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.
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.