Where the waves roll in from the open Atlantic, with no outlying islands or curving arm of land to break the force of their attack on the beach, the area between the tide lines is a difficult one for living things. It is a world of force and change and constant motion, where even the sand acquires some of the fluidity of water. These exposed beaches have few inhabitants, for only the most specialized creatures can live on sand amid heavy surf.
Animals of open beaches are typically small, always swift-moving. Theirs is a strange way of life. Each wave breaking on the beach is at once their friend and enemy; though it brings food, it threatens to carry them out to sea in its swirling backwash. Only by becoming amazingly proficient in rapid and constant digging can any animal exploit the turbulent surf and shifting sand for the plentiful food supplies brought in by the waves.
One of the successful exploiters is the mole crab, a surf-fisher who uses nets so efficient that they catch even microorganisms adrift in the water. Whole cities of mole crabs live where the waves are breaking, following the flood tide shoreward, retreating toward the sea on the ebb. Several times during the rising of a tide, a whole bed of them will shift its position, digging in again farther up the beach in what is probably a more favorable depth for feeding. In this spectacular mass movement, the sand area suddenly seems to bubble, for in a strangely concerted action, like the flocking of birds or the schooling of fish, the crabs all emerge from the sand as a wave sweeps over them. In the rush of turbulent water they are carried up the beach; then, as the wave's force slackens, they dig into the sand with magical ease, by means of a whirling motion of the tail appendages. With the ebbing of the tide, the crabs return toward the low-water mark, again making the journey in several stages. If by mischance a few linger until the tide has dropped below them, these crabs dig down several inches into the wet sand and wait for the return of the water.
As the name suggests, there is something mole-like in these small crustaceans, with their flattened, pawlike appendages. Their eyes are small and practically useless. Like all others who live within the sands the crabs depend less on sight than on the sense of touch, made wonderfully effective by the presence of many sensory bristles. But without the long, curling, feathery antennae, so efficiently constructed that even small bacteria become entangled in their strands, the mole crab could not survive as a fisher of the surf. In preparing to feed, the crab backs down into the wet sand until only the mouth parts and the antennae are exposed. Although it lies facing the ocean, it makes no attempt to take food from the incoming surf. Rather, it waits until a wave has spent its force on the beach and the backwash is draining seaward. When the spent wave has thinned to a depth of an inch or two, the mole crab extends its antennae into the streaming current. After "fishing" for a moment, it draws the antennae through the appendages surrounding its mouth, picking off the captured food. And again in this activity there is a curious display of group behavior, for when one crab thrusts up its antennae, all the others of the colony promptly follow its example.
It is an extraordinary thing to watch the sand come to life if one happens to be wading where there is a large colony of the crabs. One moment it may seem uninhabited. Then, in that fleeting instant when the water of a receding wave flows seaward like a thin stream of liquid glass, there are suddenly hundreds of little gnome-like faces peering through the sandy floor—beady-eyed, long-whiskered faces set in bodies so nearly the color of their background that they can barely be seen. And when, almost instantly, the faces fade back into invisibility, as though a host of strange little troglodytes had momentarily looked out through the curtains of their hidden world and as abruptly retired within it, the illusion is strong that one has seen nothing except in imagination—that there was merely an apparition induced by the magical quality of this world of shifting sand and foaming water.
Since their food-gathering activities keep them in the edge of the surf, mole crabs are exposed to enemies from both land and water—birds that probe in the wet sand, fish that swim in with the tide, feeding in the rising water, blue crabs darting out of the surf to seize them. So the mole crabs function in the sea's economy as an important link between the microscopic food of the waters and the large, carnivorous predators.
Even though the individual mole crab may escape the larger creatures that hunt the tide lines, the span of life is short, comprising a summer, a winter, and a summer. The crab begins life as a minute larva hatched from an orange-colored egg that has been carried for months by the mother crab, one of a mass firmly attached beneath her body. As the time for hatching nears, the mother foregoes the feeding movements up and down the beach with the other crabs and remains near the zone of the low tide, so avoiding the danger of stranding her offspring on the sands of the upper beach.
When it escapes from the protective capsule of the egg, the young larva is transparent, large-headed, and large-eyed as are all crustacean young, weirdly adorned with spines. It is a creature of the plankton, knowing nothing of life in the sands. As it grows it molts, shedding the vestments of its larval life. So it reaches a stage in which, although still swimming in larval fashion with waving motions of its bristled legs, it now seeks the bottom in the turbulent surf zone, where the waves stir and loosen the sand. Toward the summer's end there is another molt, this time bringing transformation to the adult stage, with the feeding behavior of the adult crabs.
During the protracted period of larval life, many of the young mole crabs have made long coastwise journeys in the currents, so that their final coming ashore (if they have survived the voyage) may be far from the parental sands. On the Pacific coast, where strong surface currents flow seaward, Martin Johnson found that great numbers of the crab larvae are carried out over oceanic depths, doomed to certain destruction unless they chance to find their way into a return current. Because of the long larval life, some of the young crabs are carried as far as 200 miles offshore. Perhaps in the prevailing coastwise current of Atlantic shores they travel even farther.
With the coming of winter the mole crabs remain active. In the northern part of their range, where frost bites deep into the sands and ice may form on the beaches, they go out beyond the low-tide zone to pass the cold months where a fathom or more of insulating water lies between them and the wintry air. Spring is the mating season and by July most or all of the males hatched the preceding summer have died. The females carry their egg masses for several months until the young hatch; before winter all of these females have died and only a single generation of the species remains on the beach.
The only other creatures regularly at home between the tide lines of wave-swept Atlantic beaches are the tiny coquina clams. The life of the coquinas is one of extraordinary and almost ceaseless activity. When washed out by the waves, they must dig in again, using the stout, pointed foot as a spade to thrust down for a firm grip, after which the smooth shell is pulled rapidly into the sand. Once firmly entrenched, the clam pushes up its siphons. The intake siphon is about as long as the shell and flares widely at the mouth. Diatoms and other food materials brought in or stirred from the bottom by waves are drawn down into the siphon.
Like the mole crabs, the coquinas shift higher or lower on the beach in mass movements of scores or hundreds of individuals, perhaps to take advantage of the most favorable depth of water. Then the sand flashes with the brightly colored shells as the clams emerge from their holes and let the waves carry them. Sometimes other small burrowers move with the coquinas among the waves—companies of the little screw shell, Terebra, a carnivorous snail that preys on the coquina. Other enemies are sea birds. The ring-billed gulls hunt the clams persistently, treading them out of the sand in shallow water.
On any particular beach, the coquinas are transient inhabitants; they seem to work an area for the food it provides, and then move on. The presence on a beach of thousands of the beautifully variegated shells, shaped like butterflies and crossed by radiating bands of color, may mark only the site of a former colony.
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sp; Being only briefly and sporadically possessed by the sea in those recurrent periods of the tides' farthest advance, the high-tide zone on any shore has in its own nature something of the land as well as of the sea. This intermediate, transitional quality pervades not only the physical world of the upper beach but also its life. Perhaps the ebb and flow of the tides has accustomed some of the intertidal animals, little by little, to living out of water; perhaps this is the reason there are among the inhabitants of this zone some who, at this moment of their history, belong neither to the land nor entirely to the sea.
The ghost crab, pale as the dry sand of the upper beaches it inhabits, seems almost a land animal. Often its deep holes are back where the dunes begin to rise from the beach. Yet it is not an air-breather; it carries with it a bit of the sea in the branchial chamber surrounding its gills, and at intervals must visit the sea to replenish the water. And there is another, almost symbolic return. Each of these crabs began its individual life as a tiny creature of the plankton; after maturity and in the spawning season, each female enters the sea again to liberate her young.
If it were not for these necessities, the lives of the adult crabs would be almost those of true land animals. But at intervals during each day they must go down to the water line to wet their gills, accomplishing their purpose with the least possible contact with the sea. Instead of wading directly into the water, they take up a position a little above the place where, at the moment, most of the waves are breaking on the beach. They stand sideways to the water, gripping the sand with the legs on the landward side. Human bathers know that in any surf an occasional wave will tower higher than the others and run farther up the beach. The crabs wait, as if they also know this, and after such a wave has washed over them, they return to the upper beach.
They are not always wary of contact with the sea. I have a mental picture of one sitting astride a sea-oats stem on a Virginia beach, one stormy October day, busily putting into its mouth food particles that it seemed to be picking off the stem. It munched away, intent on its pleasant occupation, ignoring the great, roaring ocean at its back. Suddenly the foam and froth of a breaking wave rolled over it, hurling the crab from the stem and sending both slithering up the wet beach. And almost any ghost crab, hard pressed by a person trying to catch it, will dash into the surf as though choosing a lesser evil. At such times they do not swim, but walk along on the bottom until their alarm has subsided and they venture out again.
Although on cloudy days and even occasionally in full sunshine the crabs may be abroad in small numbers, they are predominantly hunters of the night beaches. Drawing from the cloak of darkness a courage they lack by day, they swarm boldly over the sand. Sometimes they dig little temporary pits close to the water line, in which they lie watching for what the sea may bring them.
The individual crab in its brief life epitomizes the protracted racial drama, the evolutionary coming-to-land of a sea creature. The larva, like that of the mole crab, is oceanic, becoming a creature of the plankton once it has hatched from the egg that has been incubated and aerated by the mother. As the infant crab drifts in the currents it sheds its cuticle several times to accommodate the increasing size of its body; at each molt it undergoes slight changes of form. Finally the last larval stage, called the megalops, is reached. This is the form in which all the destiny of the race is symbolized, for it—a tiny creature alone in the sea—must obey whatever instinct drives it shoreward, and must make a successful landing on the beach. The long processes of evolution have fitted it to cope with its fate. Its structure is extraordinary when compared with like stages of closely related crabs. Jocelyn Crane, studying these larvae in various species of ghost crabs, found that the cuticle is always thick and heavy, the body rounded. The appendages are grooved and sculptured so that they may be folded down tightly against the body, each fitting precisely against the adjacent ones. In the hazardous act of coming ashore, these structural adaptations protect the young crab against the battering of the surf and the scraping of sand.
Once on the beach, the larva digs a small hole, perhaps as protection from the waves, perhaps as a shelter in which to undergo the molt that will transform it into the shape of the adult. From then on, the life of the young crab is a gradual moving up the beach. When small it digs its burrows in wet sand that will be covered by the rising tide. When perhaps half grown, it digs above the high-tide line; when fully adult it goes well back into the upper beach or even among the dunes, attaining then the farthest point of the landward movement of the race.
On any beach inhabited by ghost crabs, their burrows appear and disappear in a daily and seasonal rhythm related to the habits of the owners. During the night the mouths of the burrows stand open while the crabs are out foraging on the beach. About dawn the crabs return. Whether each goes, as a rule, to the burrow it formerly occupied or merely to any convenient one is uncertain—the habit may vary with locality, the age of the crab, and other changing conditions.
Most of the tunnels are simple shafts running down into the sand at an angle of about forty-five degrees, ending in an enlarged den. Some few have an accessory shaft leading up from the chamber to the surface. This provides an emergency exit to be used if an enemy—perhaps a larger and hostile crab-comes down the main shaft. This second shaft usually runs to the surface almost vertically. It is farther away from the water than the main tunnel, and may or may not break through the surface of the sand.
The early morning hours are spent repairing, enlarging, or improving the burrow selected for the day. A crab hauling up sand from its tunnel always emerges sideways, its load of sand carried like a package under the legs of the functional rear end of the body. Sometimes, immediately on reaching the burrow mouth, it will hurl the sand violently away and flash back into the hole; sometimes it will carry it a little distance away before depositing it. Often the crabs stock their burrows with food and then retire into them; nearly all crabs close the tunnel entrances about midday.
All through the summer the occurrence of holes on the beach follows this diurnal pattern. By autumn most of the crabs have moved up to the dry beach beyond the tide; their holes reach deeper into the sand as though their owners were feeling the chill of October. Then, apparently, the doors of sand are pulled shut, not to be opened again until spring. For the winter beaches show no sign either of the crabs or of their holes—from dime-sized youngsters to full-grown adults, all have disappeared, presumably into the long sleep of hibernation. But, walking the beach on a sunny day in April, one will see here and there an open burrow. And presently a ghost crab in an obviously new and shiny spring coat may appear at its door and very tentatively lean on its elbows in the spring sunshine. If there is a lingering chill in the air, it will soon retire and close its door. But the season has turned, and under all this expanse of upper beach, crabs are awakening from their sleep.
Like the ghost crab, the small amphipod known as the sand hopper or beach flea portrays one of those dramatic moments of evolution, in which a creature abandons an old way of life for a new. Its ancestors were completely marine; its remote descendants, if we read its future aright, will be terrestrial. Now it is midway in the transition from a sea life to a land life.
As in all such transitional existences, there are strange little contradictions and ironies in its way of life. The sand hopper has progressed as far as the upper beach; its predicament is that it is bound to the sea, yet menaced by the very element that gave it life. Apparently it never enters the water voluntarily. It is a poor swimmer and may drown if long submerged. Yet it requires dampness and probably needs the salt in the beach sand, and so it remains in bondage to the water world.
The movements of the sand hoppers follow the rhythm of the tides and the alternation of day and night. On the low tides that fall during the dark hours, they roam far into the intertidal zone in search of food. They gnaw at bits of sea lettuce or eelgrass or kelp, their small bodies swaying with the vigor of their chewing. In the litter of the tide line
s they find morsels of dead fish or crab shells containing remnants of flesh; so the beach is cleaned and the phosphates, nitrates, and other mineral substances are recovered from the dead for use by the living.
If low water has fallen late in the night, the amphipods continue their foraging until shortly before daybreak. Before light has tinged the sky, however, all of the hoppers begin to move up the beach toward the high-water line. There each begins to dig the burrow into which it will retreat from daylight and rising water. As it works rapidly, it passes back the grains of sand from one pair of feet to the next until, with the third pair of thoracic legs, it piles the sand behind it. Now and then the small digger straightens out its body with a snap, so that the accumulated sand is thrown out of the hole. It works furiously at one wall of the tunnel, bracing itself with the fourth and fifth pairs of legs, then turns and begins work on the opposite wall. The creature is small and its legs are seemingly fragile, yet the tunnel may be completed within perhaps ten minutes, and a chamber hollowed out at the end of the shaft. At its maximum depth this shaft represents as prodigious a labor as though a man, working with no tools but his hands, had dug for himself a tunnel about 60 feet deep.
The work of excavation done, the sand flea often returns to the mouth of its burrow to test the security of the entrance door, formed by the accumulation of sand from the deeper parts of the shaft. It may thrust out its long antennae from the mouth of the burrow, feeling the sand, tugging at the grains to draw more of them into the hole. Then it curls up within the dark snug chamber.
As the tide rises overhead, the vibrations of breaking waves and shoreward-pressing tides may come down to the little creature in its burrow, bringing a warning that it must stay within to avoid water and the dangers brought by water. It is less easy to understand what arouses the protective instinct to avoid daylight, with all the dangers of foraging shore birds. There can be little difference between day and night in that deep burrow. Yet in some mysterious way the beach flea is held within the safety of the sandy chamber until the two essential conditions again prevail on the beach—darkness and a falling tide. Then it awakens from sleep, creeps up the long shaft, and pushes away the sand door. Once again the dark beach stretches before it, and a retreating line of white froth at the edge of the tide marks the boundary of its hunting grounds.
The Edge of the Sea Page 15