Not all sands are inhabited by this “interstitial fauna.” Those derived from the weathering of crystalline rocks are most abundantly populated. Shell or coral sand seldom if ever contains copepods and other microscopic life; perhaps this indicates that the grains of calcium carbonate create unfavorably alkaline conditions in the water around them.
On any beach the sum of all the little pools amid the sand grains represents the amount of water available to the animals of the sands during the low-tide interval. Sand of average fineness is able to contain almost its own volume of water, and so at low tide only the topmost layers dry out under a warm sun. Below it is damp and cool, for the contained water keeps the temperatures of the deeper sand practically constant. Even the salinity is fairly stable; only the most superficial layers are affected by rain falling on the beach or by streams of fresh water coursing across it.
Bearing on its surface only the wave-carved ripple marks, the fine traceries of sand grains dropped at last by the spent waves, and the scattered shells of long-dead mollusks, the beach has a lifeless look, as though not only uninhabited but indeed uninhabitable. In the sands almost all is hidden. The only clues to the inhabitants of most beaches are found in winding tracks, in slight movements disturbing the upper layers, or in barely protruding tubes and all but concealed openings leading down to hidden burrows.
The signs of living creatures are often visible, if not the animals themselves, in deep gullies that cut the beaches, parallel to the shore line, and hold at least a few inches of water from the fall of one tide until the return of the next. A little moving hill of sand may yield a moon snail intent on predatory errand. A V-shaped track may indicate the presence of a burrowing clam, a sea mouse, a heart urchin. A flat ribbonlike track may lead to a buried sand dollar or a starfish. And wherever protected flats of sand or sandy mud lie exposed between the tides, they are apt to be riddled with hundreds of holes, marked by the sign of the ghost shrimps within. Other flats may bristle with forests of protruding tubes, pencil thin and decorated weirdly with bits of shell or seaweed, an indication that legions of the plumed worm, Diopatra, live below. Or again there may be a wide area marked by the black conical mounds of the lugworm. Or here at the edge of the tide a chain of little parchment capsules, one end free and the other disappearing under the sand, shows that one of the large predatory whelks lies below, busy with the prolonged task of laying and protecting her eggs.
But almost always the essence of the lives—the finding of food, the hiding from enemies, the capturing of prey, the producing of young, all that makes up the living and dying and perpetuating of this sand-beach fauna—is concealed from the eyes of those who merely glance at the surface of the sands and declare them barren.
I remember a chill December morning on one of Florida’s Ten Thousand Islands, with the sands wet from a recently fallen tide and the fresh, clean wind blowing handfuls of spindrift along the beach. For several hundred yards, where the shore ran in a long curve from the Gulf toward the shelter of the bay, there were peculiar markings on the dark wet sand just above the water’s edge. The marks were arranged in groups, in each of which a series of thin spidery lines radiated out from a central spot, as though unsteadily traced there by a slender stick. At first no sign of any living animal was to be seen—nothing to tell what creature had made these seemingly careless scribblings. After kneeling on the wet sand and looking at one after another of these strange insignia, I found that under each of the central spots lay the flat pentagonal disc of a serpent starfish. The marks on the sand were made by its long and slender arms, inscribing the record of its forward progress.
And then I remember wading on a June day over Bird Shoal, which lies off the town of Beaufort in North Carolina, where at low tide acres of sand bottom are covered only by a few inches of water. Near the shore I found two sharply defined grooves in the sand; my index finger could have measured their span. Between the grooves was a faint, irregular line. Step by step, I was led out across the flat by the tracks; finally, at the temporary end of the trail, I came upon a young horseshoe crab, heading seaward.
For most of the fauna of the sand beaches, the key to survival is to burrow into the wet sand, and to possess means of feeding, breathing and reproducing while lying below reach of the surf. And so the story of the sand is in part the story of small lives lived deep within it, finding in its dark, damp coolness a retreat from fish that come hunting with the tide and from birds that forage at the water’s edge when the tide has fallen. Once below the surface layers, the burrower has found not only stable conditions but also a refuge where few enemies threaten. Those few are likely to reach down from above—perhaps a bird thrusting a long bill into the hole of a fiddler crab—a sting ray flapping along the bottom, plowing up the sand for buried mollusks—an octopus sliding an exploring tentacle down into a hole. Only an occasional enemy comes through the sand. The moon snail is a predator that makes a successful living in this difficult way. It is a blind creature with no use for eyes because it is forever groping through dark sands, hunting mollusks that live as much as a foot below the surface. Its smoothly rounded shell eases its descent into the sand as it digs with the immense foot. On locating prey, it holds the animal with the foot and drills a round hole in the shell. The moon snails are voracious; young animals eat more than a third of their weight in clams each week. Some worms also are predatory burrowers; so are a few starfish. But for most predators, continuous burrowing consumes more energy than would be supplied by the prey thus found. Most of the burrowers in sand are passive feeders, digging only enough to establish a temporary or permanent home in which to lie while straining food from the water or sucking up detritus that accumulates on the sea bottom.
The rising tide sets in action a system of living filters through which prodigious quantities of water are strained. Buried mollusks push up their siphons through the sand to draw the incoming water through their bodies. Worms lying in U-shaped parchment tubes begin to pump, drawing the water in through one end of the tube, expelling it through the other. The incoming stream brings food and oxygen; the outgoing has been depleted of much of the food and bears away the organic wastes of the worm. Small crabs spread the feathery nets of their antennae like cast-nets to bring in food.
With the tide, predators come from offshore. A blue crab dashes out of the surf to seize a fat mole crab that is in the act of spreading its antennae to filter the backwash of a receding wave. Clouds of salt-water minnows move in with the tide, searching for the small amphipods of the upper beach. Launce, or sand eels, dart through the shallow water seeking copepods or fish fry; sometimes the launce are pursued by the shadowy forms of larger fish.
As the tide falls much of this extraordinary activity slackens. There is less eating and being eaten. In the wet sands, however, some animals can continue to eat even after the tide has receded. Lugworms can continue their work of passing sand through their bodies for the sake of the scraps of nutriment they contain. Heart urchins and sand dollars, lying in saturated sand, continue to sort out bits of food. But over most of the sands there is a lull of repletion—of waiting for the turn of the tide.
Although there are many places where, on quieter shores and protected shoals, such richness of life may be found, certain ones live most clearly in my memories. On one of the sea islands of Georgia is a great beach that is visited only by the most gentle surf, although it looks straight across to Africa. Storms usually pass it by, for it lies well inside the long, incurving arc of coast that swings between the Capes of Fear and Canaveral, and the prevailing winds are such that no heavy swells roll in upon it. The texture of the beach itself is unusually firm because of a mixture of mud and clay with the sand; permanent holes and burrows can be dug in it, and the streaming tidal currents carve little ripple marks that remain after the tide goes out, looking like a miniature model of the sea’s waves. These sand ripples hold small food particles dropped by the currents, providing a store to be drawn on by detritus feeders. The slope of the beach is so gentle that, when the tide falls to its lowest ebb, a quarter of a mile of sand is exposed between the high-tide line and the low. But this broad sand flat is not a perfectly even plain, for winding gullies wander across it, like creeks across the land, holding a remnant of water from the last high tide and providing a living place for animals that cannot endure even a temporary withdrawal of the water.
It was in this place that I once found a large “bed” of sea pansies at the very edge of the tide. The day was heavily overcast, a fact that accounted for their being exposed. On sunny days I never saw them there, although undoubtedly they were just under the sand, protecting themselves from the drying rays of the sun.
But the day I saw them the pink and lavender flower faces were lifted so that they were exposed at the surface of the sand, though so slightly that one could easily pass them by unnoticed. Seeing them—even recognizing them for what they were—there was a sense of incongruity in finding what looked so definitely flowerlike here at the edge of the sea.
These flattened, heart-shaped sea pansies, raised on short stems above the sand, are not plants but animals. They belong to the same general group of simple beings as the jellyfish, sea anemones, and corals, but to find their nearest relatives one would have to desert the shore and go down to some deeplying offshore bottom where, as fernlike growths in a strange animal forest, the sea pens thrust long stalks into the soft ooze.