporous world that might be a world of creation, stirring with new life.

The sense of newness is more than illusion born of the early morning light and the fog, for this is in very fact a young coast. It was only yesterday in the life of the earth that the sea came in as the coast subsided, filling the valleys and rising about the slopes of the hills, creating these rugged shores where rocks rise out of the sea and evergreen forests come down to the coastal rocks. Once this shore was like the ancient land to the south, where the nature of the coast has changed little during the millions of years since the sea and the wind and the rain created its sands and shaped them into dune and beach and offshore bar and shoal. The northern coast, too, had its flat coastal plain bordered by wide beaches of sand. Behind these lay a landscape of rocky hills alternating with valleys that had been worn by streams and deepened and sculptured by glaciers. The hills were formed of gneiss and other crystalline rocks resistant to erosion; the lowlands had been created in beds of weaker rocks like sandstones, shale, and marl.

Then the scene changed. From a point somewhere in the vicinity of Long Island the flexible crust of the earth tilted downward under the burden of a vast glacier. The regions we know as eastern Maine and Nova Scotia were pressed down into the earth, some areas being carried as much as 1200 feet beneath the sea. All of the northern coastal plain was drowned. Some of its more elevated parts are now offshore shoals, the fishing banks off the New England and Canadian coasts–Georges, Browns, Quereau, the Grand Bank. None of it remains above the sea except here and there a high and isolated hill, like the present island of Monhegan, which in ancient times must have stood above the coastal plain as a bold monadnock.

Where the mountainous ridges and the valleys lay at an angle to the coast, the sea ran far up between the hills and occupied the valleys. This was the origin of the deeply indented and exceedingly irregular coast that is characteristic of much of Maine. The long narrow estuaries of the Kennebec, the Sheepscot, the Damariscotta and many other rivers run inland a score of miles. These salt‑water rivers, now arms of the sea, are the drowned valleys in which grass and trees grew in a geologic yesterday. The rocky, forested ridges between them probably looked much as they do today. Offshore, chains of islands jut out obliquely into the sea, one beyond another–half‑submerged ridges of the former land mass.

But where the shore line is parallel to the massive ridges of rock the coast line is smoother, with few indentations. The rains of earlier centuries cut only short valleys into the flanks of the granite hills, and so when the sea rose there were created only a few short, broad bays instead of long winding ones. Such a coast occurs typically in southern Nova Scotia, and also may be seen in the Cape Ann region of Massachusetts, where the belts of resistant rock curve eastward along the coast. On such a coast, islands, where they occur, lie parallel to the shore line instead of putting boldly out to sea.

As geologic events are reckoned, all this happened rather rapidly and suddenly, with no time for gradual adjustment of the landscape; also it happened quite recently, the present relation of land and sea being achieved perhaps no more than ten thousand years ago. In the chronology of Earth, a few thousand years are as nothing, and in so brief a time the waves have prevailed little against the hard rocks that the great ice sheet scraped clean of loose rock and ancient soil, and so have scarcely marked out the deep notches that in time they will cut in the cliffs.

For the most part, the ruggedness of this coast is the ruggedness of the hills themselves. There are none of the wave‑cut stacks and arches that distinguish older coasts or coasts of softer rock. In a few, exceptional places the work of the waves may be seen. The south shore of Mount Desert Island is exposed to heavy pounding by surf; there the waves have cut out Anemone Cave and are working at Thunder Hole to batter through the roof of the small cave into which the surf roars at high tide.

In places the sea washes the foot of a steep cliff produced by the shearing effect of earth pressure along fault lines. Cliffs on Mount Desert–Schooner Head, Great Head, and Otter‑tower a hundred feet or more above the sea. Such imposing structures might be taken for wave‑cut cliffs if one did not know the geologic history of the region.

On the coasts of Cape Breton Island and New Brunswick the situation is very different and examples of advanced marine erosion occur on every hand. Here the sea is in contact with weak rock lowlands formed in the Carboniferous period. These shores have little resistance to the erosive power of the waves, and the soft sandstone and conglomerate rocks are being cut back at an annual rate averaging five or six inches, or in some places several feet. Marine stacks, caves, chimneys, and archways are common features of these shores.

Here and there on the predominantly rocky coast of northern New England there are small beaches of sand, pebbles, or cobblestones. These have a varied origin. Some came from glacial debris that covered the rocky surface when the land tilted and the sea came in. Boulders and pebbles often are carried in from deeper water offshore by seaweeds that have gripped them firmly with their “holdfasts.” Storm waves then dislodge weed and stone and cast them on the shore. Even without the aid of weeds, waves carry in a considerable volume of sand, gravel, shell fragments, and even boulders. These occasional sandy or pebbly beaches are almost always in protected, incurving shores or dead‑end coves, where the waves can deposit debris but from which they cannot easily remove it.

When, on those coastal rocks between the serrate line of spruces and the surf, the morning mists conceal the lighthouses and fishing boats and all other reminders of man, they also blur the sense of time and one might easily imagine that the sea came in only yesterday to create this particular line of coast. Yet the creatures that inhabit the intertidal rocks have had time to establish themselves here, replacing the fauna of the beaches of sand and mud that probably bordered the older coast. Out of the same sea that rose over the northern coast of New England, drowning the coastal plain and coming to rest against the hard uplands, the larvae of the rock dwellers came–the blindly searching larvae that drift in the ocean currents ready to colonize whatever suitable land may lie in their path or to die, if no such landfall is their lot.

Although no one recorded the first colonist or traced the succession of living forms, we may make a fairly confident guess as to the pioneers of the occupation of these rocks, and the forms that followed them. The invading sea must have brought the larvae and young of many kinds of shore animals, but only those able to find food could survive on the new shore. And in the beginning the only available food was the plankton that came in renewed clouds with every tide that washed the coastal rocks. The first permanent inhabitants must have been such plankton‑strainers as the barnacles and mussels, who require little but a firm place to which they may attach themselves. Around and among the white cones of the barnacles and the dark shells of the mussels it is probable that the spores of algae settled, so that a living green film began to spread over the upper rocks. Then the grazers could come–the little herds of snails that laboriously scrape the rocks with their sharp tongues, licking off the nearly invisible covering of tiny plant cells. Only after the establishment of the plankton‑strainers and the grazers could the carnivores settle and survive. The predatory dog whelks, the starfish, and many of the crabs and worms must, then, have been comparative latecomers to this rocky shore. But all of them are there now, living out their lives in the horizontal zones created by the tides, or in the little pockets or communities of life established by the need to take shelter from surf, or to find food, or to hide from enemies.

The pattern of life spread before me when I emerge from that forest path is one characteristic of exposed shores. From the edge of the spruce forests down to the dark groves of the kelps, the life of the land grades into the life of the sea, perhaps with less abruptness than one would expect, for by various little interlacing ties the ancient unity of the two is made clear.

Lichens live in the forest above the sea, in the silent intensity of their toil crumbling away the rocks as lichens have done for millions of years. Some leave the forest and advance over the bare rock toward the tide line; a few go even farther, enduring a periodic submersion by the sea so that they may work their strange magic on the rocks of the intertidal zone. In the dampness of foggy mornings the rock tripe on the seaward slopes is like sheets of thin, pliable green leather, but by midday under a drying sun it has become blackened and brittle; then the rocks look as though they were sloughing off a thin outer layer. Thriving in the salt spray, the wall lichen spreads its orange stain on the cliffs and even on the landward side of boulders that are visited by the highest tides of each moon. Scales of other lichens, sage‑green, rolled and twisted into strange shapes, rise from the lower rocks; from their under surfaces black, hairy processes work down among the minute particles of rock substance, giving off an acid secretion to dissolve the rock. As the hairs absorb moisture and swell, fine grains of the rock are dislodged and so the work of creating soil from the rock is advanced.

Below the forest’s edge the rock is white or gray or buff, according to its mineral nature. It is dry and belongs to the land; except for a few insects or other land creatures using it as pathways to the sea it is barren. But just above the area that clearly belongs to the sea, it shows a strange discoloration, being strongly marked with streaks or patches or continuous bands of black. Nothing about this black zone suggests life; one would call it a dark stain, or at most a felty roughening of the rock surface. Yet it is actually a dense growth of minute plants. The species that compose it sometimes include a very small lichen, sometimes one or more of the green algae, but most numerously the simplest and most ancient of all plants, the blue‑green algae. Some are enclosed in slimy sheaths that protect them from drying and fit them to endure long exposure to sun and air. All are so minute as to be invisible as individual plants. Their gelatinous sheaths and the fact that the whole area receives the spray of breaking waves make this entrance to the sea world slippery as the smoothest ice.

This black zone of the shore has a meaning above and beyond its drab and lifeless aspect–a meaning obscure, elusive, and infinitely tantalizing. Wherever rocks meet the sea, the microplants have written their dark inscription, a message only partially legible although it seems in some way to be concerned with the universality of tides and oceans. Though other elements of the intertidal world come and go, this darkening stain is omnipresent. The rockweeds, the barnacles, the snails, and the mussels appear and disappear in the intertidal zone according to the changing nature of their world, but the black inscriptions of the microplants are always there. Seeing them here on this Maine coast, I remember how they also blackened the coral rim of Key Largo, and streaked the smooth platform of coquina at St. Augustine, and left their tracings on the concrete jetties at Beaufort. It is the same all over the world–from South Africa to Norway and from the Aleutians to Australia. This is the sign of the meeting of land and sea.

Once below the dark film, I begin to look for the first of the sea creatures pressing up to the threshold of the land. In seams and crevices in the high rocks I find them–the smallest of the periwinkle tribe, the rock or rough periwinkle. Some–the infant snails–are so small that I need my hand lens to see them clearly, and among the hundreds that crowd into these cracks and depressions I can find a gradation of sizes up to the half‑inch adults. If these were sea creatures of ordinary habits, I would think the small snails were young produced by some distant colony and drifted here as larvae after spending a period at sea. But the rough periwinkle sends no young into the sea; instead it is a viviparous species and the eggs, each encased within a cocoon, are held within the mother while they develop. The contents of the cocoon nourish the young snail until finally it breaks through the egg capsule and then emerges from the mother’s body, a completely shelled little creature about the size of a grain of finely ground coffee. So small an animal might easily be washed out to sea; hence, no doubt, the habit of hiding in crevices and in empty barnacle shells, where often I have found them in numbers.

At the level where most of the rough periwinkles live, however, the sea comes only every fortnight on the spring tides, and in the long intervals the flying spray of breaking waves is their only contact with the water. While the rocks are thoroughly wet with spray the periwinkles can spend much time out on the rocks feeding, often working well up into the black zone. The microplants that create the slippery film on the rocks are their food; like all snails of their group, the periwinkles are vegetarians. They feed by scraping the rocks with a peculiar organ set with many rows of sharp, calcareous teeth. This organ, the radula, is a continuous belt or ribbon that lies on the floor of the pharynx. If unwound, it would be many times the length of the animal, but it is tightly coiled like a watch spring. The radula itself consists of chitin, the substance of insects’ wings and lobsters’ shells. The teeth that stud it are arranged in several hundred rows (in another species, the common periwinkle, the teeth total about 3500). A certain amount of wear is involved in scraping the rocks, and when the teeth in current use are worn down, an endless supply of new ones can be rolled up from behind.

And there is wear, also, on the rocks. Over the decades and the centuries, a large population of periwinkles scraping the rocks for food has a pronounced erosive effect, cutting away rock surfaces, grain by grain, deepening the tide pools. In a tide pool observed for sixteen years by a California biologist, periwinkles lowered the floor about three‑eighths of an inch. Rain, frost, and floods–the earth’s major forces of erosion‑operate on approximately such a scale.

The periwinkles grazing on the intertidal rocks, waiting for the return of the tide, are poised also in time, waiting for the moment when they can complete their present phase of evolution and move forward onto the land. All snails that are now terrestrial came of marine ancestry, their forebears having at some time made the transitional crossing of the shore. The periwinkles now are in mid‑passage. In the structure and habits of the three species found on the New England coast, one can see clearly the evolutionary stages by which a marine creature is transformed into a land dweller. The smooth periwinkle, still bound to the sea, can endure only brief exposure. At low tide it remains in wet seaweeds. The common periwinkle often lives where it is submerged only briefly at high tide. It still sheds eggs into the sea and so is not ready for land life. The rough periwinkle, however, has cut most of the ties that confine it to the sea; it is now almost a land animal. By becoming viviparous it has progressed beyond dependence on the sea for reproduction. It is able to thrive at the level of the high water of the spring tides because, unlike the related periwinkles of lower tidal levels, it possesses a gill cavity that is well supplied with blood vessels and functions almost as a lung to breathe oxygen from the air. Constant submersion is, in fact, fatal to it and at the present stage of its evolution it can endure up to thirty‑one days of exposure to dry air.

The rough periwinkle has been found by a French experimenter to have the rhythm of the tides deeply impressed upon its behavior patterns, so that it “remembers” even when no longer exposed to the alternating rise and fall of the water. It is most active during the fortnightly visits of the spring tides to its rocks, but in the waterless intervals it becomes progressively more sluggish and its tissues undergo a certain desiccation. With the return of the spring tides the cycle is reversed. When taken into a laboratory the snails for many months reflect in their behavior the advance and retreat of the sea over their native shores.

On this exposed New England coast the most conspicuous animals of the high‑tide zone are the rock or acorn barnacles, which are able to live in all but the most tumultuous surf. The rockweeds here are so stunted by wave action that they offer no competition, and so the barnacles have taken over the upper shore, except for such space as the mussels have been able to hold.

At low tide the barnacle‑covered rocks seem a mineral landscape carved and sculptured into millions of little sharply pointed cones. There is no movement, no sign or suggestion of life. The stony shells, like those of mollusks, are calcareous and are secreted by the invisible animals within. Each cone‑shaped shell consists of six neatly fitted plates forming an encircling ring. A covering door of four plates closes to protect the barnacle from drying when the tide has ebbed, or swings open to allow it to feed. The first ripples of incoming tide bring the petrified fields to life. Then, if one stands ankle‑deep in water and observes closely, one sees tiny shadows flickering everywhere over the submerged rocks. Over each individual cone, a feathered plume is regularly thrust out and drawn back within the slightly opened portals of the central door–the rhythmic motions by which the barnacle sweeps in diatoms and other microscopic life of the returning sea.

The creature inside each shell is something like a small pinkish shrimp that lies head downward, firmly cemented to the base of this chamber it cannot leave. Only the appendages are ever exposed–six pairs of branched, slender wands, jointed and set with bristles. Acting together, they form a net of great efficiency.

The barnacle belongs to the group of arthropods known as the Crustacea, a varied horde including the lobsters, crabs, sand hoppers, brine shrimps, and water fleas. The barnacle is different from all related forms, however, in its fixed and sedentary existence. When and how it assumed such a way of life is one of the riddles of zoology, the transitional forms having been lost somewhere in the mists of the past. Some faint suggestions of a similar manner of life–the waiting in a fixed place for the sea to bring food–are found among the amphipods, another group of crustaceans. Some of these spin little webs or cocoons of natural silk and seaweed fibers; though remaining free to come and go they spend much of their time within them, taking their food from the currents. Another amphipod, a Pacific coast species, burrows into colonies of the tunicate called the sea pork, hollowing out for itself a chamber in the tough, translucent substance of its host. Lying in this excavation, it draws currents of sea water over its body and extracts the food.

However the barnacle became what it is, its larval stages clearly proclaim its crustacean ancestry, although early zoologists who looked at its hard shells labeled it a mollusk. The eggs develop inside the parent’s shell and presently hatch into the sea in milky clouds of larvae. (The British zoologist Hilary Moore, after studying barnacles on the Isle of Man, estimated a yearly production of a million million larvae from a little over half a mile of shore.) Larval life lasts about three months in the rock barnacle, with several molts and transformations of form. At first the larva, a little swimming creature called a nauplius, is indistinguishable from the larva of all other crustaceans. It is nourished by large globules of fat that not only feed it but keep it near the surface. As the fat globules dwindle, the larva begins to swim at lower water levels. Eventually it changes shape, acquires a pair of shells, six pairs of swimming legs, and a pair of antennae tipped with suckers. This “cypris” larva looks much like the adults of another group of crustaceans, the ostracods. Finally, guided by instinct to yield to gravity and to avoid light, it descends to the bottom ready to become an adult.

No one knows how many of the baby barnacles riding shoreward on the waves make a safe landing, how many fail in the quest for a clean, hard substratum. The settling down of a barnacle larva is not a haphazard process, but is performed only after a period of seeming deliberation. Biologists who have observed the act in the laboratory say the larvae “walk” about on the substratum for as long as an hour, pulling themselves along by the adhesive tips of the antennae, testing and rejecting many possible sites before they make a final choice. In nature they probably drift along in the currents for many days, coming down, examining the bottom at hand, then drifting on to another.

What are the conditions this infant creature requires? Probably it finds rock surfaces that are rough and pitted better than very smooth ones; probably it is repelled by a slimy film of microscopic plants, or even sometimes by the presence of hydroids or large algae. There is some reason to believe it may be drawn to existing colonies of barnacles perhaps through mysterious chemical attraction, detecting substances released by the adults and following these paths to the colony. Somehow, suddenly and irrevocably, the choice is made and the young barnacle cements itself to the chosen surface. Its tissues undergo a complete and drastic reorganization comparable to the metamorphosis of the larval butterfly. Then from an almost shapeless mass, the rudiments of the shell appear, the head and appendages are molded, and within twelve hours the complete cone of the shell, with all its plates delineated, has been formed.

Within its cup of lime the barnacle faces a dual growth problem. As a crustacean enclosed in a chitinous shell, the animal itself must periodically shed its unyielding skin so that its body may enlarge. Difficult as it seems, this feat is successfully accomplished, as I am reminded many times each summer. Almost every container of sea water that I bring up from the shore is flecked with white semitransparent objects, gossamer‑fine, like the discarded garments of some very small fairy creature. Seen under the microscope, every detail of structure is perfectly represented. Evidently the barnacle accomplishes its withdrawal from the old skin with incredible neatness and thoroughness. In the little cellophane‑like replicas I can count the joints of the appendages; even the bristles, growing at the bases of the joints, seem to have been slipped intact out of their casings.

The second problem is that of enlarging the hard cone to accommodate the growing body. Just how this is done no one seems to be sure, but probably there is some chemical secretion to dissolve the inner layers of the shell as new material is added on the outside.

Unless its life is prematurely ended by an enemy, a rock barnacle is likely to live about three years in the middle and lower tidal zones, or five years near the upper tidal levels. It can withstand high temperatures as rocks absorb the heat of the summer sun. Winter cold in itself is not harmful, but grinding ice may scrape the rocks clean. The pounding of the surf is part of the normal life of a barnacle; the sea is not its enemy.